Permian Remelting and Maturity of Continental Crust Revealed by the Daqing Peraluminous Granitic Batholith, Inner Mongolia

Abstract

AbstractThe Phanerozoic granites in northeast China bear key information for studying the tectonic evolution and crustal growth or reworking in the Central Asian Orogenic Belt (CAOB). The Daqing granitic batholith widely outcrops as a high-level intrusion in the Xing’an-Mongolia Orogenic Belt, southeastern CAOB. Three types of enclaves in granites have been identified: (1) mafic magmatic enclaves (MMEs), (2) volcanic xenoliths, and (3) biotite-rich enclaves. The batholith is mainly composed of peraluminous biotite granite and granodiorite with SiO2=63.95−69.48 wt.%, A/CNK=1.15−1.27, and 2.54 to 4.30 wt.% of normative corundum. They exhibit remarkable enrichment in large ion lithophile elements (LILEs; K, Rb, Th, and Pb) and depletion in high field strength elements (HFSEs; Nb, Ta, and Ti), P, Eu, and Sr, as well as relatively enriched Sr-Nd isotopes (87Sr/86Sri=0.70530−0.70576, εNdt=–0.1−+0.2). Zircon U-Pb dating suggests that this batholith was emplaced in the Early Permian (ca. 283-282 Ma), consistent with a period of intensive magmatic activities in northern Inner Mongolia. The Nb/Ta ratios of MMEs (17.6-20.1) are higher than those of the host granites (11.4-12.5), together with the reaction rims where biotite crystals cluster around the amphibole cores, suggesting magma mixing between mantle- and crust-derived melts. Zircons from a biotite-rich enclave define a protolith age of ca. 320 Ma and an anatectic age at ca. 281 Ma. Whole-rock Sr-Nd isotopic modelling and zircon Hf isotopes reveal that the batholith was mainly produced by remelting of newly accreted continental crust with minor addition of mantle-derived materials. The geochemical compositions imply that their precursor magmas originated from a relatively high crustal level (<5 kbar) with crystallization temperatures ranging from 800 to 850°C. We suggest that the Daqing peraluminous granitoids were derived from partial melting of newly accreted crustal materials at a relatively shallow crustal depth, associated with a ridge subduction-related heat source. Such mantle-derived magmas through a slab tear window resulting from ridge subduction provide not only the heat for the widespread crustal remelting and therefore maturity but also juvenile materials for crustal growth.