Early Paleozoic collision-related magmatism in the eastern North Qilian orogen, northern Tibet: A linkage between accretionary and collisional orogenesis

Dong Fu1, Timothy Kusky1,?, Simon A. Wilde2, Ali Polat3,1, Bo Huang1, and Zhipeng Zhou1
1Center for Global Tectonics, School of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
2Department of Applied Geology, Curtin University, Perth, Western Australia, Australia
3Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
ABSTRACT 

Collision-related magmatism in accretionary-to-collisional orogens records a tectonic transition from early subduction accretionary processes to collisional orogenesis, and also plays a signifcant role in continental growth. Here, we present an integrated study of feld observations, geochemistry, whole-rock Rb-Sr and Sm-Nd isotopes, and zircon U-Pb ages and Lu-Hf isotopes for the Laohushan mafc to felsic magmatic rocks related to initial collision between the Alxa terrane and the Central Qilian block along the North Qilian orogenic belt, northeastern Tibet. The Laohushan magmatic rocks are dominated by quartz diorites (ca. 426 Ma), with minor tonalites enclosing dioritic enclaves (ca. 430 Ma) and hornblendite xenoliths (ca. 448 Ma), and some coeval dolerite dikes (ca. 427 Ma) intruded into the accretionary complex. The quartz diorites are characterized by light rare earth element (LREE)- and large ion lithophile element (LILE)-enrichment but have high feld strength element (HFSE)-depleted trace element patterns and negative initial εNd (–1.6 to –2.9) and positive zircon initial εHf (+3.0 to +6.2) values. The dioritic enclaves are also characterized by LREE-enriched and HFSE-depleted patterns and have mostly negative initial εNd (–9.2 to +0.03) but positive zircon initial εHf (+3.0 to +5.9) values. These geochemical and isotopic features, together with isotopic mixing calculations, suggest that the quartz diorites were likely derived from partial melting of the lower crust domi  nated by accreted mafc oceanic rocks with minor sediments, whereas the dioritic enclaves originated from underplated mantlederived magmas mixed with crust-derived melts. The hornblendite xenoliths have high MgO, Cr, and Ni contents, positive Th, U, and Pb anomalies, and negative Nb, Ta, and Ti anomalies. They have negative initial εNd (–2.8), near chondritic zircon initial εHf (–0.4 to +1.4) values and an Archean Nd model age (TDM = 2.74 Ga), suggesting that the hornblendites were likely produced by partial melting of subcontinental lithospheric mantle peridotite that was metasomatized by subduction-related melts beneath the Archean–Proterozoic Alxa terrane. We propose that partial melting of the lower crust of the early Paleozoic North Qilian orogenic belt was in response to slab breakoff and asthenospheric upwelling during the initial stage of collisional orogenesis. This study demonstrates that heterogeneous magma sources, involving accretionary materials (i.e., accreted oceanic crust and sediments) and various mantle-derived components, were mixed to form the collision-related magmatic rocks. It also highlights the signifcance of collisionrelated magmatism in continental growth and stabilization of newly-assembled crust in accretionary-to-collisional orogens.