Geochronology, Geochemistry, Whole Rock Sr‐Nd and Zircon Hf‐O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan, Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications,Acta Geologica Sinica-English Edition

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Geochronology, Geochemistry, Whole Rock Sr‐Nd and Zircon Hf‐O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan, Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications
Acta Geologica Sinica-English Edition ( IF 3.5 ) Pub Date : 2020-08-30 , DOI: 10.1111/1755-6724.14561
Heng ZHANG ₁ , Yanxue LIU ₁ , Xiaozhong DING ₁ , Linzhi GAO ₁ , Chun YANG ₁ , Jibiao ZHANG ₁ , Chengqiang GONG _{1,

2} , Haogang LIU _{1,

2}

Affiliation

This paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr‐Nd and zircon Hf‐O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U‐Pb dating of samples in the three areas yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 828.3 ± 8.5 Ma and 836.9 ± 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO₂ (54.08–76.80 wt%) and Al₂O₃ (12.40–21.31 wt%), low Fe₂O₃ (0.68–8.92 wt%), MgO (0.29–2.49 wt%), CaO (0.12–2.86 wt%), TiO₂ (0.10–1.59 wt%) and P₂O₅ (0.01–0.39 wt%), with variable total alkalis (K₂O + Na₂O =5.42–8.29 wt%). There exists a clear negative correlation between SiO₂ and P₂O₅. The volcanic rocks have A/CNK ratios of 1.03–2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of (La/Yb)_N values of 5.68–11.67, and with a moderate negative Eu anomaly (&Eu=0.58–0.89). Whole‐rock geochemical data shows that the Jiangshan volcanic rocks are possibly I‐type granitic rocks, even though they have some characteristics of A&S‐type granites due to the magma fractional crystallization and water‐rock interaction. Zircon δ¹⁸O values are 3.97‰–5.49‰ (average 4.50‰), 2.90‰–5.21‰ (average 4.32‰) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average δ¹⁸O values of igneous zircons in equilibrium with mantle magmas (5.3 ± 0.6‰ (2σ)), the lower δ¹⁸O value also demonstrating the presence of high temperature water‐rock interactions. The ignimbrite rocks have positive ∊_Nd(t) (4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and ∊_Nd(t) (in‐situ zircon) (4.3–14.6, a weighted mean of 8.4; 6.6–12.7, a weighted mean of 9.0; 8.1–12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle‐sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860–830 Ma volcanic rocks formed in a back‐arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan–Shaoxing suture. The first stage rift‐related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen.

中文翻译：

江南造山带东部新元古代早期火山岩的年代学，地球化学，全岩石Sr-Nd和锆石Hf-O同位素：岩石成因和构造意义的限制

本文介绍了对浙江省江山市上蜀乡上虞村，上虞镇和上虎棚村的流纹岩和火成岩进行的地质年代学，地球化学，全岩石Sr-Nd和锆石Hf-O同位素的综合研究结果。江山—绍兴缝合线的一侧。三个区域的样品的SHRIMP锆石U‐Pb测年产生的加权平均²⁰⁶ Pb / ²³⁸ U年龄为842.8±6.9 Ma和850.0±7.3 Ma，839±9 Ma和832.2±8.1 Ma，828.3±8.5 Ma和836.9±9.9麻分别。这些年龄都比分布在杭州富阳市，开化县等地区的上蜀组火山岩年龄大，大约在780 Ma。这些火山岩通常具有较高的SiO ₂含量。（54.08–76.80 wt％）和Al ₂ O ₃（12.40–21.31 wt％），低Fe ₂ O ₃（0.68–8.92 wt％），MgO（0.29–2.49 wt％），CaO（0.12–2.86 wt％），TiO ₂（0.10–1.59 wt％）和P ₂ O ₅（0.01–0.39 wt％），总碱值可变（K ₂ O + Na ₂ O = 5.42-8.29 wt％）。SiO ₂和P ₂ O ₅之间存在明显的负相关。火山岩的A / CNK比为1.03-2.77，因此呈铝质。它们的特点是富集LREE，Rb，Ba，Zr，Hf，K，Th，La，U和Nb，Sr，P，Ti贫化，并具有（La / Yb）独特的LREE和HREE分馏_N值为5.68-11.67，且Eu异常中等（＆Eu = 0.58-0.89）。全岩地球化学数据表明，江山火山岩尽管具有岩浆分步结晶作用和水岩相互作用，但具有一定的A＆S型花岗岩特征，可能为I型花岗岩。锆石δ ^18个O值是3.97‰-5.49‰（平均4.50‰），2.90‰-5.21‰（平均4.32‰），用于熔结凝灰岩分别从上戍村部，和Shanghupeng村部。它们比一般略低δ ^18个火成岩锆石O值与地幔岩浆（5.3±0.6‰（2σ）），下平衡δ ¹⁸O值也表明存在高温水岩相互作用。所述熔结凝灰岩岩石具有正ε_钕（吨）（4.02，3.37，3.91，4.74，2.85，4.39，从所述三个区域总数）和ε_的Nd（吨）（原位锆石）（三个区域中分别为4.3-14.6，加权平均值为8.4； 6.6-12.7，加权平均值为9.0； 8.1-12.0，加权平均值为9.5）。结合微量元素研究，他们指出江山火山岩的源区主要由未成年的下地壳物质和一些地幔源岩浆混合而成。详细的元素和同位素数据表明，江山火山岩是在大陆弧环境中形成的。有一系列的ca。860-830 Ma火山岩在江南造山带东部的南缘，沿江山-绍兴缝合线的西北侧，在一个弧后伸展的环境中形成。第一阶段与裂谷有关的食源性岩浆作用可能最早发生在大约江南造山带东部860 Ma。

更新日期：2020-08-30

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