BH250-72
Title
BH250-72
Subject
Rhyolite
Description
Major Mineral: quartz
Minor Minerals: glassy groundmass, plagioclase, altered biotite
BH250-72 is a rhyolite sample collected from Shovel Point, located along the North Shore of Lake Superior in Minnesota. This outcrop is part of the Midcontinent Rift System and represents one of the oldest felsic volcanic rocks in the region, dating back to approximately 1.1 billion years ago. Chemically, BH250-72 is a felsic rock with high silica content (SiO₂ >70%), setting it apart from the dominant basalt flows of the North Shore. It is enriched in alkali elements(Na₂O + K₂O), classifying it as a rhyolite, though in some flows, it verges on high-silica dacite. Trace element data show enrichment in incompatible elements such as Rb, Th, U, and light rare earth elements (LREEs). Mineralogically, BH250-72 is composed primarily of quartz, K-feldspar (orthoclase or microcline), sodic plagioclase, and devitrified volcanic glass. Minor and accessory minerals include biotite, magnetite/ilmenite, zircon, and apatite. The rock exhibits classic rhyolitic textures such as flow banding and spherulites, and contains phenocrysts of quartz and feldspar within a fine-grained or formerly glassy groundmass. The presence of rhyolite in this predominantly mafic volcanic sequence reflects significant magmatic differentiation of the parent mafic magmas, likely related to the processes that also formed the Duluth Gabbro Complex.
Minor Minerals: glassy groundmass, plagioclase, altered biotite
BH250-72 is a rhyolite sample collected from Shovel Point, located along the North Shore of Lake Superior in Minnesota. This outcrop is part of the Midcontinent Rift System and represents one of the oldest felsic volcanic rocks in the region, dating back to approximately 1.1 billion years ago. Chemically, BH250-72 is a felsic rock with high silica content (SiO₂ >70%), setting it apart from the dominant basalt flows of the North Shore. It is enriched in alkali elements(Na₂O + K₂O), classifying it as a rhyolite, though in some flows, it verges on high-silica dacite. Trace element data show enrichment in incompatible elements such as Rb, Th, U, and light rare earth elements (LREEs). Mineralogically, BH250-72 is composed primarily of quartz, K-feldspar (orthoclase or microcline), sodic plagioclase, and devitrified volcanic glass. Minor and accessory minerals include biotite, magnetite/ilmenite, zircon, and apatite. The rock exhibits classic rhyolitic textures such as flow banding and spherulites, and contains phenocrysts of quartz and feldspar within a fine-grained or formerly glassy groundmass. The presence of rhyolite in this predominantly mafic volcanic sequence reflects significant magmatic differentiation of the parent mafic magmas, likely related to the processes that also formed the Duluth Gabbro Complex.
Coverage
Location: Shovel Point, Minnesota, USA
GPS Coordinates: 47°20'22.53"N, 91°11'6.11"W
GPS Coordinates: 47°20'22.53"N, 91°11'6.11"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 72. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-72,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/82.