南阿尔金花岗岩锆石Lu-Hf同位素特征及岩石成因

Petrogenesis and zircon Lu-Hf isotopic characteristics of the granites from the southern Altun area, Northwest China

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>460Ma；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435~450Ma；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385~411Ma；第4期花岗岩组合为花岗闪长岩+二长花岗岩，时代为343~352Ma，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265Ma。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(465~469Ma)花岗岩浆活动可能与洋壳的俯冲作用有关；第2期(435~450Ma)岩浆活动可能是碰撞后环境；第3期(404~411Ma)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(343~352Ma)、第5期(265Ma)岩浆活动可能与造山带深部块体的拆沉作用有关。

On the basis of previous studies in conjunction with granite dating and geochemical study results in this paper, the SouthAltun 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 graniteand 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 ofgranite in various episodes showed that εHf(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. Basedon the results of the above studies in conjunction with regional geological characteristics, it is considered that episode-1 graniticmagma activity might be related to the subduction of oceanic crust, episode- 2 magma activity might belong to post- collisionalmagma activity, episode-3 magma activity might be related to the isostatic adjustment of orogen blocks after plate collision, andepisode-4 and episode-5 magma activity might be related to the delamination of deep blocks under the southern Altun orogen .