The discrimination of Jinningian MORB-like basalt and intra-oceanic subduction in the Dahongshan area, Northern Hubei

There are numerous mafic rocks e.g., gabbro, diabase, basalt, pillow basalt, fumarolic-amygdaloidal basalt, in the Dahongshan area, Suizhou City, northern Hubei Province. They are mainly in the form of block structurally mixed in a set of clastic rock, characterized by mélange of exotic blocks and matrix strata, suggesting a typical orogenic belt. The mafic rocks from Dahongshan area show the features of tholeiite series, and are geochemically enriched in incompatible elements such as Rb, Ba, K, Th and U and depleted in high field strength elements such as Nb and Ta, similar to features of island arc basalts. Nevertheless, the features of flat REE patterns (ΣLREE/ΣHREE=1.41-4.48, La_N/Yb_N=0.76-4.79, Zr/Y=2.65-5.38 and Ti/V=29.19-54.97) are the same as features of mid-ocean ridge basalt. Therefore, the geochemical signatures and regional geological characteristics show that these mafic rocks should be part of MORB-like/fore-arc basalts, formed along intra-ocean arc where the subduction-initiation happened. Their parent magma was produced by the nascent depleted MORB mantle and interacted with the contribution of fluids from the slab sinking plate with decompression melting. The basalts from Nanfengya and Lulinzhai yielded LA-ICP-MS U-Pb zircon ages of (816.6±7.6) Ma (MSWD=0.47) and (813.1±4.8) Ma (MSWD=0.37) respectively, interpreted as their crystallization age. Combined with the previous research results of gabbro in Yangjiapeng (947 Ma), pillow basalt in Changhe (824 Ma), and diabase in Luling (820 Ma), it is held that mass mafic rocks were formed in Jinningian period (817-947 Ma) in the Dahongshan area. They may be the products of multi-stage intra-ocean subduction. The discrimination of Jinningian ore-arc/MORB-like basalt in the Dahongshan suggests that it experienced a certain scale of ocean-ocean to ocean-continent subduction and orogeny between Yangtze block and Tongbai-Dabie block in Jinningian period, and the two blocks might have been aggregated together in late Qingbaikou period.