Generation of Triassic post-collisional granitoids in the Linxi region (Inner Mongolia, NE China) and crustal growth in the eastern Central Asian Orogenic Belt through melting of relict oceanic crust

Abstract

The Linxi area is located in the eastern segment of the Central Asian Orogenic Belt (CAOB), which is the largest accretionary orogen on Earth. However, it remains highly debatable about the mechanism for crustal growth through the tectonic evolution history, in particular after the closure of the paleo-Asian Ocean. Early Triassic post-collisional granitoids (241–249 Ma) from the Linxi area in Inner Mongolia, NE China are distributed along the Xar Moron fault and comprise mainly granodiorite and monzogranite. These felsic rocks show peraluminous (aluminum index, A/CNK = 1.10–1.18, A/CNK = Al2O3/(CaO + Na2O + K2O) in molar ratio) and magnesian affinities, and are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth element (LREEs) but depletion in Nb-Ta. They also have radiogenic Nd (whole-rock εNd(t) = +1.7–+3.3) and Hf (zircon εHf(t) = +8.9–+15.2) and mantle-like O isotopic compositions (zircon δ18O = 5.4–6.8‰). Compared with their Paleozoic and Cretaceous counterparts, the early Triassic granitoids have higher values of whole-rock εNd(t) and zircon εHf(t), which requires a significant contribution of juvenile crustal components during their genesis. The combined results indicate that the early Triassic granitoids originated from melting of a mixed source consisting predominantly of mafic crust with minor pelagic sediment. These crustal components represent a lithologic assemblage typically observed in relict oceanic crust. The compositional similarities between the early Triassic igneous rocks and bulk continental crust, together with their spatial distribution along the Solonker-Xar Moron-Changchun suture, suggest that melting of relict oceanic crust may have been an important mechanism for Mesozoic crustal growth after continental collision.