Petrogenesis of the Middle Triassic Erenhot granitoid batholith in central Inner Mongolia (northern China) with tectonic implication for the Triassic Mo mineralization in the eastern Central Asian Orogenic Belt

Abstract

Voluminous post-orogenic magmatism holds a key to monitoring important geodynamic processes and crustal evolution within an extensional accretionary orogen. The Middle Triassic Erenhot granitoid batholith from central Inner Mongolia, northern China, is tectonically located in the eastern Central Asian Orogenic Belt. This batholith consists of monzogranite, syenogranite, alkali-feldspar granite and albite granite with minor mafic to felsic enclaves and dykes. The formation of this composite batholith involved distinct mafic and felsic magma end-members, concomitant episodic mafic-felsic interaction and intra-felsic magma evolution. Mafic (diabasic) and high-Mg intermediate (dioritic) dykes show enrichment in large ion lithophile elements, depletion in high field strength elements and heterogeneous isotopic compositions, indicative of a metasomatized lithospheric mantle source inherited from prior oceanic slab subduction. The monzogranites and aplites have arc-like elemental features and variable isotopic compositions (ISr(t) = 0.706132, εNd(t) = −0.4, zircon εHf(t) = 2.8 to 7.4, zircon δ18O = 5.32 to 6.74‰), clearly suggesting that the felsic magma end-member of a dual heritage was derived from dominant newly-underplated mafic crust and subordinate ancient lower crust. Hybridization processes between mantle-derived mafic magma and crust-derived felsic magma result in the formation of monzonitic enclaves (ISr(t) = 0.704850, εNd(t) = −0.1) with an isotopic resemblance to the host granites. Fractional crystallization of the monzogranitic magmas endows the differentiated syenogranites and alkali-feldspar granites with highly evolved elemental characteristics. We further deduce that the magmatic immiscibility between K-rich and Na-rich residual melts in a deep reservoir produces albite granites with distinct deformation imprints, variable K2O contents and wide range of crystallization temperatures. Combining with coeval regional igneous episodes as well as concomitant sedimentary and structural events in the northern China-Mongolia tract, the Triassic Erenhot composite batholith can be considered as post-orogenic magmatic products possibly within a coupled geodynamic scenario between lithospheric dripping and concurrent gravitational collapse. Therefore, the documentation of this large-scale Triassic post-orogenic magmatism could not only lead to the characterization of the eventual closure of the Paleo-Asian Ocean, but also capture a snapshot into the tectonic background for the widespread porphyry-type Mo mineralization in the eastern Central Asian Orogenic Belt during early Mesozoic.