BH250-118
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Description
BH250-118, BH250-118b, and BH250-118c are samples collected from the Morton Gneiss in Morton, Minnesota. The Morton Gneiss is one of the oldest known rocks in North America and is a widely studied example of high-grade metamorphic rock. It is prominently exposed in southwestern Minnesota, particularly around the town of Morton, from which it takes its name. Here, active quarries reveal striking outcrops of the gneiss, displaying its distinctive banding. Historically, the rock was quarried for use in tabletops, building walls, and tombstones, and quarrying continues today under the ownership of a company based near St. Cloud. The site has also been of academic interest, with recent Carleton College graduate Emma Watson (Class of 2023) completing her comprehensive thesis on the Morton Gneiss.
The Morton Gneiss is Archean in origin, with its protolith (original rock) forming around 3.6 billion years ago. Since its formation, it has undergone multiple episodes of metamorphism and deformation, most notably during the Penokean orogeny (~1.85 billion years ago) and later thermal/metamorphic overprinting during the Proterozoic. It is a banded, migmatitic gneiss, showing evidence of both solid-state metamorphism and partial melting (migmatization). The rock features interlayered felsic bands (BH250-118), mafic bands (BH250-118b), and amphibolite (BH250-118c), each representing different compositions, origins, and metamorphic histories.
The felsic portion (BH250-118) is light pink to gray in color and composed mainly of quartz, K-feldspar (orthoclase and microcline), plagioclase, and biotite, with minor zircon. It is coarse-grained and foliated, often displaying leucosome veins that formed from partial melting. Its origin is thought to be granitic or granodioritic protoliths that underwent high-grade metamorphism and partial melting. In contrast, the mafic portion (BH250-118b) forms dark gray to black bands and is composed of hornblende, biotite, and more calcic plagioclase. These bands are finer-grained and more resistant to weathering than the felsic layers and are interpreted as amphibolitized mafic intrusions or original mafic layers within the crust that were metamorphosed alongside the felsic material.
BH250-118b is the mafic part of the Morton Gneiss which is 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.
Amphibolite (BH250-118c) is present as lenses or layers within the Morton Gneiss and represents metamorphosed mafic rocks that may have originated as basaltic or gabbroic intrusions, or as volcanic sediments. Amphibolite in this setting is dominated by hornblende and plagioclase, typically lacking quartz. It can occur conformably with the gneissic layering or as discordant bodies, suggesting emplacement as dikes during or before metamorphism.
Geologically, the Morton Gneiss is significant because it preserves a record of early Earth crustal evolution, including crust formation, tectonism, and high-grade metamorphism. It forms part of the Minnesota River Valley subprovince, which is itself a key component of the Superior Province of the Canadian Shield. Interestingly, in the past, outcrops of the Morton Gneiss were considered as potential storage sites for radioactive waste, and remnants of that period—including test wells and excavated pits—can still be seen in certain locations.
The Morton Gneiss is highly weathered in places, and a saprolite has developed on it at several sites near Redwood Falls and Granite Falls, Minnesota. Saprolite samples BH250-100, 100b, 100c, and 100d were collected from these locations. Several student projects have investigated this topic—please refer to the papers, abstracts, and posters available on the Geochemistry of Natural Waters website for more information.
There are large samples that we brought from Moton in the rock garden.
Coverage
GPS Coordinates: 44°33'7.12"N, 94°59'29.41"W