摘要
东昆仑造山带出露有一套奥陶纪基性岩墙,对研究东昆仑造山带早古生代活动大陆边缘与原特提斯洋相关的俯冲过程具有重要意义.本文对出露于东昆仑东段塔妥—清水泉地区的基性岩墙进行锆石U-Pb年代学、全岩地球化学及Sr-Nd同位素等研究.两件辉绿岩样品的锆石U-Pb测年结果表明该基性岩墙形成年龄为486～483 Ma,形成时代为早奥陶世.全岩地球化学结果表明该套基性岩墙具有低SiO2(50.06％～51.68％)、中等MgO(5.08％～6.77％)和TiO2(0.99％～1.20％)含量的特征,属于拉斑玄武岩系列.在微量和稀土元素方面,岩石呈现大离子亲石元素(Rb、Sr、Ba、Th、K)富集和高场强元素(Nb、Ta、Ti)为负异常的特点;球粒陨石标准化稀土元素配分图呈现轻稀土元素弱富集、重稀土元素亏损以及弱的Eu正异常特征.本研究中的样品具有高(87Sr/86Sr)i值(0.7126～0.7133)和相对低的εNd(t)同位素组成(-9.80～-4.31),综合研究表明该基性岩浆起源于俯冲带流体交代的富集地幔,形成于大洋俯冲带上盘的岩浆弧(伸展)构造环境.结合区域地质资料,本文认为东昆仑原特提斯洋在早古生代早期向北俯冲,并于早奥陶世俯冲大洋岩石圈板块后撤,进而导致俯冲带上盘出现伸展构造背景.
Abstract
Early Ordovician mafic dykes exposed in thenorth Kunlun are of great significance for understanding the subduction processes and dynamic background of the paleo-active continental margin.In this paper,we present detailed petrology,zircon U-Pb chronology,whole-rock geochemistry and Sr-Nd isotopes of the mafic dykes.Zircon U-Pb dating results show that the zircon U-Pb ages of these dykes range from 486 to 483 Ma,suggesting an Early Ordovician age.Whole-rock geochemical results show that the mafic dykes have low SiO2(50.06％～51.68％),medium MgO(5.08％～6.77％)and TiO2(0.99％～1.20％)content,and belong to the tholeiitic basalt series.Regarding trace and rare earth elements,the rocks are enriched in large-ion lithophile elements(e.g.,Rb,Sr,Ba,Th,K)and depleted in high-field-strength elements(e.g.,Nb,Ta,Ti).The chondrite-normalized REE diagrams are featured by weak enrichment of LREE and depletion of HREE,with weak Eu anomaly.In addition,the dykes show high(87Sr/86Sr)i(0.7126～0.7133)and relatively low εNd(t)isotopic composition(-9.80～-4.31)of the enriched lithospheric mantle.The petrogenesis research indicates that the basic magma originated from the enriched lithospheric mantle,was metamorphosed by fluids in the subduction zone,and subsequently experienced fractional crystallization mainly of olivine and clinopyroxene during the diagenetic processes.Based on the regional geological data,we argue that the Kunzhong Ocean subducted northward in early stages of the Early Paleozoic and formed a magmatic arc system.It may have experienced the retreat of the subducting oceanic slab at 486 Ma,leading to an extensional tectonic setting in the upper plate of the subduction zone and then generation of mafic dykes.
维普
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