3.1 Intrusives
3.1.1 Overview
Intrusives in the survey area are mainly formed in the Mesozoic acid to intermediate-acid magmatism, which belongs to the Shunxi–Huzhou tectono-magmatic subbelt, north Zhejiang Province. There are 39 plutons of different sizes in the Anji–Chun’an area and some span the provinces of Zhejiang and Anhui, which are mainly stock and apophysis, followed by bosse and a few batholith. Distribution of the intrusives is closely related to structures, and regional faults and folds axial zone provided space for magmatic emplacement. In addition, the contact zone along the Mesozoic volcanic rock and the Paleozoic sedimentary rock is also a favorable emplacement location. The formation time of intrusives can be roughly divided into the Late Jurassic and Early Cretaceous, the former a total of 13 places accounting for one-third of the total number of plutons, the latter about 26 places accounting for two-thirds of the total number. However, based on the study of modern isotope geochronology, there is no obvious temporal interval between both periods, often in a transitional relationship, indicating that there were frequent magma activities from Late Jurassic to Early Cretaceous in northwestern Zhejiang. Intrusives are mainly distributed in the northern part of the survey area (the Anji–Lin’an belt). The outcroppings are less in the southern part of the survey area (the west Chun’an belt), only a small number of granodiorite plutons, but geophysical exploration data indicate that there may be concealed plutons under the overlying strata. The intrusives in the survey area are mainly granodiorite, monzonitic granite, and syenogranite, etc., and secondly quartz (porphyritic)-syenite and quartz monzonite, etc. Metallogenesis is mainly closely related to granodiorite, monzonitic granite, and syenogranite. The contact zone between plutons and wall rocks often developed alterations such as hornfelsic, silicification, pyritization, skarnization, and marbleization, while alterations such as albitization, potassic, greisenization, and sericitization were usually visible on plutons.
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Distribution Characteristics
Geological characteristics of intrusives in the survey area
Name | Geographical location | Area (km2) | Geological features | Lithological combination | Zircon U-Pb age (Ma) | Lithologic symbol | |
|---|---|---|---|---|---|---|---|
Lithology | Mineral assemblage | ||||||
Ma’anshan | Northeast: 119°27′30″ 30°40′00″ Southwest: 119°25′40″ 30°34′52″ | 83.86 | In the strike of northeast, distributed in Shangshanling–Tongkengcun–Lingxi, its southeast side contacts by the NE50°–60° faults with the Paleozoic strata, the fault occurrence is 300°–310°∠60°–80°; its east side from Longxianshan to Xiayangcun is fine-grained syenogranite and megacrystic porphyritic monzonitic granite, in intrusive contact with the Ordovician strata; the border in the northeast side is not seen in the survey area, and based on previous study, it is in intrusive contact with the Silurian strata, and the contact surface occurrence is 300°–340°∠70°–80°, locally in fault contact, and the contact surface between plutons and strata has hornfelsic alteration | Pegmatitic porphyritic monzonitic granite | Quartz 35%, K-feldspar 50%, plagioclase 15%, a few biotite, and hornblende, etc., apatite and zircon occasionally seen; minerals grain size was generally <2 mm; quartz is in the anhedral granular shape, feldspar in the shape of block and strip, quartz and feldspar are distributed alternately in equal size | 132.2 ± 1.6 SHRIMP | ηγ (W)K12 |
Phenocryst-porphyritic monzonitic granite | Phenocryst: plagioclase 5–8%, K-feldspar 2–7%, quartz 2%, with grain size of 1–3 cm; matrix: quartz 25%, K-feldspar 20%, plagioclase 20%, biotite 10–20%, a handful of hornblende and metallic minerals, with grain size of 2–5 mm in general, and >5 mm for a handful | 127.7 ± 1.2 SHRIMP | ηγ (G)K12 | ||||
Fine-grained syenogranite | Phenocryst: plagioclase 5–10%, K-feldspar 2–4%, and a handful of quartz 3–5%, generally the grain size >3 cm; matrix: quartz 25%, K-feldspar 20%, plagioclase 20%, biotite 10–20%, a handful of hornblende 1–2%, and the grain size is 3–8 mm | 128.3 ± 1.1 SHRIMP | γ (x) K12 | ||||
Tangshe | North: 119°19′20″ 30°32′09″ East: 119°21′10″ 30°31′02″ Southwest: 119°15′57″ 30°29′34″ | 18.90 | In the strike of northeast, intruded in the anticline core. The north part of the medium-coarse-grained syenogranite in Bijia Mount–Tali Mount is in intrusive contact with the Nanhua–Sinian strata, the contact zone has developed strong skarnization and hornfelsic, with the dip angle of 60°–80°; medium-grained monzonitic granite in Nanshan–Tangshe, in its south is in intrusive contact with the Nanhua–Cambrian strata, the contact zone has developed hornfelsic, or locally strong skarnization; in the southeast side of main pluton, there are developed small stocks such as medium-grained syenogranite, fine-grained syenogranite, and fine-grained monzonitic granite | Medium-grained monzonitic granite | Quartz 20%, K-Na-feldspar 30%, plagioclase 35%, biotite 5–10%, hornblende 5%, a handful of pyrite, and magnetite, etc., zircon and apatite visible occasionally; minerals have smaller grain sizes, generally 2–3 mm or few 0.5–2 mm | 140.9 ± 3.4 LA-ICP-MS | ηγ (z) K11 |
Medium-coarse-grained syenogranite | K-feldspar 40%, plagioclase 15%, quartz 30%, biotite 5–10%, hornblende 3%, a handful of metallic minerals, and zircon occasionally seen; minerals have a bigger grain size, generally 3–9 mm, up to 1–2 cm for some K-feldspar and >3 cm for a handful of them | 132.2 ± 1.6 LA-ICP-MS | γ (c) K12 | ||||
Fine-grained syenogranite | Quartz 30%, K–Na-feldspar 45%, plagioclase 20%, biotite 5%, a handful of metallic minerals, zircon, and apatite occasionally seen; general grain size is 0.5–2 mm, and a handful of grain size up to 2–3 mm | 125.0 ± 2.0 LA-ICP-MS | γ (x) K12 | ||||
Tonglizhuang | North: 119°29′44″ 30°30′40″ South: 119°28′56″ 30°29′57″ | 1.54 | It partially outcrops near Baofu Town in the survey area. Fine-grained syenogranite at the north side of the pluton is in intrusive contact with the Cambrian–Ordovician strata, the contact surface occurrence is 300°–330°∠60°–80° and it has developed hornfelsic alteration. In the south side of the pluton at Shimendong, medium-coarse-grained porphyritic quartz syenite is in intrusive contact with the first member of the Huangjian Formation volcanic strata. The medium-grained monzonitic granite in the southeast side also intrude into the first member of the Early Cretaceous Huangjian Formation, with the intrusive contact surface occurrence of 320°∠70° | Coarse-medium-grained porphyritic quartz syenite | Phenocryst: K–Na-feldspar 25%, plagioclase 5%, and dark-colored minerals 2%, minerals have bigger grain sizes, generally over 2 mm, even up to 6–7 mm individually; a handful of plagioclase phenocryst has eroded into clay minerals. Matrix: feldspar 48% and quartz 12%, with grain size below 0.5 mm in general | 126.0 ± 3.0 LA-ICP-MS | ξo (z) K13 |
Fine-grained syenogranite | K-feldspar 40–50%, plagioclase 20–25%, quartz 20–30%, and a handful of biotite 5%, with grain size of 0.2–1 mm; K-feldspar is mainly orthoclase, wide plate euhedral crystal, Carlsbad twin visible in some of them; plagioclase is in the shape of subhedral wide plate and plate-column, albite bicrystal visible in some of them | About 130 | γ (x) K12 | ||||
Medium-grained monzonitic granite | Quartz 25%, K–Na-feldspar 30%, plagioclase 30%, biotite 5–10%, hornblende 5%, zircon and apatite occasionally seen; grain size is 2–3 mm and a handful of 1–2 mm | 142.3 ± 1.8 LA-ICP-MS | ηγ (z) K11 | ||||
Zhinan Mount | 119°34′22″ 30°22′10″ | 0.7 | The NW-strike small apophysis intruded in the rhyolitic tuff lava of the second member of the Huangjian Formation, about 1 km long and 700 m wide | Medium-grained quartz diorite | Quartz 22%, K–Na-feldspar 5%, plagioclase 50%, hornblende 20%; minerals’ grain size is generally 0.4–3 mm. Quartz is granular and distributed pretty evenly; plagioclase is plate-column, most weathered, based on the refractive index it is mainly andesine—oligoclase; hornblende is column | 130.5 ± 1.7 LA-ICP-MS | δο (z) K13 |
Xianxia | Northeast: 119°25′43″ 30°29′20″ Southwest: 119°15′05″ 30°20′18″ | 84.18 | In the NE-strike, it spreads in Xianxia–Zhangcun, the southeast side of the pluton is mainly in NE30°–40° fault contact with the Paleozoic strata, with the occurrence of 320°–330°∠60°–80°, and locally intruded into the Cambrian–Ordovician strata; its northwest side is in intrusive contact with the Nanhua–Ordovician strata, with the contact surface occurrence of 310°–350°∠60°–90° and fault contact exists locally | Medium-coarse-grained quartz syenite | K-Na-feldspar 85%, plagioclase 10%, quartz 5%, biotite 5–10%; grain size is 3–10 mm, and a handful of minerals’ grain size is up to 1 cm; quartz is granular; feldspar is mostly plate and short strip, locally is arranged directionally, and individual plagioclase 1–2 cm | About 130 Ma | ξo (c) K13 |
Fine-grained syenogranite | Quartz 30%, K-feldspar 55%, plagioclase 10%, biotite 5%; grain size 0.5–2 mm | 132.4 ± 2.4 LA-ICP-MS | γ (x) K12 | ||||
Medium-coarse-grained and coarse-medium-grained syenogranite | Quartz 30%, K-feldspar 45%, plagioclase 20%, biotite (5%); grain size generally 5–10 mm for internal-facies minerals and up to 2 cm for a few, and locally it becomes medium-coarse-grained syenogranite, the grain size of margin minerals is 3–6 mm | 132.9 ± 3.3 LA-ICP-MS | γ (c) /(z) K12 | ||||
Phenocryst and medium-coarse-grained porphyritic monzonitic granite | Phenocryst: feldspar and quartz, in the content of 10–15%, grain size up to 1–3 cm; Matrix: quartz 20%, K–Na-feldspar 20%, plagioclase 30%, biotite 5–10%, hornblende 5% the grain size is 2–5 mm, occasionally 5–8 mm | 144.2 ± 1.0 SHRIMP | ηγ (G)/(c) K11 | ||||
Medium- and fine-grained (porphyritic) monzonitic granite | Quartz 15%, K-feldspar 30%, plagioclase 40%, biotite 5–10%, hornblende 5%; minerals’ grain size is generally 2–5 mm, a handful of plagioclase is bigger in grain size, up to about 1 cm and dark-colored minerals increased in the margin of the pluton | 145.1 ± 1.2 SHRIMP | ηγ (z) K11 | ||||
Wushanguan | Northwest: 119°38′04″ 30°29′59″ Southwest: 119°36′38″ 30°25′03″ Southeast: 119°43′18″ 30°24′29″ | 64.00 | It spreads in the NW-strike, its west side is mainly in fault contact with the Huangjian Formation volcanic strata, the fault’s strike is NW 320°–330° and NE 40°, it dips toward southwest and northwest, its dip angle varies between 65°–80°; locally in intrusive contact with volcanic strata (or the Cambrian Yangliugang Formation). Its northeast and south sides are in intrusive contact with the Huangjian Formation volcanic strata; its southeast side is in intrusive contact with the Cambrian strata, the intrusive contact surface is not constant in attitude and dip, with dip angle of 60°–90°, and hornfelsic and skarnization are often seen near the intrusive contact zone | Coarse-medium-grained syenite | K–Na-feldspar 85%, plagioclase 10%, quartz 5%, biotite 5–10%; the grain size is 3–8 mm, but up to 1 cm for a handful of minerals; quartz is granular; feldspar is mostly plate and locally short strip, it is arranged directionally, and individual plagioclase 1–2 cm | About 130 | ξ (z) K13 |
(Porphyritic) fine-grained syenogranite | K-feldspar 60–65%,quartz 25–35%, plagioclase 5–10%; the grain size is mainly fine, about 0.5–1.5 mm | 128.1 ± 0.82 SHRIMP | γ (x) K12 | ||||
Phenocryst-porphyritic monzonitic granite | Phenocryst: feldspar and quartz, in the content of 10–15%, grain size up to 1–3 cm; matrix: quartz 20%, K–Na-feldspar 20%, plagioclase 30%, biotite 5–10%, hornblende 5%, the grain size is generally 2–5 mm, occasionally 5–8 mm | About 130 | ηγ (G) K12 | ||||
Medium-coarse- and medium-grained syenogranite | K-feldspar 45–55%, plagioclase 15–25%, quartz 25–35%, biotite 1%; the grain size is 2–7 mm with few 7–10 mm, and locally, K-feldspar is approximately porphyritic (1–2 cm × 1–1.5 cm); from southeast to northwest, it evolves from medium-coarse-grained to coarse-medium-grained | 132.0 ± 3.0 LA-ICP-MS | γ (z) K12 | ||||
Medium-grained monzonitic granite | Quartz 20–30%, K-feldspar 15–30%, plagioclase 30–40%, biotite 2–6%, very few zircon and apatite; the grain size is 0.5–4 mm. Inside the lithology there are developed mafic micro-granular enclaves (MMEs) such as biotite or biotite, plagioclase, in the ellipse, long strip and irregular shapes, in the sized of 1–30 cm | 136.3 ± 1.2 SHRIMP | ηγ (z) K11 | ||||
Dongkeng | 119°35′34″ 30°24′02″ | 1.2 | Ellipse-like small apophysis in NEE strike, intruded in the contact zone between the second–third member of the Huangjian Formation, from outside to inside they are coarse-medium-grained quartz monzonite, medium-coarse-grained syenogranite, and fine-grained syenogranite | Fine-grained (porphyritic) syenogranite | Phenocryst: K–Na-feldspar 10%, plagioclase 4%; the grain size is 0.1–1 mm and 0.5–2 mm, but 2–6 mm for very few of it; matrix: K–Na-feldspar 46%, plagioclase 8%, quartz 30%, a handful of metallic minerals, and the grain size is 0.1–1 mm | 127.6 ± 1.2 LA-ICP-MS | γ (x) K13 |
Medium-coarse-grained syenogranite | Quartz 40%, K–Na-feldspar 54%, plagioclase 6%, a handful of biotite; the grain size is 3–8 mm, and 8–12 mm for a handful | 127.9 ± 1.3 LA-ICP-MS | γ (c) K13 | ||||
Fine-medium-grained porphyritic quartz monzonite | Phenocryst: quartz (very few), plagioclase 20%, K–Na-feldspar 8%, hornblende 4%; the grain size is generally 1–3 mm, few 3–4 mm; matrix: quartz 12%, K–Na-feldspar 35%, plagioclase 20%, and the grain size is generally 0.2–0.5 mm | About 130 | ηο (z) K13 | ||||
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Rock Classification
3.1.2 Main Plutons
3.1.2.1 Ma’anshan Composite Pluton
- 1.
Geological Features
Regional geological sketch map of Ma’anshan composite pluton. 1. Pegmatitic porphyritic monzonitic granite in second phase, Early Cretaceous; 2. Early Cretaceous Period-2 Phenocryst–porphyritic monzonitic granite in second phase, Early Cretaceous; 3. Early Cretaceous Period-2 fine-grained syenogranite in second phase, Early Cretaceous; 4. Granite porphyry vein; 5. Cambrian Yangliugang Formation; 6. Cambrian Huayansi Formation; 7. Cambrian–Ordovician Xiyangshan Formation; 8. Ordovician Ningguo Formation–Huangnigang Formation; 9. Ordovician Changwu Formation; 10. Ordovician Wenchang Formation; 11. Geological boundary; 12. Fault; 13. Lithofacies boundary; 14. Sampling location
- 2.
Petrological Features
(Pegmatitic and megacrystic) porphyritic monzonitic granite
Field pictures and petrographical microscopic pictures of Ma’anshan composite pluton. a K-feldspar phenocryst in pegmatitic porphyritic monzonitic granite; b dioritic enclaves developed in pegmatitic porphyritic monzonitic granite; c biotite concentrates in strips at the margin of phenocryst-porphyritic monzonitic granite and fine-grained syenogranite; d fine-grained syenogranite; e biotite (Bt), plagioclase (Pl) and quartz (Qtz) in phenocryst-porphyritic monzonitic granite; f plagioclase (Pl) containing K-feldspar (Kf) core grown in phenocryst-porphyritic monzonitic granite; g biotite (Bt) gets crystallized at the contact edge of quartz (Qtz), plagioclase (Pl) and K-feldspar (Kf) in pegmatitic porphyritic monzonitic granite; h plagioclase (Pl) wrapping K-feldspar (Hbl) in pegmatitic porphyritic monzonitic granite; i quartz (Qtz), K-feldspar (Kf), and hornblende (Hbl) in fine-grained syenogranite
Fine-grained syenogranite
Features of internal-facies zones of megacrystic porphyritic monzonitic granite in Ma’anshao composite pluton
- 3.
Wall-rock Alteration
Fine-grained syenogranite is weakly weathered, a handful of feldspar probably developed epidotization along cracks, and the contact zone developed joints structure and silicified quartz vein. The Silurian sandstone strata in the southeast side of the pluton mainly have developed hornfelsic alteration. Since the contact surface’s dip angle is gentle, the hornfelsic zone is generally 1–3 km wide, the inner zone is about 1.5 km wide and mainly contains dark purple gray hornfels, and the outer zone is about 1.2 km wide and mainly contains dark purple gray and hornfelsic siltstone. The Ordovician calcareous siltstone around the Yonghe Forest Farm in its southeast which is interbedded with micro-crystal limestone, and the strata there have developed silicification, poorly skarnization, and marbleization.
- 4.
Geochemical Features
Harker diagram of SiO2 in Ma’anshan composite pluton (its legends are the same as those in Fig. 3.5)
A/CNK-A/NK diagram (a) and SiO2–K2O diagram (b) for Ma’anshan composite pluton
Chondrite-normalized REE distribution mode (a) and primitive mantle-normalized trace element spider diagram (b) for Ma’anshan composite pluton (the normalized values of chondrite and primitive mantle come from Sun and McDonough 1989)
Pegmatitic porphyritic monzonitic granite (86Sr/87Sr)i is 0.70825, εNd(t) is −6.77 and the two-stage model ages (TDM2) is 1.47 Ga; fine-grained syenogranite (86Sr/87Sr)i is 0.69215, εNd(t) is −6.59, and TDM2 is 1.46 Ga; both have similar features, indicating their same magma evolution.
- 5.
Isotope geochronology
Main zircon cathodoluminescence (CL) imaging and dating spots diagram of samples from Ma’anshan composite pluton
Zircon U–Pb concordant diagrams for Ma’anshan composite pluton
Based on the contact relationships, geochronological and geochemical features, Ma’anshan composite pluton experienced two stages of magmatism, the early stage is pegmatitic and megacrystic porphyritic monzonitic granite (132.2 ± 1.6 Ma—127.7 ± 1.2 Ma) and the later stage is fine-grained syenogranite (128.3 ± 1.1 Ma).
3.1.2.2 Tangshe Composite Pluton
- 1.
Geological Features
Regional geological sketch map of Tangshe composite pluton. 1. medium-grained monzonitic granite; 2. medium-coarse-grained syenogranite; 3. medium-fine-grained syenogranite; 4. the Xiuning Formation; 5. the Nantuo Formation; 6. the Piyuancun Formation; 7. the first member of Lantian Formation; 8. the second member of Lantian Formation; 9. the third member of Lantian Formation; 10. the fourth member of Lantian Formation; 11. the first member of Hetong Formation; 12. veins; 13. sampling location; 14. faults
- 2.
Petrological Features
Medium-grained monzonitic granite
Outcrop and microscopic petrographical pictures of Tangshe composite pluton. a medium-grained monzonitic granite; b medium-coarse-grained syenogranite; c fine-grained syenogranite; d plagioclase (Pl), biotite (Bt), and quartz (Qtz) in medium-grained monzonitic granite; e K-feldspar (Kf), plagioclase (Pl) showing polysynthetic twin, quartz (Qtz), and biotite (Bt) in coarse-grained syenogranite; f plagioclase (Pl) showing polysynthetic twin, K-feldspar (Kf), and quartz (Qtz) in fine-grained syenogranite
Medium-coarse-grained syenogranite
Fine-medium-grained monzonitic granite
- 3.
Wall-rock Alteration
- 4.
Geochemical Features
SiO2–Na2O + K2O diagram for Tangshe composite pluton (its legends are the same as those in Fig. 3.12)
A/CNK-A/NK diagram (a) and SiO2-K2O diagram (b) for Tangshe composite pluton
Chondrite-normalized REE distribution mode (a) and primitive mantle-normalized trace-element spider diagram (b) for Tangshe composite pluton (the normalized values of chondrite and primitive mantle come from Sun and McDonough 1989)
Medium-coarse-grained and coarse-medium-grained monzonitic granite is high in content of ∑REE (179.39 × 10−6–229.56 × 10−6), and the chondrite-normalized REE patterns (Fig. 3.13) show a feature of weakly dipping toward right, LREE and HREE differentiate pretty obvious, LaN/YbN is 5.92–16.45 and δEu is 0.22–0.32, Eu showing strong negative anomaly. Similarly, rocks are enriched in K, Th, U, and Rb, weakly depleted in LILE such as Ba and Sr; weakly depleted in HFSE such as P, Nb, Ta, and Ti. (86Sr/87Sr)i value is 0.703,60, εNd(t) value is −5.11 and TDM2 is 1.34 Ga.
Fine-grained syenogranite is high in content of ∑REE (214.70 × 10−6), and the chondrite-normalized REE patterns (Fig. 3.13) show a feature of “V” shape, light, and heavy rare earth differentiates unobviously, LaN/YbN is 6.24 and δEu is 0.10, Eu showing strongly negative anomaly. Similarly, the rocks are also enriched in LILE such as K, Th, U, and Rb, strongly depleted in LILE such as Ba and Sr; in terms of HFSE, strongly depleted in P and Ti, weakly depleted in Nb and Ta.
- 5.
Isotope geochronology
CL images, analysis location ,and U–Pb age of some zircon points at Tangshe composite pluton
Zircon U–Pb concordant diagrams for Tangshe composite pluton
Zircon in medium-coarse and coarse-medium-grained syenogranite (D0004 and D0033) is basically in consistent form, euhedral–subhedral platy, the grain size is 80–150 μm, its length-width ratio is 2:1–1:1, and almost all zircon have developed magmatic oscillatory zoning. There are totally 14 dating points for D0004. The ages from point 6 which is rather large and point 13 which is rather small are removed. The 206Pb/238U weighted mean age of the remaining 12 points is 131.7 ± 3.2 Ma (MSWD = 2.2), representing the crystallization age of medium-coarse-grained syenogranite. There are totally 20 dating points for D0033, and the 206Pb/238U weighted mean age is 132.2 ± 1.6 Ma (MSWD = 1.19), representing the crystallization age of medium-grained syenogranite. Th/U values of 12 zircons in sample D0004 and 20 zircon in sample D0033 are 0.37–0.76 and 0.30–0.83, respectively, featuring magmatic orgin.
In fine-grained syenogranite (D0035), the zircon grain size is 60–120 μm, its length-width ratio is 2:1–1:1, euhedral–subhedral, and zircon has well-developed magmatic oscillatory zoning, and the phenomenon of decrystallization may be seen in a few zircons. There are totally 18 dating points for D0035, of which the 206Pb/238U age of point 15 is 113 ± 3 Ma, greatly smaller than those of other dating points, which may represent a thermal event at the end of crystallization stage. The age data of point 17 are problematic, possibly resulting from a mistake during the experimental operation. After the ages of point 15 and 17 are removed, the 206Pb/238U weighted mean age of the remaining 16 points is 132.2 ± 2.0 Ma (MSWD = 2.0), representing the crystallization age of fine-grained syenogranite. Th/U value of 16 dating zircons in sample D0035 is 0.55, and in combination with the features of CL image, it is considered that the zircon in fine-grained granite is magmatic orgin.
In conclusion, Tangshe composite pluton has conspicuously experienced three stages of magmatism, the early stage is medium-grained monzonitic granite (140.4 ± 3.3 Ma), the middle stage is medium-coarse-grained and coarse-medium-grained syenogranite (132.2 ± 1.6 Ma–131.7 ± 3.2 Ma), and the later stage is fine-grained syenogranite (125.0 ± 2.0 Ma).
3.1.2.3 Xianxia Composite Pluton
- 1.
Geological Features
Regional geological sketch map of Xianxia composite pluton
- 2.
Petrological Features
Lithological composition and geological features of the Xianxia composite pluton
Lithology | Contact relationship | Mineral assemblage | Fabric feature |
|---|---|---|---|
Medium- grained monzonitic granite | The periphery of southwest part and the central top cap is in intrusive contact with wall-rock strata (345°∠20°), and locally fault contact. | Plagioclase (35–40%), K-feldspar (25–35%), quartz (20–25%), biotite (5–10%), a handful of magnetite, apatite, and zircon, etc. | Light gray, fine-medium-grained subhedral granular texture, generally 1–3 mm in size, occasionally 3–6 mm, locally seen biotite-bearing MMEs and megacrystic plagioclase, and plagioclase has developed zoning texture |
Phenocryst-porphyritic monzonitic granite | The central part of its southwest section is in gushing intrusive contact with medium-grained monzonitic granite at the margin. | Phenocryst: plagioclase (3–5%), K-feldspar (5–7%); matrix: plagioclase (30–35%), K-feldspar (20–25%), quartz (25–30%), and biotite (5–10%) | Light gray–light pink, in facies-change zonation with medium-grained monzonitic granite, megacrystic porphyritic is subhedral granular, and phenocryst is about 1–2 cm in size; matrix is about 1–3 mm in size |
Coarse-medium-grained porphyritic monzonitic granite | Mainly distributed in southwest section at the intersection of medium-grained granite and megacrystic porphyritic monzonitic granite, or distributed near the fault edge in southeast section | Plagioclase (35–40%), K-feldspar (25–35%), quartz (20–25%), a handful of apatite, and zircon, etc. | Light pink, coarse-medium-grained porphyritic granitoid texture; the grain size is 3–8 mm, feldspar is generally long column-like and had a certain directionally arrangement, and rocks at the margin minerals such as quartz have developed cataclastic texture due to later fractures |
Coarse-medium-grained syenogranite | Mainly distributed in the middle and northeast sections of the pluton, and in intrusive contact with monzonitic granite in southwest section | Quartz (25–35%), K-feldspar (35–40%), plagioclase (15–20%), biotite (5–10%) | Light pink, coarse-medium-grained texture, with 1–7 mm in size, and from southwest to northeast, the grain size gradually decreased |
Fine-grained syenogranite | Small apophysis or vein, intruded in coarse-medium-grained syenogranite in northeast section. | Quartz (30–35%), K-feldspar (35–40%), plagioclase (15–20%), biotite (3–5%) | Light pink, fine-grained texture, grain size 0.5–2 mm |
Coarse-grained quartz syenite | Small apophysis, intruded in coarse-medium-grained syenogranite | Quartz (10–15%), K-feldspar (65–70%), plagioclase (5–10%), biotite (1–3%) | Coarse-grained granitic texture, grain size 5–8 mm |
Contact relationship between medium-grained monzonitic granite and megacrystic porphyritic monzonitic granite in Majiafan area
Medium-grained monzonitic granite
Outcrops and micrographs images showing lithological features of the Xianxia composite pluton. a biotite-bearing MMEs in medium-grained monzonitic granite; b K-feldspar megacryst in megacrystic porphyritic monzonitic granite; c medium-coarse-grained porphyritic monzonitic granite long-strip shaped feldspar; d medium-coarse-grained syenogranite; e fine-grained syenogranite vein intruded in medium-grained monzonitic granite; f coarse-grained quartz syenite; g zoning texture plagioclase in medium-grained monzonitic granite; h feldspar phenocryst in megacrystic porphyritic monzonitic granite (K-feldspar wrapping plagioclase); i K-feldspar inside plagioclase in medium-coarse-grained porphyritic monzonitic granite, and quartz cataclastic phenocryst affected by later fracture
Megacrystic porphyritic monzonitic granite
Coarse-medium-grained granite
- 3.
Wall-rock Alteration
- 4.
Geochemical Features
SiO2–K2O diagram for Xianxia composite pluton (its legends are the same as those in Fig. 3.20)
SiO2–Na2O + K2O and A/CNK-A/NK diagrams for Xianxia composite pluton
Chondrite-normalized REE distribution mode (a) and primitive mantle-normalized trace element spider diagram (b) for the Xianxia composite pluton (the normalized values of chondrite and primitive mantle come from Sun and McDonough 1989)
In the Xianxia composite pluton, (86Sr/87Sr)i is 0.70988–0.70455 and εNd(t) is −5.14 to −8.87, and from the early monzonitic granite to the late syenogranite both have a trend of decrease, and TDM2 is low (1.36–1.65 Ga); they are similar to (86Sr/87Sr)i (0.71030–0.70613) and εNd(t) (−3.75 to −6.4) of the Mogan Mount granite pluton in the northern Zhejiang Province (Zhang et al. 2012) or slightly lower than (86Sr/87Sr)i (0.71010–0.70960) and εNd(t) (−6.28 to −7.32) of Jinde granodiorite pluton in the southern Anhui Province (Zhou et al. 2014).
- 5.
Isotope geochronology
Typical zircon CL images, dating points, and ages of the rocks in the Xianxia composite pluton
Zircon U–Pb concordant diagrams of main rocks of the Xianxia composite pluton
3.1.2.4 Wushanguan Composite Pluton
- 1.
Geological Features
- 2.
Petrological Features
Medium-grained monzonitic granite
Medium-coarse-grained and medium-grained syenogranite
Megacrystic porphyritic monzonitic granite
Fine-medium-grained granite. It intrudes in medium-grained monzonitic granite
Sketch showing the internal features of megacrystic porphyritic-medium-grained monzonitic granite and intrusive medium-coarse-grained granite contact relationship of the Wushanguan pluton
Contact relationship between medium-coarse-grained syenogranite and megacrystic porphyritic monzonitic granite in the Wushanguan pluton. a MMEs (dark, biotite, and plagioclase bearing) developed in medium-grained monzonitic granite; b MMEs developed in megacrystic porphyritic monzonitic granite; c contact borderline of megacrystic porphyritic monzonitic granite and coarse-grained syenogranite; d K-feldspar phenocryst in megacrystic porphyritic monzonitic granite; e and f coarse-grained syenogranite wrapped in megacrystic porphyritic monzonitic granite
Fine-grain syenogranite
Contact relationship of fine-grained syenogranite intrusion in medium-grained monzonitic granite at Zhaojiatang in the Wushanguan pluton. a fine-grained syenogranite intruded in early stage medium-grained monzonitic granite, with clear intrusion border line; b fine-grained syenogranite at the margin intruded as vein shape in early medium-grained monzonitic granite; c the early stage medium-grained monzonitic granite cognate xenolith in fine-grained syenogranite
Medium-grained syenite
- 3.
Wall-rock Alteration
- 4.
Geochemical Features
A/CNK-A/NK diagram (a) and SiO2-K2O diagram (b) for the Wushanguan pluton (legends are shown in Fig. 3.29)
REE chondrite-normalized distribution mode and primitive mantle-normalized trace element spider diagram for Wushanguan composite pluton (the normalized values of chondrite and primitive mantle come from Sun and McDonough 1989)
- 5.
Isotope geochronology
Main Zircon CL images, dating location, and ages of the rocks in the Wushanguan composite pluton
Zircon U–Pb concordant curve of main lithologies of the Wushanguan composite pluton
3.1.2.5 Tonglizhuan Composite Pluton
- 1.
Geological Features
Regional geological sketch of the Tonglizhuang composite pluton (based on the 1:200,000 Lin’an Mapsheet). 1. Nahuaian; 2. Sinian; 3. Cambiran; 4. Ordovician; 5. the lower Silurian; 6. the lower Cretaceous; 7. Quaternary; 8. medium-grained monzonitic granite; 9. fine-grained syenogranite; 10. medium-grained quartz orthophyre; 11. fine-grained granitic vein; 12. granitic aplite vein; 13. granitic porphyry vein; 14. granodiorite-porphyry vein; 15. quartz vein; 16. normal fault; 17. geological boundary; 18. sampling location
- 2.
Petrological Features
Outcrop and micrograph images showing the lithological features in the Tonglizhuang pluton. a medium-grained monzonitic granite; b fine-grained syenogranite; c medium-grained quartz orthophyre; d K-feldspar (Kf) and plagioclase (Pl) in medium-grained monzonitic granite; e plagioclase (Pl) containing K-feldspar (Kf) core growth in medium-grained monzonitic granite; f plagioclase (Pl), K-feldspar (Kf), and quartz (Qtz) in fine-grained syenogranite
Fine-grained syenogranite is pink–light pink, fine-grained texture, is mainly composed of K-feldspar (40–50%), plagioclase (20–25%), quartz (20–30%), and a handful of biotite (5%), in the grain size of 0.2–1 mm. K-feldspar is mainly orthoclase, wide plate, growth Carlsbad bicrystal; plagioclase is hypidiomorphic wide plate and plate-column, growth Na-feldspar bicrystal; quartz is hexagonal bipyramid and in form of fine granular aggregation; biotite is reddish brown and plate or laminated, and its margin is often surrounded by fine biotite aggregation and irony points (Fig. 3.33b, f).
Medium-grained quartz orthophyre is pink–light pink and medium-grained porphyaceous texture. Phenocryst is mainly composed of K-feldspar (25–30%), plagioclase (5–10%), and a handful of quartz and biotite, in the grain size of 1–5 mm; while matrix is micro-grained texture, and mainly composed of K-feldspar (40–45%), quartz (5–10%), and a handful of plagioclase (5–10%) as well as accessory minerals such as undeterminated metallic minerals, apatite, and zircon, with the grain size of 0.05–0.1 mm; K-feldspar is mainly orthoclase while plagioclase is mainly oligoclase (Fig. 3.33c).
- 3.
Wall-rock Alteration
- 4.
Geochemical Features
REE chondrite-normalized distribution mode and primitive mantle-normalized trace element spider diagram for Tonglizhuang composite pluton (the normalized values about chondrite and primitive mantle come from Sun and McDonough 1989)
The later medium-grained quartz orthophyre is low in SiO2 content (66.25%), enrichment in alkali (Alk = K2O + Na2O, 10.02%), high in K2O/Na2O ratio (1.66); low in content of MgO (0.67%), P2O5 (0.10%), and TiO2 (0.37%). A/CNK is 0.94, Rittmann Index (σ) is 4.32 and is metaluminous and shoshonite features.
Both are low in ∑REE content (127.02 × 10−6–183.02 × 10−6), LREE and HREE differentiated rather clear, LREE/HREE = 11.71–9.12, LaN/YbN = 15.67–9.58, δEu = 0.72–0.79, showing weak negative Eu anomaly, the chondrite-normalized REE patterns show the feature of weakly dipping rightward; enrichment in K, Rb, Th, and U, weakly depleted in LILE such as Ba and Sr; weakly depleted in HFSE such as Nb, Ta, P, and Ti. Both have similar REE and trace elements features, indicating that both may have the same magma source.
- 5.
Isotope geochronology
Zircon U-Pb concordant diagrams for the Tonglizhuang composite pluton
The Paleozoic strata and the volcanic rocks in Mount Tianmu are intrusive contact between the two sides of the Tonglizhuang composite pluton. The zircon U–Pb dating results indicated that crystallization age of the early medium-grained monzonitic granite is 142.3 ± 1.4 Ma. By comparing the lithological types and features of intrusive rocks in the survey area, the crystallization time of the middle stage fine-grained syenogranite was close to the fine-grained syenogranite in the Ma’anshan pluton and the Xianxia pluton (129.6–128.3 Ma). The forming time of the later stage medium-grained quartz orthophyre was 125.3 ± 1.5 Ma. Magmatism in the early and middle stages was likely to be related to the translational extensional fractures developed in the late Yanshanian in northern Zhejiang and southern Anhui, while the late-stage magmatism was likely to be related to regional volcanic eruption activities.
3.1.2.6 Zhinanshan-Dongkeng Volcanic-Intrusive Complexes
- 1.
Geological Features
Sketch geological map of the Zhinanshan–Dongkeng volcanic-intrusive complex. 1. the second member of the Huangjian Formation; 2. the third member of the Huangjian Formation; 3. fine-grained granite; 4. coarse-grained granite; 5. fine-grained quartz monzonite; 6. medium-grained quartz diorite; 7. andesite; 8. andesitic porphyrite
Geological section map of the Dongkeng volcanic-intrusive complex
The south side of the Zhinanshan-Dongkeng complexes is widely in intrusive contact with the second member of the Huangjian Formation (K1h2) rhyolitic tuff lava, while the north side is in intrusive contact with the third member of the Huangjian Formation (K1h3) rhyodacitic gravel-bearing vitroclastic ignimbrite. Quartz diorite, granitic porphyry, felsite, and andesite veins are presented inside and outside of the contact zone between the complexes and the second–third member of the Huangjian Formation.
- 2.
Petrological Features
Outcrop images showing the contact relationship between the main rock types in the Zhinanshan–Dongkeng volcanic-intrusive complexes. a Quartz diorite (porphyry) in the Zhinanshan complex; b quartz diorite(porphyry)-cemented andesite breccia in the Zhinanshan complex; c clinkdered strip occurred in rhyolitic tuff lava, which is the wall rocks of quartz diorite (porphyry) in the Zhinanshan complex; d the early stage medium-grained quartz monzonite in the Dongkeng complex; e the lump medium-coarse-grained syenogranite intruded in the early stage medium-grained quartz monzonite; f medium-coarse-grained, fine-grained lump syenogranite intruded in the early stage medium-grained quartz monzonite
Based on the field contact relationship, from early to late stage, the Dongkeng volcanic-intrusive complex is medium-fine-grained (porphyritic) quartz monzonite → fine-grained syenogranite (porphyry) → medium-coarse-grained syenogranite.
Medium-fine-grained (porphyritic) quartz monzonite in the Dongkeng complex is light gray and medium-fine-grained (porphyritic) texture. Phenocryst contains plagioclase (20%), K–Na-feldspar (8%), hornblende (4%), and very little quartz. The grain size of the phenocryst is 1–3 mm, but a small amount up to 3–4 mm; and locally K-feldspar is 5–10 mm in size; plagioclase is 1–4 mm in size, but a small amount up to 4–12 mm, and epidotization occurred somewhere; hornblende is 0.5–3 mm in size. Matrix contains quartz (12%), K–Na-feldspar (major), and plagioclase (minor) (55%) in the grain size of 0.2–0.5 mm. Some gray-green ellipse-like irregular enclaves are seen in the size of 1–8 cm × 1–8 cm. The light pink micro-grained to fine-grained irregular rhyolitic tuff lava xenoliths are also presented, in the size of 40 cm × 30 cm with no clear border. The coarse-medium-grained syenogranite and fine-grained syenogranite (porphyry) inclusions are developed in the medium-fine-grained quartz monzonite near the central of the complex, which are irregular and varies in size about 10–150 cm (length) × 10–150 cm (width). Few oval fluidal structure enclave is visible, about 40 cm × 30 cm in size. The border of syenogranite inclusions and quartz monzonite is irregular but clear, lack of burning-off or alteration, etc. Andesite veins, 1–2 m wide, intruded in quartz monzonite, the contact surface is irregular bending, with the occurrence of 100°∠80°.
Fine-grained syenogranite (porphyry) is light pink, granitic (porphyritic) texture, and matrix is micro-grained texture. Phenocryst mainly quartz (16%), K–Na-feldspar (10%), plagioclase (2%), and a handful of biotite, with the general size of 1–2 mm and very little of 2–6 mm, epidotization occurred to a handful of plagioclase, not evenly distributed. Matrix mainly contains feldspar (50%) and quartz (20%), about 0.1–0.2 mm in grain sizes; quartz is granular, feldspar is plate and mainly K–Na-feldspar with a handful of plagioclase. In the fine-grained syenogranite (porphyry), light pink micro-grained to fine-grained syenite veins and light pink coarse-medium-grained granite lumps can be seen locally. Two groups of joints are seen in outcrop of the complexes, with the occurrences of 322°∠50° and 138°∠40°.
Medium-coarse-grained syenogranite is light pink, medium-coarse-granular texture, mainly consisting of quartz (32%), K-Na-feldspar (56%), and plagioclase (12%), with the general grain size of 3–8 mm or a few of 8–12 mm. The large lump-like syenogranite intruded in fine-grained syenogranite and medium-fine-grained (porphyritic) quartz monzonite, while the lump varies in size ranging from 5–150 cm × 5–200 cm, with clear borderline and lack of clear burning-off alteration.
3.2 Geochemical Features
Quartz diorite in the Zhinanshan complex, and the medium-fine-grained quartz monzonite, the fine-grained syenogranite (porphyry), as well as the medium-coarse-grained syenogranite in the Dongkeng complex have similar geochemical features, possibly due to the differentiation evolution passes from the same magma origin.
SiO2–Na2O + K2O, A/CNK-A/NK, SiO2–K2O, REE chondrite-normalized diagram, and trace element primitive mantle-normalized diagrams for the Zhinanshan–Dongkeng volcanic-intrusive complexes
All rocks in the Zhinanshan-Dongkeng complexes have medium ∑REE content (135.25 × 10−6–202.71 × 10−6), LREE and HREE differentiated rather obvious, LREE/HREE = 4.49–8.06, LaN/YbN = 3.42–9.24, δEu = 0.15–0.64, having strong negative Eu anomalies, the chondrite-normalized REE patterns show the feature of weakly dipping rightward, enrichment in K, Rb, Th, and U, weakly depleted in LILE such as Sr and Ba; depleted or weakly depleted in HFSE such as Ti, P, Nb, and Ta.
From fine-medium-grained quartz diorite → medium-fine-grained quartz monzonite → fine-grained syenogranite → medium-coarse-grained syenogranite, SiO2 and A/CNK increased gradually, and contents of TiO2, Al2O3, MgO, CaO, P2O5, MnO, LREE, Sr, and Ba as well as δEu decrease gradually. For fine-grained and medium-coarse-grained syenogranite, (86Sr/87Sr)i = 0.70554–0.70636, εNd(t) = −5.73 to −4.19 and the TDM2 = 1.26–1.39 Ga.
- 3.
Isotope geochronology
Zircon U–Pb concordant curve of the main types of the rock from the Zhinanshan–Dongkeng volcanic-intrusive complexes
Zircons in the fine-grained syenogranite and medium-coarse-grained syenogranite of the Dongkeng complex, U content is 146 × 10−6–558 × 10−6 and 85 × 10−6–1445 × 10−6, respectively, Th content is 101 × 10−6–426 × 10−6 and 61 × 10−6–978 × 10−6, respectively, and Th/U ratio is 0.70–1.11 and 0.58–0.94, respectively, a typical feature of magma origin. Sample locations mostly are projected on or near the concordant curve, and 206Pb/238U weighted mean age is 127.6 ± 1.2 Ma (MSWD = 1.8) and 127.9 ± 1.3 Ma (MSWD = 1.7), representing the crystallization age of the Dongkeng complex.
3.2.1 Origin, Evolution of Magma, and Its Relationship with Regional Metallogeny
3.2.1.1 Origin and Evolution Series of Magma
- 1.
Evolution Series of Magma
Diagram showing lithological compositions and evolution series of magmatism in the Early Cretaceous
- (1)
Intrusive rock series
Stage-2 (K12): Zircon U–Pb age is 133.9–125.0 Ma. According to the contact relationship, the time sequence followed by coarse-medium-grained to medium-coarse-grained syenogranite (133.9–131.2 Ma) of Tangshe, Xianxia, and Wushanguan; pegmatitic and megacrystic porphyritic monzonitic granite (132.2–127.7 Ma) of Ma’anshan and Wushanguan; fine-grained syenogranite (130.8–125.0 Ma) of Ma’anshan, Tangshe, Xianxia, Tonglizhuang, and Wushanguan.
- (2)
Volcanic-intrusive series
- 2.
Petrogenetic types
Discrimination diagrams of intrusive rock and volcanic-intrusive rock types
For the volcanic-intrusive rock series, SiO2 content (58.30–75.25%), alkali content (7.33–10.02%), FeOT/MgO ratio (1.81–4.38), and DI (62.05–94.58) are varied greatly, and mostly metaluminous and shoshonitic. However, all the features above increased gradually, followed by quartz diorite → quartz monzonite → syenogranite.
Some researches demonstrate that solubility of apatite is very low in the metaluminous–weakly peraluminous magma, and would be even lower when SiO2 content increase during the process of magmatic differentiation. But in strongly peraluminous magma, the solubility of apatite increases with the increase of SiO2 content; therefore, the changing trend of P2O5 and SiO2 content with the evolution of magmatic differentiation can be used to distinguish between the I-type and A-type granites and S-type granitoid. In the survey area, the intrusive rocks have lower P2O5 content with an average of 0.04–0.17%. From early stage monzonitic granite to late-stage syenogranite, the P2O5 content decreased with the SiO2 content increased, similar to the evolution trend of I-type granite. (86Sr/87Sr)i and εNd(t) also are similar to those of I-type granite in South China. As a result, intrusive rock in the survey area may be highly differentiated I-type granite. Together with A-type granite, both developed in the Early Cretaceous strata in northwestern Zhejiang and neighboring areas.
- 3.
Magma Sources
(87Sr/86Sr)i–εNd(t) and (87Sr/86Sr)i–t(Ma) diagrams of intrusive rock and volcanic-intrusive rock (legends same as Fig. 3.42)
Metamorphic basement in south Yangtze area (Li et al. 2003); MORB (mid-ocean ridge basalt) and OIB (oceanic island basalt) (Xia et al. 2004); lithosphere-enriched mantle cited from (Zhang et al. 2008); Yangtze lithospheric mantle, Yangtze lower crust, and crust in the east of Jiangnan orogen (Jiang et al. 2011; Wang et al. 2012); Yangtze upper crust (Wang et al. 2004); Tongcun Granodiorite porphyry (Zhu et al. 2014); Jinde granodiorite (Zhou et al. 2013); Mogan Mount granite (Zhang et al. 2012).
3.2.1.2 Tectonic Environment of Intrusive Rock
In Middle Jurassic, South China transformed from Tethys tectonic regime to Pacific tectonic regime. Many scholars investigated its tectonic settings profoundly and mainly the following ideas were obtained: (1) Lithosphere began extension in Middle Jurassic, and happened periodically until lithosphere regionally extended in Cretaceous (Chen et al. 2002; Li et al. 2007); (2) In Middle and Late Jurassic, subduction, compression, and orogenic environment of active continental margin in the Pacific tectonic regime (Xing et al. 2008; Zhang et al. 2009; Li et al. 2013) or intracontinent orogeny dominated by paleo-Pacific plate oblique-subduction (Mao et al. 2014), but extension also happens locally (Xing et al. 2008), and after collision orogeny the intracontinent extension took place in the beginning of the Early Cretaceous (Li et al. 2013; Mao et al. 2014); (3) from Middle and Late Jurassic to the beginning of Early Cretaceous, multiple blocks compressed strongly intracontinent orogeny from multiple directions (165 ± 5 Ma–136 Ma), in the beginning of Early Cretaceous, extensional collapses and the lithospheric thinning (135–100 Ma), in the Late–EarlyCretaceous, weak compressional deformation took place (100–83 Ma) (Dong et al. 2007), stressing constraints of interactions of multiple blocks (Wang et al. 2013). Comprehensive research results show that in the northwest Zhejiang and even in South China, in Middle Jurassic (165 ± 5 Ma–145 ± 5 Ma), the tectonic settings were generally in an compressional environment, and transformed into extensional environment in the beginning of Early Cretaceous (145 ± 5 Ma–125 Ma), which was triggered by the oblique and shallow subduction direction of the Izanagi plate replaced by the straight and steep subduction of the Pacific plate (Zhu et al. 2010).
Y + Nb–Rb and R1–R2 diagrams of intrusive rock and volcanic-intrusive rock (legends same as Fig. 3.42)
3.2.1.3 Features of Magma Emplacement and Metallogenesis
- 1.
Magma Emplacement Mechanism and Metallogenesis
- 2.
Pattern of Emplacement Growth and Metallogenesis of Plutons
Sketch showing the model for accretion growth ways of pluton in the survey area
3.3 Volcanic Rocks
3.3.1 Overview
In the survey area, volcanic rocks are mainly located in the east half of Xianxia Mapsheet and Chuancun Mapsheet with outcropping of 556 km2, distributed a normal trapezoid. Its northern side, bordering with Baofuzhen, Anji–Wushanguan, Yuhang District, is in angular unconformable contact with Paleozoic sedimentary strata and in intrusive contact with Wushanguan composite pluton; its southern side, bordering with Majiafan–Gaolingcun–Linbeicun, Lin’an City, is in angular unconformable or fault contact with Paleozoic sedimentary strata; its western side, bordering with Dengcun, Lin’an City–Zhangcunzhen, Anji County, in NE-strike fault contact with Paleozoic sedimentary strata an Xianxia composite pluton; it eastern side, bordering with Xianbaikengcun–Langjiacun, Yuhang District, is in intrusive or NW-strike fault contact with Wushanguan composite pluton.
In the survey area, volcanic rock belongs to Yianmu Mount–Mugan Mont volcanic structural depression of Tingzi Mount–Tianmu Mount–Mogan Mount sunzone of the active volcanic zone in northern Zhejiang. With many types of rocks and complex lithologies, volcanic rocks are characterized by great changes in lithofacies combinations, developed volcanic apparatus and multiple eruptions and migrations. On the basis of volcanic rock’s distribution features, lithological and lithofacies combination as well as regional comparison, in the survey area, volcanic rockfalls within Mesozoic Huangjian Formation, and can be divided into four eruption rhythms (stratum unit at member level) by volcanic eruption from early to late and from strong to weak.
3.3.2 Rock Types and Features
In the survey area, volcanic rock has a number of rock types, can be mainly divided into four categories: volcanic clastic rocks, volcanic sedimentary rocks, lavas, and subvolcanic rocks, and rocks are named mainly by referencing to classification and naming schemes for volcanic clastic rocks in Research Report on Volcanic Structure—Lithology and Lithofacies—Volcanic Strata Mapping Methods, and other categories of rock are named as per this.
3.3.2.1 Volcanic Clastic Rock
In the survey area, there are various types of volcanic clastic rock with main lithology, including rhyolitic agglomerate breccia, rhyolitic agglomerate breccia tuff, rhyolitic/dacitic (breccia-bearing) crystal-vitric tuff, rhyolitic/dacitic (breccia-bearing) crystal-vitric ignimbrite, and rhyolitic/dacitic vitric ignimbrite, etc.
- 1.
Rhyolitic daciticacitic tuff (ignimbrite)
- (1)
Rhyolitic dacitic (breccia-bearing) crystal-vitric ignimbrite
- (2)
Rhyolitic dacitic agglomerate-breccia-bearing (vitric) ignimbrite
- 2.
Rhyolitic ignimbrite
Rock varies greatly in color, dark red, light purple, gray purple, gray, offwhite, and gray green, (breccia-bearing) ignimbritic texture and false-rhyolitic structure. In the rock, breccia is 5–10% in content, containing mainly gray green, dark gray and gray yellow ignimbrite, tuff or cryptocrystalline, some breccia are directionally arranged, in the size of 2–15 mm for most, 20–30 mm for some and up to 80 mm for a few (agglomerate), in the subangular–subrounded shape; crystal pyroclast is mainly light pink K-feldspar (5–10%), quartz (5–10%), and plagioclase (5%), in the size 0.5–2 mm, and 3–4 mm for a few, cemented with tuffaceous. K-feldspar is most in the angular shape, less in the shape of plate-like and column and occasionally Carlsbad twin visible; plagioclase is mostly in the shape of subhedral plate and column, Na-feldspar bicrystal often visible and annular-belt structure occasionally visible; quartz is mostly in the shape of angle and hexagonal bipyramid, occasionally in corrosion curved shape at the margin; dark minerals are mainly hornblende and biotite, which has been altered to be clay minerals. In the rock, plastic vitric pyroclast and volcanic ash are 60–82% in content, plastic vitric pyroclast is in the shape of bar, rod, strip, and fine stripe, and has been devitrified to be felsitic felsic minerals; plastic debris content is 1–10%, in the shape of lenticles, ripped, and striation, after devitrification it becomes felsitic texture and crystal micro-granular texture, K-feldspar, plagioclase, and quartz phenocryst; plastic vitric pyroclast and debris are clearly directionally distributed forming rhyolitic structure.
- 3.
Rhyolitic tuff
- (1)
Rhyolitic agglomerate-bearing breccia(-bearing) crystal-vitric tuff
- (2)
Rhyolitic agglomerate-bearing breccia
3.3.2.2 Volcanic Sedimentary Rock
It is the main lithology at the upper part of the Member #1 of Huangjian Formation’s (K1h1), additionally, a handful of it is distributed at the lower part of the Member #1 of Huangjian Formation (K1h1) and the Member #3 of Huangjian Formation (K1h3) in the form of interbeddings (for instance, Tianhuangping and Lichanglong Mount), and lithology includes tuffaceous glutenite, tuffaceous (gravel-bearing) packsand-siltstone, and tuffite, etc.
- 1.
Tuffaceous Glutenite
- 2.
Tuffaceous (gravel-bearing) fine-siltstone
Rock at Shenxi Village Canyon is purple red, medium–thin-laminar structure, has minute rhythmic bedding, gravel-bearing coarse-fine-silt texture, in the interbedded shape; purple-red tuffaceous gravel-bearing gritstone stratum, purple gritstone stratum, purple packsand stratum, and dark purple silty mudstone are 10–20 cm, 10–30 cm, 5–10 cm, and 5–8 cm in thickness, respectively, with the attitude of 180°∠20°. Rocks contain a handful of plagioclase whose content is 3–5% and size is 1–3 mm; a handful of dark-black and gray-green angular tuff breccia locally seen, in the content of 2–3% and size of 3–10 mm; the lithology at the top is gray-black carbonaceous argillaceous siltstone. Terrigenous clast is 80–85% in content and mainly is silty-fine sand (65–70%), argillaceous (15%), and irony (5%). Silty-fine sand is mainly quartz, and a handful of debris, in the angular shape, evenly distributed; argillaceous is cryptocrystalline; irony is limonite, in rendered shape. Volcanic clastic is 10–15% in content, mainly crystal pyroclast (mainly plagioclase, and a handful of quartz), and a handful of volcanic cinder, pretty evenly distributed in the terrigenous clastics.
At Tianhuangping area, tuffaceous gravel-bearing packsand or siltstone is interbedded with sedimentary breccia crystal-vitric tuff, purple red and brown, medium–thin lamellar, locally interlined with medium–thin-lamellar tuffite, about 5–30 cm thick per layer; in the shape of intermittent long strips and bubbles, tuffaceous packsandy texture, composed of volcanic clastics (25%), silt and argillaceous (75%), volcanic ash and cemented with argillaceous. Gravel is in the subangular shape and in the grain size of 0.1–0.25 mm, mainly feldspar and secondly quartz.
- 3.
Tuffite
3.3.2.3 Lava
It the main lithology of the Member #2 of Huangjian Formation, there are complex and various rocks, mainly massive rhyolitic porphyry, rhyolitic tuff lava, rhyolitic agglomerate breccia lava, porphyritic rhyolitic, and bubble rhyolite.
- 1.
Massive rhyolitic porphyry
- (1)
(breccia-bearing) vitric rhyolitic porphyry
- (2)
Felsitic massive rhyolitic porphyry
- (3)
Felsitic nevadite
- 2.
Porphyritic (bubble) rhyolite
Porphyritic rhyolite, light gray, porphyritic texture, bubble-rhyolitic structure, the content of phenocryst varies greatly, more rocks in the central, and less in the margins, mainly K-feldspar (5–20%), and a handful of plagioclase (1–2%) and biotite (1–2%). K-feldspar and plagioclase are in granular and short strip shapes, in the size of 1–3 × 1–2 mm, some of it has been epidotized; biotite is in the form of schistose agglomerate, in the size of 1–3 mm × 1–3 mm. Rocks at margins have developed bubble in the content of 5–10% and in the size of 3–15 mm, and rocks in the central have less or not bubble, Rocks have developed rhyolitic strip structure and eddy-like structure. K-feldspar and plagioclase are in the shape of subhedral plate and column, the former surface is argillization of different degrees, the latter surface has altered into sericite and some has Na-feldspar bycrystal; quartz is in the shape of hexagonal bipyramid and some show cataclastic phenomenon; dark minerals have been fully decomposed into chlorite, sericite, and carbonatite, etc., but on the basis of their features such as preserved appearance and transection, it is known they are mainly hornblende, biotite, or augite. Matrix is 75–93% in content, cryptocrystalline, felsitic, and crystal micro-granular textures, micro-graphic, spherulitic, and cataclastic textures sometimes, mainly felsic minerals, in the partizle size of 0.005–0.05 mm.
Bubble rhyolite, of which the most developed is at the mountain top of Shifosi, Shanchuang Town, and bubble is 50–60% in content and in the size of 2–15 mm, but 15–80 mm for a few.
- 3.
Rhyolitic tuff lava
- 4.
Rhyolitic agglomerate-breccia-bearing lava
3.3.2.4 Subvolcanic Rock
It is distributed in Shenxi Village Canyon pitching up Tianpingshan ding–Dongling, Tianhuangping Reservoir, and Liwaichanglongshan–Linjiatang, and its lithology is mainly (quartz) monzonitic porphyry, dacite–porphyrite, and rhyolitic porphyry.
- 1.
(Quartz) monzonitic porphyry
- 2.
Dacite–porphyrite
- 3.
Rhyolite (porphyry)
3.3.3 Facies of Volcanic Rocks
3.3.3.1 Division of Volcanic Rock Facies and Their Main Features
Volcanic rock lithology and lithofacies division in the survey area
Stratigraphic unit | Lithofacies | Lithology | |
|---|---|---|---|
Huangjian Formation (K1h) | Explosive facies | Fallout facies | Rhyolitic/dacitic (bearing-) crystal-vitric tuff, breccia (-bearing) crystal-vitric tuff, and breccia(-bearing) crystal pyroclast tuff |
Clastic flow facies | Rhyolitic/dacitic crystal-vitric ignimbrite, breccia(-bearing) crystal-vitric ignimbrite, breccia(-bearing) vitric ignimbrite, breccia(-bearing) crystal pyroclast ignimbrite, and debris ignimbrite | ||
Surging facies | Its lower part is interbedding of rhyolitic crystal pyroclast tuff and tuffaceous sandstone, and its upper part is rhyolitic crystal pyroclast tuff | ||
Volcanic debris-flow facies | Thick-lamellar–massive rhyolitic agglomerate breccia tuff, rhyolitic/dacitic agglomerate rock, agglomerate breccia, and breccia | ||
Eruptive sedimentary facies | Tuffaceous glutenite, tuffaceous gravel-bearing sandstone, tuffaceous sandstone, and tuffite | ||
Extrusive overflow facies | Vitric (breccia-bearing)rhyolitic porphyry, felsitic massive rhyolitic porphyry, and felsitic massive nevadite | ||
Eruptive spill facies | Bubble rhyolite (porphyre) and fluidal rhyolite (porphyre) | ||
Explosive spill facies | Rhyolitic tuff lava | ||
Volcanic vent facies | Agglomerate breccia lava | ||
Subvolcanic rock facies | Monzonitic porphyry, granite porphyry, diorite porphyry, and dacite–porphyrite | ||
- 1.
Explosive Facies
- (1)
Fallout accumulative facies
Thin-medium-layered dacitic tuff in the mid-part of the Member #1 of Huangjian Formation (K1h1), Nanwu Forest Farm, Zhangcun
- (2)
Debris-flow accumulative facies
1. Dacitic breccia-bearing crystal-vitric strong ignimbrite of the Member #3 of Huangjian Formation (K1h3) at the peak of East Tianmu Mount. 2. Dacitic tuff crystal-vitric strong ignimbrite in the lower part of the Member #1 of Huangjian Formation (K1h1). 3 and 4. Rhyolitic magmatic-fragment crystal-vitric strong ignimbritein the lower part of the Member #2 of Huangjian Formation (K1h2), Shenxi Canyon, Baofu Town
With the stable thickness and mostly massive structure, strata in these areas have developed hexagonal, pentagonal, and quadrangular columnar joints and massive joints (e.g., the peak of East Tianmu Mount Scenic Area). Where volcanic debris-flow cooling unit or flow unit is completely developed, their facies sequence structures are: the lower part is rocks of surging accumulative facies (In the survey area, it is only seen at Xikou, Lin’an District, as mentioned below), the mid-part is rock of debris-flow accumulative facies (ignimbrite), and the upper part is rocks of fallout accumulative facies, the debris-flow accumulative facies is in gradually transitional relationship with the surging accumulative facies while in abrupt change relationship with the fallout accumulative facies with clear borderlines (e.g., Shenxi Canyon).
- (3)
Surging accumulative facies
Lower part at Xikou Village, Lin’an District, is interbedding of medium–thick-layered light gray rhyolitic crystal pyroclast tuff and dark gray tuffaceous siltstone (upward, gradually become offwhite tuffaceous sandstone, and rhyolitic crystal pyroclast tuff in a single layer has developed reverse and normal graded)
Grade change of volcanic rocks of Surging facies at Xikou, Lin’an
Tuffaceous siltstone, dark gray, tuffaceous silty texture, and medium–thick-lamellar structure, about 5–40 cm thick per layer; mainly composed of normal sediments and volcanic debris, argillaceous and volcanic ash cements. Normal sediments are 55–60% in content mainly silty and argillaceous, etc. The silty is finer (0.03–0.05 mm) and contains feldspar and quartz, not evenly distributed, more at local places. The volcanic debris is 40–45% in content, mainly vitric fragment, a handful of crystal pyroclast and volcanic ash, etc. Inside a single layer, normal graded bedding structure is developed, and grain size is transitional gradually.
Tuffaceous sandstone, offwhite, mainly sand grains, and argillaceous, etc. Sand is in the angular shape and in the grain size of 0.1–0.2 mm, mainly feldspar and quartz, etc. The volcanic debris is less than normal sediments in content, about 40%, mainly vitric fragment, a handful of crystal pyroclast and volcanic ash, etc. Based on analysis on debris, such rocks experienced short movement and are very likely to accumulation at near places.
For the surging facies at Xikou Village, Lin’an, the ratio of interbedding of both lithologies in the lower is 6:4, the contact surface is slightly wave, in general, the stratum at the lower is thicker and the one at the upper gets thinner; parallel bedding developed at the upper and oblique bedding at the lower; stratum’s attitude is 62°∠42°.
- 2.
Volcanic debris-flow accumulative facies
- (1)
Volcanic debris varies greatly in size, volcanic ash coexists with agglomerate, in general, since debris-flow accumulation there is a certain sorting from bottom to top but the sorting is poor in a single stratum.
- (2)Agglomerate and breccia had been partly rounded in the transfer process, angular agglomerate coexists with subangular, subrounded, and rounded coarse debris (Fig. 3.52), and on the surface of some agglomerate breccia various impressions due to friction and collision are visible.
Fig. 3.52Bottom feature of Huangjian Formation’s Member #2 (K1h2) at East Tianmu Mount Scenic Area
- (3)
Most of the accumulations are volcanic debris and cement is mainly tuffaceous.
- 3.
Eruptive Sedimentary Facies
Interbeddings of tuffaceous gravel-bearing packstone and tuffaceous siltstone in the top of the Member #1 of Huangjian Formation, Nanfu Forest Farm
- 4.
Extrusive overflow facies
There exists gradually transitional relationship between various facies zones in the massive rhyolitic porphyry of Shenxi Village Canyon, and the zonation phenomenon is clear. Horizontally, it is divided into vitric (breccia-bearing) massive rhyolitic porphyry at the margin, felsitic massive rhyolitic porphyry in the central and felsitic block nevadite, and meanwhile vein-like intrusion of Early Cretaceous subquartz monzonitic porphyry in the center (Yantianping). The contact relationship between block rhyolitic porphyry and the ignimbrite at the margin is irregular, at some local places steeply dipping vein-like intrusion occurs (see Yangtianping revived caldera for details) and at other local places it dips gently and overlies ignimbrite; affected by later NE-strike fault, it is in fault contact with rhyolitic tuff lava of the magmatic-liquation-type explosive spill facies in the southeastern, and it is presumed that they were in gradually transitional relationship during early eruption stage.
Feature of massive rhyolitic porphyry at Shenxi Canyon
- 5.
Eruptive spill facies
Porphyritic rhyolite in West Tiannu Mount–Longwangshan
- 6.
Explosive spill facies
Rhyolitic tuff lava at Dongkeng Village, West Tianmu Mount, Lin’an District
- 7.
Subvolcanic rock facies
Intrusive features of quartz monzonitic porphyry at Yangtianping
It is often filled in radial and annular fracture or fissures between strata, minerals in rocks are complex and many agglomerate breccias such as andesite wrapped inside.
3.3.3.2 Combination of Volcanic Facies
Combination of volcanic facies is the volcanic facies generated during one eruption in the history of a volcano and its generation sequence, and the type of combination of volcanic facies reflects comprehensively the features and rules of volcanic activities. There are temporal sequences and spatial superimposition relationships in lithofacies produced from multiple eruptions, research on combination types of volcanic facies help identify the source and direction of volcanic matters, determine the location of volcanic vent or volcanic eruption center, and restore the type and activity history of ancient volcanos. According to research on volcanic facies in the survey area, there are mainly three types of basic combinations as below.
- 1.
Explosive facies series (debris-flow accumulative facies–fallout accumulative facies)–eruptive sedimentary facies combination
Lithofacies combination in the lower of Yangtianping revived caldera
- 2.
Explosive facies series (debris-flow accumulative facies–fallout accumulative facies)–extrusive overflow facies combination
Combination of debris-flow accumulative facies–eruptive spill facies–explosive spill facies in the Member #2 of Huangjian Formation in the western of East Tianmu Mount
- 3.
Explosive facies series (surging accumulative debris–flow accumulative facies–fallout accumulative facies)–eruptive spill facies–explosive spill facies combination
Combination of volcanic debris-flow facies–fallout accumulative facies–eruptive spill facies in Huangjian Formation’s Member #2 of Tianmu Mount Scenic Area
3.3.4 Volcanic Eruption Rhythms and Cycles
3.3.4.1 Volcanic Eruption Rhythm
Volcanic eruption rhythm means the cyclical changes of volcanic eruption, and such cyclical changes include regular changes in erupted material components, eruption intensity, eruption ways, and erupted thickness, etc. Generally speaking, a rhythm is composed of a or a few layers of rocks some of which could be very thin, just dozens of centimeters while others may be very thick, about hundreds of meters.
Overall histogram on volcanic eruption rhythm and lithofacies formation in the survey area
Eruption rhythm #1 is mainly the evolution of the debris-flow accumulative facies (locally interbedded with erupted sedimentary facies) → the fallout accumulative facies → the eruptive sedimentary facies, with the total thickness of > 855 m, mainly distributed along Zhangcun Town in the survey area.
Eruption rhythm #2 is mainly evolutions of the volcanic debris-flow facies and the surging accumulative facies (local) → the debris-flow accumulative facies → the extrusive overflow facies → the eruptive spill facies → the explosive spill facies → the subvolcanic rock facies, with total thickness of 7274–9202 m, an important part of volcanic rocks in the survey area. The combination of main rhythmic lithofacies varies in different areas, of these the Baofu Town–Zhangcun Town is mainly the evolution of the debris-flow accumulative facies → the extrusive overflow facies → the subvolcanic facies, the East Tianmu Mount is evolution of the volcanic debris-flow facies → the debris-flow accumulative facies → the eruptive spill facies → the explosive spill facies → the subvolcanic facies while Xikou Village is evolution of the surging accumulative facies → the debris-flow accumulative facies → the eruptive spill facies → the explosive spill facies.
Eruption rhythm #3 is mainly evolution of the debris-flow accumulative facies (locally interbedded with the eruption sedimentary facies) → the eruptive sedimentary facies → the subvolcanic facies, mainly developed in the north Tianhuangping–Linjiatang of the survey area, with the total thickness of > 341–2701 m.
Eruption rhythm #4, prettily single, mainly composed of the porphyritic rhyolite and bubble rhyolite in eruptive spill facies, is mainly developed at West Tianmu Mount–Longwangshan and South Tianmu Mount of the survey area, with total thickness >500 m.
3.3.4.2 Volcanic Eruption Time Limits and Cycles
- 1.
Time limit for eruption
Zircon U–Pb concordia diagrams and age histograms for mainly volcanic rocks in the survey area
- 2.
Eruption Cycles
- (1)
In the survey area, the Huangjian Formation is the only stratum unit of volcanic rocks which has four eruption rhythms.
- (2)
Though the volcano erupts in multiple stages for a few times in the survey area, volcanic activities are basically continuous (135.2–125.4 Ma), with no regional structural unconformable surface.
- (3)
In the survey area, despite non-consistence in volcanic rock types and lithofacies combinations between eruption stages, chemical and geochemical features of the volcanic rocks are prettily similar (as mentioned below).
- (4)
In the survey area, the volcano acts in the Yianmu Mount–Mogan Mount volcanic depression where the structural environment is unchanged, and spatial distribution landscape and types of volcanic structures are pretty single.
3.3.5 Volcanic Structures
The distribution pattern of the Late Mesozoic volcanic rocks in Jixi County–Anji County, an area adjacent to the survey are in Zhejiang and Anhui Province indicates the NE-strike regional fracture plays a controlling role in volcanic activities. In the survey area, volcanic rocks are widely distributed, up to 140 km in length, the northwest border of the strip shaped Changhua–Tianmu Mount–Mogan Mount volcanic rock zone is under control of the NE-strike Jixi fracture zone and Xuechuan–Huzhou Fault to form a series volcanic eruption areas where plutonic intrusions probably are intermittently distributed (three eruption areas: Changhua, Tianmu Mount, and Mogan Munt), manifesting the relationships between volcanic activities and regional structures.
In the survey area, the main body of volcanic structures is a Tianmu Mount volcanic depression (basin) (Level-IV) in the shape of a normal trapezoid, and its south and north sides are in EW strike or NE strike fault or unconformable contact; its west side borders with NE-strike Maotan–Luocun facture and its east side is in fault or intrusive contact with Wushanguan pluton, which is the area accumulation products of Early Cretaceous volcanic activities in the regional fault-depressed structural basin, 30–37 km long from east to west and about 23 km wide from south to north; its inside developed the eruptive sedimentary rocks, which is intermittently periclinal and dips inward, rock strata are good in stratification, with gentle attitude, for instance at attitude is 95°–115°∠13°–15° for rock strata at Zhangcun Town in the west, 195°∠27° for rock strata at Shenxi Village Canyon in the north, and 50°∠35° for rock strata at East Tianmu in the south. The border of the volcanic depression is under control of nearly EW-strike buried faults and the NE-strike fault. The nearly EW-strike buried fault, possibly formed in Caledonian or earlier, mainly dipping southward, constituting the north margin of the volcanic rock basin; a large wide and gentle synclinorium formed in the Early Indosinian, i.e., regionally, buried syncline, forming the beginning shape of the volcanic basin, and forming the NE-strike Maotan-Luocun fault (Xuechuan–Huzhou fault) constituting the west margin of the volcanic basin; lifting occurs in Yanshanian so that the fault early formed at the basin border was activated, and the early syncline depression settled down along the border, and intense volcanic eruption and granite intrusion intruded.
Division of volcanic apparatus in the survey area
3.3.5.1 (Revived) Caldera
Caldera is a large volcanic apparatus formed after a volcano or a group of volcanoes collapsed, one of the main types of volcanic apparatuses in the survey area. Revived caldera means that after a caldera takes shape there are still volcanic clastic rock and lava erupting out of caldera, but mainly piedmont accumulation, collapsed accumulation at the caldera wall and lacustrine sediments; in some large caldera composed of acidic rocks volcanic activity often recurred to be developed into revived caldera; revived dome left some or all previously sunken fault blocks to uplift and ascent, so that rock strata previously horizontal or dipping inward possibly dip outward at the dip angle from a few degrees to dozens of degrees.
- 1.
Yangtianping revived caldera
- (1)
Morphological feature
- (2)
Lithological and lithofacies features
Cross-section profile of Yangtianping revived caldera structure
Images showing field features of Yangtianping revived caldera
Images showing (rhythmic) unconformable contact relationship between volcanic eruptions in two stages and horizontal joints developed in vitric rhyolitic porphyry in the lower part of late eruption
- (3)
Structural features
- (4)
Time limit for eruption
Diagram showing evolution process of the Yangtianping revived caldera
- 2.
Tianhuangping Caldera
Structural geological map of the Tianhuangping caldera
- (1)
Morphological feature
- (2)
Lithological and lithofacies features
Tianhuangping caldera cross-section profile
- (3)
Structural features
Within 1–3 km in the south and east sides of the caldera there are developed subvolcanic rocks such as irregular rhyolitic porphyry and monzonitic porphyry, and veins such as andesite, felsite, and diorite porphyrite.
- (4)
Time limit for eruption
- 3.
Changlongshan–Linjiatang Caldera
- (1)
Morphological feature
- (2)
Lithological and lithofacies features
Structural geological map of the Changlongshan–Linjiatang caldera
Cross-section profile map of the Changlongshan–Linjiatang caldera
- (3)
Structural features
- (4)
Time limit for eruption
- 4.
East Tianmu Mount–Caotanggang Revived Caldera
- (1)
Morphological feature
Structural geological map of East Tianmu Mount–Caotanggang Revived Caldera
- (2)
Lithological and lithofacies features
- (3)
Structural features
Cross-section profile of East Tianmu Mount–Caotanggang Revived Caldera
- (4)
Time limit for eruption
3.3.5.2 Dome-Shaped Volcano
Dome-shaped volcanic apparatus is a dome-shaped body formed by intrusive bodies and spilled lava. When high-viscosity lava materials extruded out of the volcanic vent, they accumulated around or above the exit and do not overflow to form intrusive bodies, but at the same time lava that overflew slightly earlier is hunched up due to increased lava pressure so that it becomes the shape of a dome which is steep at the margin, gentle in the center and has cupola and lava dome in the peak. Main rocks constituting a dome-shaped volcanic apparatus are acid and neutral-acid rocks such as rhyolite and dacite, as well as trachyte and andesite.
- 1.
West Tiannu Mount–Longwang Mount dome-shaped volcano
- (1)
Morphological feature
- (2)
Lithological and lithofacies features
Structural geological map of West Tianmu Mount–Longwang Mount dome-shaped volcano
- (3)
Structural features
Structural cross-section profile of West Tianmu Mount–Longwang Mount dome-shaped volcano
- (4)
Time limit for eruption
- 2.
South Tianmu Mount Dome-shaped Volcano
- (1)
Form and shape feature
- (2)
Lithological and lithofacies features
Structural geological map of South Tianmu Mount–Longwang Mount dome-shaped volcano
Structural cross-section profile of South Tianmu dome-shaped volcano
- (3)
Time limit for eruption
- 3.
Dashulin Dome-shaped Volcano
- (1)
Morphological feature
Structural plane of Dashulin dome-shaped volcano
- (2)
Lithological and lithofacies structural features
Structural profile of Dashulin dome-shaped volcano
- (3)
Time limit for eruption
3.3.5.3 Regularity on Volcano Structural and Product Migration
In the survey area, volcanic apparatuses typically have the feature of bead-stream combinations, for instance: in the west side, West Tianmu Mount -Longwang Mount, South Tianmu Mount, and Nayushan dome-shaped volcanos, and Yangtianping revived caldera have clearly NE-strike arrangement feature; in the east side, South Tianmu Mount dome-shaped volcano, and Tianhuangping, Linjiatang–Changlong Mount and Yaotianfan calderas have the spatial combinations of NW-strike bead-stream-like arrangement; in the north side, South Tianmu Mount, Nanyushan and Shifo Temple dome-shaped volcanos have the feature of nearly EW-strike arrangement; in the south side, West Tianmu Mount–Longwan Mount and East Tianmu Mount–Caotanggang dome-shaped volcanos and Yaotianfan caldera also have the feature of nearly EW-strike arrangement, all these indicate the controlling effect of the deep basement structure on volcanic apparatuses, constituting a normal-trapezoid volcanic basin. In general, in the survey area, volcanic structures feature migration from the central to east, north, and south, and the volcanism shows an evolution feature that is dominated by eruption in the early stage, extrusive overflow, eruptive spill, and explosive spill in the mid-stage, and eruption in the late stage.
3.3.6 Geochemical Characteristics and Tectonic Setting of the Volcanic Rocks
3.3.6.1 Geochemical Features
- 1.
Volcanic rocks
SiO2–Na2O + K2O for volcanic rocks in the survey area
REE chondrite-normalized diagram and trace element primitive mantle-normalized diagram for main volcanic rocks in the survey area
Volcanic rocks are medium or high in ∑REE content (151.99 × 10−6–296.83 × 10−6), and the chondrite-normalized REE patterns show the feature of dipping rightward. The light and heavy rare-earths differentiate rather clear, LaN/YbN is 6.32–11.92 and δEu is 0.17–0.65, showing strong negative Eu anomaly. From early to late, the LREE and HREE differentiation weakens and the negative Eu anomaly gets slightly stronger. Enriched in K, Th, U, and Rb, strongly depleted in such LILE as Ba, and from early to late stage the depleted features weakens; weakly depleted in Nb and Ta, strongly depleted in HFSEs such as Sr, P, and Ti.
(86Sr/87Sr)i is 0.70029–0.70751, εNd(t) is −6.24 to −4.69, and TDM2 is 1.31–1.43 Ga.
- 2.
Subvolcanic Rocks
Subvolcanic rock is high in content of ∑REE (202.73 × 10−6–224.14 × 10−6), and the chondrite-normalized REE patterns show a feature of weakly dipping toward right, light, and heavy rare-earth differentiate pretty obvious, LaN/YbN is 9.78–10.32 and δEu is 0.51–0.68, Eu showing medium negative anomaly. Enrichment in K, Th, U, and Rb, etc., and depletion in Ba, Sr, P, Nb, Ta, and Ti, etc., weaken compared with volcanic rock.
(86Sr/87Sr) is 0.70733–0.70847, εNd(t) is −5.07 to −4.55, and TDM2 is 1.30–1.33 Ga, like other volcanic rocks in the survey area.
3.3.6.2 Magmatic Source and Tectonic Setting
Discrimination diagrams of volcanic rock types and tectonic setting in the survey area
For (sub) volcanic rock series, (86Sr/87Sr)i is 0.70029–0.70847 with average value of 0.70566; εNd(t) is −6.24 to −4.55, with average value of −5.05, and TDM2(Nd) is 1.30–1.43 Ga; εNd(t) is significantly higher than εNd(t) (−8.12 to −9.06) of lithospheric mantle in the north margin of Yangtze block (Xue et al. 2009); the TDM2(Nd) also significantly lower than the peak age (1.6–1.7 Ga) of crust-source-type granite of Yanshanian in South China (Li 1993) and the TDM2(Nd) of basement metamorphic rocks in southwest of Yangtze block (2.0 Ga) and Cathaysian orogeny (1.8–2.2 Ga) (Che and Jiang 1999), indicating the magmatic source area may be the place where the lithospheric mantle of Yangtze block and ancient crust materials are mixed because there are clear crust–mantle interaction and addition of juvenile mantle materials in volcanic rock of Tianmu Mount.
Similar to the intrusive rock series and the volcanic-intrusive rock series in the survey area, intrusive (sub) volcanic rock series in the survey area belongs to the post-collision granite in the Rb-Y + Rb diagram (Fig. 3.45), and also largely belongs to the later orogeny and post-orogeny areas in the R1–R2 diagram. The (sub) volcanic-intrusive rock series are the product of intracontinent stretching after subduction orogeny in Cretaceous epoch, according to the geochemical and chronological features.
3.3.7 Volcanic Activity and Metallogenesis
In the survey area, volcanism and intrusion are developed well, and metallogenesis related to volcanic activity is much weaker compared to intrusive activities. The reasons are that (1) volcanic activity mainly acts within the early depression basin where there is no wall rocks related to metallogenesis; (2) the product of volcanic eruption crystallizes fast, and there is no plenty of ore-forming fluids. Therefore, in the field survey, a handful of Zn–Pb and fluorite mineralization developed in fractures are casually seen. However, in the survey area, uranium ore occurrences are developed in rhyolitic porphyry of Mogan Mount, in the east periphery of the survey area. Volcanic structures and plenty of lava outcrops are developed, so in the future exploration and research should be focused on metallogenesis associated with volcanic structures. In addition, in the survey area, exogenous mineral resources associated with volcanic rock are mainly bentonite, kaolin, and clay minerals, which developed in ignimbrite and lava, in lamellar shape, and are worth prospecting.
3.4 Vein Rocks
3.4.1 Distribution Features
In the survey area, vein rocks are widely distributed, and areas for sedimentary strata, intrusive rocks and volcanic rocks have developed veins to different degrees, and there are various types of attitudes, mainly NW-strike and NW-strike, secondly NNE-strike, nearly SN strike and nearly EW strike, etc.; there are totally 194 veins, of which 59 are in sedimentary rock areas, 60 in intrusive rock areas, and 75 in volcanic rock area.
3.4.2 Rock Types
Main geological features of vein rocks in the survey area
Area | Lithology | Intrusive age | Lithological features |
|---|---|---|---|
Ma’anshan | Granite porphyry | 127.3 ± 1.8 Ma LA-ICP-MS Zircon U–Pb | NE-strike of 40°–60°, mostly dipping northwest, and a few dipping southeast, at the dip angle of 50°–80°; 1–10 km long and 10–100 m wide per vein. Porphyritic structure, phenocryst: quartz (10%), K-feldspar (10%), plagioclase (5%), and a handful of biotite, with grain size of 0.6–1.5 mm, and up to 2 mm for a few. Matrix: feldspar and quartz, mostly in the micro-gained shape, generally with the grain size <0.5 mm |
Xianxia | Diabase | 128.3 ± 2.4 Ma LA-ICP-MS Zircon U–Pb | Veins are prettily developed, granitic vein is the most common, vein strikes vary from NE to SW differently, more veins are NE-strike of 20°–60°, mostly dipping southeast, less dipping northwest, dip angle of 50°–80°; veins are mainly 1.5–3 m wide, a few up to 50 m wide; it is 0.2–1 km long per vein |
West Tianmu | Diabase | 130.3 ± 1.1 Ma LA-ICP-MS Zircon U–Pb | NW-strike or nearly EW strike intruded in porphyritic rhyolite, it is generally 10–100 m long, 0.1–1 m wide, dipping 0–10°, and at the dip angle of 75°–80° |
Granitic porphyry
a, b Outcropping sketch and zonation diagram of granite pegmatite inside Ma’anshan composite pluton; c fine-grained syenogranite vein cut off by granite porphyry inside Ma’anshan composite pluton
Pegmatite
Dioritic porphyrite
Outcropping features of veins inside Xianxia composite pluton
Diabase
3.4.3 Geochronological Features
Zircon LA-ICP-MS U–Pb geochronological research is conducted on granitic porphyry vein (D0034) in Ma’anshan pluton, diabase veins in Xianxia pluton (D0013) and West Tianmu Mount porphyritic rhyolite (D4235). In granitic porphyry, zircon is mostly irregular long and short column, with the length of 50–200 μm, and the length-width ratio is about 3:1–1:1, showing obvious oscillatory zoning. In diabase, zircon is mostly irregular long and short columns, with the length of 60–100 μm, the length-width ratio of 1.5:1–1:1, and also contains oscillatory zoning.
Zircon U–Pb concordia diagrams and age histograms for vein rocks in the survey area
3.4.4 Geochemical Features and Origin of the Vein Rocks
- 1.
Geochemical Features
Granitic porphyry
- (1)
The content of SiO2, MgO, CaO, Na2O + K2O, P2O5, and TiO2 is 76.74%, 0.21%, 0.40, 8.04%, 0.02%, and 0.07, respectively.
- (2)
Rittmann Index (σ) is 1.92, within the range of high K–Ca alkaline series.
- (3)
Peraluminous, A/CNK is 1.14.
- (4)
∑REE is low, ∑REE = 122.78 × 10−6, LREE/HREE = 3.47, and the chondrite-normalized REE patterns show a feature of “V” shape. The light and heavy rare-earth differentiate unobviously, and Eu anomaly is obvious (δEu = 0.21).
- (5)The trace element spider diagram indicates enrichment in LILEs such as K, Th, U, and Rb, but strongly depleted in elements such as Ba; strongly depleted in HFSEs such as Sr, P, Nb, and Ti (Fig. 3.86).
Fig. 3.86Geochemical feature and discrimination diagram of main rock veins in the survey area
Diabase
- (1)
The content of SiO2, MgO, and CaO is 47.66–48.44%, 3.32–3.53%, and 5.41–6.19%, belonging to trachy basalt and tephrite in the TAS diagram, alkaline basalt in the Nb/Y–Zr/(10000TiO2) diagram.
- (2)
Na2O + K2O content is 5.49–6.87%, Rittman index (σ) is 5.54–10.13, within the area for shoshonite series.
- (3)
Metaluminous, A/CNK is 0.84–0.86, the content of P2O5 and TiO2 is 0.41–0.54% and 2.14–2.23%.
- (4)
∑REE is moderate, ∑REE = 174.98 × 10−6–179.48 × 10−6, showing enrichment in LREE, and LREE and HREE differentiate obviously, LREE/HREE = 7.93–7.95. The chondrite-normalized REE patterns generally show the feature of dipping rightward, nearly no Eu anomaly (δEu = 0.91–0.95) and Ce anomaly (δCe = 0.95–0.96).
- (5)
The trace element spider diagram indicates enrichment in LILE such as Rb and K, and slightly weakly depleted in elements such as Ba, Th, U, and Sr.
- 2.
Origin of the vein rocks
Geochemical discrimination diagram of selective diabase veins in the survey area
Meanwhile, a series Early Cretaceous tensile red fault depression basins were developed in southeast China, indicating since the Early Cretaceous it had been regionally in the background of lithosphere expansion. In south China, from early to late stage (140–70 Ma) of the Late Mesozoic, diabase veins had been widespread for multiple periods, which also indicates the then background of intense crust tension, and such crust tension has the feature of episodic activity that is transitioned from local regions in the early stage (e.g., 140 Ma mainly happened in north Guangdong and west Fujian) to holistic tension in the late stage (since 94 Ma). Diabase vein rocks formed under the expansional background are widely distributed in west Fujian, coastal areas of Fujian, Hainan, Jiangxi, and Zhejiang, etc., a product from the joint melting of lithospheric mantle and convective asthenosphere.
3.5 Comparison Between Intrusion and Volcanism
3.5.1 Time and Space Relationship
According to the patterns of magmatism, magmatite in the survey area is divided into three series: intrusive rock, volcanic-intrusive rock, and (sub)volcanic rock, and their metallogenic time limits are 145.1–125.0 Ma, 130.5–127.7 Ma, and 135.2–125.4 Ma, respectively, all in the beginning of Early Cretaceous. Intrusive rocks are mainly developed at the margins of NE-strike faults, folds, and volcanic depressions, Ma’anshan pluton is mainly under control of the NE-strike, Tongkengcun–Qiguancun fault and Tangshe pluton are mainly under control of the NE-strike, Wangjia-Tangshecun anticline fold while Xianxia, Tonglizhuang, and Wushanguan plutons intruded around the margin of volcanic depressions, and their formation is closely related to basin-control structures in volcanic depressions. Volcanic-intrusive rock is mainly developed at the margin of early intrusive rock and volcanic depressions as well as in volcanic rocks; volcanism–subvolcanism is mainly under control of volcanic depressions. Overall, in the survey area, intrusion is a little earlier than volcanism, featuring migration from west to east, and there is close time-space relationship in terms of rock-control and basin-control structures.
3.5.2 Origin Relationship
Feature comparison of magmatism of the Early Cretaceous in the survey area
Series | Intrusive rock series | Volcanic-intrusive rock series | (Sub) volcanic rock series |
|---|---|---|---|
Location and rock-control structure | Paleozoic stratum area; NE-strike fault, folds, and the margin of volcanic depressions | Margin of early intrusive rocks and volcanic depressions, as well as inside of volcanic lava | Inside of volcanic depressions, and fracture at the basin margin |
Lithological combination | Monzonitic granite –syenogranite | Quartz diorite–quartz syenite–quartz monzonite–syenogranite | Ignimbrite, lava, quartz monzonitic porphyry and rhyolitic porphyry |
Major elements geochemical features | ① Acid ② Metaluminous ③ High K calc-alkaline series ④ Low → high differentiation | ① Intermediate acid ② Weakly peraluminous ③ Shoshonite series ④ Moderate differentiation | ① Intermediate acid → acid ② Weakly peraluminous ③ Shoshonite series ④ Low-high differentiation |
Trace element geochemical features | Medium and high ∑REE, weakly dipping-right “V”-shape rare-earth curve, negative Eu anomaly. Enrichment in K, Th, U, and Rb, and deficit in Ba, Sr, P, Nb, Ta, and Ti, etc. From monzonitic granite to syenogranite, ∑REE, Rb content, and negative Eu anomaly trend to increase, and (La/Yb)N ratio, Sr/Y ratio and Sr content trend to decrease while deficit in Ba, Nb, Ta, Sr, P, Eu, and Ti, etc., trend to enhance | Medium and high ∑REE, weakly dipping-right “V”-shape rare-earth curve, negative Eu anomaly. Enrichment in K, Th, U, and Rb, and loss in Ba, Sr, P, Nb, Ta, and Ti, etc. From quartz diorite to syenogranite, ∑REE, Rb content, and negative Eu anomaly trend to increase, and (La/Yb)N ratio, Sr/Y ratio, and Sr content trend to decrease while deficit in Ba, Nb, Ta, Sr, P, Eu, and Ti, etc., trend to enhance | Medium and high ∑REE, weakly dipping-right rare-earth curve, negative Eu anomaly. Enrichment in K, Th, U, and Rb, and deficit in Ba, Sr, P, Nb, Ta, and Ti, etc. From volcanic rock to subvolcanic rock, ∑REE, Rb content, negative Eu anomaly, and loss in Ba, Nb, Ta, Sr, P, and Ti, as well as (La/Yb)N ratio, all trend to lower |
Type of formation causes | I-type granite | I-type granite | I-type granite |
Magmatic source | Mainly remodification of ancient crust materials, and a handful of lithospheric mantle materials of Yangtze block | Mixture of materials in lithospheric mantle of Yangtze block and ancient crust | Mixture of materials in lithospheric mantle of Yangtze block and ancient crust |
Magmatic evolution | Differentiation crystallization → partial melting | Partial melting | Partial melting |
Formation time limit (Ma) | 145.1–125.0 | 130.5–127.7 | 135.2–125.4 |
Metallogenesis | Skarn-type Zn-Fe polymetallic and fluorite; quartz fine-vein tungsten; hydrothermal vein-type fluorite, Sb | Volcanic-subvolcanic hydrothermal Zn-Pb, fluorite mineralization, Uranium? |
3.5.3 Comparison with Magmatite in Neighboring Areas
Based on the reliable dating data in recent 10 years, it is known that Yanshanian intrusive rocks in northwestern Zhejiang and neighboring areas were mainly formed in Middle Jurassic–Early Cretaceous (168–124 Ma), concentrated in three stages: 168–163 Ma, 150–147 Ma, and 142–124 Ma. The age trends to become young toward northeast, and in Late Cretaceous, small-scale magmatic activities occurred at 118–115 Ma locally.
Geochemical features and discrimination diagrams for magmatite in the survey area and neighboring areas
For volcanic basins developed in Late Mesozoic in northwestern Zhejiang and its surrounding, the main body of its volcanic rock strata is the lower volcanic rock series, and a handful of upper volcanic rock series, and bordering with the nearly EW-strike Changhua-Hangzhou Fault, it can be divided into two volcanic rock areas: Shunxi-Huzhou and Changshan-Tonglu. In Shunxi-Huzhou volcanic rock area, outcropped volcanic rock is mainly Huangjian Formation (K1h) and a handful of Laocun Formation (K1l) of lower volcanic rock series Jiande Group, and on the basis of zircon U–Pb age analysis on volcanic basins of Tianmu Mount, Huangjian Formation is erupted in 135–125 Ma, the beginning of Early Cretaceous. In Changshan–Tonglu volcanic rock area outcropped strata are mainly Laocun Formation (K1l), Huangjian Formation (K1h), Shouchang Formation (K1s), and Hengshan Formation (K1hs) of lower volcanic rock series Jiande Group, and a handful of Zhongdai Formation (K2z) of upper volcanic rock-system Qujiang Group, and in Jiande Group, the zircon U-Pb age is 134–115 Ma (Li et al. 2011). Overall, the period of lower volcanic rock series in Tianmu Mount and Shouchang, Jiande, northwest Zhejiang, is similar to the time limit for eruption of volcanic rock in its northwest neighborhood area, the middle and lower Yangtze River, (which is 135–123 Ma in Early Cretaceous) (Zhou et al. 2011).
Distribution of eruption time limits of volcanic rocks in Zhejiang and neighboring areas
In Fujian Province, of the lower volcanic rock series of Late Mesozoic, Nanyuan Formation volcanic rock is the most widely distributed rock types, with large era span, i.e., the end of Late Jurassic–the beginning of Early Cretaceous (Regional Geology in Fujian 2011), and the representative age of volcanic rock is 162.3–149.8 Ma (Late Jurassic) and 142.3–130.1 Ma (Early Cretaceous) (Xing et al. 2008), and the age of volcanic rock in Xiadu Formation, Bantou Formation, and Xiaoxi Formation of the lower volcanic rock series is 127–116 Ma; the age of volcanic rock in Shimaoshan Group of the upper volcanic rock series is 103–102 Ma. Regarding the tectonic setting of Late Jurassic volcanic rock in Nanyuan Formation, previous research determined that it formed under a different tectonic setting compared that under which the Early Cretaceous volcanic rock and both are products, respectively, before and after the end of great transformation of the tectonic regime in later Mesozoic (Xing et al. 2008; Li et al. 2009), and it is unsuitable to include the Late Jurassic volcanic rock in the lower volcanic rock series (Duan et al. 2013). Recently, some Late Jurassic volcanic rock strata were also discovered in Zhejiang and Anhui, for instance, Shiling Formation in Tunxi, south Anhui (154.7 Ma, Yu et al. 2016; 156–152 Ma, Tang et al. 2016), Huangjian Formation in Shouchang, north Zhejiang (150.9 Ma, Li et al. 2011), Dashuang Formation in Dagangtou, Lishui, southeast Zhejiang (155–152 Ma, Zhejiang Institute of Geological Survey 2013), and in Longquan (162.7–148.7 Ma, Zhejiang Institute of Geological Survey 2017). All of these ages indicate a volcanic eruption event happened in South China in Late Jurassic (163–150 Ma) but with a small scale, which may be a prelude of a large-scale volcanic activities of Early Cretaceous, or product of volcanic activities under different tectonic settings, which should be further researched in the future.
By combining comparative analysis on regional strata and chronology, it is known that Late Mesozoic volcanic activity in Zhejiang and Fujian should begin in 162 Ma (Mid to Late Jurassic); the Early Cretaceous lower volcanic rock series erupts in some 142–118 Ma in large scale, reaching peak in 135–125 Ma, which is a set of high K calc-alkaline volcanic eruptive rocks, most of which are intermediate acidic–acidic. While the upper volcanic rock series volcanic activities weaken significantly in 115–92 Ma, a set of basalt-rhyolite dual-peak combination and red layer sedimentary rocks; the regional unconformable surface between the lower and upper volcanic rock series (i.e., Minzhe movement) should have formed in about 118–115 Ma.
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