Geochemical characteristics of whole rock and minerals from the Late Cretaceous granitic rocks of the Rokko Mountains

Abstract

The Rokko Mountains consist primarily of late Cretaceous granitic rocks. These are recognized as Nunobiki Granodiorite, Yahatadani Granite, Dobashi Quartz Diorite, and Rokko Granite. Based on grain size and texture, the Nunobiki Granodiorite is subdivided into a porphyritic and equigranular facies, whereas the vertically zoned Rokko Granite pluton is differentiated into fine-grained, medium -to coarse-grained, and micrographic facies. The fine-grained facies occurs at high altitude, and the medium -to coarse-grained granite is found at an intermediate to low altitude within the Rokko Mountains. The micrographic facies intrudes both of these varieties. The granitic rocks were analyzed for trace and rare earth elements and mineral composition. The fine and micrographic facies of the Rokko Granite have higher Rb, Nb, and Y, and lower Sr and Zr when compared with the medium -to coarse-grained facies equivalents and have strong negative Eu anomalies in chondrite normalized REE patterns. In the Rokko Granite the Rb content decreases with an increase in Sr. Whole-rock chemistry indicates that the Rokko granitic magma formed a zoned magma chamber due to plagioclase crystal fractionation. Results of the least-square mass balance calculation for the major elements are in good agreement with the analysis data for Eu. The Rokko granitic magma was rich in the volatile components, SiO2, and K2O at the top of the magma chamber, whereas the base was more intermediate in chemical composition. The fine-grained facies has a low An content, and biotite in the fine and micrographic facies is rich in Fe and poor in Mg. These chemical characteristics indicate that two facies are formed from a more differentiated magma than the medium -to coarse-grained facies.