Parent magma of the Cretaceous Northern Kyushu batholith and heat source of large-scale igneous activity: Petrological constraints on magmatism of the Northeastern Asian continental margin in the Cretaceous period, example for the Kita-Taku mafic complex in northern Kyushu, SW Japan

Abstract

The Japanese island in the Northeastern Asian continental margin is located in a subduction zone, and granitoid covers about 30% of the island arc. The Cretaceous Northern Kyushu batholith (CNKB) is classified as 19 bodies based on previous studies and consists of a granitic body and a small amount of associated mafic bodies. Of these, the Kita-Taku mafic complex body (KMC) mainly consists of gabbroic rocks and can be classified into cumulus textures (Cumulus group: Types 1–4) and those without (Non-cumulus group: Types 5–7), which can provide information on the heat source of the large-scale igneous activity in northern Kyushu from the Northeastern Asian continental margin at Cretaceous period.Melt compositions inferred from the high Mg# pyroxene compositions of the olivine-bearing pyroxene hornblendite (Type 1) member of KMC were estimated using melt-pyroxene partition coefficients. The clinopyroxene-bearing equilibrium melts are similar to the Sanukitic high-Mg andesite (HMA) in multi-element and rare-earth element patterns. Besides, the trace element and rare earth element patterns of the Cretaceous high-Mg diorites (HMDs) in northern Kyushu have similar patterns to the clinopyroxene-bearing equilibrium melts of the KMC and Sanukitic HMA magma. This implies that Sanukitic HMA magmas were substantially activated in northern Kyushu during the Cretaceous period.Analysis of the parent magma (heat source) and the small-scale plutonic bodies that consist of CNKB indicates that the essential granitic magma is formed by partial melting of the lower crust and mixing with mantle-derived Sanukitic HMA magma. The Sanukitic HMA magma could be the parent magma for the heat source and crustal growth. The lithofacies of CNKB can be reconstructed from the examination of small-scale plutonic bodies due to the mixing and assimilation of various end-member magmas and crustal materials, and their associated differentiation and accumulation processes. As the Northeastern Asian continental margin during the Cretaceous is assumed to be situated within the extensional setting due to slab rollback, the results of this study provide new insights into the complicated batholith formation processes and heat source.