Nucleation and Initial Growth of Garnet in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt, Kanto Mountains, Japan

Mutsuko Inui,Yumenosuke Wakai,Hiirou Sakuragi

doi:10.3390/min10030292

Abstract

The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.

Highlights

The beginning of recrystallization within a subducting oceanic plate is of particular interest because it records the initiation of dehydration within the subduction zone
Crystal size distribution analyses of garnet performed in this study suggested that the majority of the garnet grains were formed by the same event since they showed log-normal size distribution
Crystal size distribution and chemical zoning of garnet grains in the pelitic schists of the Nagatoro area were investigated to discuss the beginning of the recrystallization in the Sanbagawa metamorphic belt

Summary

Introduction

The beginning of recrystallization within a subducting oceanic plate is of particular interest because it records the initiation of dehydration within the subduction zone. Garnet is widespread in metamorphic rocks because it is stable over a wide range of pressures and temperatures for the bulk compositions most commonly found in sedimentary rocks [2]. Garnet is useful to petrologists because compositional growth zoning can be used to quantify metamorphic conditions under the assumption of chemical equilibrium [3,4]. Zoning profiles have been used as an indicator of the history of garnet growth and metamorphism [5,6,7]. The extensive use of garnet in thermobarometric studies has motivated investigations of its crystallization process in addition to its thermodynamic properties, such as controls by bulk rock chemistry [8], controls by fluids [9], the extent of equilibrium during growth [10,11], and crystal size distribution [12].

Methods

Results

Discussion

Conclusion