南阿尔金花岗岩锆石Lu-Hf同位素特征及岩石成因

摘要

On the basis of previous studies in conjunction with granite dating and geochemical study results in this paper, the South Altun granite can be divided into five episodes:episode 1, characterized by the granite association of quartz diorite+granodiorite+monzogranite, with affinities of I-type granite and an age of >460 Ma; episode 2, characterized by the granite association of granodiorite+monzogranite+syenogranite, with geochemical affinities of I-type granite and an age of 435-450 Ma;episode 3, characterized by the granite association of monzogranite+syenogranite+alkali feldspar granite, with affinities of A-type granite and an age of 385-411 Ma;episode 4, characterized by the granite association of granodiorite+monzogranite with an age of 343-352 Ma, with geochemical affinities of S-type granite;and episode 5, characterized by the granite association of quartz diorite+monzogranite+syenogranite, with geochemical affinities of I-type granite and an age of 265 Ma. Zircon Lu-Hf isotope analysis of granite in various episodes showed thatεHf(t) values are positive in most cases and negative for a small number of inherited zircons, indicating that their source rocks were mainly juvenile crust mixed with a minor amount of ancient continental components. Based on the results of the above studies in conjunction with regional geological characteristics, it is considered that episode-1 granitic magma activity might be related to the subduction of oceanic crust, episode-2 magma activity might belong to post-collisional magma activity, episode-3 magma activity might be related to the isostatic adjustment of orogen blocks after plate collision, and episode-4 and episode-5 magma activity might be related to the delamination of deep blocks under the southern Altun orogen .%提在以往工作的基础上，结合本文的岩石地球化学研究，将南阿尔金花岗岩划分为5期，第1期花岗岩组合为石英闪长岩+花岗闪长岩+二长花岗岩，具有I型花岗岩的属性，时代>460 Ma；第2期花岗岩组合为花岗闪长岩+二长花岗岩+正长花岗岩，也具有I型花岗岩的特征，时代为435~450 Ma；第3期花岗岩组合为二长花岗岩+正长花岗岩+碱长花岗岩，具有A型花岗岩的属性，时代为385~411 Ma；第4期花岗岩组合为花岗闪长岩+二长花岗岩，时代为343~352 Ma，具有S型花岗岩的地球化学属性；第5期花岗岩组合为石英闪长岩+二长花岗岩+正长花岗岩，具有I型花岗岩的地球化学特征，时代为265 Ma。各期花岗岩锆石Lu–Hf同位素分析表明，εHf(t)值大多数为正值，少数继承性锆石为负值，反映了它们的源岩以新生地壳为主，同时，也混有少量的古大陆壳的成分。根据年代学和岩石地球化学研究结果，结合区域地质特征，我们认为第1期(465~469 Ma)花岗岩浆活动可能与洋壳的俯冲作用有关；第2期(435~450 Ma)岩浆活动可能是碰撞后环境；第3期(404~411 Ma)岩浆活动可能与板块碰撞后造山带块体均衡调整、伸展有关，第4期(343~352 Ma)、第5期(265 Ma)岩浆活动可能与造山带深部块体的拆沉作用有关。