南秦岭东段耀岭河群、陨西群、武当山群火山岩和基性岩墙群岩石成因

Petrogenesis of the Yaolinghe Group,Yunxi Group,Wudangshan Group Volcanic Rocks and Basic Dyke Swarms from Eastern Part of the South Qinling Mountains

南秦岭东段新元古代中—晚期(676～833Ma)耀岭河群、陨西群、武当群火山岩和基性岩墙群产生于大陆板内裂谷环境。根据岩石地球化学数据,南秦岭东段新元古代中—晚期裂谷基性熔岩和基性岩墙总体上属于低Ti/Y(<500)岩浆类型。元素和同位素数据表明,南秦岭东段新元古代中—晚期裂谷基性熔岩和基性岩墙的化学变化不是由一个共同的母岩浆结晶分异作用所产生,它们极有可能是源于地幔柱源(εNd(t)≈+5,Mg#≈0.7,La/Nb≈0.7)。大陆地壳或大陆岩石圈混染作用对于南秦岭东段新元古代中—晚期裂谷基性熔岩和基性岩墙的形成有重要贡献。我们的研究揭示,南秦岭东段新元古代中—晚期火山岩和基性岩墙存在空间上的地球化学变化。它们总体上是产生于幔源石榴子石稳定区。而西北部镇安地区耀岭河群基性熔岩的母岩浆则是形成于幔源尖晶石-石榴子石过渡带。碱性熔岩是产生于部分熔融程度较低(<10%)的条件下,拉斑玄武质熔岩是产生于部分熔融程度较高(10%～30%)的条件下。武当山地区的武当山群和耀岭河群基性熔岩的母岩浆经受了浅层位辉长岩质(cpx+plag±ol)分离作用,而其他地区基性熔岩和基性岩墙的化学演化则是受控于单斜辉石(cpx)±橄榄石(ol)分离作用。

The mid-late Neoproterozoic （676 - 833 Ma） Yaolinghe Group, Yunxi Group, Wudangshan Group volcanic rocks and basic dyke swarms from eastern part of South Qinling Mountains produced an intracontinental rift setting. On the basis of geochemical data, the mid-late Neoproterozoic rift-related basic lavas and basic dykes from eastern part of South Qinling Mountains generally belong to a low Ti/Y （〈500） magma type. Elemental and isotopic data suggest that the chemical variations of the mid-late Neoproterozoic rift-related basic lavas and basic dykes from eastern part of South Qinling Mountains can not be explained by crystallization from a common parental magma. They most likely originated from a mantle plume source （εNd （t） ≈ ＋ 5, Mg #≈ 0. 7, La/Nb ≈ 0. 7） . Continental crustal or continental lithosphere contamination has also contributed significantly to the formation of the mid-late Neoproterozoic rift-related basic lavas and basic dykes from eastern part of the South Qinling Mountains. Our data indicate that the spatial geochemical variations existed in the mid-late Neoproterozoic rift-related volcanic rocks and basic dykes from eastern part of South Qinting Mountains. They were generally generated in a mantle source within the garnet stability field. In contrast, in the northwestern Zhen＇ an area the parental magmas of the Yaolinghe Group basic lavas were generated from a mantle around the spinel-garnet transition zone. The alkaline lavas were generated by lower degrees （〈10%） of partial melting and the tholeiitic lavas were generated by higher degrees （10 % - 30%） of partial melting. The parental magmas of the Wudangshan Group and Yaolinghe Group basic lavas from Wudangshan area were then subjected to shallow level gabbroic （cpx ＋ plag ± ol） fractionation and the chemical evolution of the basic lavas and basic dykes from other areas was controlled by a clinopyroxene （cpx）± olivine （ol） fractionation.