A geochemical study of syn-subduction and post-collisional granitoids at Muzhaerte River in the Southwest Tianshan UHP belt, NW China

Abstract

A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock elements and Sr-Nd isotopes was carried out for two granitic intrusions at Changawuzi and Alasan in Muzhaerte River of Southwest Tianshan (NW China). SHRIMP zircon U-Pb dating gave two 206Pb/ 238U ages of 333±3Ma and 326±3Ma for the Changawuzi pluton. LA-ICP-MS zircon U-Pb dating yielded two 206Pb/ 238U ages of 293±3Ma and 294±2.2Ma for the Alasan pluton. The two groups of ages predate and postdate the UHP metamorphic event at 320-305Ma in this region, respectively, registering to syn-subduction and post-collisional magmatism. The Changawuzi pluton is composed of gneissic porphyritic tonalite and garnet-bearing gneissic granitoids, having SiO 2 of 55.1-64.5wt.%, K 2O+Na 2O of 2.58-5.07wt.%, MgO of 2.07-4.32wt.%, and Al 2O 3 of 17.4-19.9wt.% with A/CNK ratios of 0.99 to 1.27. They show arc-like geochemical affinity, with enrichment of LILE (K, Rb, Sr and Ba) and depletion of HFSE (Nb, Ta and Ti) and P, and moderate initial 87Sr/ 86Sr ratios of 0.7062-0.7071 and neutral e Nd(t) values of -1.5 to 1.6. Except for one zircon with a negative e Hf(t) value of -3.5, the others have positive e Hf(t) values of 2.5-11.4 and single-stage Hf model ages of 509 to 869Ma. The Changawuzi pluton would be formed by partial melting of possible juvenile arc-derived rocks with minor involvement of ancient continental rocks during subduction of the paleo-South Tianshan oceanic crust and subsequent arc-continent collision. The Alasan pluton consists of medium-grained and porphyritic granitoids. The medium-grained granitoids are composed of granodiorite, quartz monzodiorite and quartz monzonite, displaying SiO 2 of 59.37-65.49wt.%, K 2O+Na 2O of 6.24-8.2wt.% and MgO of 1.45-3.06wt.%, with A/CNK ratios of 0.87-1.04. They have relatively high initial 87Sr/ 86Sr ratios of 0.7082-0.7103, negative e Nd(t) values of -5.9 to -5.6 and negative e Hf(t) values of -4.5 to -1.1. The porphyritic granitoids consist of granite, granodiorite and quartz monzonite, with relatively high SiO 2 (65.94-75.27wt.%) and K 2O+Na 2O (7.55-9.13wt.%), low MgO (0.18-1.22wt.%), and A/CNK ratios of 0.82-1.11. They also have relatively high initial 87Sr/ 86Sr ratios of 0.7073-0.7103, negative e Nd(t) values of -6.0 to -0.5 and neutral e Hf(t) values of -1.4 to 2.8. Both of them also display arc-like patterns of REE and trace element distribution. They belong to post-collisional granitoids and were produced by partial melting of the Paleo-Mesoproterozoic crust of obducted Central Tianshan block with minor involvement of juvenile crust. The geological occurrences of both the syn-subduction and post-collisional granitoids in this region suggest differential tectonic processes during the Late Paleozoic collision between the Tarim and Yili-Central Tianshan Blocks. It is inferred that slab breakoff occurred subsequent to the UHP metamorphism, resulting in partial melting of the subducted upper crust and the overlying lower crust and thus formation of voluminous granitoids like the Alasan pluton. © 2011 Elsevier B.V.