Was Triassic Continental Subduction Solely Responsible for the Generation of Mesozoic Mafic Magmas and Mantle Source Enrichment in the Dabie-Sulu Orogen?

摘要

Mesozoic mafic rocks in the Dabie-Sulu orogen (DSO) are enriched in large-ion lithophile elements (LILE, e.g., Rh, K, Sr, Ba), relatively depleted in high-field strength elements (HFSE, e.g., Nb, Ta, P, Ti), and have strongly fractionated rare-earth elements (REE) and highly "enriched" Sr-Nd isotope ratios. These "arc-type" geochemical signatures have been generally ascribed to mantle source enrichment beneath the DSO by fluids/melts released from deeply subducted Triassic continental crust, i.e., part of the South China block (SCB). However, contemporaneous mafic rocks are widespread in the North China block (NCB) and possess similar elemental and Sr-Nd isotopic characteristics. Given that most of the NCB mafic rocks were emplaced at a considerable distance from the DSO, their arc-type geochemical features are unlikely to have been caused by Triassic continental subduction. Therefore, we argue that subduction of the SCB is a potential, but nut necessarily the only, mechanism responsible for mantle enrichment beneath the DSO. We propose instead that the lithospheric mantle beneath the DSO and NCB was enriched prior to Triassic subduction, and that generation of the mafic rocks in both regions was due to a shared, regional-scale geological event, which we attribute to post-collisional delamination of the mantle lithosphere.