BH250-118b
Title
BH250-118b
Subject
Amphibolite
Description
Major Minerals: amphiboles, biotite, quartz, plagioclase
Minor Minerals: sericite, chlorite, apatite, zircon
BH250-118, 118b, and 118c are samples collected from the Morton Gneiss near Morton, Minnesota. The Morton Gneiss is one of the oldest known rocks in North America and a classic example of high-grade metamorphic crust. It is prominently exposed in southwestern Minnesota, especially around the town of Morton, where active and historic quarries reveal spectacular banded outcrops. These exposures have been quarried in the past for use as tabletops, building veneers, and tombstones, and the resource is still being worked today by a company near St. Cloud, Minnesota.
The Morton Gneiss is Archean in origin, with its protolith forming around 3.6 billion years ago. Since its formation, the rock has undergone multiple episodes of metamorphism and deformation, most notably during the Penokean Orogeny (~1.85 Ga), followed by additional thermal and metamorphic overprints during the Proterozoic. Recent work, including the comprehensive thesis research by Carleton graduate Emma Watson (Class of 2023), has helped clarify the complex history recorded in these rocks.
Texturally and compositionally, the Morton Gneiss is typically described as a migmatitic, banded gneiss, exhibiting both evidence of high-grade solid-state metamorphism and partial melting (migmatization). BH250-118 represents the felsic portion: light pink to gray in color, coarse-grained and foliated, and dominated by quartz, K-feldspar (orthoclase and microcline), plagioclase, and biotite, with minor zircon. It frequently displays leucosome veins, indicating partial melting, and is interpreted to derive from granitic to granodioritic protoliths that were strongly reworked during high-grade metamorphism.
BH250-118b is the mafic part of the Morton Gneiss, characterized by dark gray to black bands. Its mineralogy includes hornblende, biotite, and more calcic plagioclase, and it is finer-grained and generally more resistant to weathering than the felsic layers. This mafic material is interpreted as amphibolitized mafic intrusions or original mafic crustal layers that were metamorphosed contemporaneously with the felsic components.
BH250-118c is an amphibolite lens or layer within the gneiss, representing metamorphosed mafic rock—either originally intruded as basaltic/gabbroic material or deposited as volcanic protoliths. Its composition is dominated by hornblende and plagioclase and often lacks quartz. These amphibolites may occur conformably with the gneissic fabric or as discordant bodies, possibly reflecting late-stage dike intrusions during or prior to metamorphism.
Geologically, the Morton Gneiss records early Earth crustal evolution, including formation, deformation, partial melting, and high-grade metamorphism. It is part of the Minnesota River Valley subprovince, itself a component of the Superior Province of the Canadian Shield. At one time, some outcrops of Morton Gneiss were evaluated for use in radioactive waste storage; remnants of test wells and exploratory quarrying for that purpose can still be observed in certain locations.
Minor Minerals: sericite, chlorite, apatite, zircon
BH250-118, 118b, and 118c are samples collected from the Morton Gneiss near Morton, Minnesota. The Morton Gneiss is one of the oldest known rocks in North America and a classic example of high-grade metamorphic crust. It is prominently exposed in southwestern Minnesota, especially around the town of Morton, where active and historic quarries reveal spectacular banded outcrops. These exposures have been quarried in the past for use as tabletops, building veneers, and tombstones, and the resource is still being worked today by a company near St. Cloud, Minnesota.
The Morton Gneiss is Archean in origin, with its protolith forming around 3.6 billion years ago. Since its formation, the rock has undergone multiple episodes of metamorphism and deformation, most notably during the Penokean Orogeny (~1.85 Ga), followed by additional thermal and metamorphic overprints during the Proterozoic. Recent work, including the comprehensive thesis research by Carleton graduate Emma Watson (Class of 2023), has helped clarify the complex history recorded in these rocks.
Texturally and compositionally, the Morton Gneiss is typically described as a migmatitic, banded gneiss, exhibiting both evidence of high-grade solid-state metamorphism and partial melting (migmatization). BH250-118 represents the felsic portion: light pink to gray in color, coarse-grained and foliated, and dominated by quartz, K-feldspar (orthoclase and microcline), plagioclase, and biotite, with minor zircon. It frequently displays leucosome veins, indicating partial melting, and is interpreted to derive from granitic to granodioritic protoliths that were strongly reworked during high-grade metamorphism.
BH250-118b is the mafic part of the Morton Gneiss, characterized by dark gray to black bands. Its mineralogy includes hornblende, biotite, and more calcic plagioclase, and it is finer-grained and generally more resistant to weathering than the felsic layers. This mafic material is interpreted as amphibolitized mafic intrusions or original mafic crustal layers that were metamorphosed contemporaneously with the felsic components.
BH250-118c is an amphibolite lens or layer within the gneiss, representing metamorphosed mafic rock—either originally intruded as basaltic/gabbroic material or deposited as volcanic protoliths. Its composition is dominated by hornblende and plagioclase and often lacks quartz. These amphibolites may occur conformably with the gneissic fabric or as discordant bodies, possibly reflecting late-stage dike intrusions during or prior to metamorphism.
Geologically, the Morton Gneiss records early Earth crustal evolution, including formation, deformation, partial melting, and high-grade metamorphism. It is part of the Minnesota River Valley subprovince, itself a component of the Superior Province of the Canadian Shield. At one time, some outcrops of Morton Gneiss were evaluated for use in radioactive waste storage; remnants of test wells and exploratory quarrying for that purpose can still be observed in certain locations.
Coverage
Location: Morton, Minnesota, USA
GPS Coordinates: 44.5532, -94.992694
GPS Coordinates: 44.5532, -94.992694
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-118b. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-118b,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/321.