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俯冲陆壳部分熔融形成埃达克质岩浆

ISSN号：1006-7493
期刊名称：《高校地质学报》
时间：0
分类：P588.121[天文地球—岩石学;天文地球—地质学]
作者机构：[1]中国地质大学地质过程与矿产资源国家重点实验室和地球科学学院,武汉430074, [2]西北大学大陆动力学国家重点实验室,西安710069
相关基金：国家自然科学基金（40234052和40521001）和教育部长江学者和创新团队发展计划（IRT0441）项目资助

关键词：关山花岗岩, 埃达克质岩, 岩石成因, 俯冲陆壳, 祁连山, Guanshan granite, adakitic rock, petrogenesis, subducted continental crust, Qilian Orogenic Belt

中文摘要：

在岛弧背景,埃达克质岩浆形成于俯冲洋壳板片的部分熔融已得到共识,但在大陆碰撞背景,埃达克质岩浆是否形成于俯冲陆壳的部分熔融尚未有研究报导.对祁连山东南部关山花岗岩（229 Ma）的地球化学和岩石成因研究提供了俯冲陆壳部分熔融形成埃达克质岩浆的一个实例.关山花岗岩以高K（K2O=4.12%～5.16%,K2O/Na2O=0.97～1.64）、高Sr/Y比值（13.6～84.1）、低Y（6.8×10^-6～15.7×10^-6）和低HREE（eg.Yb=0.62×10^-6～1.31×10^-6）为特征,并具有强分异的稀土元素组成模式[（La/Yb）N=17.5～41.6]和演化的Sr-Nd同位素组成[初始^87Sr/^86Sr=0.70587～0.70714,εNdd（t）=-10.9～-5.16,tDM=1.10～1.49 Ga].这些地球化学特征表明关山花岗岩属于大陆型（C型）埃达克质岩石,而明显不同于俯冲洋壳板片或底侵玄武质下地壳部分熔融形成的埃达克岩.关山花岗岩Pb-Sr-Nd同位素组成与商丹断裂北侧的祁连山前寒武纪基底岩石、早古生代火山岩和花岗岩类存在显著差异,但类似于商丹断裂南侧秦岭早中生代花岗岩类的Pb-Sr-Nd同位素组成,由此认为具有埃达克质的关山花岗岩的岩浆来自于南部俯冲陆壳物质的部分熔融,并提出了大陆碰撞背景中埃达克质岩浆产生的一个新的地质模型.

英文摘要：

Present produced by partial geological models for adakitic magma generation assume that most adakitic magmas were melting of subducted oceanic slabs. However, it is also possible that the adakitic magma may be generated by partial melting of subducted continental crust in continental collisional settings. Here, we present a case study of the Guanshan granite （229 Ma） from the southeastern part of the Qilian Orogenic Belt. The Guanshan granite is characterized by high K （K2O：4.12～5.16 %; K2O/Na2O： 0.97～1.64） and Sr/Y （13.6～84.1）, low Y （6.8×10^-6～15.7×10^-6） and HREE （eg. Yb： 0.62×10^-6～1.31×10^-6）, strongly fractionated REE patterns with （La/Yb）N = 17.5～41.6 and evolved Sr-Nd isotopic compositions （^87Sr/^86Sr）229Ma = 0.70587～0.70714, εNd（229 Ma） = -10.9～-5.16, tDM = 1.10～1.49 Ga）. These geochemical signatures show that the Guanshan granite has an affinity with continent-type adakitic rocks, but are distinct from the adakites, produced by partial melting of subducted oceanic slabs and/or underplated basaltic lower crust in arc settings. Pb-Sr-Nd isotopic compositions of the Guanshan granite are quite distinct from those of the Precambrian basement rocks, the Early Paleozoic volcanic rocks and the Early Paleozoic granitoids in the Qilian belt of the southern margin of the North China plate, but are similar to those of the lower crust of the northern margin of the Yangtze （South China） plate, as probed by the Early Mesozoic granites of the northern margin of the Yangtze plate. It is proposed that the magma for the Guanshan granite was derived from partial melting of the subducted Yangtze continental crust during the Triassic continental collision between the North China and Yangtze plates. Our result provides a new model that adakitic magma can be generated by partial melting of subducted continental crust in continental collisional settings.