Petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites of the Songnen Block, NE China: Implications for the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans

Abstract

The petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites in NE China offers new insights into the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans. This study reports new LA–ICP–MS zircon UPb ages, geochemical data, and zircon Hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the Songnen Block, NE China. Zircon UPb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca. 319 and 120 Ma, respectively. The syenogranites have high K2O contents of 4.15–4.79 wt% and (K2O + Na2O)/CaO ratios of 33–102, and low MgO (0.06–0.20 wt%) and P2O5 (0.01–0.02 wt%) contents. Their remarkably high HREE contents and Y/Nb ratios (2.07–2.40), significantly negative Eu anomalies (Eu/Eu⁎ = 0.15–0.28), and low Sr contents (29.7–85.8 ppm) indicate that the syenogranites are A-type granites. They have positive zircon εHf(t) values of 8.4–12.1 and Neoproterozoic Hf TDM2 ages of 795–561 Ma, suggesting a juvenile lower crustal source. The quartz diorite porphyries have relatively high Na2O and MgO, and moderate K2O contents. Their A/CNK values (0.95–1.18) indicate a transition from being metaluminous to weakly peraluminous. They are enriched in LILEs (e.g., Rb, K, and Ba) and depleted in HFSEs (e.g., Nb, Ta, Ti, and P). Their low Y (11.1–14.4 ppm) and Yb (0.98–1.34 ppm) contents and high Sr (589–1131 ppm), Cr, and Ni contents and Sr/Y ratios indicate that they were generated by partial melting of oceanic crust. Based on these data and regional geological investigations, we propose that the syenogranites on the northern margin of the Songnen Block were formed in a post-collision extensional setting, likely due to closure of the Paleo-Asian Ocean between the Xing'an and Songnen blocks. Formation of the Early Cretaceous quartz diorite porphyries may have been triggered by partial melting of the Paleo-Pacific flat slab. Apart from rollback of the Paleo-Pacific oceanic slab, the main reason for the eastward migration of magmatism during the Cretaceous was the trench-ward dehydration, eclogitization, and sinking of the residual Paleo-Pacific flat slab.