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

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 that ε_Hf(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.