摘要
祁漫塔格构造带是东昆仑造山带的重要组成部分,在青藏高原北部早古生代汇聚板块边缘地质研究中具有重要意义。祁漫塔格构造带在早古生代通常被认为是弧或弧后盆地,但其早期形成阶段的地质记录尚未报道,初始俯冲的时代也有待进一步限定。本文选择了祁漫塔格东段拉陵高里河地区祁漫塔格群火山岩和闪长岩进行岩石学、地球化学、锆石U-Pb年龄学和Hf同位素研究,探讨了岩石成因和构造意义。火山岩以安山岩为主,安山岩SiO 2含量中等(54.02%~63.99%),具有低TiO 2(0.19%~0.24%)、Al 2O 3(10.67%~12.86%)、P 2O 5(0.03%~0.05%)和低FeO T/MgO比值(0.80%~1.04)的特征,具有高的MgO(7.86%~11.44%)和Mg#(63~69);闪长岩SiO 2含量为53.62%~55.81%,也具有低TiO 2(0.19%~0.25%)、Al 2O 3(10.58%~11.52%)、P 2O 5(0.02%~0.04%)和低FeO T/MgO比值(0.9~1.0)的特征,同样具有高的MgO(9.86%~11.63%)和Mg#(64~66)的特点。高镁安山岩和闪长岩微量元素组成相近,Th(0.22~1.23μg/g)、Rb(4.11~16.11μg/g)和Ba(66.7~166μg/g)含量低,Cr(286~668μg/g)和Ni(57~181μg/g)含量高,δEu=0.91~1.47,Sr/Y比值(19.7~36.1)较低,∑REE含量介于11.8~27.3μg/g,(La/Yb)N=1.3~4.3,轻重稀土分馏弱。在微量元素原始地幔标准化图解上大离子亲石元素Sr、Ba和Pb具明显的正异常,高场强元素Ta、Nb和Ti具有明显的负异常。高镁安山岩LA-ICP-MS锆石U-Pb同位素年龄为519 Ma;闪长岩LA-ICP-MS锆石U-Pb同位素年龄为520 Ma和516 Ma,两者锆石Hf同位素组成相似,εHf(t)值分布在+10.5~+15.8。上述研究结果表明,祁漫塔格东段拉陵高里河地区高镁安山岩/闪长岩源于亏损的地幔楔的部分熔融,形成于原特提斯洋俯冲早期的岛弧环境。本研究进一步证明祁漫塔格构造带古洋盆俯冲时代早于520 Ma,该认识也为恢复东昆仑造山带原特提斯洋早期俯冲演化历史提供了重要地质资料。
The Qimantagh belt is an important component of the East Kunlun orogen and plays a key role in reconstructing the early Paleozoic convergent history of the northern Tibetan Plateau.The Qimantagh belt used to be interpreted as part of early Paleozoci arc or back-arc basin,however,the geological records of the early-stage of subduction have seldom been reported and the age of subduction initiation still remains poorly constrained.In this paper,we studied the petrology,geochemistry,zircon U-Pb ages and Hf isotopes of volcanic rocks and diorite from the Qimantagh Group of the Lalinggaolihe area and discussed their petrogenesis and tectonic significance.Volcanic rocks are dominated by andesites with moderate SiO 2 contents(54.02%63.99%),low TiO 2(0.19%0.24%),Al 2O 3(10.67%12.86%)and P 2O 5(0.03%0.05%)contents and low FeO T/MgO ratios(0.801.04),high MgO contents(7.86%11.44%)and Mg#(6369).Diorites have moderate SiO 2 contents(53.62%55.81%),low TiO 2(0.19%0.25%),Al 2O 3(10.58%11.52%)and P 2O 5(0.02%0.04%)contents and low FeO T/MgO ratios(0.91.0)with high MgO contents(9.86%11.63%)and Mg#(6466).The high-Mg andesites and diorites share similar trace-elements compositions and have low Th(0.221.23μg/g),Rb(4.1116.11μg/g)and Ba(66.7166μg/g)contents,high Cr(286668μg/g)and Ni(57181μg/g)contents,moderateδEu(0.911.47)and low Sr/Y ratios(19.736.1).They also have low total REE contents(11.827.3μg/g)and(La/Yb)N values(1.34.3),indicating weak LREE/HREE fractionation.The primitive mantle-normalized trace-element diagram shows positive Sr,Ba and Pb anomalies and negative Ta,Nb and Ti anomalies.The LA-ICP-MS U-Pb zircon ages of high-Mg andesites and dorites are 519 and 520/516 Ma,respectively.Both zircons share similar Hf isotopic compositions withεHf(t)of+10.5 to+15.8.These results indicate that the andesites and diorites from the Lalinggaolihe area originated from partial melting of depleted mantle wedge in island arc setting during the early stage of subduction of the Proto-Tethyan Ocean.This study further demonstrates that the subduction of paleo-ocean basin in the Qimantagh belt predated 520 Ma,which improves the understanding on the early-stage subduction history of the Proto-Tethys Ocean in the East Kunlun orogen.
作者
王秉璋
潘彤
任海东
王涛
赵志逸
封建平
张金明
WANG Bingzhang;PAN Tong;REN Haidong;WANG Tao;ZHAO Zhiyi;FENG Jianping;ZHANG Jinming(Key Laboratory of the Northern Qinghai-Tibet Plateau Geological Processes and Mineral Resources,Qinghai Geological Survey Institute,Xining 810012,China;Bureau of Geological Exploration and Development of Qinghai Province,Xining 810001,China;Department of Geological Engineering,Qinhai University,Xining 810016,China)
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2021年第1期318-333,共16页
Earth Science Frontiers
基金
国家自然科学基金项目(41762006,41862011)
第二次青藏高原综合科学考察研究(STEP)项目(2019QZKK0702)。