JOURNAL OF PETROLOGY VOLUME 0 NUMBER 0 PAGES1^34 2013 doi:10.1093/petrology/egt057 Journal of Petrology Advance Access published November 13, 2013 The P^T History of Anatectic Pelites of the Northern East Humboldt Range, Nevada: Evidence for Tectonic Loading, Decompression, and Anatexis Downloaded from BENJAMIN W. HALLETT* AND FRANK S. SPEAR DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE, JONSSON^ ROWLAND SCIENCE CENTER, 1W19, 110 8TH STREET, TROY, NY 12180, USA http://petrology.oxfordjournals.org/ RECEIVED NOVEMBER 1, 2012; ACCEPTED SEPTEMBER 18, 2013 The migmatitic lower plate of the Ruby Mountains^East Hum- INTRODUCTION boldt Range (RM^EHR) metamorphic core complex of northeast- In the lower to middle crust, partial melting of metasedi- ern Nevada represents a deep section of the hinterland of the Sevier mentary rocks dominantly occurs as a result of dehydra- orogenic belt. New major and trace element (Zr-in-rutile, Ti-in- tion melting reactions of hydrous phases, most commonly at Rensselaer Polytechnic Institute on November 14, 2013 biotite) thermobarometry is aided by garnet zoning analysis and a micas, in conjunction with quartz and plagioclase. thermodynamic modeling approach involving melt reintegration.The Anatexis and migmatization require some combination of P^T history for rocks from the Winchell Lake nappe (WLN) is heating and/or decompression across the effective solidus characterized by a phase of high-temperature, nearly isothermal, (e.g. Groppo et al., 2012), which has a steeply positive slope probably tectonic loading followed by decompression and melting in pressure^temperature (P^T) space. The slope of the during continued heating from 675 to 7408C. Rocks of the Liz- P^T path crossing this solidus is critical to the interpret- zies Basin Block (LBB) below the emplacement fault for the ation of the tectonic evolution that led to anatexis, as well WLN allochthon record no evidence of a nearly isothermal phase of as the tectonic implications of the production of significant tectonic loading and associated metamorphism. Hence a different volumes of partial melt. P^T path characterized by more widespread melting during pro- The presence of even a small volume of distributed par- grade heating and contraction is compiled for the LBB. Different in- terpretations are presented for the exhumation history and the tial melt (several per cent) can have a profound effect on context of partial melting in the WLN versus the LBB. Anatexis in the bulk rheological properties of a crustal section of rock pelites of the WLN occurred as a result of dehydration melting reac- (Arzi, 1978; Paquet et al., 1981; Hollister & Crawford, 1986; tions that took place in a decompressional phase of the prograde Rushmer, 1996; Rosenberg & Handy, 2005; Vanderhaeghe, P^T path, in contrast to dehydration melting during contraction in 2009). Anatexis and leucogranite generation are demon- the LBB.The emplacement of the WLN must have been a signifi- strated to play a significant role in crustal differentiation cant event in the tectonic history of the RM^EHR. These results (e.g. Brown,1994), in addition to contributing to a decrease have important implications regarding the relationship between crus- in bulk-rock strength. Studies in the Himalaya have tal thickening, syncontractional exhumation, and anatexis within pointed to anatexis and leucogranite magmatism as both continental orogenic systems. a trigger (e.g. Searle et al., 2010) and a result of exhumation of a mid-crustal section or ‘channel’ (Patin‹ o Douce & Harris, 1998; Harris et al., 2004). Decompression of hot KEY WORDS: migmatite; garnet; partial melting; metamorphic core crust was, at least in part, a result of syncontractional complex; zirconium; rutile normal-sense motion on the South Tibet Detachment ß The Author 2013. Published by Oxford University Press. All *Corresponding author. Telephone: 518-276-3358. Fax: 518-276-2012. rights reserved. For Permissions, please e-mail: journals.permissions@ E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 0 NUMBER 0 MONTH 2013 system (Harris & Massey, 1994; Patin‹ o Douce & Harris, inherently linked (Batum, 1999; McGrew et al., 2000; Lee 1998; Harris et al., 2004; Searle et al., 2010). et al., 2003). Rocks of the hinterland of the Sevier Orogenic belt in Protoliths of the RM^EHR metamorphic infrastructure western North America record tectonism related to crustal include Archean^Paleoproterozoic basement gneisses and thickening and contraction along an Andean-style contin- Neoproterozoic^Paleozoic miogeosynclinal sedimentary ental margin. The nexus of crustal thickening occurred rocks including siliciclastic, calcareous, and pelitic rocks west of the preserved Sevier fold and thrust belt at the (Howard, 1980; Lush et al., 1988; McGrew et al., 2000). western edge of the North American craton, in a region Two Jurassic granitoid plutons are preserved in the Ruby that has since experienced protracted Cenozoic magma- Mountains (Hudec, 1992; Jones, 1999b; Howard et al., 2011), tism and extension. Exposures of the Sevier hinterland yet no similar intrusions are recognized in the East are limited to exhumed footwall blocks in a series of meta- Humboldt Range. A suite of granitoid intrusive rocks of morphic core complexes stretching from Mexico to the Cretaceous to Early Oligocene age is ubiquitous through- Downloaded from Canadian Cordillera (Fig. 1; Armstrong, 1982). Mo de r n out the RM^EHR infrastructure. In the Ruby Mountains, surface expressions of this mid-crustal belt are much more these rocks include possibly five generations of leucogra- limited south of the Snake River Plain. Miocene to present nites (c. 92 Ma, possibly 83 Ma, 69 Ma, 38 Ma, and 29 Basin and Range extension has exposed the footwall Ma), an Eocene biotite^hornblende^quartz diorite, and block of the Ruby Mountains^East Humboldt Range Oligocene biotite monzogranites (U^Pb zircon and mona- http://petrology.oxfordjournals.org/ (RM^EHR) metamorphic core complex in northeastern zite ages; Wright & Snoke, 1993; MacCready et al., 1997; Nevada. The RM^EHR represents a section of the middle Howard et al., 2011). All of these intrusions are penetra- to lower crust affected by Sevier tectonism and associated tively deformed by top-to-the-WNW mylonitic shearing magmatism (Fig. 2). The P^T and melting history of these rocks has significant bearing on the interplay of major deformation and anatexis as they relate to the evolution of batholith an orogenic system. metamorphic This study presents new petrological data as well as core complex Canada prominent major and accessory phase thermobarometric analysis to U.S.A. Laramide fault improve constraints on P^T paths for migmatites of the Cordilleran/ northern East Humboldt Range. In addition, the prograde Sevier thrust belt at Rensselaer Polytechnic Institute on November 14, 2013 and retrograde reaction history based on complex major and trace element zoning in garnet and thermodynamic modeling is presented. This contribution addresses the cause and significance of partial melting as it relates to decompression and heating. The results have implications regarding the nature of thickening and the mechanisms of exhumation of continental crust with parallels to active contractional orogenic systems such as the Himalaya and Snake RiverARG Plain Andes. GEOLOGICAL SETTING The Ruby Mountains^East Humboldt Range meta- RM–EHR SR morphic core complex (Davis & Coney, 1979), like other Cordilleran core complexes, is characterized by unmeta- morphosed supracrustal rocks separated from metamor- phic and intrusive rocks by a low-angle, top-to-the-NNW, mylonitic ductile shear zone and associated brittle normal fault system. The infrastructure, or footwall of the core complex shear zone, contains abundant leucogranitic N dikes and sills among other Jurassic to Tertiary intrusions. 200 km The leucogranite dikes and sills are concordant with, and cross cut the preserved subhorizontal metamorphic fabric. Fig. 1. Sketch map of part of the western North American Cordillera At least one generation of leucogranites has been showing the Ruby Mountains^East Humboldt Range (RM^EHR) interpreted to be genetically linked with distinct in situ metamorphic core complex (black box), one of few exposures of the root of the Sevier orogenic belt in the Great Basin; ARG, Albion^ leucosomes in gneiss and metapelite, suggesting that the Raft River^Grouse Creek complex; SR, Snake Range complex (after magmatic and metamorphic history of this range are Davis & Coney,1979; Foster et al., 2007). 2 HALLETT & SPEAR P^T HISTORY, EAST HUMBOLDT RANGE 115°30’ Clover Hill 91–72Ma z RM-9 published ge U-Pb sample ogn leucosome an R A’ 29±0.5Ma m,z RM-5 t ld 41° bt monzogr ogn o Wood b Structural Symbols m Hills Normal fault u H t Winchell Low-angle s detachment fault a A Lake nappe E z Normal-sense 40±3Ma RM-19 Downloaded from mylonitic fabric hbl-bt-qtz dioritic ogn Trend and direction of plunge of major fold z 32Ma z RM-11 84.8±2.8Ma RM-18 leucogr http://petrology.oxfordjournals.org/ 40°45’ bt-ms ogn m 37Ma RM-21 38±2 Ma z RM-13 leucogr ogn bt-hbl-qtz diorite 39Ma m RM-24 29–33Ma z RM-20 leucogr ogn bt monz ogn 84 Ma m RM-12 35±3 Ma z RM-7 grt-ms leucogr 39±1Ma m RM-3 bt granodio gneiss grt-ms leucogn Lamoille Rock Units at Rensselaer Polytechnic Institute on November 14, 2013 30±4Ma z RM-15 Canyon Hanging wall bt monzogranite Tertiary sedimentary and volcanic rocks 40°30’ 83Ma(?) m RM-4 69 Ma z WRP02-7 Paleozoic to Triassic grt-bt-ms sedimentary rocks leucogr Footwall 26±8Ma z RM-17 ~36 Ma Harrison Northernbt Ruby monz Mountains ogn Pass pluton Igneous and high-grade metamorphic rocks 10 km Mixed Jurassic and N 115°15’ Cretaceous granites 115° Fig. 2. Geological map of the Ruby Mountains^East Humboldt Range (RM^EHR) after Sullivan & Snoke (2007).