Early Permian A-type granites from central Inner Mongolia, North China: Magmatic tracer of post-collisional tectonics and oceanic crustal recycling

Abstract

A-type granitoids constitute volumetrically minor but genetically important building blocks of most post-Archean continental landmasses on Earth and their diversified genesis encapsulates key information for ascertaining critical geodynamic controls on continental crustal formation and differentiation. This zircon U–Pb dating and geochemical study documents the Early Permian peralkaline granites from central Inner Mongolia of China, eastern Central Asian Orogenic Belt (CAOB). These granites show an A-type magmatic affinity, with typical enrichment in alkalis, Ga, Zr, Nb and Y, depletion in Sr and P and fractionated REE patterns showing strong negative Eu anomalies. They have meaningful initial 87Sr/86Sr ratio of 0.70594, εNd(t) of +4.25 to +6.05, zircon εHf(t) from +9.8 to +16.0 and δ18O values from 2.41 to 4.72‰. These elemental and isotopic characters argue for parental magmas from partial melting of a newly trapped charnockitic lower crustal protolith and their subsequent fractional crystallization. With an inherent hydrothermally altered oceanic crustal heritage and possible tectonic linkage to post-collisional slab breakoff, this kind of A-type granites could not only serve as an insightful temporal milestone for monitoring the amalgamation between various constituent terranes in the CAOB, but also present a spatial magmatic proxy for marking the maturation stage of juvenile crustal reworking during the final phase of a large orogenic event.