Doctoral Dissertations

Date of Award

8-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Theodore C. Labotka

Committee Members

Harry Y. McSween, Lawrence A. Taylor, Jeffrey D. Kovac

Abstract

The Late Archean to Early Proterozoic terrain exposed in the Black Hills of South Dakota, records a history of deformation, metamorphism, and igneous activity extending from ~2.5 to 1.7 Ga. Metamorphism began during crustal thickening; however, maximum temperatures were attained after crustal thickening during a regional low-pressure, high-temperature event coinciding with the emplacement of the Harney Peak granite at 1.7 Ga. Metamorphic grade increased over a distance of 10 to 15 km from garnet grade in the northern portion of the terrain, to sillimanite + K-feldspar grade in the southern portion. Isograds mapped in the terrain include the first appearance of staurolite + biotite, andalusite + biotite, sillimanite, sillimanite + biotite + garnet, and sillimanite + K-feldspar. In the extreme western exposure of the terrain, kyanite + biotite occurs instead of andalusite + biotite. Geobarometry indicates that metamorphic pressures were between 2.0 and 4.4 kbar. Geothermometry indicates that temperature increased from 469-500 °C in the garnet zone to 528-555 °C in the sillimanite zone on the northern flank of the Harney Peak granite. Higher temperatures are recorded along the southern flank of the granite in the sillimanite + biotite + garnet zone (585 °C) and in the sillimanite + K-feldspar zone (≤ 662 °C). The distributions of isograds, isotherms, and major fold axes indicate that the Harney Peak granite was at the locus of a regional thermal high and that doming during granite emplacement locally deformed preexisting isograds and fold axes. Pressure-temperature paths based on compositionally zoned garnets from garnet-grade and staurolite-grade rocks indicate that prograde metamorphism from 400 °C to 500 °C occurred under almost isobaric conditions (± 0.5 bars). PT paths based on compositionally zoned garnets also have a clockwise orientation. Because of volume diffusion and retrograde decomposition, prograde PT paths could not be determined from sillimanite and sillimanite + K-feldspar zone garnets; however, reaction textures in these rocks strongly suggest clockwise decompression and cooling paths. Homogenization of garnets at a temperature of ~565 °C indicates heating and cooling occurred at a minimum of -4x107 years. Low-pressure, high-temperature metamorphism in the Black Hills followed crustal thickening associated with continental collision and the suturing of the Archean aged Superior and Wyoming provinces. One- and two-dimensional numerical models were constructed to examine the thermal consequences of rapid homogeneous crustal thickening and slow uplift. The uplift rate (0.014 mm/year) was estimated from the amount of uplift determined from geobarometry (14 to 16 km) and the time interval between peak metamorphism at 1.7 Ga and the age of the Cambrian (~0.6 Ga) lithologies unconformably overlying the Proterozoic crystalline complex. This value requires that uplift was constant and continuous. At an uplift rate of 0.014 mm/year, low-pressure, high-temperature metamorphism occurs with a moderate mantle heat flux (33 mW/m2) and a measured crustal radiogenic heat production rate of 3.3 μW/m3. The mantle heat flux is consistent with present-day reduced heat flux measurements and the rate of radiogenic heat production is estimated from the concentration of U, Th, and K in major lithologic units from the Black Hills. Estimated PTt paths are consistent with nearly isobaric heating, clockwise decompression, and with a thermal perturbation that lasts on the order of 107 years.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS