Abstract Title

Geology of a Paleogene Garnet Bearing Rhyolite Intrusion, Northern Black Hills Igneous Province, South Dakota

Abstract

The Northern Black Hills Igneous Province consists of a series of 58-50Ma intrusive centers that trends W-NW from Bear Butte in South Dakota to Devils Tower and the Missouri Buttes in Wyoming. The intrusive bodies include laccoliths, sills, and dikes, as well as breccia pipes and diatremes. Hence, this area provides an excellent opportunity to study mechanisms of shallow level magma intrusions and associated volcanism. This study involves investigating a small intrusion located within Precambrian basement rocks. Eleven rock samples were examined in thin section for petrography and microprobe mineral chemistry, and whole rock chemistry using ED-XRF. Petrography and geochemistry classify the rocks as rhyolite (70-74% SiO2) containing up to 2% phenocrysts of plagioclase feldspar, biotite, and garnet in an aphanitic quartzo-feldspathic groundmass. Texturally, the phenocrysts are euhedral to subhedral and are 1-2 mm in diameter. Electron microscope analysis of garnets revealed their composition to be primarily almandine and show no zoning or systematic compositional variability. Metamorphic garnets are usually zoned with respect to Iron, Magnesium, and Manganese and have inclusions. Based on these observations we conclude that these garnets formed by crystallization from magma and therefore are not metamorphic xenocrysts. Almandine garnets form in high temperature and pressure environments and therefore are rare in surface or near-surface rhyolites. We hypothesize a scenario where the garnets were crystallizing in magma at lower crustal depth (720 km) and are preserved in the rhyolite due to rapid ascent to the surface before they had time to equilibrate and dissolve into the magma.

Modified Abstract

The Northern Black Hills Igneous Province consists of a series of 58-50Ma intrusive centers that trend W-NW from South Dakota to Wyoming. This area provides an excellent opportunity to study mechanisms of shallow magma intrusions and associated volcanism. Eleven samples were examined in thin section for petrography, microprobe mineral chemistry, and whole rock chemistry. The results classify the rocks as rhyolite containing up to 2% phenocrysts of plagioclase, biotite, and garnet in an aphanitic groundmass. Electron microscope analysis of garnets revealed their composition to be primarily almandine which form under high temperature and pressure and therefore are rare in surface or near-surface rhyolites. We hypothesize the garnets crystallized in a low crustal depth magma and were preserved in rhyolite during rapid ascent to the surface.

Research Category

Geology/Geography

Primary Author's Major

Geology

Mentor #1 Information

Dr. David

Hacker

Presentation Format

Poster

Start Date

April 2019

Research Area

Earth Sciences | Geology | Physical Sciences and Mathematics

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Apr 9th, 1:00 PM

Geology of a Paleogene Garnet Bearing Rhyolite Intrusion, Northern Black Hills Igneous Province, South Dakota

The Northern Black Hills Igneous Province consists of a series of 58-50Ma intrusive centers that trends W-NW from Bear Butte in South Dakota to Devils Tower and the Missouri Buttes in Wyoming. The intrusive bodies include laccoliths, sills, and dikes, as well as breccia pipes and diatremes. Hence, this area provides an excellent opportunity to study mechanisms of shallow level magma intrusions and associated volcanism. This study involves investigating a small intrusion located within Precambrian basement rocks. Eleven rock samples were examined in thin section for petrography and microprobe mineral chemistry, and whole rock chemistry using ED-XRF. Petrography and geochemistry classify the rocks as rhyolite (70-74% SiO2) containing up to 2% phenocrysts of plagioclase feldspar, biotite, and garnet in an aphanitic quartzo-feldspathic groundmass. Texturally, the phenocrysts are euhedral to subhedral and are 1-2 mm in diameter. Electron microscope analysis of garnets revealed their composition to be primarily almandine and show no zoning or systematic compositional variability. Metamorphic garnets are usually zoned with respect to Iron, Magnesium, and Manganese and have inclusions. Based on these observations we conclude that these garnets formed by crystallization from magma and therefore are not metamorphic xenocrysts. Almandine garnets form in high temperature and pressure environments and therefore are rare in surface or near-surface rhyolites. We hypothesize a scenario where the garnets were crystallizing in magma at lower crustal depth (720 km) and are preserved in the rhyolite due to rapid ascent to the surface before they had time to equilibrate and dissolve into the magma.