东昆仑造山带三叠纪富Nb基性岩墙的岩石成因及其对古特提斯洋俯冲过程的启示

Petrogenesis of Triassic Nb-rich mafic dykes in the east Kunlun orogen:Implications for the subduction of paleo-Tethyan ocean

富Nb基性岩在大地构造重建中扮演着重要角色。本文以首次厘定的东昆仑三叠纪富Nb基性岩为研究对象,通过岩石学、年代学和地球化学研究,探讨其岩石成因和地球动力学背景。结果显示,岩石以辉绿岩岩墙群形式产出,其LA-ICP-MS锆石U-Pb年龄为251 Ma;岩石的SiO_(2)(48.15%~54.41%)、MgO(4.33%~9.74%)和Al_(2)O_(3)(13.38%~16.41%)含量较高,属于钙碱性玄武岩系列岩石;岩石轻重稀土分馏较显著(LaN/YbN为3.43~6.85),具有弱负Eu异常(δEu为0.71~0.91),富集LILEs(Th、U和K等),亏损HFSEs(Nb、Ta和Ti等),Nb含量显著高于岛弧玄武岩(10.92×10^(-6)~20.88×10^(-6));岩石具有富集的锆石Lu-Hf同位素组成[ε_(Hf)(t)为-1.4~1.3]。综合区域资料,本文认为东昆仑早三叠世富Nb基性岩墙群形成于东昆仑古特提斯洋俯冲消减阶段的弧后伸展背景。

The Nb-rich mafic rock plays an important role in the tectonic reconstruction.In this paper,the petrology,chronology and geochemistry of Triassic Nb-rich mafic rocks in the East Kunlun orogen have been studied in order to explore their petrogenesis and geodynamic setting.The results show that the mafic rocks occurred in the form of diabase dyke swarm,with LA-ICP-MS zircon U-Pb age of about 251 Ma,have relatively high contents of SiO_(2)(48.15%~54.41%),MgO(4.33%~9.74%)and Al_(2)O_(3)(13.38%~16.41%),belonging to the calc-alkaline basalt series.The samples have significant light and heavy REE differentiation(LaN/YbN=3.43~6.85),with slightly negative Eu anomalies(δEu=0.71~0.91),enriched LILEs(such as Th,U and K),and depleted HFSEs(Nb,Ta and Ti).Especially,their Nb contents are significantly higher than that of arc basalts(Nb=10.92×10^(-6)~20.88×10^(-6)).The rocks have enriched zircon Lu-Hf isotopic compositions[ε_(Hf)(t)=-1.4~1.3].Combined with the regional geological data,it is considered that the Early Triassic Nb-rich mafic dyke swarms in the East Kunlun were formed in the back-arc extensional setting during the subduction of the paleo-Tethyan Ocean.