Diversity of Felsic Rocks in Oceanic Crust: Implications From the Neoproterozoic Plagiogranites Within the Northeast Jiangxi Ophiolite, Southern China

Abstract

AbstractSilicic igneous rocks in the oceanic crust provide insights into the formation of proto‐continental crust, which can be investigated by studying rare silicic rocks within ophiolite complexes. This study focuses on plagiogranites that crop out as lenses and blocks in the northeast Jiangxi ophiolite (NJO), located in the eastern segment of the Neoproterozoic Jiangnan Orogen, southern China. The plagiogranites yield a mean SHRIMP U‐Pb zircon crystallization age of ca. 995 Ma and also record a late thermal event at ca. 220 Ma. The plagiogranites have high Na, very low K, and highly variable rare earth element (REE) concentrations from extremely depleted total REEs to strong enrichment in light REEs. Some samples have adakitic characteristics. Zircons from the plagiogranites have positive εHf(t) values (9.96 to 16.6) and mantle‐like δ18O values (5.13 ± 0.21‰), indicating a juvenile crustal or mantle source. Based on REE modeling, we propose a two‐stage model for the formation of the geochemically variable NJO plagiogranites: (1) High‐REE adakitic magmas plagiogranites were generated by 20% partial melting of basaltic oceanic crust; and (2) subsequent fractional crystallization (up to 50%) of these magmas and crystal accumulation produced the low‐REE plagiogranites. Our results show that granitic magmas can become highly differentiated in an oceanic setting. The diversity of silicic rocks in the oceanic crust is mainly the result of partial melting of subducted basaltic rocks and subsequent differentiation rather than just fractional crystallization of basaltic magmas.