Thermobarometric constraints on the tectonothermal evolution of the East Humboldt Range metamorphic core complex, Nevada Allen J. McGrew* Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 Mark T. Peters† Department of Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 James E. Wright Department of Geology and Geophysics, Rice University, Houston, Texas 77251 ABSTRACT tural evidence, and previously published and rheological condition of the crust during tec- thermochronometric results, we interpret this tonism? (3) What pressure-temperature-time path The East Humboldt Range of Nevada pro- trend as a P-T-t path for Late Cretaceous to (P-T-t path) did this terrane follow through its vides a record of deep-crustal tectonic proc- Oligocene time. At least 2 kbar of this decom- history, and how does this path constrain the tim- esses in a classic Cordilleran metamorphic pression (equivalent to at least 7 km of de- ing, rate, and mechanism of tectonic processes core complex located in the hinterland of the nudation) occurred in Late Cretaceous to early such as burial, exhumation, and magmatic addi- Late Cretaceous to early Tertiary Sevier oro- Tertiary time. This interpretation supports the tion to the crust? genic belt. New constraints reported here on idea that tectonic exhumation of deep-crustal The disentanglement of the polyphase meta- the metamorphic history of this terrane sug- rocks in northeastern Nevada began during or morphic history of the East Humboldt Range pre- gest an overall clockwise pressure-tempera- immediately after the closing stages of the Se- sents a formidable challenge that requires careful ture-time (P-T-t) path that began with deep vier orogeny. Finally, the steepness of the pro- petrography and integration of available thermo- tectonic burial and metamorphism at kyanite posed P-T-t path implies a thermal evolution chronometric data. + staurolite + garnet grade before Late Creta- from a colder to a much hotter geotherm, a cir- This paper reports on one in a series of studies ceous time (possibly in Late Jurassic time?). cumstance that probably requires plastic thin- detailing the tectonothermal evolution of the East Subsequently, higher-temperature Late Creta- ning of the lower plate in addition to brittle at- Humboldt Range. In other papers, McGrew and ceous peak metamorphism overprinted this tenuation and removal of the upper plate Snee (1994) have discussed 40Ar/39Ar thermo- event, resulting in widespread partial melting during Tertiary extension. chronometric results, and Peters and Wickham and leucogranite injection contemporaneous (1994) have discussed the metamorphic history with emplacement of a large-scale recumbent INTRODUCTION as recorded by marble and calc-silicate units. The fold (the Winchell Lake nappe). A new thermobarometric results reported here come 207Pb*/206Pb* date of 84.8 ± 2.8 Ma on syntec- Delineating spatial and temporal variations in primarily from deep-seated metapelites and, to a tonic leucogranite from the hinge zone of this metamorphic conditions is essential to under- lesser extent, from metabasites in the northern fold constrains the age of this major phase of standing the tectonic evolution of any crustal- and central part of the East Humboldt Range. tectonism. Metamorphism at this time proba- scale orogenic system. Quantitative thermoba- bly reached the second sillimanite isograd, at rometry offers an important tool for constraining REGIONAL SETTING least at deep structural levels, with peak P-T such relationships, especially when linked to de- conditions of 800 °C and >9 kbar. High-grade tailed thermochronology and careful, field-based The Ruby Mountains and East Humboldt conditions persisted during extensional tec- structural investigation. In this paper, we report Range lie in the northeastern part of the Basin tonic denudation throughout much of Tertiary thermobarometric results from the East Hum- and Range province in the hinterland of the time. In conjunction with previously published boldt Range, Nevada, which represents the north- Mesozoic Sevier fold-and-thrust belt and in the work, the petrologic and thermobarometric ern part of the Ruby Mountains–East Humboldt immediate foreland of the Antler orogenic belt results reported here for the northern East Range metamorphic core complex. As in other (Fig. 1). Like many metamorphic core com- Humboldt Range delineate a steeply decom- metamorphic core complexes in the western plexes, the Ruby–East Humboldt terrane pressional P-T trend that extends from ~9 kbar North American Cordillera, the East Humboldt records a complicated tectonic history, includ- and 800 °C to 5 kbar and 630 °C. In the light of Range presents a number of important tectonic ing at least two major orogenic events in Meso- decompressional reaction textures, microstruc- problems that are best addressed through petro- zoic time before tectonic exhumation during logic and thermobarometric investigation. Key Tertiary extension. Oligocene tectonism has in- *Present address: Department of Geology, Univer- questions include the following: (1) From what creasingly obscured older metamorphic and de- sity of Dayton, 300 College Park, Dayton, Ohio 45469- crustal levels did the East Humboldt Range orig- formational events at the deepest structural lev- 2364; e-mail: [email protected]. inate? (2) What thermal conditions accompanied els in the Ruby–East Humboldt complex, but †Present address: Los Alamos National Laboratory, 1180 Town Center Drive, Las Vegas, Nevada 89134. deformation, and what was the thermal structure exposures at shallower structural levels docu- Data Repository item 20001 contains additional material related to this article. GSA Bulletin; January 2000; v. 112; no. 1; p. 45–60; 5 figures; 3 tables. 45 MCGREW ET AL. Quartzite and schist, undivided (Cambrian? and Neoproterozoic?) ment an intense and protracted pre-Oligocene Hudec, 1992). Farther north, Cretaceous and large-scale deformation (Snoke et al., 1979; tectonic history. younger deformations largely obscure the Late Snoke and Miller, 1988). In addition, 40Ar/39Ar In the central Ruby Mountains, polyphase Jurassic tectonic history. In the northern Ruby muscovite and biotite cooling ages from the folding and upper-amphibolite-facies metamor- Mountains and the southern East Humboldt Wood Hills to the east suggest that peak meta- phism occurred synkinematically with em- Range, U-Pb monazite ages on leucogranites morphism there was also Late Cretaceous in placement of Late Jurassic two-mica granite and biotite schist document widespread Late age and that final exhumation of the Wood Hills (Kistler et al., 1981; Hudec and Wright, 1990; Cretaceous migmatization, metamorphism, and probably began by early Eocene time (Fig. 1) 46 Geological Society of America Bulletin, January 2000 TECTONOTHERMAL EVOLUTION, EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX, NEVADA Figure 1. Geologic map of the northern part of the East Humboldt 1. Geologic map of the northern Figure Range, Nevada, Inset to in this study. sample localities referred showing the general geology of Ruby–East Humboldt metamorphic map shows et complex (after Snoke core al., tectonic setting of 1990) and the regional to the Roberts Mountain allochthon west and relative the study area belt to the east. orogenic the late Mesozoic Sevier (Thorman and Snee, 1988; Camilleri and along the western flank of the Ruby Mountains in age from 29 to 40 Ma, documenting an Oligo- Chamberlin, 1997). and East Humboldt Range for >150 km along cene or younger age for mylonitization (Wright Overprinting and variably obscuring the older strike, with the mylonites generally recording and Snoke, 1993). 40Ar/39Ar hornblende and mica tectonic history is the gently west-dipping, >1- west-northwest–directed, normal-sense shear cooling ages and fission-track zircon, sphene, and km-thick, normal-sense mylonitic shear zone that (Fig. 1) (Snoke and Lush, 1984; Snoke and Miller, apatite cooling ages record diachronous unroof- exhumed the Ruby Mountains and East Hum- 1988; Snoke et al., 1990; McGrew, 1992). Mylo- ing of the Ruby Mountains and East Humboldt boldt Range. This shear zone is well exposed nitic fabrics overprint Tertiary granitoids ranging Range from east-southeast to west-northwest be- Geological Society of America Bulletin, January 2000 47 MCGREW ET AL. tween 35 and 20 Ma (Kistler et al., 1981; Dall- stretching lineation (McGrew, 1992; Peters and dolomite marble, calc-silicate gneiss, metapelite, meyer et al., 1986; Dokka et al., 1986; McGrew Wickham, 1994). metapsammite, and metaquartzite. Within these and Snee, 1994). In addition, previously published thermobaro- lithologies there are few systematic spatial varia- The northern East Humboldt Range exposes metric results exist for several surrounding areas, tions in mineral assemblage. A relict subassem- a diverse suite of migmatitic upper-amphibolite- including the southwestern East Humboldt blage including kyanite (and rarely also staurolite) facies rocks arrayed as a stack of allochthons Range, the Ruby Mountains, the Wood Hills, and occurs only on the upper limb of the Winchell emplaced during the polyphase tectonic history Clover Hill, a small terrane located between the Lake nappe. Muscovite is also much more abun- (Fig. 1). One of these allochthons is a southward- northern East Humboldt Range and the Wood dant on the upper limb of the nappe, and assem- closing recumbent isoclinal fold, the Winchell Hills (Fig. 1) (Kistler