Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota

Abstract

This paper evaluates thermotectonic models for metamorphism and leucogranite generation during the Proterozoic Trans-Hudson orogeny, as recorded in rocks exposed in the Black Hills, SD. Intrusion of the Harney Peak Granite and associated pegmatites at ∼1715 Ma occurred at the waning stages of regional deformation and staurolite-grade regional metamorphism. Published Consortium for Continental Reflection Profiling (COCORP) results indicate that Proterozoic sedimentary rocks were thrust over the Archean Wyoming province during the Trans-Hudson collision. Isotopic compositions of the Harney Peak Granite suggest that the exposed Proterozoic and Archean metasedimentary rocks in the Black Hills represent source rocks of the granites. Numerical simulations of the regional metamorphism and Harney Peak Granite generation, assuming crustal thickening by thrusting coupled with erosion, show the following: (1) Doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust; (2) a 35-km drop-off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level; it is, however, insufficient to produce staurolite, and melting can occur only in the deepest parts of the crust; (3) temperatures in crust with stable 70 km thickness for ∼40 Ma and 35 km drop-off length for heat production could become sufficient to produce staurolite at the current erosion level, and subsequent rapid denudation of the crust could potentially trigger decompression-melting of lower crustal rocks. Although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation, it is inconsistent with melting of upper crustal Proterozoic source rocks that is indicated by isotopic compositions of the granites, with lack of evidence for rapid denudation of the Trans-Hudson orogen, and with confinement of the leucogranites to the deformed Proterozoic metapelitic rocks. Production of the Harney Peak Granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the Wyoming province and overthrusted sedimentary rocks. We suggest that with reasonable rheologic properties of metapelites and rates of plate convergence, shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust. Modeling rheology of the lithologically stratified thickened crust, with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny, which explains regional folding of the schists and predicts ductile shear zones in the granite source region. Because of the lithologic stratification, the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists. A weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens.