TECTONICS, VOL. 11, NO. 1, PAGES 154-164, FEBRUARY 1992 THERMAL EVOLUTION OF A PORTION OF THE SEVIER HINTERLAND: THE NORTHERN RUBY MOUNTAINS-EAST HUMBOLDT RANGE AND WOOD HILLS, NORTHEASTERN NEVADA K. V. Hodges Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology Cambridge, Massachusetts A. W. Snoke and H. A. Hurlow 1 Department of Geology andGeophysics, University of Wyoming,Laramie, Wyoming Abstract. Conventional thermobarometry and Gibbs' Method modeling have been usedto obtainnew information pertaining to the Mesozoic-Tertiary thermal evolution of the Ruby Mountains-East Humboldt Range and Wood Hills metamorphic complexes, northeastern Nevada.Kyanite-grade assemblages fromClover Hill, in the extreme northeastern part of the East Humboldt Range, were produced at temperatures of 780-810 K and depthsin excess of 35 km; previously published geochronologic data suggest anEarlyCretaceous or Late Jurassicage for this event. The Clover Hill area subsequently experienced a majorunroofing interval priorto Late Cretaceous time,during which it was brought roughly 20 km closer to thesurface. The lackof stratigraphic evidence for substantial Cretaceous erosion in theregion andthetopology of thepressure-temperature paths for theCloverHill samples suggest thatthe unroofing mechanism may have been tectonic denudation related to the relaxationof large topographic gradients produced by crustal thickening in the hinterland of the Sevier orogen. Metamorphic rocksat Clover Hill and withintheWoodHills core complex equilibrated at 820-900 K and500-640MPa (18-24km) in Cretaceous time, priorto 115 Ma. Other parts of the Ruby Mountains-East Humboldt Range metamorphic complex, presently stmcturally separated from the Clover Hill sequence, were intruded extensively by Late Cretaceous granitic magmas and attainedsubstantially higher temperatures than the Clover Hill-Wood Hills block. Sillimanite-grade assemblages from theseareasapparently underwent roughly10 km of unroofing and 130 K of cooling prior to final equilibration at 820-920 K and 360-430 MPa (13-16 km) during Oligocene extension. INTRODUCTION Most metamorphic corecomplexes in thehinterland of the North American Cordillera show evidence of poly- 1Now at Department of Geological Sciences, University of Washington, Seattle, Washington Copyright 1992by the American Geophysical Union. Paper number 91TC01879. 0278-7407/92/91TC-01879510.00 metamorphism as a consequence of superposed Mesozoic contractile and Tertiary extensional tectonics [Armstrong, 1982]. Unraveling the prolonged metamorphic history of these terrains is a difficult task, butpressure-temperature-time (P-T-t) pathsestimated for the Mesozoic-Tertiary interval provide our only real insight into the thermal evolution of the hinterland of the Sevier orogen through thetransition froma compressional to an extensional regime. The Ruby Mountains, EastHumboldt Range, andWood Hills of northeastern Nevada comprise one of the largest tracts of metamorphic rocks in theeastern Great Basin (Figure 1). Existing U-Th-Pb, Rb-Sr, 40Ar/39Ar, and fission track geochronologic data indicate that the Ruby-EastHumboldt core complex was subjected to at leastthreedistinct thermal pulses of LateJurassic, Cretaceous, and Oligo-Miocene ages [Snokeet al., 1979; Dallmeyer et al., 1986; Dokka et al., 1986; Wright and Snoke, 1986; Hudec and Wright, 1990]. Hornblende and mica 40Ar/39Ar cooling ages for the Wood Hills suggest a complex Mesozoic thermal history overprinted by an Eocene heating event [Thorman and Snee, 1988]. Petrographic studies in the two metamorphic complexes [Howard, 1966; Thorman, 1970; Snoke and Lush, 1984] revealed that high- and medium-grade metamorphic assemblages were produced during each of thesethermal pulses. We havestudied eightrepresentative peliticsamples from the northern Ruby Mountains, East Humboldt Range, and Wood Hills using conventional thermobarometry and Gibbs' Method modeling [Spear andSelverstone, 1983]in an effortto explore thefirst-order pressure-temperature history of these metamorphic terrains. Combining our resultswith published geochronologic data, we present in this paper a preliminary analysis of thethermal evolution of themiddle and lowercrust in this partof theCordilleran orogen. GEOLOGIC SETTING The Ruby Mountains andEastHumboldt Range consist of a high-grade metamorphic core structurally and stratigraphically overlainby low-grade to unmetamorphosed sedimentarystrata of Paleozoic-Triassic and Tertiary age (Figure 1). The stratigraphy and structural characteristics of thesepackages have been described in detail by Snoke and Lush [1984], Snoke and Miller [1988], and Snoke et al. [1990]; only those points salient to the metamorphic evolution of the area are reviewed here. The metamorphic core includesorthogneisses, pelitic schists and paragneisses, quartzites, marbles, calc-silicate rocks, and amphibolitescontaining mineral assemblages indicative of middle to upper amphibolite facies metamorphism. Although the degree of metamorphism complicates correlation of these rocks with the known stratigraphy of this part of the North American Cordillera, units ranging from Archean crystalline basement to upper Paleozoic miogeoclinal strata havebeenidentified [Howard, 1971; Snoke and Lush, 1984; Lush et al., 1988; Snoke, 1989]. Theserockshave beenintruded by numerous dikes, sills, and plutonic bodies ranging from quartz gabbroto granite; compositions corresponding to granodiorite and monzogranite are most common. In the central Ruby Mountains, Late Jurassic two-micagranitoids haveintruded previously metamorphosed and deformed late Precambrian to