Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan

Yuki Nobe,Kohei Tominaga,Hiroshi Mori,Hidetoshi Hara

doi:10.3390/min11080854

Abstract

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

Highlights

Asian continent during the Late Cretaceous to Paleocene [2]. This metamorphism formed the Sanbagawa metamorphic complex, which is classified into chlorite, garnet, and biotite metamorphic zones based on the mineral assemblages [3,4]
We focus on carbonaceous material in pelitic rocks and quartz fabrics in siliceous rocks for the estimation of the temperature condition under low-grade metamorphism
To investigate the relationship between the quartz diameter and metamorphic grade, we compared the quartz diameter derived from the root mean square (RMS) data with to the temperature derived from the Raman spectroscopy of carbonaceous material (RSCM) data obtained from adjacent localities (Figure 1; Table 2)

Summary

Introduction

The metamorphic complex and the accretionary complex distributed in the Japanese Islands have been formed by the subduction of the oceanic plate since the Paleozoic. The Sanbagawa regional high P–T metamorphism resulted from the subduction of the Izanagi oceanic plate beneath the eastern. Asian continent during the Late Cretaceous to Paleocene [2]. This metamorphism formed the Sanbagawa metamorphic complex, which is classified into chlorite, garnet, and biotite metamorphic zones based on the mineral assemblages [3,4]. The chlorite zone, which corresponds with the pumpellyite–actinolite metamorphic facies, is broadly recognized in the Cretaceous Sanbagawa metamorphic complex and in the Late Jurassic

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