BH250-52
Title
Subject
Description
Major Minerals: Quartz, plagioclase, albite, biotite, hypersthene
Minor Minerals: Garnet, clinopyroxene
Optical Features: Biotite displays bird's-eye maple interference pattern and strong pleochroism
Texture: Strong foliation; pre-kinematic (pre-tectonic) garnet growth
BH250-52 is a garnet-bearing schist, part of the high-grade metamorphic suite exposed in the Minnesota River Valley, including the area around Granite Falls. These rocks occur within a Precambrian migmatitic terrane composed of granitic gneisses, amphibolitic gneisses, and paragneisses. Many of these metamorphic rocks contain garnet, biotite, and hypersthene, indicative of deep crustal metamorphism.
The schist protolith is pelitic in composition and likely formed during early Archean crustal development, with many surrounding gneissic bodies dated to 3.2–3.6 billion years old. Metamorphism occurred under upper amphibolite to granulite facies conditions, with estimated peak temperatures of 650–750 °C and pressures of 4.5–7.5 kbar.
Typical garnet schists here feature garnet porphyroblasts set in a matrix of biotite and other micas, quartz, and feldspar, with possible inclusions of cordierite or sillimanite. Foliation bands and migmatitic textures are well developed, visible both in hand sample and in thin section.
Coverage
GPS Coordinates: 44 48 06.07 N, 95 32 26.18 W