Compositional variation of the late Cretaceous–Paleogene plutons from southwest Japan and its implication for ore genesis and continental growth

Abstract

During the late Cretaceous–Paleogene period, the fluids liberated from the subducting slab along Japanese-arc subduction zone could have triggered the partial melting of the mantle wedge, whose composition was similar to the fertile primitive mantle. Underplating of the produced basaltic magmas beneath the continental crust could further facilitate the partial melting of the lower continental crusts. The primary magmas produced by the mixing of resultant partial melts underwent various degrees of fractional crystallization processes to produce the late Cretaceous–Paleogene plutons in southwest Japan, and have contributed to the growth of continental crust with new, non-recycled materials during the Phanerozoic.The factor analysis of chemical data from ore-barren Ryoke and W–Sn-ore-rich Sanyo plutonic rocks identifies three major groups of elements: the mafic group (Al, Fe, Mg, Mn, Ca, Sr, Zn, Co, V, P, Cu, Ni, and Cr); the felsic group (Si, K, Rb, Cs, Tl, Pb, Y, Nb, Ta, Hf, Ge, Sn, W, Th, U, and HREE); and the LREE+Zr group (Ba, La, Ce, Pr, Nd, Eu, Zr, and Hf). Mafic group and felsic group elements are inversely correlated. The concentrations of LREE+Zr group elements increase first, then, decrease with increasing SiO2 content. Therefore, the high silica samples of the Ryoke+Sanyo belts are low in LREE+Zr group elements. In the Ryoke samples Mo behaves like Zr, which results in the lowest Mo content in the Ryoke samples with high silica. The high Mo and Si contents in some Sanyo samples suggest that the zircon-compatible Mo4+ may be converted to incompatible Mo6+ under a relatively high oxygen fugacity condition at the late stage of magmatic differentiation. W and Sn behave as the felsic group elements in the Ryoke+Sanyo samples. Thus, their concentrations are high in the high silica granitoids, especially rock samples from the Sanyo belt, which are often associated with W–Sn ores. The factor analysis of the Sanin plutonic samples indicates that W still belongs to the felsic group, but Sn no longer correlates with W and behaves more like Zr due to the conversion of incompatible Sn2+ to zircon-compatible Sn4+ under a relatively high oxygen fugacity condition in the Sanin belt. Therefore, Sn content is low in high silica granitoids from Sanin belt. The high silica Sanin samples are often enriched in Mo, again suggesting the conversion of zircon-compatible Mo4+ into incompatible Mo6+ in the magmas of those samples. The high Mo granitoids from the Sanin belt are probable source rocks for the associated Mo ores in the area.