The ~500 Ma Asaji ultramafic–mafic intrusion in Kyushu, southwest Japan: Implications for boninitic magmatism in a late Cambrian nascent arc

Abstract

It is thought that the continental crust that ultimately became Japan (i.e., proto-Japan) formed during the early Cambrian near the margin of the South China block following its separation from the Australian block. However, the onset of timing of subduction and details of the magmatic evolution of the region are poorly understood, due mainly to the sporadic occurrence of Cambrian rocks. A high-temperature metamorphic complex is distributed throughout eastern Kyushu island, Japan. At the northeastern end of the complex, a narrow zone of ultramafic–mafic rocks includes a late Cambrian ophiolitic assemblage, namely the Asaji ultramafic–mafic intrusion. The ultramafic cumulates in the intrusion consist of harzburgite (spinel–olivine–orthopyroxene), lherzolite–olivine websterite (olivine–orthopyroxene–clinopyroxene ± spinel), and websterite (orthopyroxene–clinopyroxene ± olivine), which formed in that order during differentiation of a parental magma. The chemical compositions of clinopyroxene and spinel from the cumulates show strongly depleted features (e.g., spinel Cr# [= atomic Cr/(Cr + Al)] ≤ 0.91 and TiO2 < 0.3 wt%, and clinopyroxene YbN < 2) and comparable to those of spinel in boninites. In addition, the calculated trace element composition of melts in equilibrium with clinopyroxene from the ultramafic rocks closely resembles the composition of the North Tongan boninite. These petrological and geochemical features suggest that the Asaji ultramafic–mafic intrusion consists of cumulates from a boninitic magma. The presence of boninitic magmatism suggests that the Asaji ultramafic–mafic intrusion was the result of subduction initiation and the infiltration of water into hot asthenosphere during the formation of nascent arc crust.