Tectonophysics 342 (2001) 371–388 www.elsevier.com/locate/tecto Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota Peter I. Nabelek*, Mian Liu, Mona-Liza Sirbescu Department of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA Accepted 20 June 2001 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 * Corresponding author. E-mail address: [email protected] (P.I. Nabelek). 0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-1951(01)00171-8 372 P.I. Nabelek et al. / Tectonophysics 342 (2001) 371–388 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. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Shear heating; Leucogranites; Numerical modeling; Rheology; Black Hills; Anatexis; Metamorphism 1. Introduction phism has been focused on the Himalayas where leucogranites constitute an integral part of the orogen The source of heat leading to leucogranite gen- (e.g., Le Fort et al., 1987; Harris and Massey, 1994; eration from crustal rocks in thickened convergent Treloar, 1997; Harrison et al., 1998; Huerta et al., orogens is a major unresolved issue. Although under- 1998; Vance and Harris, 1999). However, analogous plating of the crust or intrusion of mafic magmas leucogranites in terms of composition, mode of em- could potentially trigger crustal anatexis, there is a placement, source and host-rock compositions, and lack of chemical and physical evidence for intrusion structural context occur in other regions where crus- of mantle-derived magmas into the source regions of tal collisions have occurred, including the Appala- leucogranites (Le Fort et al., 1987; Scaillet et al., chian Mountains of Maine (Tomascak et al., 1996; 1990; Krogstad and Walker, 1996; Tomascak et al., Pressley and Brown, 1999) and the Black Hills, SD 1996; Nabelek and Bartlett, 1998; Pressley and (Redden et al., 1990; Nabelek et al., 1992a; Krogstad Brown, 1999). Furthermore, partial melting of crustal and Walker, 1996; Nabelek and Bartlett, 1998). This protoliths requires intrusion of at least an equivalent suggests that there may be a common process lead- mass of basalt, which is likely to lead to hybrid- ing to leucogranite generation during crustal colli- ization (Grunder, 1995). Without intrusion of mafic sion. In this paper, we explore possible models for magmas, thermal relaxation within thickened crust generation of the Harney Peak Granite (HPG) in the with typical concentration of radioactive elements Black Hills during the Proterozoic Trans-Hudson cannot by itself give temperatures necessary to melt orogeny, which was responsible for coalescence of metasedimentary source rocks by dehydration-melt- much of the North American craton. Previously ing reactions, except in lower parts of the crust (e.g., published geological, geochemical, thermobaromet- England and Thompson, 1984; Thompson and Con- ric, and chronological data for the metamorphism nolly, 1995). Although pressure–temperature–time and granite generation in the Black Hills provide ( P–T–t) paths in thick orogens may intersect wa- stringent constraints for numerical models of leucog- ter-present solidus of metapelites during exhumation, ranite generation. We conclude that shear heating of thermometry, compositions, and phase relationships pelitic schists during synorogenic thrusting was most of leucogranites suggest that most were high-temper- likely responsible for generation of the HPG. The ature ( > 750 °C) magmas that formed by muscovite similarity of scales and processes in the Trans-Hud- or biotite dehydration-melting reactions in metasedi- son orogen to other large orogens suggests that shear mentary rocks (Harris and Inger, 1992; Nabelek et heating may be important for petrogenesis of leucog- al., 1992b; Nabelek and Bartlett, 1998; Patin˜o-Douce ranites in collisional settings. and Harris, 1998). Therefore, to explain the leucog- ranites, modifications of simple crustal thickening- erosion models, including decompression melting of 2. Metamorphism in the Black Hills lower-crustal rocks or deep burial of heat-producing lithologies, have been proposed (e.g., Harris and The Proterozoic Trans-Hudson orogen extends over Massey, 1994; Ruppel and Hodges, 1994; Huerta et several thousand kilometers from the southern edge of al., 1998; Jamieson et al., 1998). the Wyoming craton to northern Quebec. Following Much of the debate about the heat source for erosion and covering by Phanerozoic sediments, part leucogranite generation and associated metamor- of it was uplifted during the Laramide orogeny and is P.I. Nabelek et al. / Tectonophysics 342 (2001) 371–388 373 now exposed in the core of the Black Hills (Fig. 1a). calated gabbro sills and felsic tuffs (Redden et al., The orogenic events that are recorded by the Pre- 1990). It is likely that these sequences represent what cambrian rocks in the Black Hills have traditionally Baird et al. (1996) inferred to be a wedge of arc rocks been ascribed to collision of the Archean Superior that were thrust over the Wyoming province (Fig. 1). province with the Wyoming province. However, a The Precambrian terrane also includes exposures of a recent Consortium for Continental Reflection Profil- small Archean leucogranite body and metapelites at ing (COCORP) transect across the orogen south of Bear Mountain along the western margin of the terrane the U.S.–Canada border indicates instead that there and of a highly deformed Archean Little Elk Creek may have been a small crustal block, named the granite near the eastern margin of the terrane to the Dakota block, that collided directly with the Wyom- north of the area shown in Fig. 2 (Redden et al., 1990). ing province (Baird et al., 1996; Fig. 1b). It is thus evident that the Archean rocks, probably Metamorphic rocks and leucogranites in the Black belonging to the Wyoming province, were imbricated Hills are the products of events that occurred during with the Proterozoic formations. the orogeny. The metamorphic rocks are dominated by The metasedimentary rocks have undergone two quartzite, metapelite, and metagraywacke (Fig. 2) that regional deformation events (Redden et al., 1990). The originated as platform to deep-marine sequences de- first event resulted in northeast-trending F1 folds