南秦岭地体北部武当群和耀岭河群的形成时代及地质意义

Eastern Qinling orogenic belt, which is located between the North and South Bolcks, has undergone complicated evolution histories before early Triassic. Neoproterozoic is an critical period when terrains and (micro-)blocks were interacting actively for the tectonic evolution of the eastern Qinling orogenic. In South Qinling terrain, Neoproterozoic volcanic-sedimentary rocks record regional magma events and tectonic background and would offer significant geological proofs to settle dispute in the eastern Qinling orogen evolution history from Neoproterozoic to Paleozoic. This project selects the Neoproterozoic Wudang and Yaolinghe Groups in Pingli-Zhuxi (juncture of Shaanxi and Hubei Provinces) areas, which are mainly composed of meta volcanic - sedimentory sequences, as main subjects. Zircon U-Pb chronology, Hf isotopic compositions, petrogeochemistry and isotope geochemistry would help to solve age, source and tectonic setting problems. In comparation with other (micro-)blocks, we plan to construct a whole Neoproterozoic magma-tectonic event sequence and to discuss the crustal evolution history for the South Qinling terrains, and then to state objectively the evolution history of the eastern Qinling orogen in Neoproterozoic.

华北和华南陆块之间的东秦岭造山带在三叠纪早期最终形成之前，经历了复杂的构造演化历史。新元古代是东秦岭造山带及其相邻块体相互作用的重要时期，也是其构造演化的关键时期。南秦岭地体新元古代火山-沉积岩系很好的记录了区域构造-岩浆事件，可以为解决东秦岭造山带新元古代-古生代的演化争议提供重要的地质依据。本项目选取东秦岭造山带南秦岭地体北部陕西平利至湖北竹溪地区出露的新元古代武当群和耀岭河群火山-沉积岩系为研究对象，通过锆石U-Pb年代学、Hf同位素和地球化学特征研究，厘定该岩系的形成时代、物质来源及源区特征。并通过与相邻地体的对比研究，建立南秦岭地体新元古代构造-岩浆事件的年代学框架，探讨其在南秦岭地体地壳演化中的作用及大地构造意义，揭示东秦岭造山带新元古代构造演化历史。

南秦岭地体是东秦岭造山带的重要组成部分，新元古代之前一直被认为与扬子北缘有亲缘关系。新元古代耀岭河群火山-沉积岩是研究该地体构造-岩浆演化的一个重要窗口。基于不同的时空分布和地球化学特征，耀岭河群火山岩揭示了三个不同的岩浆期次。第一期次以拉斑玄武岩为主，形成时代为847 ± 8 Ma，具有大离子亲石元素(LILE, 如Th, U)的轻稀土元素(LREE)富集，高场强元素亏损(HFSE, 如Nb, Ta, Zr, Hf, Ti)的特征，这与典型岛弧玄武岩的性质类似。初始Nd同位素比值143Nd/144Nd =0.51162 - 0.51172，εNd(t) =+1.6 - +3.5，表明岩浆源区为与俯冲作用相关的亏损地幔。第二期次显示完整的玄武岩-安山岩-流纹岩组合，形成时代集中于735Ma和770Ma，微量元素同样具有岛弧火山的的LILE富集和HFSE亏损的特征。其中玄武岩具有较低的初始Sr同位素比值87Sr/86Sr = 0.7030 to 0.7052 和较高的初始Nd同位素比值143Nd/144Nd =0.51172 - 0.51194，εNd(t) =+0.9-+5.3，中酸性火山岩初始Nd同位素比值较低，143Nd/144Nd =0.51150 - 0.51177，εNd(t) =-4.5 +1.9，表明这套火山岩形成过程中俯冲板片流体/熔体交代岩石圈地幔使其发生富集。第三期次岩浆活动以耀岭河群最年轻的火山岩及同时代的基性侵入岩为代表，具有HFSE亏损不明显和LILE、Ti微弱富集的特征，显示板内玄武岩特征。后者的SIMS U-Pb年龄为651 ± 5 Ma，显示较高的初始143Nd/144Nd（0.51251 to 0.51271），较低的206Pb/204Pb (16.96-17.45) 和中等的87Sr/86Sr (0.7043-0.7076)，εNd(t) = +0.2 - +3.3，表明这些新元古代晚期岩浆活动应该与弧后环境下形成的陆内裂解有关。耀岭河群火山岩和基性侵入岩的年代学和地球化学特征揭示攀西-汉南-米苍山弧岩浆体系向北延伸到南秦岭地体，扬子北缘在新元古代经历了俯冲-裂解岩浆作用。