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LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 1 M cK ay _L 64 2 2 nd p ag es / 1 o f 1 9 RESEARCH Prolonged metamorphism during long-lived terrane accretion: Sm-Nd garnet and U-Pb zircon geochronology and pressure-temperature paths from the Salmon River suture zone, west-central Idaho, USA Matthew P. McKay1, Elizabeth M. Bollen2, Keith D. Gray3, Harold H. Stowell2, and Joshua J. Schwartz4 1DEPARTMENT OF GEOGRAPHY, GEOLOGY AND PLANNING, MISSOURI STATE UNIVERSITY, 901 S. NATIONAL AVENUE, SPRINGFIELD, MISSOURI 65897, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF ALABAMA, 2003 BEVILL BUILDING, TUSCALOOSA, ALABAMA 35487, USA 3DEPARTMENT OF GEOLOGY, WICHITA STATE UNIVERSITY, GEOLOGY BUILDING, 1845 FAIRMOUNT STREET, WICHITA, KANSAS 67260, USA 4DEPARTMENT OF GEOLOGICAL SCIENCES, CALIFORNIA STATE UNIVERSITY, LIVE OAK 1202, 18111 NORDHOFF STREET, NORTHRIDGE, NORTHRIDGE, CALIFORNIA 91330, USA ABSTRACT The Salmon River suture zone of western Idaho (USA) records mid-crustal metamorphism and deformation associated with orogenesis during Mesozoic accretion of volcanic arc terranes to western Laurentia. We present petrographic and microstructural observations, garnet geochemistry, pressure-temperature isochemical phase diagrams, and Sm-Nd garnet and U-Pb zircon ages to investigate the timing and conditions of metamorphism in the Salmon River suture zone. The Salmon River suture zone is comprised of three thrust sheets: from east to west, the amphibolite facies Pollock Mountain plate, upper greenschist to amphibolite facies Rapid River plate, and greenschist facies Heavens Gate plate. The Pollock Mountain plate was isothermally loaded from 6 to >8 kbar at ~700 °C between 141 and 124 Ma during northwest-southeast crustal shortening. The underlying Rapid River plate was isothermally loaded from 7 to ~10 kbar at 600–650 °C during ca. 124–112 Ma metamorphism, which is contemporaneous with late- to post-peak metamorphism and ca. 118 Ma exhumation of the overly- ing Pollock Mountain plate. In the Rapid River plate, thrust sheet emplacement induced high-strain ductile deformation and led to regional development of linear-planar fabrics. The 206Pb/238U zircon ages for syndeformational to postdeformational magmatism record ca. 117 Ma or younger juxtaposition of the two plates on the southeast-dipping Pollock Mountain thrust fault. Coeval 124–112 Ma metamorphism of the Rapid River plate, ca. 118 Ma exhumation of the Pollock Mountain plate, and ca. 117 Ma or younger movement along the Pollock Mountain fault suggest that metamorphism of the Rapid River plate was possibly driven in part by thrust juxtaposition and loading along the Pollock Mountain fault. In this context, we interpret that metamorphism records diachronous thrust stacking during prolonged (>30 m.y.) accretion- ary orogenesis in western Idaho. LITHOSPHERE GSA Data Repository Item 2017234 doi:10.1130/L642.1 INTRODUCTION Continental assembly and crustal growth along active margins are dominated by accretion of allochthonous terranes and magmatic addi- tions from the underlying mantle (e.g., Coney et al., 1980; Scholl et al., 1986). The collision of displaced terranes with cratonic margins is often accompanied by contractional deformation and crustal thickening, which generally leads to anatexis and regional metamorphism (Chamberlain and Karabinos, 1987). The age and pressure-temperature (P-T) conditions dur- ing metamorphism in the roots of arc-continent collision zones are criti- cal for understanding collisional processes including the time scales and mechanisms of thermal perturbations, the extent and driving forces behind the metamorphism and partial melting, and the magnitude and timing of deformation in the middle to lower crust. In orogenic belts where laterally extensive tracts of deep-crustal rocks are exposed (e.g., Western Gneiss region of Norway and Pamir Plateau of Central Asia), metamorphic and thermochronologic studies indicate that widespread chemical and ther- mal equilibrium is not uniformly achieved during collisional orogenesis (Spencer et al., 2013; Stearns et al., 2015). Determining the age of meta- morphism and conditions during synmetamorphic deformation provide crucial insight into the tectonic processes at work. In this study we present Sm-Nd garnet and U-Pb zircon ages along with integrated isochemical phase diagram models and thermobarometry estimates to construct P-T- time paths that reflect the evolution of orogenic belts roots through time. The Salmon River suture zone of western Idaho (USA) exposes mid- crustal rocks across a complex island arc–continent collision zone and provides an ideal opportunity to resolve the duration, magnitude (i.e., P-T conditions through time), and spatial patterns of metamorphism during accretionary orogenesis. The Salmon River suture zone is located in the central North American Cordillera, and is dominated by metasedimen- tary and metavolcanic rocks of island arc affinity that record progressive deformation and metamorphism during Late Jurassic to Early Cretaceous accretion of the Blue Mountains province to western Laurentia (Fig. 1; Silberling et al., 1984; Lund and Snee, 1988; Avé Lallemant, 1995; Snee © 2017 Geological Society of America. For permission to copy, contact
[email protected] Matthew McKay http://orcid .org /0000 -0002 -7756 -6489 http://orcid.org/0000-0002-7756-6489 http://orcid.org/0000-0002-7756-6489 http://orcid.org http://orcid.org/0000-0002-7756-6489 http://www.geosociety.org MCKAY ET AL. 2 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE M cK ay _L 64 2 2 nd p ag es / 2 o f 1 9 et al., 1995; Wyld and Wright, 2001; Wyld et al., 2006; Blake et al., 2009; Gray et al., 2012). Historically, the Salmon River suture zone has played a fundamental role in tectonic models describing the evolution of the central North American Cordillera (Oldow et al., 1989; Burchfield et al., 1992; McClelland et al., 2000; Wyld and Wright, 2001; Giorgis et al., 2005). Early studies postulated that metamorphism in the suture zone (Hamilton, 1963; Onasch, 1977) was related to intrusion of the 100–54 Ma Idaho batholith (Gaschnig et al., 2010) to the east. More recent tec- tonic models incorporating Sm-Nd garnet geochronology have proposed that island arc amalgamation and subsequent accretion were recorded by distinct metamorphic events that occurred ca. 144 Ma and ca. 128 Ma (Selverstone et al., 1992; Getty et al., 1993). One problem associated with garnet age interpretation is that isoto- pic measurements typically require large sample masses to achieve high precision, introducing the possibility that isotopic garnet analyses may reflect a mixed value between two different age domains within a single crystal, and fail to resolve long-duration garnet growth. Complicated zoning, particularly in polymetamorphic garnet (e.g., Stowell and Gold- berg, 1997), adds complexity because precise ages for each event requires sampling separate zones for individual isotope analyses. In addition, the possible adverse effects of inclusions must be considered (e.g., Prince et al., 2000). The resulting lack of age resolution presents problems in relating garnet growth to discrete tectonothermal events. In this study we have attempted to minimize the impacts of age mixing by microsampling distinct chemical and textural zones within garnet from amphibolites. We combine new Sm-Nd garnet ages with additional geochronologic data (U-Pb zircon, Ar-Ar hornblende), P-T estimates, and fabric element analyses (microscopic to map scale) to provide insights into the processes of regional metamorphism and deformation associated with arc-continent collision during protracted subduction-driven orogeny (Selverstone et al., 1992). Based on these new data, we present a tectonic model for the Salmon River suture zone, wherein long-lived regional metamorphism is controlled by diachronous thrust stacking. In this model, protracted metamorphism and contractional deformation occurred over a period of 30 m.y. or more, during which the Blue Mountains province was incor- porated into the Laurentian margin. GEOLOGIC BACKGROUND Blue Mountains Province The North American Cordillera largely consists of an assemblage of terranes that were progressively accreted to western Laurentia during Pha- nerozoic subduction of the Farallon plate (Coney et al., 1980; Engebretson TcrTcr TcrTcr RigginsRiggins hcc (PMp) pma (PMp) RRp HGp 7D Po llo ck Mt t hru st f au lt He av en s G at e fa ult LGCCLGCC N PcpgPcpg R ap id Ri ve r t hr us t f au lt Salm on R . ID23ID23 Sm-Nd garnet locality EXPLANATION Ar/Ar hbl locality U-Pb zircon locality 522*522* PRCPRC Fig. 2a Fig. 2b ID03ID03 ID42ID42 ID04ID04 ID48ID48 ID07ID07 PRC01PRC01 ID58ID58 ID26ID26 R34**R34** R18**R18** ? ? ? ? ? 598*598* R30**R30** Fold axis kmkm 550 30' 25' 20' 15' 10' 5'116° W 116° W 25 ' 45 ° N 20 ' 45 ° N 15 ' 45 ° N Wallowa Terrane Baker Terrane Izee Basin Olds Ferry N 50 0 km EXPLANATION SRSZ tectonites Idaho Batholith > 0. 70 6 > 0. 70 6 46o 45o -117o -116o-118o Id ah o Ba th ol ith Id ah o Ba th ol ith < 0.="" 70="" 4="">< 0. 70 4 inset inset87sr/86sri line or id mt ut wa oregon idaho figure 1. geologic map of the salmon river suture zone (srsz). 7d—seven devils volcanics of wallowa terrane; hgp—heavens gate plate; rrp—rapid river plate; pmp—pollock mountain plate; kh—hazard creek complex; lgcc—little goose creek com- plex; tcr—columbia river basalt group; pcp—permian chair point pluton. fold symbols mark the axes of the riggins synform and lake creek antiform (after hamil- ton, 1969; lund, 2004; gray, 2013). locations of figures 2a and 2b are shown. sample localities from this study (id03, id07, id23, id26, id48) and previously published geochronology and thermochro- nology results (* denotes 40ar/39ar hornblende ages from getty et al., 1993; ** denotes 40ar/39ar horn- blende ages from lund and snee, 1988; 598—119 ± 2 ma; r18—106.5 ± 1.4 ma; r30—118.0 ± 0.6 ma; r34 = 109.1 ± 0.6 ma). lithosphere | volume 9 | number 4 | www.gsapubs.org 3 m ck ay _l 64 2 2 nd p ag es / 3 o f 1 9 prolonged metamorphism during terrane accretion: salmon river suture zone | research et al., 1985). in west-central idaho, the initial 87sr/86sr = 0.706 isopleth is subparallel to longitude ~116°w and marks the geochemical boundary between rocks with cratonic affinity to the east and accreted terranes to the west (armstrong et al., 1977; fleck and criss, 2004) (fig. 1). this isotopic boundary is within the salmon river suture zone and defines the eastern edge of the blue mountains province (silberling et al., 1984, 1987). the blue mountains province is comprised 0.="" 70="" 4="" inset="" inset87sr/86sri="" line="" or="" id="" mt="" ut="" wa="" oregon="" idaho="" figure="" 1.="" geologic="" map="" of="" the="" salmon="" river="" suture="" zone="" (srsz).="" 7d—seven="" devils="" volcanics="" of="" wallowa="" terrane;="" hgp—heavens="" gate="" plate;="" rrp—rapid="" river="" plate;="" pmp—pollock="" mountain="" plate;="" kh—hazard="" creek="" complex;="" lgcc—little="" goose="" creek="" com-="" plex;="" tcr—columbia="" river="" basalt="" group;="" pcp—permian="" chair="" point="" pluton.="" fold="" symbols="" mark="" the="" axes="" of="" the="" riggins="" synform="" and="" lake="" creek="" antiform="" (after="" hamil-="" ton,="" 1969;="" lund,="" 2004;="" gray,="" 2013).="" locations="" of="" figures="" 2a="" and="" 2b="" are="" shown.="" sample="" localities="" from="" this="" study="" (id03,="" id07,="" id23,="" id26,="" id48)="" and="" previously="" published="" geochronology="" and="" thermochro-="" nology="" results="" (*="" denotes="" 40ar/39ar="" hornblende="" ages="" from="" getty="" et="" al.,="" 1993;="" **="" denotes="" 40ar/39ar="" horn-="" blende="" ages="" from="" lund="" and="" snee,="" 1988;="" 598—119="" ±="" 2="" ma;="" r18—106.5="" ±="" 1.4="" ma;="" r30—118.0="" ±="" 0.6="" ma;="" r34="109.1" ±="" 0.6="" ma).="" lithosphere="" |="" volume="" 9="" |="" number="" 4="" |="" www.gsapubs.org="" 3="" m="" ck="" ay="" _l="" 64="" 2="" 2="" nd="" p="" ag="" es="" 3="" o="" f="" 1="" 9="" prolonged="" metamorphism="" during="" terrane="" accretion:="" salmon="" river="" suture="" zone="" |="" research="" et="" al.,="" 1985).="" in="" west-central="" idaho,="" the="" initial="" 87sr/86sr="0.706" isopleth="" is="" subparallel="" to="" longitude="" ~116°w="" and="" marks="" the="" geochemical="" boundary="" between="" rocks="" with="" cratonic="" affinity="" to="" the="" east="" and="" accreted="" terranes="" to="" the="" west="" (armstrong="" et="" al.,="" 1977;="" fleck="" and="" criss,="" 2004)="" (fig.="" 1).="" this="" isotopic="" boundary="" is="" within="" the="" salmon="" river="" suture="" zone="" and="" defines="" the="" eastern="" edge="" of="" the="" blue="" mountains="" province="" (silberling="" et="" al.,="" 1984,="" 1987).="" the="" blue="" mountains="" province="" is="">