东昆仑印支晚期埃达克质花岗岩的成因和地质意义

Petrogenesis and geological significance of the Late Indosinian adakitic granites in the East Kunlun Orogen

东昆仑造山带印支期的碰撞造山过程目前尚存在争议,在东昆仑小南川地区新发现的三叠纪埃达克质花岗岩为约束碰撞造山演化提供了新的地质证据。研究通过对小南川地区出露的磨石沟和本头山2个花岗岩体开展岩石学、地球化学、锆石U-Pb和Lu-Hf同位素分析,探讨其岩石成因和构造环境,并结合以往东昆仑印支晚期岩浆作用和沉积作用的研究成果,初步讨论了东昆仑印支造山带的碰撞造山过程。磨石沟岩体岩性为花岗闪长岩和二长花岗岩,形成时代为209~208 Ma;本头山岩体岩性为花岗闪长岩,形成时代为201~200 Ma。2个岩体的花岗岩含较高的SiO_(2)和Al_(2)O_(3),富碱且相对富钠,同时含较高的Sr(398×10^(-6)~613×10^(-6))和Sr/Y比值(50~97),亏损重稀土,无Eu异常,表现出埃达克质花岗岩的地球化学特征。磨石沟花岗岩具有负的全岩ε_(Nd)(t)值(-3.60^(-)3.34)和变化的锆石ε_(Hf)(t)值(-1.3~5.9),表明其来源于加厚下地壳的部分熔融。本头山花岗岩具有负的全岩ε_(Nd)(t)值(-1.65^(-)1.55)和正的锆石ε_(Hf)(t)值(+3.4^(+)7.3),表明其来源于变质基性岩组成的加厚下地壳,残留相为榴辉岩。磨石沟岩体和本头山岩体花岗岩形成于东昆仑印支造山带碰撞后伸展的构造环境。综合分析表明,东昆仑造山带在晚三叠世处于碰撞和碰撞后阶段,而碰撞后阶段的岩浆活动可以进一步划分为晚三叠世早期和晚三叠世晚期—早侏罗世初期2个阶段。

[Objective]The Indosinian collision process of the East Kunlun Orogenic Belt remains a subject of debate.The newly discovered Triassic adakitic granites in the Xiaonanchuan area of East Kunlun provide new geological evidence that constrains the evolution of collisional orogenesis.[Methods]This study conducted petrological,geochemical,zircon U-Pb,and Lu-Hf isotopic analyses of the Moshigou and Bentoushan granitic intrusions in the Xiaonanchuan area to investigate their petrogenesis and tectonic settings.By integrating previous research on magmatism and sedimentation during the late Indosinian period within the East Kunlun Orogenic Belt,a preliminary discussion was conducted on collisional orogensis process.[Results and Conclusion]The Moshigou intrusion consists of granodiorite and monzogranite with zircon U-Pb ages of 209–208 Ma.The Bentoushan intrusion is composed of granodiorite with zircon U-Pb ages of 201–200 Ma.These granitoids have high SiO_(2) and Al_(2)O_(3) contents and are rich in sodium.They also have high Sr contents(398×10^(−6)–613×10^(−6))and Sr/Y ratios(50–97)and are depleted in heavy rare earth elements without Eu anomalies,exhibiting typical geochemical characteristics of adakitic rocks.The Moshigou granitoids have negative whole-rock ε_(Nd)(t)(−3.60 to−3.34)and variable zirconεHf(t)(−1.3 to+5.9),indicating their derivation from the partial melting of the thickened lower crust.The Bentoushan granitoids have negative whole-rock ε_(Nd)(t)(−1.65 to−1.55)and positive zircon ε_(Hf)(t)(+3.4 to+7.3),suggesting their origin from meta-basic rock-dominated thickened lower crust with eclogite residue.Significance These results suggest that they were formed in a post-collisional extension setting.A comprehensive analysis indicates that the East Kunlun Orogenic Belt was in the collision and post-collision stages during the Late Triassic.The post-collision stage can be further divided into two phases of magmatic activity:early and late phases of the Late Triassic.