Continental crustal growth and differentiation in intraoceanic arcs: Insights from plagiogranites in the forearc ophiolite of the southeastern Central Asian Orogenic Belt, China

Oceanic plagiogranites in subduction initiation zones record key information of continental crustal growth and differentiation. However, their petrogenesis remains controversial, due to the lack of complete subduction initiation records in most of ophiolites. In this study, we report a newly recognized plagiogranite suite (including intrusive plagiogranite porphyries and extrusive rhyolites) from the Diyanmiao forearc ophiolite in Inner Mongolia, southeastern section of the Centra Asian Orogenic Belt, where a relatively complete forearc magmatic succession is preserved. In the Diyanmiao suite, the plagiogranite porphyries, occur as small irregular dykes or pods intruding pillowed transitional lavas, and rhyolite lavas within or overlying the upper boninitic lavas, from bottom to top. Zircon U-Pb dating of two plagiogranite porphyry samples and one rhyolite sample yields mean ages of 322.5 +/- 3.6, 319.9 +/- 1.9, and 319.6 +/- 2.6 Ma respectively. Similar to other oceanic plagiogranites, the plagiogranite porphyries and rhyolites in our study exhibit high SiO2 and Na2O contents and low K2O. They have chondrite-normalized flat to slightly depleted rare earth element patterns, and significantly negative Nb, Ta, and Ti anomalies similar to arc magmatic rocks. They also display high positive epsilon Hf(t) values (+12.5 - +18.7) and epsilon Nd(t) values (+6.9 - +10.0), comparable to the values of their host rocks. Petrographic characteristics and Rhyolite-MELTS modeling suggest that the plagiogranite porphyries and rhyolites were generated by high degree of crystal fractionation from parent tholeiitic basalt magmas. Our results suggest that melt migration triggered by fractional crystallization of mafic magmas played a vital role in the growth of continental crust in intraoceanic arcs in the southeastern Paleo-Asian Ocean.