Further Study on Geochemical Characteristics and Genesis of the Boninitic Rocks from Bikou Group, Northern Yangtze Plate

Compared with the major and trace elements of typical boninite, the metabasalts collected from the Nanfanba （南范坝）-Miaowanli （庙湾里） region in the Bikou （碧口） block could be treated as boninite characterized by low-Si, low-Ti, low-P, high-Mg^2 and high Al2O3/TiO2, consistent with geochemical features of boninite. The normal mid-ocean ridge basalt （N-MORB） normalized spider diagram displays fairly depleted high field strong elements （HFSE） （Zr, Y, Ti）. Enriched refractory elements （Cr, Co, Ni） as well as light rare earth elements （LREE）-depleted chondrite-normalized REE distribution patterns suggest the boninitic magmas are derived from an extremely depleted mantle wedge in the presence of a hydrous fluid, meanwhile signifying the source region had previously undergone a high degree partial melting process yielding primary magmas with enriched large ion lithophile elements （LILE）. In addition, almost all the samples in the Nb-Zr-Y and Ti-Zr-Y discrimination diagrams were plotted in the island arc basalt （IAB） field. Coupled with the island arc tholeiitic （IAT） basalt in the study region, therefore, the geochemical characteristics of the studied rocks indicate the meta-basalts probably occurred in a fore-arc subduction setting. This conclusion may be of great significance for the further study of the tectonic background of the Bikou volcanism.