Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

Toru Takeshita

doi:10.3390/min11101038

Abstract

The Sambagawa metamorphic rocks in central Shikoku, southwest Japan consist of an inverted metamorphic sequence from the upper chlorite to oligoclase-biotite zones at the lower structural level (LSL), which is overlain by a normal metamorphic sequence consisting of the albite-biotite and garnet zones at the upper structural level (USL). These sequences form a large-scale recumbent fold called the Besshi nappe. To unravel the mechanism of recrystallization and physical conditions in quartz, and their relation to exhumation tectonics, microstructures of recrystallized quartz grains in quartz schist from the Asemi-Saruta-Dozan River traverse were analyzed. The recrystallized quartz grain size increases with increasing structural level from 40 µm in the upper chlorite zone to 160 µm in the garnet zone of the USL. Further, the mechanism of dynamic recrystallization of quartz changes from subgrain rotation to grain boundary migration with increasing structural level across the uppermost garnet zone of the LSL. From these data, the deformation temperatures in quartz schist are calculated to increase with increasing structural level within the range between 300 and 450 °C using paleopiezometers and experimental flow laws. It could be interpreted that a rapid cooling of the Besshi nappe from above is responsible for the deformation temperatures recorded in quartz schist.

Highlights

Quartz is a major constituent mineral in the Earth’s crust, and its rheological properties are important for understanding geotectonic processes in the upper part of the crust and orogenic belts (e.g., [1])
In order to analyze the dislocation creep regime of quartz from the Sambagawa metamorphic rocks and their exhumation tectonics, we reviewed the existing data on the size, aspect ratio, and c-axis Crystallographic preferred orientations (CPOs) of deformed and recrystallized quartz grains in quartz schist from the Asemi-Saruta-Dozan River traverse, central Shikoku, southwest
The nappe consists of the upper chlorite, garnet, and albite-biotite zones of the lower structural level (LSL); the oligoclase-biotite zone; and the albite-biotite and garnet zones of the upper structural level (USL) in structural ascending order

Summary

Introduction

Quartz is a major constituent mineral in the Earth’s crust, and its rheological properties are important for understanding geotectonic processes in the upper part of the crust and orogenic belts (e.g., [1]). The recrystallized grain size–stress relation known as paleopiezometer for quartz was first shown by [15], which was later applied to deformed and recrystallized quartz in mylonites (e.g., [16,17,18]) It was experimentally shown by [19] for the first time that recrystallized quartz grains with oblate (S-type) and equant (P-type) shape form at a relatively faster strain rate and lower temperature, and slower strain rate and high-temperature conditions. Later, it was shown by [20] that the microstructural change from S- to P-type occurred with increasing deformation temperature in quartz from the Sambagawa quartz schist, southwest

Objectives

Findings

Discussion

Conclusion