西大别造山带新元古代中期双峰式火山岩的发现对扬子陆块北缘构造演化的约束
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本文为中国地质调查局地质调查项目(编号DD20242892)和湖北省自然科学基金项目(编号2020CFB344)联合资助的成果


Identification of middle Neoproterozoic bimodal volcanic rocks in the West Dabie Orogenic Belt, Central China: Constrains for the tectonic evolution of the Yangtze Block
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    摘要:

    扬子陆块北缘新元古代中期的构造机制(是俯冲- 增生造山、还是后造山- 裂谷)是华南新元古代构造演化研究中长期争论的重要科学问题。本文在西大别造山带红安桃花地区新识别出双峰式火山岩,为进一步揭示扬子陆块新元古代中期岩浆- 构造演化过程和动力学机制提供了新的制约信息。SIMS和LA- ICP- MS锆石U- Pb定年结果表明,桃花双峰式火山岩形成年龄为750~740 Ma。基性火山岩为拉斑玄武质,显示由正常洋中脊型玄武岩向弧型玄武岩演化的微量元素特征,且具有变化的全岩εNd(t)值(0. 05~+5. 96)。这些基性火山岩可能是亏损的软流圈地幔在减压条件下部分熔融的产物,岩浆经历了结晶分异和围岩混染。酸性火山岩中,徐门寨样品富硅(SiO2≥75%)和碱(K2O+Na2O≥7%),富集REE和HFSE(如Zr、Hf、Nb、Y),其Zr+Ce+Y+Nb≥600×10-6,10000Ga/Al≥2. 6,与分异A型花岗岩类相似。它们同时具有富集的Hf- Nd同位素组成,其锆石εHf(t)值主要集中在12. 32~1. 88,而全岩εNd(t)值为13. 29~9. 60,可能是区域上新元古代早期的岩浆岩在高温低压条件下部分熔融形成的产物。综合本文和前人的资料表明,南秦岭- 桐柏- 大别- 苏鲁造山带在新元古代中期(780~635 Ma)的双峰式岩浆组合形成于一个软流圈上涌、岩石圈拉张减薄的大陆裂谷环境;高温- 低压条件下的软流圈、岩石圈地幔和地壳的熔融及其相互反应,是这一时期不同性质岩浆形成的主要原因。

    Abstract:

    The Neoproterozoic tectonic evolution of the northern Yangtze Block remains controversial, with ongoing debate whether it transitioned from compressional to extensional at 820~800 Ma or persisted as a subduction setting until around 700 Ma. This study presents new data on the Taohua bimodal volcanic rocks in the West Dabie orogenic belt, providing novel constraints on the Neoproterozoic tectonic evolution and dynamic mechanisms of the Yangtze Block. SIMS and LA- ICP- MS zircon U- Pb dating of these bimodal volcanic rocks yields consistent ages of 750~740 Ma. The basic volcanic rocks display tholeiitic characteristics and exhibit N- MORB- like to arc- like trace element patterns, with variable whole- rock εNd(t) values ranging from 0. 05 to +5. 96. These observations suggest that these basic rocks were probably generated by decompression melting of a depleted asthenospheric mantle, followed by fractional crystallization and wall- rock contamination. The acid volcanic rocks from the Xumengzhai area are enriched in silicon (SiO2≥75%), alkalis (K2O+Na2O≥7%), REE and HFSE (Zr, Hf, Nb, Y), with high Zr+Ce+Y+Nb (≥600×10-6) and 10000×Ga/Al ratios (≥2. 6), showing a close affinity to fractionated A- type granites. Their evolved zircon εHf(t) (mostly 12. 32 to 1. 88) and whole- rock εNd(t) (13. 29 to 9. 60) values indicate an evolved crustal source, potentially generated by partial melting of Early Neoproterozoic magmatic rocks in the region, followed by fractional crystallization. The integration of these new findings with previous studies of Neoproterozoic rocks in the South Qinling- Tongbai- Dabie- Sulu orogenic belt leads to the proposition that the 780~635 Ma bimodal magmas in this belt were probably formed in a continental rift environment characterized by episodic extension- related asthenosphere upwelling and lithosphere thinning during the middle Neoproterozoic. This scenario implies that melting of the asthenosphere, lithospheric mantle, and crust under high- temperature and low- pressure conditions, coupled with interactions between the asthenosphere and the lithosphere, played a prominent role in the formation of the 780~635 Ma bimodal- type volcanic rocks and their diverse magmatic geochemical features.

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徐扬,金鑫镖,邓新,刘浩,张维峰,王令占,杨振宁,李宁宁.2024.西大别造山带新元古代中期双峰式火山岩的发现对扬子陆块北缘构造演化的约束[J].地质学报,98(7):2128-2155.
XU Yang, JIN Xinbiao, DENG Xin, LIU Hao, ZHANG Weifeng, WANG Lingzhan, YANG Zhenning, LI Ningning.2024. Identification of middle Neoproterozoic bimodal volcanic rocks in the West Dabie Orogenic Belt, Central China: Constrains for the tectonic evolution of the Yangtze Block[J]. Acta Geologica Sinica,98(7):2128-2155.

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  • 收稿日期:2023-07-20
  • 最后修改日期:2023-09-11
  • 录用日期:2023-09-15
  • 在线发布日期: 2024-07-24
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