Geochemical and geochronologic constraints for Paleozoic magmatism related to the orogenic collapse in the Qimantagh-South Altyn region, northwestern China

Voluminous and discrete early Paleozoic bimodal magmatic suites are thought to be the result of post-collisional extension following the amalgamation of East Kunlun and Altyn Tagh. In this paper, four representative magmatic units were studied for their geochemical fingerprint in conjunction with geochronological studies. The 467-445 Ma Mangya mafic suite shows E-MORB type rare earth element (REE) patterns that are the result of asthenospheric interaction with a metasomatized subcontinental lithosphere. High-K calc-alkaline granodiorites, intruded at ca. 450 Ma, are characterized by high Mg#, the least fractionated REE pattern without an Eu anomaly, as well as high Sr and low Rb/Sr ratio. We interpret these geochemical signals to result from lower crustal melting of garnet amphibolite at pressures between 16 and 22 kbar. A 430-420 Ma A-type granite is interpreted to result from the melting of metaigneous rocks at middle to lower crustal depths. Lastly, a late magmatic pulse occurs between 400 and 380 Ma and is represented by the Alk granite. The Alk granite is interpreted to be a product of metapelite melts and is associated with a smaller volume of mafic melts. U-Pb zircon geochronology of Paleozoic igneous rocks of the Qimantagh-South Altyn reveals that most of the magmatic episodes are either coeval with, or post, extensional deformation. This phase of extension is supported by the exhumation of HP/UHP metamorphic rocks and crustal anatexis. Collectively, the evolutionary stages documented in this study correspond to a succession of post-collisional, postorogenic and, ultimately, within plate magmatic episodes. The overall features support orogenic collapse via removal of a thickened lithospheric root beneath the East Kunlun-Altyn Tagh collisional orogen during early Paleozoic. (C) 2014 Elsevier B.V. All rights reserved.