Chemical contrast of the Late Cretaceous granitoids of the Sanyo and Ryoke Belts, Southwest Japan: Okayama-Kagawa Transect

摘要

Late Cretaceous granitoids (41 samples) and gabbroids (6 samples) of the Inner Zone of SW Japan are studied chemically and microscopically in the southern Okayama - Kagawa Prefectures. The plutonic rocks are divided into the Sanyo Belt at north and the Ryoke Belt at south by distribution of the Ryoke metamorphic rocks and tungsten deposits. The former is composed of coarse-to medium-grained, massive granodiorite to monzogranite of batholithic units, which often contain pink K-feldspar, and fine-grained granitoids of various composition. The latter consists of coarse- to medium-grained, massive or stressed granodiorite to monzogranite of batholithic units, and fine-grained biotite monzogranite stocks of Aji granitoids. As compared with the Ryoke granitoids, the Sanyo granitoids are higher in silica and alkali contents, especially of K_2O, and poorer in Al_2O_3 and lower in alumina saturation index (ASI). Rb is also rich in the Sanyo granitoids, particularly of muscovite-biotite leucogranite, and poor in the Aji (hornblende-)biotite granitoids. Sr contents show a reverse correlation with Rb. Therefore the Rb/Sr ratio is the highest in the leucogranite (approx 39) and lowest in the Aji granitoids (0.2). The Rb/Sr ratio implies degree of mgmatic fractionation. The fractionated leucogranite is also high in trace amount of Y, W, Sn, U, Th, Ta and Nb; many of these elements are concentrated in the surrounding ore deposits. Chondrite normalized REE patterns can be grouped into (1) those with intermediate values on LREE, which become low from Eu to HREE; observed on the Sanyo granitoids and Ryoke granitoids, (2) those having higher values in all the elements than (1) with moderate Eu anomalies; observed typically on the pink Mannari Granite, and (3) flat pattern rich in HREE with strong Eu anomalies, observed not on the Aji Granite but on the fractionated leucogranite. The first pattern can be explained by the presence of garnet and amphibole in the protolith; the third pattern may be due to plagioclase fractionation. The Ryoke granitoids, the Aji granitoids and garnet-muscovite-biotite granite in particular, have higher ASI and delta~(18)O value but lower K_2O content than the Sanyo granitoids, implying higher percentage of sedimentary rocks without abundant illites incorporated in the protolith, although both belong to magnetite-free, ilmenite series. Bulk compositions of the Ryoke granitoids are more mafic than the Sanyo granitoids. Mafic igneous rocks are rare in the Sanyo Belt, but are common in the Ryoke Belt where magma mingling and mixing textures are observed at many places. The mafic magmas injection from the upper mantle have occurred almost continuously in the Ryoke Belt during the granitic magmatism, an early phase of which resulted in the regional metamorphism of the Ryoke Belt, and also mingling and mixing of the mafic magmas from depth and felsic magmas from the crust made the bulk composition more mafic in the Ryoke Belt than in the Sanyo Belt.