Deep Crustal Xenoliths from Central Montana, USA: Implications for the Timing and Mechanisms of High-Velocity Lower Crust Formation

Investigations of North America as EarthScope Reaches Its Maturity themed issue

Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation

Katherine R. Barnhart1, Kevin H. Mahan1, Terrence J. Blackburn2, Samuel A. Bowring2, and Francis O. Dudas2 1Department of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Avenue, Boulder, Colorado 80309, USA 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

ABSTRACT INTRODUCTION seismic velocities (Vp) of >7.0 km/s. Such high-velocity lower crustal layers have been Integration of petrologic, chronologic and Continental crust provides an integrated observed in a variety of settings, particularly petrophysical xenolith data with geophysical record of crustal growth and differentiation in shields and platforms, continental arcs, and observations can offer fundamental insights through time. The lower crust is central to rift zones (Fig. 1) where the material is com- into understanding the evolution of conti- this evolution and its composition and degree monly considered a discrete product of mantle- nental crust. We present the results of a of hetero geneity can have a profound infl u- derived magmatic underplating or intraplating deep crustal xenolith study from the north- ence on the rheology of the lithosphere, yet (Warner, 1990; Kay and Mahlburg-Kay, 1991; ern Rocky Mountain region of the western it is commonly the least constrained part of Eaton, 2006; Karlstrom et al., 2005; Crowley U.S., where seismic experiments reveal an the lithospheric column, because data density et al. 2006; Cornwell et al., 2010; Ridley and anomalously thick (10–30 km), high seismic and seismic constraints generally decrease Richards, 2010). Imbrication of oceanic crust velocity (compressional body wave, Vp > 7.0 with depth. Although the average composi- has also been proposed in some tectonic set- km/s) lower crustal layer, herein referred tion of the lower crust is generally considered tings (e.g., New Zealand; Okaya et al., 2007). to as the 7.x layer. Xenoliths exhumed by to be mafi c, debate continues on this topic as Although high seismic velocity layers are Eocene minettes from the Bearpaw Moun- a result of rare exposure and an incomplete not uncommon, worldwide compilations show tains of central Montana, within the Great and potentially biased record made available that they are generally relatively thin, averaging Falls tectonic zone, include mafi c and inter- through lower crustal xenoliths (e.g., Rudnick 3–8 km (Fig. 1; Christensen and Mooney, 1995; mediate garnet granulites, mafi c hornblende and Taylor, 1987; cf. Hacker et al., 2011). Dis- Rudnick and Fountain, 1995). In contrast, sev- eclogite, and felsic granulites. Calculated crete layers of suspected mafi c lower crust are eral seismic experiments in the Rocky Moun- pressures of 0.6–1.5 GPa are consistent with identifi able as having high compressional wave tain region of Montana and Wyoming (USA), derivation from 23–54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories Northern Shields & Wyoming Continental Extended including prograde burial and episodes of Orogens Platforms Craton Arcs Rifts Crust decompression. Samples with barometrically 0 determined depths consistent with residence Figure 1. Worldwide compila- within the seismically defi ned 7.x layer have tion of average crustal veloci- 10 calculated bulk P-wave velocities of 6.9–7.8 ties (Vp—compressional body km/s, indicating heterogeneity in the layer. wave) and depths to the Moho Shallower samples have markedly slower 20 by tectonic settings (modifi ed velocities consistent with seismic models. from fi g. 7 in Christensen and New monazite total U-Th-Pb data and a vari- Mooney, 1995). The crustal col- 30 ety of additional published geochronology Moho

umn observed by Deep Probe Depth (km) indicate a prolonged and episodic metamor- (Gorman et al., 2002) in central phic history, beginning with protolith ages as Moho Montana (northern Wyoming 40 Moho old as Archean and followed by metamorphic Moho craton) is shown for compari- and deep crustal fl uid-fl ow events ca. 2.1 Ga, son to illustrate the unusual Moho 1.8–1.7 Ga, and 1.5–1.3 Ga. We suggest that 50 thickness of the 7.x layer (black the 7.x layer in this region owes its character fi ll). Moho to a variety of processes, including magmatic 60 underplating and intraplating, associated <5.7 5.7–6.4 6.4–6.8 with multiple tectonic events from the Neo- Vp (km/s) 6.8–7.0 >7.0 >7.8 archean to the Mesoproterozoic.

Geosphere; December 2012; v. 8; no. 6; p. 1408–1428; doi:10.1130/GES00765.1; 12 ﬁ gures; 6 tables; 1 supplemental ﬁ le. Received 11 November 2011 ♦ Revision received 24 July 2012 ♦ Accepted 24 July 2012 ♦ Published online 16 November 2012

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and Alberta (Canada) reveal a distinctly thick 115°0′0″W 110°0′0″W 105°0′0″W A Trans-Hudson (10–30 km) lower crustal layer with Vp between Figure 2. (A) Simpliﬁ ed geo- Hearne Province 7.0 and 7.8 km/s (Henstock et al., 1998; Snel- logic of Montana and adja- Orogen

son et al., 1998; Clowes et al., 2002; Gorman cent states and provinces. Late BC et al., 2002; Eaton, 2006; Schutt et al., 2008). Cretaceous–early Tertiary age

50°0'0"N Vulcan Low SK The unusual thickness of this high-velocity Precambrian-cored uplifts, 19 Canada N

layer, which constitutes as much as half of the the location of seismic experi- AB ″

U.S.A. 0 Medicine ′ 40–60 km crustal column, makes it arguably ments, tectonic boundaries, and Extent of Figure 3 one of the most anomalous regions of continen- the numbers of geochronologic Hat Block 50°0 tal crust in North America. Initial observations constraints on the formation of WA 8,16 indicate that Archean and/or Proterozoic mag- the 7.x layer compiled in Table 1 10 MT 3,9,15,&17 matic underplating events may have played a role are shown. GFTZ—Great Falls ID in contributing to the thickness of high-velocity tectonic zone, LBM—Little Belt OR LBM N

material (Gorman et al., 2002: Chamber lain Mountains, TRM—Tobacco ″ GFTZ 0 Root Mountains, BTM—Bear ′ 4 &13 et al., 2003). However, the age and character 5 BTM 45°0 18 & 20 of the lower crust in this region have until now Tooth Mountains, MR—Madi- TRM Wyoming N ″

1&14 0 remained largely unknown. son Range, BM—Bighorn Province ′ Xenoliths afford a unique opportunity to Mountains, TR—Teton Range, TR 2 BM 45°0 characterize the physical and chemical charac- BH—Black Hills, WRR—Wind 11 teristics of the lower crust, and the data can be River Range, LM—Laramie linked with modern geophysical studies (e.g., Range, SAREX—Southern WRR BH LM Rudnick and Taylor, 1987; Kay and Mahlburg- Alberta Refraction Experiment, 6 12 Kay, 1991). As extensive geophysical data sets TA—Transportable Array. Map NV WY 7 NE become available for the western United States is modifi ed from the Montana 115°0′0″W 3 UT 110°0′0″W 105°0′0″W through large projects such as the National Sci- State Geologic Map (Vuke et al., ence Foundation EarthScope program, it will 2007) and the U.S. Geological Deep Probe Shot Point Billings Array be increasingly important to link these observa- Survey Integrated Geologic SAREX Shot Point CD ROM Recievers tions to compositional, petrologic, and chrono- State Map Database (http:// Deep Probe receivers USARRAY TA stations SAREX receivers Pre Cambrian Exposures logic information in order to both better refi ne pubs.usgs.gov/of/2005/1351/). Geochronologic Constraints from Table 1 the regional tectonic history and the range of (B) Simplifi ed interpretation of composition and petrology corresponding to a the Deep Probe seismic refrac- B particular set of seismic properties. Although tion line with compressional South GFTZ MHB North observations from xenoliths are incomplete, body wave (Vp) speeds (in km/s) Wyoming VL Hearne 0 perhaps unpredictably biased, and depth con- and the location of the 7.x lower straints have limitations, xenoliths commonly crustal layer. The colors and 20 provide the best method of directly sampling velocity thresholds are the same 40 7.x layer relatively modern lower crust. as in the crustal columns of Fig- 60 Moho f Here we report data from crustal xenoliths ure 1. Figure modifi ed from Fig- 80 1 exhumed in Eocene minettes in the Great ure 10 in Gorman et al. (2002). 100 f2 MHB—Medicine Hat block, 120 Falls tectonic zone in central Montana. The VE 5:1 data include thermobarometry and calculated VL—Vulcan low, f1 and f2— (km) Depth Below MSL 140 bulk seismic velocities as well as petrology fl oating refl ectors interpreted –400 –200 0 200 400 600 and monazite geochronology documenting to be relic subducted oceanic Distance North from 49th Parallel (km) a dynamic and polyphase Paleoproterozoic crust. MSL—mean sea level. <5.7 5.7–6.4 6.4–6.8 Vp (km/s) history. We emphasize the heterogeneity in 6.8–7.0 >7.0 >7.8 physical properties and history in the xenolith record, and explore the possible means of assembly for the high-velocity lower crustal layer in this region. features of this region that are relevant to this that the upper crystalline crust of the Medicine study are described in more detail in the follow- Hat block is made of ca. 3.27–2.65 Ga meta- GEOLOGIC SETTING ing (and in the Discussion). plutonic gneiss (Villeneuve et al., 1993). This block is bounded to the north by the aeromag- The Rocky Mountains of Montana and Wyo- Medicine Hat Block, Wyoming Craton, and netically defi ned Vulcan low, which is thought ming record a protracted history of crustal Great Falls Tectonic Zone to represent a Proterozoic collisional boundary growth and reactivation. This region consists of with the Hearne craton (Ross, 2002). Possible Archean cratons amalgamated in the Protero- The Medicine Hat block is completely cov- ages of collision are based on very sparse drill zoic and accreted Proterozoic terranes, both of ered by the Western Canada Sedimentary Basin core data, including a single discordant ca. which have been covered by Phanerozoic sedi- and has been studied exclusively by geophysi- 2.1 Ga U-Pb analysis of titanite (Villeneuve mentary rocks and late Mesozoic and Cenozoic cal methods, xenoliths, and drill core samples et al., 1993) and ca. 1.8 Ga K-Ar biotite dates volcanic rocks (Figs. 2A and 3). The principal (e.g., Ross, 2002). Drill core samples reveal (Burwash et al., 1962).

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Montana Alkalic Province and Crustal Xenoliths

The projected trace of the Great Falls tectonic zone and the Wyoming craton are intruded by suites of potassic and alkali volcanics and kimberlites, including the Highwood Mountains, Sweet Grass Hills, Eagle Buttes, Little Rocky Mountains, Crazy Mountains, and the Williams and Homestead kimberlites. Many of these localities contain upper mantle and lower crustal xenoliths (Collerson et al., 1989; Hearn, 1989; Hearn et al., 1989; Joswiak, 1992; Carlson and Irving, 1994; Downes et al., 2004; Bolhar et al., 2007; Facer et al., 2009; Blackburn et al., 2010, 2011). Crustal xenoliths described in this study are from the Robinson Ranch and Little Sand Creek localities and were exhumed by Eocene minettes (54–50 Ma) of the Bearpaw Mountain volcanic fi eld in north-central Montana (Marvin et al., 1980; Hearn, 1989; MacDonald et al., Figure 3. Geologic map of southwestern and central Montana. Late Cretaceous–early Ter- 1992) and within the Great Falls tectonic zone tiary Precambrian cored uplifts, exposed and subsurface extent of Belt Supergroup sedi- (Figs. 2A and 3). ments, Cretaceous and Tertiary volcanics and intrusives, tectonic boundaries, and xenolith Xenoliths from Little Sand Creek yielded localities are shown. Map modifi ed from the Montana State Geologic Map (Vuke et al., U-Pb SHRIMP (sensitive high-resolution ion 2007) and the U.S. Geological Survey Integrated Geologic State Map Database (http://pubs microprobe) dates on zircon that range from ca. .usgs.gov/of/2005/1351/). 3.0 to 1.8 Ga (Bolhar et al., 2007). Northwest of the Bear Paw Mountains at the Sweet Grass Hills xenolith locality (Fig. 3), granulite xeno- The basement rocks of the Wyoming craton and high-grade contractional tectonism sug- liths yield U-Pb zircon dates of ca. 1.8 Ga and are observed in discrete Laramide-age uplifts gesting a major collisional boundary (Mueller Nd isochrons (garnet, clinopyroxene, and whole and through xenolith studies, and contain three et al., 2002; Harms et al., 2004). The timing rock) ranging from 1.7 to 1.5 Ga and are inter- main domains that share a common isotopic of deformation and metamorphism attributed preted to refl ect modifi cation of the lower crust fi ngerprint but have different Archean histories to activity along the Great Falls tectonic zone (Davis et al., 1995). Davis et al. (1995) postu- (Chamberlain et al., 2003; Frost, 1993; Mueller in southwestern Montana extends to as late as lated that the mafi c granulites represent either and Frost, 2006). The Montana metasedimen- 1.71 Ga (Roberts et al., 2002; Harms et al., metamorphosed Archean crust or an addition to tary province contains ca. 3.3–3.1 Ga igneous 2004). The craton is bounded to the south by the lower crust ca. 1.8 Ga. rocks and intercalated metasediments (Mueller the ca. 1.78–1.75 Ga Cheyenne belt (Karlstrom et al., 1996). The Beartooth-Bighorn magmatic and Houston, 1984), and both cratonic blocks Modern Crustal Structure zone is primarily ca. 2.9–2.7 Ga metaplutonic are bounded to the east by the ca. 1.83–1.72 Ga tonalite-trondhjemite-granodiorite (Frost et al., Trans-Hudson orogen (Bickford et al., 1990; Crustal thickness in Montana and Wyoming 2006a). The southern accreted terranes were Dahl et al., 1999). varies from 49 to 60 km, based on the Deep joined to the rest of the Wyoming craton by Probe active source seismic experiment (Snel- terrane accretion and arc magmatism at 2.68– Belt Basin Development son et al., 1998; Gorman et al., 2002), although 2.50 Ga (Frost et al., 2006b). Dike swarms (ca. more recent passive source seismic studies 2.1 Ga) are reported from exposed rocks at both Mesoproterozoic sediments of the Belt Basin indicate somewhat lower crustal thickness esti- the northern and southern margins of the Wyo- cover portions of the northwestern Wyoming mates of 39–50 km for central Montana (e.g., ming craton and are interpreted as rift related craton and western Medicine Hat block. The Bensen et al., 2009; Gilbert, 2012). Much of the (Premo and Van Schmus, 1989; Cox et al., 2000; Belt Basin reaches 15–20 km in thickness and region is underlain by a layer of anomalously Mueller et al., 2004). defi nes a north-northwest–oriented rift basin thick and seismically fast lower crust, referred The Wyoming craton and Medicine Hat block near the western margin of North America (e.g., to here as the 7.x layer (Fig. 2B; Gorman et al., are sutured together by the ca. 1.86–1.71 Ga Harrison, 1972; Link et al., 1993). Rocks of the 2002). Originally identifi ed in the analysis of Great Falls tectonic zone (Giletti, 1966; O’Neill basin have also been identifi ed in the subsurface COCORP seismic lines (Morel-a-l’Huissier and Lopez, 1985; Mueller et al., 2002; Harms of central Montana (Fig. 3). The sedimentary et al., 1987), the active source seismic refrac- et al., 2004). Some workers have speculated sequence is thought to have been deposited tion experiments Deep Probe, Lithoprobe, and that the Great Falls tectonic zone may be an between 1.47 and 1.37 Ga (Evans et al., 2000). SAREX (Southern Alberta Refraction Experi- intracontinental tectonic feature (e.g., Boerner Mafi c dikes emplaced across the central Wyo- ment) further refi ned the north-south extent and et al., 1998). However, more recent geologi- ming craton between 1.47 and 1.45 Ga are inter- thickness variation of the 7.x layer (Snelson cal studies have identifi ed ca. 1.85 Ga igneous preted to refl ect an eastern rift arm of the Belt et al., 1998; Clowes et al., 2002; Gorman et al., rocks with arc affi nities (Mueller et al., 2002), Basin (Chamberlain et al., 2003). 2002). It is a 10–30-km-thick layer that forms

1410 Geosphere, December 2012

the lower crust of the northern two-thirds of the TABLE 1. GEOLOGIC CONSTRAINTS SHOWN IN FIGURE 2 Wyoming craton, the Great Falls tectonic zone, Age Constraint (Ma) Location Nature, interpretation Reference* and much of the Medicine Hat block (Gorman Archean et al., 2002). Within the Wyoming craton, the 1 2705 ± 4 Stillwater Complex, MT Posttectonic plutonism 1 7.x layer has Vp values of 7.0–7.7 km/s that 2 ca. 2900–2750 Bighorn subprovince, WY Restite from magmatism 2 3 2092 ± 9 Sierra Madre Range, WY Rift-related metagabbro 3 generally increase northward and an average ca. 2100 thickness of 25 km. Within the Medicine Hat 4 2060 ± 6 Tobacco Root Mountains, MT Mafic dikes 4 block, the layer has higher average Vp (7.6–7.9 5 2081 ± 30 Ruby, MT Anatexis 5 6 2170 ± 8 Wind River Range, WY Mafic sill 6 km/s), appears thinner (16 km thick), and may 7 2010 ± 10 Laramie Range, WY Mafic dikes 7 display more upper surface topography. 8 2167 ± 13 Little Sand Creek, MT Zircon rims in xenolith 8 Crust and upper mantle tomography from 9 ca. 2000–2100 Robinson Ranch, MT Monazite in xenoliths this study ca. 1500 the Billings (Montana) seismic array is consis- 10 1469 ± 2.5, Belt Basin, MT Near-surface sills 9 tent with the presence of high-velocity lower 1457 ± 2 11 ca. 1460 Wind River Range, WY Mafic dikes 1 0 crust as thick as 19 km east of the Deep Probe 12 ca. 1460 Granite Mountains, WY Mafic dikes 1 0 line (Schutt et al., 2008). To the west, the high- 13 1470–1450 Tobacco Root Mountains, MT Mafic dikes 1 1 velocity lower crustal layer extends to the 14 1470–1450 Beartooth Mountains, MT Mafic dikes 1 2 15 ca. 1500 Robinson Ranch, MT Thermal resetting of rutile in xenoliths 13 Yellowstone Caldera (Stachnik et al., 2008). ca. 1300 In southern Wyoming, the high-velocity lower 16 1268 ± 34 Little Sand Creek, MT Zircon rims in xenoliths 14 crustal layer is observed to end to the north of 17 ca. 1300 Robinson Ranch, MT Monazite in xenoliths this study 18 1379 ± 1 Belt Basin, ID Zircon in rift-related magmatic complex 15 the Cheyenne belt (Rumpfhuber et al., 2009). 19 1468 ± 2 Belt Basin, BC Mafic sills 1 6 Various mechanisms and ages have been 20 1364 ± 9 Belt Basin, ID A-type megacrystic granite 17 proposed for the formation of the 7.x layer Note: MT—Montana; WY—Wyoming; ID—Idaho; BC—British Columbia. *Age References: 1—Premo et al., 1990; 2—Chamberlain et al., 2003 (and references therein); 3—Premo and (Table 1), and these are explored further in the van Schmus, 1989; 4—Mueller et al., 2004; 5—Alcock and Muller, 2010; 6—Harlan et al., 2003; 7—Cox et al., Discussion . 2000; 8—Bolhar et al, 2007; 9—Sears et al, 1998; 10—Chamberlain and Frost 1995; Chamberlain et al., 2000; 11—Wooden et al., 1978; 12—Mueller et al., 1982; 13—Blackburn et al., 2012b; 14—Scherer et al., 2000; 15— Doughty and Chamberlain, 1996; 16—Anderson and Davis, 1995; 17—Evans and Zartman, 1990. SAMPLE DESCRIPTIONS

Analytical Methods consideration of effective bulk compositions for size, and textural characteristics (Table 2). All Modal mineralogy was quantifi ed by auto- petrologic modeling (e.g., Stüwe, 1997). of the analyzed samples are garnet and feldspar mated scanning electron microscope analysis Quantitative mineral compositions were bearing but with a wide range of abundances (Quantitative Evaluation of Minerals by SCAN- determined using a JEOL JXA-8600 electron (e.g., garnet abundance ~12%–46%, feldspar ning, herein referred to as QEMSCAN) at microprobe at the University of Colorado. ~5%–50%). Pyroxene and/or quartz are pres- the Advanced Mineralogy Research Center at the Before quantitative analysis, X-ray maps were ent in only about half the samples. Some sam- Colorado School of Mines. This technique used to evaluate potential zoning in phases such ples contain deformation-related features such employs a combination of simultaneously col- as garnet and feldspar. For mapping, beam con- as gneissic layering defi ned by garnet-rich or lected energy-dispersive spectroscopy (EDS) ditions were 15 kV and 100 nA. Dwell time was quartzofeldspathic bands (Fig. 4; ROB5 and spectra and calibrated backscatter electron 40 ms per pixel. X-ray maps collected Mg Kα ROB7) and foliation defi ned by aligned biotite intensity to assign mineral identity to each and Al Kα on TAP crystals, Ca Kα on a PET and/or amphibole (Fig. 4; ROB4 and ROB6), mapped pixel (Pirrie et al., 2004; Hoal et al., crystal, and Mn Kα on a LIF crystal. For quanti- whereas others have generally granoblastic tex- 2009). The instrument is a Carl Zeiss EVO50 tative analysis, beam size was ~1 μm (focused) tures (Fig. 4; LSC04). Most samples preserve scanning electron microscope with four energy for all phases except feldspar (5 μm beam) and secondary products that are interpreted to have dispersive spectrometers. Instrument condi- column conditions were 15 kV and 20 nA. A developed during volcanic transport from the tions for full thin section mapping were 25 kV common suite of natural and synthetic stan- lower crust to the surface. These include glassy accelerating voltage, 5 nA sample current, a dards was used. Full mineral compositions are kelyphitic rims and very fi ne grained and local- beam size of 0.25–0.5 μm, pixel step sizes of reported in the Supplemental File1. ized symplectite at grain boundaries (Padovani 20–30 μm, and a variable dwell time necessary and Carter, 1977; Rudnick and Taylor, 1987; to accrue 1000 counts. The resulting fi eld image General Textural Observations, Bulk Messiga and Bettini, 1990). In addition, some maps (Fig. 4) are used to make general textural Compositions, and Mineral Assemblages samples contain porphyroblast-inclusion rela- observations and to determine the textural set- tionships or reaction textures that are interpreted ting of accessory phases. Because the xenoliths We analyzed 11 xenolith samples: 8 from to represent an important part of the pre-erup- are too small for whole-rock measurements, the Robinson Ranch suite (ROB) and 3 from the tion evolution of the rocks (described herein). mineral modes and compositions were used Little Sand Creek suite (LSC). The samples dis- With one exception (LSC18; Fig. 4), late-stage to estimate bulk major element chemistry for play a diverse range of modal mineralogy, grain surface alteration is minor. general characterization, petrological model- Bulk major element compositions were cal- ing, and bulk seismic velocity calculations. This 1Supplemental File. Excel fi le of seven supple- culated from modal proportions and major and method also allows avoidance of complications mental mineral composition tables and one monazite minor mineral compositions (Table 3). For sim- chemistry table. If you are viewing the PDF of this due to decompression during eruption and late- paper or reading it offl ine, please visit http://dx.doi plicity, the samples are divided into mafi c (<52 stage alteration effects on the bulk properties .org/10.1130/GES00765.S1 or the full-text article on wt% SiO2), intermediate (>52 wt% and <57 wt%

(e.g., Rudnick and Jackson, 1995) and allows www.gsapubs.org to view the Supplemental File. SiO2), and felsic (>57 wt% SiO2) granulites .

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A ROB1 B ROB3 C ROB4 D ROB5

E ROB6 F ROB7 G ROB8 H ROB9

I LSC04 J LSC06 K LSC18 Mineral Key Garnet Biotite Orthopyroxene Clinopyroxene Hornblende Plagioclase K-Feldspar Quartz Rutile Ilmenite Apatite Titanite Alteration Hole in Section 5000 μm

Figure 4. Simpliﬁ ed QEMSCAN (automated scanning electron microscope analysis) mineralogy of full slide scans from eight samples from Robinson Ranch and three samples from Little Sand Creek. Each color represents a different mineral determined using a combination of energy dispersive spectrometry and backscatter electron number. (Full-resolution images of individual samples are available from the authors upon request.)

1412 Geosphere, December 2012

TABLE 2. MODES FROM QEMSCAN DATA Robinson Ranch Little Sand Creek Sample ROB1 ROB3 ROB4 ROB5 ROB6 ROB7 ROB8 ROB9 LSC04 LSC06 LSC18 Qz 5.21 0.0 0.55 26.76 0.0 29.86 <0.1 0.0 0.0 0.0 0.14 Grt 16.36 45.94 35.75 17.88 14.56 4.65 39.23 11.72 43.26 23.51 23.43 Pl*†# 12.42 10.17 14.39 29.53 38.49 24.39 11.95 0.64 21.39 27.21 20.17 Cpx†§ 50.65 0.0 33.27 0.0 0.0 0.0 36.51 42.20 35.05 45.83 0.00 Opx* 0.0 0.0 0.0 0.0 39.28 0.0 0.0 0.0 0.0 0.0 0.00 Hbl 0.0 0.0 0.0 0.0 0.0 0.0 0.0 32.17 0.0 0.0 0.00 Oamph 0.0 0.0 0.0 0.0 3.18 0.0 0.0 2.69 0.0 0.0 0.00 Bt 2.02 26.14 9.91 4.50 2.12 0.27 1.97 4.07 0.0 0.0 1.76 Kfs 12.56 16.34 4.73 21.31 0.0 40.78 7.11 4.57 0.0 0.0 12.79 Spl 0.0 0.03 0.0 0.0 1.28 0.0 0.0 0.0 0.0 0.0 0.00 Rt 0.52 0.92 0.51 0.0 0.45 0.0 0.20 0.16 0.15 0.21 0.33 Ilm** 0.02 0.46 0.21 0.02 0.64 0.04 2.23 0.20 0.15 1.12 0.07 Ap 0.20 0.00 0.21 0.00 0.0 0.01 0.36 1.15 0.0 0.83 0.32 Ttn 0.04 0.0 0.46 0.0 0.0 0.0 0.44 0.42 0.0 0.24 0.03 Mag/Hem 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.06 0.00 Alteration†† 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 40.97 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Note: Mineral abbreviations after Whitney and Evans, 2010. QEMSCAN—automated scanning electron microscope analysis. *Consists of two generations in ROB6. †Consists of two generations in ROB9. §Consists of two generations in LSC06. #Consists of three generations in LSC06. **Includes both primary ilmenite and secondary ilmenite rims on rutile. ††Secondary alteration is minor in most samples and not included. The one exception is LSC18, where >40% of the sample is altered to dominantly pectolite and apophyllite, but also minor amounts of carbonate and zeolite.

TABLE 3. MAJOR ELEMENT COMPOSITIONS CALCULATED FROM MODAL AND MINERAL COMPOSITIONS Locality Robinson Ranch Little Sand Creek Sample ROB1 ROB3 ROB4 ROB5 ROB6 ROB7 ROB8 ROB9 LSC04 LSC06 LSC18*

SiO2 55.0 44.0 48.1 67.2 53.1 73.4 47.8 46.1 47.6 49.9 53.8 TiO2 0.9 2.3 1.5 0.3 0.9 0.0 1.8 1.9 0.4 1.2 0.5 Al2O3 11.3 22.0 14.6 16.2 16.7 14.6 14.6 11.2 17.6 13.1 13.9 MnO 0.1 0.2 0.2 0.1 0.2 0.0 0.3 0.5 0.2 0.0 0.1 MgO 9.2 7.8 8.6 2.6 12.5 0.5 7.2 7.9 8.7 6.3 2.4 FeO(t) 6.6 16.0 13.1 5.3 11.4 1.4 14.1 13.8 10.7 12.3 7.2 CaO 12.9 1.4 10.3 2.1 1.2 1.3 11.2 14.6 12.6 14.0 15.2

Na2O 1.9 0.9 1.8 3.1 3.8 2.7 1.7 1.7 2.2 2.8 3.5 K2O 2.0 5.6 1.8 3.1 0.3 6.1 1.1 1.9 0.1 0.1 3.2 P2O5 0.1 0.0 0.1 0.0 0.0 0.0 0.2 0.5 0.0 0.4 0.1 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 *Alteration phases in LSC18 are arbitrarily split 50/50 between pectolite and apophyllite.

2 The mafi c granulites are ROB3, ROB4, ROB8, only one of the fi ve mafi c granulites. However, of Opx2 + Spl + Pl2 symplectite (Figs. 6B and ROB9, LSC04, and LSC06. Intermediate gran- rare small rounded quartz inclusions occur in 7B) (described in further detail herein). The pri- ulites are ROB1, ROB6, and LSC18, and the the margins of garnet porphyroblasts in ROB8, mary assemblage in LSC18 is Grt + Pl + Kfs + two felsic granulites are ROB5 and ROB7. On suggesting that silica activity is near one in this Bt + Qz, but it is heavily altered such that ~40% a total alkali versus silica diagram, the samples sample as well. Plagioclase is interpreted as a of the matrix is pectolite, apophyllite, and minor generally plot in the basalt-basaltic andesite or stable component of the peak assemblage in all carbonate and zeolite, likely representing prod- dacite-rhyolite fi elds (Fig. 5). The plotted range but one of the mafi c granulites. In ROB9, plagio- ucts of near-surface alteration. The two felsic is similar to crustal xenoliths collected from clase occurs as small rounded inclusions in some granulites (ROB5 and ROB7) also have the peak the nearby Eagle Buttes (Joswiak, 1992) and of the larger clinopyroxene grains (Fig. 6C) and assemblage Grt + Pl + Kfs + Bt + Qz ± Rt ± Ilm. from southern Wyoming and northern Colorado as a fi ne-grained component of the recrystal- Ilmenite is present in all samples. However, (Farmer et al., 2005; Mirnejad and Bell, 2008) lized margins of clinopyroxene, but is otherwise in some samples where ilmenite is not consid- with perhaps a tendency for lower alkali content absent from the matrix. The sixth mafi c granu- ered part of the peak metamorphic assemblage, it

in more mafi c xenoliths. lite (ROB3) has <2 wt% CaO and 22 wt% Al2O3 occurs as either early inclusions in other miner- Five of the six mafi c granulites have >10 wt% (sample is 46% garnet by mode) and its inter- als (e.g., garnet) or more commonly as secondary bulk CaO (Table 3). The primary (peak) meta- preted peak mineral assemblage is Grt + Bt + rims on rutile grains. Accessory zircon occurs in morphic mineral assemblage in these high-Ca Pl + Kfs + Rt. all samples, monazite is recognized in at least six mafi c granulites, as well as in one of the interme- The remaining two intermediate granulites diate granulites with similar CaO, FeO, and MgO (other than ROB1) have distinctly different bulk 2We use a numeral as a subscript to a mineral ab- (ROB1), is Grt + Cpx + Rt ± Pl ± Qz ± Ilm ± Hbl compositions and mineral assemblages. ROB6 breviation to denote different generations of mineral ± Bt ± Kfs (mineral abbreviations herein after is a Grt + Opx + Oamph + Bt + Pl + Rt granulite growth in polymetamorphic samples. Thus, an early Whitney and Evans, 2010). Quartz is a clear and gneiss. An extensive late reaction zone occurs generation of plagioclase associated with the earliest observed metamorphic event M is denoted as Pl and signifi cant component of the peak assemblage in between primary garnet and Opx that consists 1 1 1 a subsequent generation of plagioclase is denoted Pl2.

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14 samples (see Monazite Geochronology discussion), and apatite and titanite occur in most of 12 the higher bulk Ca samples (Fig. 4; Table 2). Figure 5. Total alkali versus Trachyte silica diagram for samples from 10 Mineral Compositions Robinson Ranch and Little Sand Trachy- andesite Garnet is generally unzoned with the com- Creek (this study) as well as 8 regional samples (from Joswiak, mon exception of a slight increase in Ca and Dacite Rhyolite decrease in Mg# [X /(X + X ) x 100] toward 1992; Farmer et al., 2005; O (wt. %) Mg Mg Fe 2 6 Mirnejad and Bell, 2008). The grain edges that is interpreted to represent dif- samples from Robinson Ranch Robinson Ranch fusional exchange with matrix phases during O + K 3 and Little Sand Creek range in 2 4 Andesite Little Sand Creek isobaric cooling (Fig. 7A) . Thus, core compo-

Na Joswiak/Eagle Buttes sitions were typically used for bulk composition bulk composition, but are com- Farmer/State Line parable to the range of composi- 2 estimation and pressure-temperature (P-T) cal- Farmer/Leucite Hills culations. Garnet compositions in the high CaO tions previously observed. Basalt Mirnejad Leucite Hills 0 maﬁ c granulites and in the high Ca intermediate 40 45 50 55 60 65 70 75 80 granulite ROB1 are relatively rich in grossular SiO (wt. %) (XGrs = 0.16–0.34) and Mg#s are variable among 2 the samples (19–47) (Table A1 [see footnote 1]). All other samples have signiﬁ cantly lower gros-

sular contents (XGrs = 0.01–0.06) and less variable Mg# (32–46). A ROB3 B ROB6 Plagioclase grains are generally characterized by broad unzoned interiors with gradational Pl decreases in anorthite content toward extreme Opx 1 Pl margins, commonly spatially associated with garnet. This is interpreted to refl ect diffusional exchange with garnet during isobaric cooling Bt Rt Grt from high temperature. In most samples, plagioclase interiors have a relatively restricted composition of X between 0.23 and 0.41, where Grt An Figure 10C the range in each sample is generally <0.03 (Table A2 [see footnote 1]). The lowest anor- Spl thite content (XAn = 0.14) occurs in the low bulk Opx +Spl 2 Ca intermediate granulite ROB6 and as early Pl1 inclusions in coarse clinopyroxene in the horn- 500 μm 500 μm blende eclogite ROB9. K-feldspar occurs in the primary assemblage of several samples and as C ROB9 D ROB9 a common secondary phase rimming corroded plagioclase in others, although little systematic Cpx 2 compositional variation is observed. The latter may refl ect high-temperature interaction with Cpx 2 K-bearing fl uids during a ca. 1.8 Ga metaso- matic event identifi ed in mantle xenoliths from Cpx2 this region (Carlson and Irving, 1994; Carlson et al., 2004; Rudnick et al., 1999; Downes et al., Hbl Pl1 2004). K-feldspar in the hornblende eclogite (ROB9) contains ~2.5% BaO. All the high-Ca mafi c granulites and one

Cpx3+ high-Ca intermediate granulite (ROB1) contain Pl 3 clinopyroxene (diopside or sodian diopside by the classiﬁ cation of Morimoto, 1988), whereas 500 μm 500 μm

3We use X with a subscript to denote the pro- Figure 6. Photomicrographs of preserved assemblages and reaction textures in Robinson portion of mineral end-member components. For

Ranch samples. (A) Spinel preserved in the core of garnet in ROB3. Monazite ROB3m2 is example, XMg = 0.25 indicates that 25% of the min- preserved within this spinel grain (inset box shows location of Fig. 10D). (B) Reaction tex- eral is the Mg-endmember of the solid-solution. The ⇒ subscript may also be an abbreviation for the named ture of Grt + Opx1 Opx2 + Spl + Pl2 in ROB6 (mineral abbreviations after Whitney and end-member mineral (e.g., XGrs = 0.25 indicates that Evans, 2010). See Figure 7B for related X-ray map. (C) Pl1 preserved within Cpx2 in ROB9. 25% of the garnet composition is grossular, which is ⇒ (D) Reaction texture between high-Na Cpx2 low-Na Cpx3 + Pl3 in ROB9. the Ca-endmember of the series).

1414 Geosphere, December 2012

A ROB5 Ca Kα one intermediate granulite (ROB6) contains pri- + Oamph + Bt + Pl + Rt with a strong gneissic mary Opx. Pyroxene Mg#s range from 61 to 82 foliation deﬁ ned by the distribution of garnet, with generally little zoning or internal variation pyroxene, and feldspar and a shape-preferred within each sample (Table A3 [see footnote 1]). orientation of biotite and amphibole (gedrite).

Three exceptions where early pyroxene is exten- A symplectite intergrowth of Opx2, hercynitic

sively recrystallized and a later pyroxene has a spinel, and Pl2 is extensively developed along

distinctly different composition are described grain boundaries between Grt1 and Opx1 (Fig. further in the following discussion. Amphi- 6B). The new Opx is distinctly lower in Mg# Pl 2 bole occurs in two of the xenoliths (Table A4 (51) and higher in total Al2O3 (9–10 wt%)

[see footnote 1]). ROB6 contains an unzoned than the peak Opx1 (Mg# 73, Al2O3 6–7 wt%; orthoamphibole that is gedrite (classiﬁ cation of Table A3; see footnote 1). Development of these Grt Leake et al., 1997). ROB9 contains unzoned cal- reaction domains prior to eruption in the vol- cic amphibole that is ferropargasite and minor canic host is suggested by (1) the large spatial anthophyllite pseudomorphs that may have extent and grain size of the reaction domains 700 μm replaced orthopyroxene. All but two samples (several tens to hundreds of μm wide, grain B ROB6 Mg Kα contain primary biotite, although with signiﬁ - sizes to several tens of μm) compared to the cantly variable abundances (Table 2). Mg#s substantially narrower widths and grain sizes

range from 53 to 78 and TiO2 contents range of kelyphite rims that are attributed to rapid from 3.3–6.3 wt% with little variation in each heating and decompression during exhumation, sample (Table A5 [see footnote 1]). (2) variations in Fe and Mg between reactants and products that correlate with proximity to Polymetamorphic Samples grain boundaries and suggest late diffusional Grt exchange of these elements, a process that is Of the 11 examined xenoliths, 4 contain not expected to occur over signifi cant distances either early or late assemblages and reaction during the short residence times (hours to days) Opx2+Spl textures that provide additional insight into the in the erupting magma (Morin and Corriveau, tectonometamorphic history of the deep crust 1996; Sparks et al., 2006; Rutherford, 2008), in this region. ROB3 is a low-CaO (1.4 wt%), and (3) the presence of a distinct Proterozoic high-Al O (22 wt%) quartz-free mafi c granu- population of monazite spatially associated Opx 2 3 1 lite that contains 46% garnet by volume. The with the reaction domains (see Monazite Geo- 70 μm high modal proportions of garnet and biotite chronology discussion ). suggest that this xenolith may have undergone ROB9 and LSC06 are high-Ca mafi c granu- C ROB9 Na Kα multiple episodes of melt extraction. For exam- lites that contain Grt, Cpx (sodian diopside), and

ple, the SiO2-poor and [Al2O3 – (FeO + MgO + Rt as part of their peak assemblages. Plagioclase

TiO2) – K2O] rich bulk composition is similar to is not present as a matrix phase in ROB9. How- diatexite residuum from extremely migmatized ever, this sample contains plagioclase in two

metasedimentary rocks in western Maine (Solar distinct pre-peak (M1) and post-peak (M3) meta- Cpx and Brown, 2001). The primary assemblage is morphism textural settings. First, small rounded 2 Grt + Bt + Pl + Kfs + Rt, but inclusions in larger inclusions of plagioclase (Pl ; X = 0.14) com- Cpx + 1 An 3 garnets preserve a distinct earlier assemblage. monly occur in the cores of larger clinopyrox- Pl 3 Two main garnet morphologies are present. ene grains (Cpx2; Fig. 6C). Second, the margins

Large (to 7 mm) anhedral grains contain abun- of peak Cpx2 (XJd = 0.17–0.18) are extensively Hbl dant inclusions of ilmenite, biotite, and Zn-bear- recrystallized to symplectitic intergrowths of a ing hercynitic spinel (Fig. 6A). Smaller euhedral less sodic Cpx3 (XJd = 0.05) and Pl3 (XAn = 0.24) 50 μm garnet grains, generally without inclusions, (Figs. 6D and 7D). This sample also contains occur throughout the matrix. However, similar substantial hornblende (ferropargasite), biotite, Figure 7. X-ray maps showing key observa- compositions of both garnet morphologies and and K-feldspar that occur in textural equilibrium tions from Robinson Ranch samples. Min- the lack of signiﬁ cant major element zonation, with the peak assemblage, as well as antho- eral abbreviations after Whitney and Evans other than that interpreted as having developed phyllite pseudomorphs that may have replaced (2010). (A) Ca Kα map from ROB5 showing during retrograde diffusion, indicate homogeni- orthopyroxene (Table 2). Although plagioclase Ca enrichment at garnet edges commonly zation of garnet compositions at relatively high occurs in the matrix of LSC06 and is considered observed in the sample suite. This pattern temperature. The inclusion suite indicates an part of the peak assemblage, peak Cpx1 (XJd = is interpreted to represent isobaric cooling. early metamorphic episode characterized by the 0.12) is similarly recrystallized to a less sodic α (B) Mg K map showing the reaction texture assemblage Grt + Bt + Pl + Spl + Ilm. A dis- Cpx (XJd = 0.05) and a new Pl (XAn = 0.26). ⇒ 2 2 of Grt + Opx1 Opx2 + Spl + Pl2 preserved tinct population of monazite is also associated Similar arguments to those here for ROB6 can in ROB6. (C) Na Kα map of ROB9 showing with this early assemblage (see Monazite Geo- be made for development of these reaction tex- ⇒ the reaction between high-Na Cpx2 low-Na chronol ogy discussion). tures prior to exhumation in the volcanic host.

Cpx3 + Pl3. Operating conditions for X-ray ROB6 is a low-CaO (1.2 wt%), high-MgO For example, both recrystallized Cpx (locally map collection were a voltage of 15 kV, cur- (13 wt%) quartz-free intermediate granulite whole grains are reconstituted) and very ﬁ ne rent of 100 nA, and a dwell time of 40 ms. gneiss. The primary assemblage is Grt + Opx grained (glassy) kelyphite rims occur in sample

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LSC06, the latter locally appearing to have partly consumed the former. x x In summary, some samples in this xenolith x suite preserve evidence for a dynamic and poly- Pseudosection**

metamorphic history in the lower crust. ROB3 # x contains evidence for an early spinel-bearing x

assemblage that may have initially developed Ti-in-Bt at relatively low pressure prior to attaining peak §

conditions. Reaction textures and assemblages xx

in ROB6 suggest that this sample underwent Ab-Jd-Qz

a relatively late reheating event. The high Ca § maﬁ c granulite ROB9, a hornblende eclogite x x (sensu lato), initially and ultimately resided in x a P-T ﬁ eld where plagioclase was stable, but Grt-Pl-Opx-Qz

reached eclogite facies without stable plagio- §

clase at its peak in the intervening period. d e s u x x x x x x x x x x d o

THERMOBAROMETRY AND P-T PATHS h t e Grt-Pl-Cpx-Qz m r §

Pressure and temperature conditions were o s n x x x

determined to further constrain the metamorphic o i t c Eastonite history and potential residence depths of the a 1; http://gsc.nrcan.gc.ca/sw/twq_e.php). e † xenoliths. Calculations used the program TWQ R 2.34 (Berman, 1991; http://gsc.nrcan.gc.ca/sw x x x /twq_e.php) and the internally consistent thermo- ples with pressures < 0.8 GPa.

dynamic database of Berman and Aranovich Grt-Opx(Al) †

(1996, updated in 2007; Berman et al., 2007). x

In general, only well-calibrated reactions that Grt-Sp are recommended in the TWQ program were † used. Absolute errors are considered to be x ~±50 °C and ±0.1 GPa (Berman, 1991). Cal- Opx-Sp † culations for equilibrium P-T conditions were x also made for three garnet granulite xenoliths Grt-Opx

from the Eagle Buttes (Fig. 3) using mineral † x x x x x x x x x compositions from Joswiak (1992) to allow for x

direct comparison with the new xenolith data Grt-Cpx presented here. Isochemical phase assemblage † x x x x x diagrams (pseudosections) were calculated for x Grt-Bt

a subset of samples in order to better determine THERMOBARAMETRIC CONDITIONS 4. CALCULATED TABLE ) 9 . a 0 0 7 0 9 3 8 7 3 0 2 7 4 8 P-T stability limits for a particular phase (e.g., 0 7 3 9 7 6 0 1 4 3 1 3 5 4 7 - P ...... 6 0 1 0 1 1 1 1 0 1 1 1 0 0 1 . G ( 0 spinel stability in the garnet granulite ROB3; < Pressure plagioclase stability in the hornblende eclogite 0

ROB9). These calculations were made using 0 2 0 0 0 0 0 0 5 0 0 5 0 0 0 0 1 7 6 5 0 2 1 3 5 0 3 8 7 0 2 – 6 8 8 8 7 8 8 7 8 7 8 7 7 PerpleX07 (Connolly and Petrini, 2002) and 8 (°C) 0 0

the 2007 updated version of the internally con- 9 sistent database of Holland and Powell (1998). Temperature ) m l d I e +

The calculation results, along with the speciﬁ c r t t e t R R 2 l + + l reactions used and whether these data were sup- a z z ( P t t Q Q + n t R plemented with pseudosection calculations, are R t + + z + s s + B T l f f z z +Spl+Pl + Q ± l P K K shown in Table 4. The Ti-in-biotite thermometer 2 h ± Q Q + P m + + p l t t t t t + + I + t m s B R B B R l m ± x

of Henry et al. (2005), which is calibrated for f I B 2 + + + + + a p l l l l l l l K + + t O s + P P P P P P O P f 0.4–0.6 GPa, was also used to estimate tem- t B + +Pl + + + + + + + + K B 2 + x x x x x x x x x l e + + p l p p p p p p p p l l peratures for samples with pressures <0.8 GPa. p b O C P P C C C C C C C S a + + + + + + + + + + + + t t t t t t t t t t t t The absence of observed graphite may mean t s r r r r r r r r r r r r l G G G G G G G G G G G G P : : : : : : : : : : : that these temperatures are minimum estimates : : k k k k k k k k k k k y l e a a a a r a a a a a a a t e e e e e a e e e e e e

(Henry et al., 2005). Pressures calculated with a P P P P E P L P P P P P reactions involving quartz are maximum esti- P * * 5 1 - mates for samples where quartz is not observed. * 2 3 8 8 4 6 - - 6 1 3 8 9 5 4 1 1 0 0 8 8 - B B B B B B B Thermometer reactions as calculated in TWQ 2.34. All are Fe-Mg exchange reactions except for Al in Opx thermometer (Berman, 199 All are Fe-Mg exchange reactions except for TWQ 2.34. Thermometer reactions as calculated in TWQ 2.34. Barometer reactions calculated in in Biotite empirical thermometer from Henry et al. (2005), calibrated for 0.4-0.6 GPa. Used to estimate temperatures sam Ti Calculated temperatures may be minimum 8 8 C C C 6 *Mineral assemblages and compositions from Joswiak (1992) recalculated. † § # **Calculations made in PerpleX07 (Connolly and Petrini, 2002). O O O O O O O B S 2 C S S L ROB3 Peak: Grt+Pl+Bt+Kfs+Rt 700 1.30 x R 6 R L R ROB9 Peak: Grt+Cpx+Hbl+Kfs+Bt+Rt 800 1.7–2.0 x x ROB7 Peak: Grt+Pl+Bt+Kfs+Qz+Rt 800 0.60 R R ROB6 Late: Grt + Opx -> R Sample Key assemblage E L LSC06 Late: Grt+Cpx L estimates due to some degree of diffusional R

1416 Geosphere, December 2012

exchange of Fe and Mg during cooling (Frost indicated by the presence of this phase in BULK SEISMIC VELOCITY and Chacko, 1989; Spear and Florence, 1992). inclusions in Cpx and its absence in the peak Calculated peak pressures range from 0.6 to assemblage. An isochemical phase diagram The calculated peak pressures are interpreted >1.7 GPa and calculated temperatures are gen- for this bulk composition places the plagio- to approximate the depths from which the sam- erally in the range of 700–900 °C (Table 4; Fig. clase-out boundary at ~1.7 GPa at 800 °C ples were derived during exhumation, with the 8A). In addition, four of the samples preserve (Fig. 8A). From TWQ calculations, garnet- exceptions of the three polymetamorphic sam- evidence for multiple assemblages where por- pyroxene and garnet-biotite temperatures are ples with evidence for late-stage decompres- tions of a P-T path can be estimated or inferred. near 800 °C whereas a pressure of 2.0 GPa sion described above. Retrograde pressures for First, isochemical phase assemblage modeling at that temperature is calculated using the samples with late decompression textures are indicates that spinel stability for the ROB3 bulk Jd + Qtz = Ab barometer and the albite com- interpreted to indicate residence depths before composition is limited to pressures below ~0.6 position from coexisting K-feldspar. This is exhumation. The calculated pressures range GPa, which extends to lower pressures with considered a maximum pressure due to the from 0.6 to 1.53 GPa, which correspond to ~23 increasing temperature (Fig. 8A). The Fe-Mg absence of free quartz in the assemblage. to ~54 km depths using the average crustal den- exchange reaction between garnet and spinel is Restabilization of plagioclase, and thus some sity proﬁ le of Christensen and Mooney (1995). calculated at 800 °C with TWQ, although the likely decompression (Fig. 8B), is indicated These data are consistent with 8 of the 14 sam- compositions may have been modiﬁ ed dur- by pyroxene margins that are recrystallized ples shown in Figure 9 derived from within the

ing younger metamorphic events. Despite this to Cpx3 + Pl3. Estimated conditions for this modern seismically deﬁ ned 7.x layer as pro- uncertainty in the temperature of early metamor- late stage are 730 °C and 1.37 GPa. A similar jected from the Deep Probe proﬁ le. The remain- phism, ROB3 apparently underwent relatively decompression stage in LSC6 is suggested by ing samples are midcrustal and derived from

low pressures early in its history prior to being extensively recrystallized Cpx2 + Pl2 domains. depths shallower than the 7.x layer. taken to pressures as high as 1.3 Ga during peak Peak and retrograde conditions in this sample Bulk seismic velocities of each sample were metamorphism (Fig. 8B). An increase in pres- are estimated as 730 °C, 1.42 GPa and 720 °C, calculated using the physical properties spread- sure also predicts a transition from ilmenite to 1.18 GPa (Table 4; Fig. 8B). Extensive reac- sheet of Hacker and Abers (2004) to establish rutile stability, which is consistent with the for- tion between peak garnet and orthopyroxene in the context of the samples within the seismic mer as inclusions in garnet and the abundance of ROB6 (estimated conditions 850 °C, 1.2 GPa) observations. Velocities were calculated for the

rutile in the matrix. to a new Opx2 + Sp assemblage (estimated as equilibration pressure at three temperatures: An increase in pressure is also inferred for >900 °C and 0.6–0.9 GPa) suggests a reheating the calculated equilibrium temperature, 25 °C ROB9 from the destabilization of plagioclase event accompanied by some decompression. for comparison with laboratory measurements

ep AB<52% SiO early 80 -eclogite Hbl- 2 out MORB 52> x <57 % SiO2 crustal thickening peak >57% SiO during subduction late 2 and collision in 2.0 GFTZ ROB9 amphibolite ROB9 peak eclogite ROB9 pl-out 1.8-1.78 Ga? ROB9 pl-out blueschist -eclogite present Moho .7 Ga? LSC4 LSC6 LSC6 peak ROB9 ROB8 1.75–1ROB9 ROB1 ROB4 ROB3 ROB3 Pl3+Cpx3 high-P granulite ROB9 peak ROB6 40 DeepProbe DeepProbe LSC6 ROB6 7.x layer LSC6 peak 1715 ±35 Ma early Pl Pl +Cpx 1.0 EB88-21 2 2

LC88-3-5 1784±10 Ma Depth (km) a ROB6 LSC18 M 626-18 ROB5 ROB6 1 Pressure (GPa) Pressure Opx2-Spl ROB3 Sp ROB3 Spl-out ROB7 l-out granulite 1793 ±1 ROB3 advective heat from intraplating amphibolite ROB-3 early Grt-Spl magmas? advective heat from 2.1 Ga intraplating magmas? 0.0 0 400 600 800 1000 400 600 800 1000 Temperature (°C) Temperature (°C)

Figure 8. (A) Pressure-temperature (P-T) estimates of xenolith samples. Table 4 describes methods used to calculate P-T for each sample.

The size of each sample box represents the error on the P-T estimate. The shade of each box represents the SiO2 content. Petrogenetic grid for rocks of roughly basaltic composition is modiﬁ ed from Figure 2 of Ernst (2010). Location of hornblende stability limit for mid-oceanic ridge basalt (MORB) is from Figure 4 of Ernst (2010) (mineral abbreviations after Whitney and Evans, 2010). (B) P-T-time paths of polymetamorphic samples ROB3, ROB6, ROB9, and LSC6. Color and shape of sample symbols indicates early, peak, or late assemblage based on petrographic observations. GFTZ—Great Falls tectonic zone. Both A and B show the depth of the Moho and vertical extent of the seismogenically deﬁ ned 7.x layer (after Gorman et al., 2002) as well as the boundaries of spinel breakdown in ROB3 and plagioclase breakdown in ROB9.

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Pressure Depth (GPa) Vs VRH (km/s) Vp VRH (km/s) Vp/Vs VRH (km) 0 0 Source Locality B C A Robinson Ranch Little Sand Creek Eagle Buttes 0.5 19 ROB7 ROB7 ROB7 LSC-18 ROB5 Top 7.x central MT 1 38

LSC-06 LSC-06 ROB3 ROB3 ROB8 ROB3 ROB8 1.5 57 LSC-04 Composition LSC-04 Deep Probe Moho ~58 km <52 wt% SiO2

52–57 wt % SiO2 >57 wt% SiO 2 2 76 3 3.5 4 4.5 5 6 6.5 7 7.5 8 1.625 1.75 1.875 2.00

Figure 9. Bulk seismic velocity plotted against depth for samples from Robinson Ranch and Little Sand Creek (this study), and from Joswiak (1992). Vp, Vs (compressional, shear body wave velocities), and Vp/Vs are calculated at 500 °C, the baseline temperature for modern lower crust in this region (Blackburn et al., 2012a). Horizontal error bars represent variation in calculated seismic properties based on thin section–scale compositional heterogeneity. Vertical error bars represent uncertainty in the pressure estimate (Table 4; Fig. 8). Samples without error bars have uncertainty within the area of the symbol. MT—Montana. VRH—Voigt-Reuss-Hill average. The depth of the Moho and extent of the seismogenically deﬁ ned 7.x layer are shown after Gorman et al. (2002).

commonly reported at room temperature, and arrangement of minerals is beyond the scope of MONAZITE GEOCHRONOLOGY 500 °C, which is a baseline temperature for mod- this contribution. However, the results of Naus- ern lower crust in this region (Fig. 9; Blackburn Thijssen et al. (2011) suggest that variations Monazite geochronology was undertaken for et al., 2012b). The latter perhaps provides a bet- from the calculated VRH velocities due to micro- fi ve samples in order to further constrain the ter comparison to modern seismic observations. structure are not likely to exceed those described origin and metamorphic history of the Robin- Aside from uncertainties regarding the modern above based on modal mineralogy variations. son Ranch xenolith suite. Slow diffusion of Pb geothermal gradient, the most readily quantifi - The results suggest a distinct increase in in monazite makes it a robust chronometer for able source of error in the calculated seismic bulk seismic velocities with residence depths high-temperature events (Parrish, 1990; Cher- properties is due to variations in modal compo- at ~35 km and deeper and are consistent with niak et al., 2004), and the sensitivity of monazite sition, which was evaluated by making similar the seismically determined high-velocity layer to recrystallization makes it particularly useful calculations over halves of the full thin sections (Fig. 9). At these depths, calculated veloci- for elucidating the history of polymetamorphic (Table 5; Fig. 9). Typical variations in the mode ties (VRH average) at 500 °C vary consider- rocks (Williams et al., 2007). However, this of a major phase are in the range of 5%–10% of ably from 6.97 to 7.85 km/s (7.16–8.02 km/s at same sensitivity to recrystallization commonly the full section quantity, which results in errors of 25 °C). These data compare well with the 7.0– results in complexly zoned grains that may <1% on the compressional velocities (Table 5). 7.7 km/s velocities modeled for the lower crust require high spatial resolution. Fortunately, the The largest modal variations are for gneissic in central Montana from the Deep Probe study generally high Th and radiogenic Pb content of samples (e.g., to ±30% variation from full section (Gorman et al., 2002). The interpreted resi dence monazite allows utility of the high spatial reso- garnet mode in ROB5), which result in 1%–2% depths are also consistent with estimated crustal lution capability of the Th–U–total Pb method, variation in calculated velocities. Table 5 pre- thicknesses from the Deep Probe study, but do which is capable of analyzing compositional sents both averages calculated by the Hacker and not preclude somewhat thinner crust, as sug- domains as small as 2 μm (Jercinovic et al., Abers (2004) spreadsheet, the Voigt-Reuss-Hill gested by recent passive source studies (e.g., 2008; Mahan et al., 2010). (VRH) and Hashin-Shtrikman (H-S); the for- Bensen et al., 2009; Gilbert, 2012), particularly mer does not account for the geometric arrange- since the highest residence pressure calculated Analytical Methods ment of mineral grains, but has nonetheless been (for LSC04) is a maximum estimate due to the shown to be a remarkably robust scheme for absence of quartz. The calculated velocities also Identifi cation of monazite and its textural set- evaluating elastic properties and is thus the most signifi cantly exceed the 6.40–7.00 km/s range ting were performed using an automated map- commonly used in Earth science literature (e.g., measured for garnet-free lower crustal xeno- ping routine using QEMSCAN at the Colorado Mainprice and Humbert, 1994), whereas the liths (at 25 °C and 1.0 GPa) from the Leucite School of Mines similar to that described herein latter explicitly assumes a statistically random Hills in southernmost Wyoming (Farmer et al., for major phases. The routine is specifi cally structure (Bunge et al., 2000). As the anisot- 2005), which is consistent with the absence of calibrated to rapidly target monazite at a step ropy of physical properties is not considered an imaged 7.x layer in that part of the craton size of 4 μm. The subsequent protocol for mona- here, further consideration of the microstructural (Gorman et al., 2002; Rumpfhuber et al., 2009). zite geochronology followed that of Williams

1418 Geosphere, December 2012

et al. (2006). Select grains were X-ray mapped

s at the University of Massachusetts-Amherst e t t

u using a Cameca SX-50; U Mβ, Th Mα, Nd Lα, B e l and Ca Kα were measured on PET crystals and g

a α

E Y L was measured on a TAP crystal with an accelerating voltage of 15 kV, a beam current of 200 nA, and a pixel dwell time of 80 ms. These X-ray maps were used to deﬁ ne compositional domains and direct the analytical strategy. U–Th–total Pb monazite dates were pro-

k duced on a modiﬁ ed Cameca SX-100 (Ultra- e e

r chron) at the University of Massachusetts C

d (UMASS). Appendix A of Dumond et al. n a

S (2008) detailed current analytical procedures e l t

t and standard compositions used at the UMASS i L , at 25 °C for comparison with laboratory measurements

eq microprobe facility, including a list of all spec- T trometers, column conditions, count times, standards, and overlap corrections. In addition to the overlap corrections listed in Dumond et al. (2008), an overlap of Nd Lβ on Eu Lα was made. Acceptance or rejection of points follows the criteria of Dumond et al. (2008). For each domain, a weighted average of accepted analyses was calculated after Williams et al., 2006. ackburn et al., 2012a).Vp, Vs—compressional, shear body wave velocities. The 2σ error reported for each date (Table 6; Table B1 [see footnote 1]) is the larger of either the error calculated by propagating the analytical uncertainty on trace element compositions through the age equation plus an estimated 1% uncertainty in background intensities (Williams et al., 2006) or two times the standard error of the mean. The consistency standard used was the Moacyr Brazilian pegmatite monazite, which has the following weighted mean isotope

h dilution–thermal ionization mass spectrom- c n

a etry (ID-TIMS) dates: 506.4 ± 1.0 (2σ, mean R

n square of weighted deviates, MSWD = 0.6) for o

s 208 232 n

i Pb/ Th, 506.7 ± 0.8 Ma (MSWD = 0.83) b

o 207 235

TABLE 5. CALCULATED SEISMIC PROPERTIES AND DENSITY SEISMIC PROPERTIES 5. CALCULATED TABLE for Pb/ U, and 515.2 ± 0.6 Ma (MSWD = R 0.36) for 206Pb/238U (W.J. Davis [Geological Survey of Canada], 2007, personal commun.). a baseline temperature for modern lower crust in this region (Bl Samples and Results

All xenoliths in this study except the high-Ca mafi c granulites contain monazite. We report data from fi ve samples: the low-Ca mafi c granulite ROB3, two intermediate granulites ROB1 and ROB6, and the two felsic granulites ROB5 and ROB7. In all, 48 compositional domains in 33 grains from 5 samples were analyzed (Table 6; full chemistry for each domain is reported in Table B1 [see footnote 1]). —temperature; VRH—Voigt-Reuss-Hill average; H-S—Hashin-Shtrikman average. Velocities were calculated at three temperatures: average; H-S—Hashin-Shtrikman average. Velocities —temperature; VRH—Voigt-Reuss-Hill ROB1 contains rare monazite that only ; T occurs in secondary textural settings along grain boundaries. The grains are small (10– 50 μm), unzoned, and commonly have high ) 3.26 0.02 3.42) 0.11 3.45 2.94 3.22 0.07 0.02 3.24 3.39) 0.11 2.71 3.42 0.04 3.55 2.91 3.19 0.07 0.02 3.40 3.20 3.37 0.02 0.11 2.69 3.48 3.38 0.04 3.42 3.51 2.90 0.06 3.02 3.35 3.15 0.03 0.02 2.67 3.44 3.17 0.05 3.37 3.48 3.22 2.99 3.32 0.03 0.02 3.41 3.14 3.35 3.18 2.97 0.03 3.11 3.15 —pressure 3 3 3 eq (°C) 780 700 820 750 1000 800 815 800 855 720 760 800 870 T (Gpa) 1.37 1.30 1.38 0.90 0.80 0.60 1.40 1.40 1.53 1.18 0.77 1.00 0.90 eq aspect ratios (to 1:10; Figs. 10A, 10B). All y 25 °C t T P 500 °C i l

a three dated grains contain relatively little Th, c Note: P o rho (g/cm Vp (km/s) H-SVs (km/s) H-S 7.52 4.25 0.04 0.02 7.10 3.86 0.13Vp (km/s) H-S 0.08Vs (km/s) H-S 7.58 4.23 6.62 7.33 3.87 4.11 0.10 0.04 0.07 0.02 7.18 6.91 4.09 3.74 0.13 6.41Vp (km/s) H-S 0.08 3.75Vs (km/s) H-S 7.38 0.04 4.09 0.03 7.83 6.46 7.20 4.39 3.73 4.02 0.10 0.03 7.55 0.06 0.02 4.23 6.94 6.81 0.01 3.91 3.68 0.02 0.12 6.27 0.07 8.02 3.63 7.22 0.03 4.51 3.98 0.03 7.62 7.64 6.37 4.28 4.25 3.64 0.11 6.93 7.36 0.06 3.83 4.10 6.63 0.04 0.02 3.68 0.04 0.02 6.17 7.84 3.54 7.32 0.02 4.38 4.07 0.03 7.43 7.49 7.29 4.14 4.14 4.09 6.77 7.22 3.69 4.01 0.05 0.02 0.04 0.01 7.68 7.14 4.26 3.93 7.32 7.13 4.06 3.96 6.67 3.60 0.06 0.06 7.00 3.83 6.97 3.84 rho (g/cm Vp/Vs H-SVp (km/s) VRHVs (km/s) VRHVp/Vs VRH 7.53 1.77 4.25 0.04 0.00 0.02 7.16 1.77 1.84 3.85Vp/Vs H-S 0.12 0.00 0.00Vp (km/s) VRH 0.07 7.60 1.86Vs (km/s) VRH 1.79 4.21Vp/Vs VRH 0.01 6.68 7.35 1.71 1.78 3.88 1.81 4.11 0.12 0.03 0.00 0.00 0.07 0.02 7.21 1.72 6.97 1.79 1.76 1.85 4.09 3.73 0.00 0.11 0.00 6.43 0.00 1.71Vp (km/s) VRH 0.06 3.76 1.76 7.40 1.87Vs (km/s) VRH 0.05 1.80 0.01 4.07Vp/Vs VRH 0.04 0.00 7.85 1.71 6.53 7.22 1.79 1.73 4.38 3.74 1.82 4.01 0.00 0.12 0.02 7.55 0.00 1.78 0.07 0.02 4.23 1.79 6.97 1.75 6.87 0.02 1.80 1.77 0.00 3.90 3.67 0.02 0.00 0.11 0.00 1.79 6.29 1.73 0.06 8.02 3.64 1.79 7.25 1.78 1.87 0.00 0.05 0.01 4.50 3.96 0.04 0.00 7.63 7.66 1.73 6.43 1.78 1.80 4.27 4.24 1.78 3.66 1.83 0.00 0.11 7.00 7.37 1.81 1.79 0.07 3.85 4.09 1.79 1.81 6.67 1.76 0.05 0.02 0.03 0.00 3.67 0.03 0.02 0.00 1.82 1.80 6.19 7.85 7.32 3.54 1.82 1.79 1.80 0.03 0.00 0.04 4.37 4.07 0.03 7.44 7.52 1.75 1.79 7.30 1.78 4.12 4.13 1.80 1.80 4.08 0.01 6.84 7.23 1.83 3.71 4.00 1.81 1.82 0.06 0.02 1.79 0.03 0.04 0.01 1.84 1.81 7.69 7.14 1.82 0.03 0.00 4.25 3.93 7.33 7.13 1.80 4.04 1.81 1.82 3.95 6.74 3.63 1.81 0.06 1.81 0.06 1.86 7.00 0.03 3.82 6.97 1.83 3.84 1.82 rho (g/cm commonly reported at room temperature, and 500 °C, which is Vp/Vs H-S 1.79 0.00 1.85 0.00 1.81 1.75 0.00 1.80 1.74 0.01 1.81 1.80 0.00 1.80 1.80 1.85 0.03 1.83 1.82 SamplePeak ROB-1 ± ROB-3 ± ROB-4 ROB-5 ± ROB-6 ROB-7 ± ROB-8 ROB-9 ± LSC-04 LSC-06 LSC-18 ± LC88-3-5 EB88-21 L Peak U, and Pb (<2.0 wt% Th; <1000 ppm U and Pb)

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TABLE 6. MONAZITE TRACE ELEMENT COMPOSITIONS AND CALCULATED DATES Trace element concentrations Date (ppm) Date 2σ Sample analyzed* Grain Setting Domain Y1σ Th 1σ Pb 1σ U1σ Th/U N (Ma) (Ma) ROB1 7/3/2009 m1 bdr: Grt, Ap whole 159 20 11482 35 835 9 735 28 16 3(6) 1297 70 ROB1 7/3/2009 m4 incl. in alt. Cpx w/ Rt whole 181 16 16710 36 1131 7 743 21 22 5(6) 1279 44 ROB1 7/3/2009 m6 incl. in Cpx w/ Grt whole 152 11 1928 13 238 5 578 19 3 9(9) 1312 99 ROB3 Population 1: low Th (<5000 ppm), low U (<500 ppm), and low Y (<600 ppm) grains (15 mm), typically occurring in early textural settings ROB3 7/29/2008 m2 incl. in Spl in Grt core 536 17 3055 17 442 6 418 23 7 5(5) 2042 130 ROB3 7/1/2009 m2 incl. in Spl in Grt core 540 18 3298 20 423 7 342 26 10 4(4) 1980 151 ROB3 7/29/2008 m2 incl. in Spl in Grt rim 525 19 4364 20 566 7 312 24 14 4(5) 2170 128 ROB3 6/30/2009 m19 incl. in Pl whole 113 13 4642 16 510 5 188 15 25 7(7) 2039 154 ROB3 Population 2: low Th (<1.0 wt% Th), low Y (<600 ppm) relatively high U (1500–3300 ppm), typically occurring in early textural settings ROB3 7/29/2008 m3 incl. in Grt rim 536 22 4886 24 902 9 1466 34 3 3(6) 1879 80 ROB3 7/1/2009 m3 incl. in Grt rim 342 21 6415 28 1198 9 2277 35 3 3(6) 1752 56 ROB3 7/2/2009 m9 incl. in Grt core 540 18 1678 19 1105 8 2931 31 1 4(4) 1890 55 ROB3 6/30/2009 m10 incl. in Grt core 431 18 3627 20 1048 8 2331 31 2 4(4) 1843 58 ROB3 7/2/2009 m20 incl. alt. Pl/Kfs core 345 15 9309 23 1804 7 3305 25 3 6(7) 1809 29 ROB3 Population 3: high but variable Th concentrations (1–8 wt% Th), typically found within garnet or in the matrix, some grains have distinct cores and rims ROB3 7/29/2008 m1 alt core 476 17 27681 54 3173 8 2721 24 10 5(7) 1813 21 ROB3 7/1/2009 m4 incl. in Grt core 339 18 37113 79 3386 9 892 19 42 4(6) 1806 22 ROB3 7/1/2009 m6 Chl alt. core1 292 20 64669 158 5727 14 1114 20 58 3(3) 1794 20 ROB3 6/30/2009 m6 Chl alt. core2 4038 22 44734 77 4292 8 1824 19 25 6(6) 1798 16 ROB3 6/30/2009 m7 incl. in Bt whole 211 14 9184 22 832 6 235 14 39 6(6) 1785 49 ROB3 6/30/2009 m8 incl. in Grt w/ Bt core1 1991 18 75804 129 6904 11 1865 17 41 6(6) 1799 16 ROB3 6/30/2009 m8 incl. in Grt w/ Bt core2 1767 17 61172 104 5189 9 438 8 140 6(6) 1781 14 ROB3 6/30/2009 m8 incl. in Grt w/ Bt rim 207 14 75633 129 6481 10 806 11 94 6(6) 1778 14 ROB3 7/1/2009 m11 incl. in Grt core 4208 22 48152 83 5259 9 4284 24 11 6(6) 1775 15 ROB3 6/30/2009 m13 incl. in Bt w/Zrn core 1971 22 45333 96 3887 10 296 9 153 4(6) 1804 19 ROB3 6/30/2009 m13 incl. in Bt w/Zrn rim 254 14 5836 18 638 6 471 20 12 6(6) 1814 72 ROB3 7/2/2009 m15 incl. in Grt core1 3344 21 48807 84 5319 9 4298 23 11 6(6) 1776 14 ROB3 6/30/2009 m21 incl. in Bt core 1894 18 55014 94 4810 9 437 9 126 6(6) 1829 15 wtd mean 1793 11 MSWD 4.2 ROB3 7/1/2009 m3 incl. in Grt core 174 15 73367 137 5985 11 84 2 873 5(5) 1752 13 ROB3 6/30/2009 m6 Chl alt. rim 173 16 13331 31 1154 7 251 15 53 5(5) 1747 40 ROB3 7/2/2009 m15 incl. in Grt core2 624 17 66118 123 5417 10 307 7 215 5(6) 1738 14 ROB3 7/1/2009 m16 incl. in Grt whole 190 14 71220 121 5934 10 500 8 142 6(6) 1752 13 wtd mean 1748 7 MSWD 0.96 ROB5 Population 1: variable compositions, typically occurring in early textural settings ROB5 7/3/2009 m2 incl. in Grt core 15236 42 41541 72 8667 12 11559 32 4 6(6) 2156 70 ROB5 7/3/2009 m2 incl. in Grt rim 2833 20 62497 107 8974 13 6467 26 10 6(6) 2191 94 ROB5 7/3/2009 m6 incl. in Grt core 13821 43 40363 77 7522 12 10975 34 4 5(6) 1979 50 ROB5 7/3/2009 m6 incl. in Grt rim 1369 18 63847 119 8183 13 5142 26 12 5(6) 2098 40 ROB5 7/3/2009 m12 incl. in Bt whole 514 15 1231 15 197 5 182 19 7 6(6) 2177 298 ROB5 7/3/2009 m7 Grt-Kfs bdy whole 478 18 3109 20 374 7 304 19 10 4(6) 1895 176 (continued)

and yield imprecise but consistent ca. 1.3 Ga ranging from 2170 ± 128 Ma to 2039 ± 154 Ma. 1794 ± 20 Ma and 1798 ± 16 Ma, but an outer dates (1312 ± 99 Ma to 1279 ± 44 Ma; Table 6). The second population (5–50 μm) also tends to rim with a date of 1747 ± 40 Ma (Figs. 10E and U-Pb TIMS dates for three zircon grains from occur in relatively early textural settings such as 11). Using a distinct 23 m.y. gap in the dates this sample were reported in Blackburn et al. inclusions within garnet, and contains low Th among these 17 domains, we distinguish two (2012b); two grains are <1% discordant with (<1.0 wt% Th) and Y (<600 ppm) but relatively groups. The older group of 13 domains yield 207Pb/206Pb dates of 1683 ± 2 Ma and 1699 ± high U (1500–3300 ppm). Thus, Th/U ratios in a weighted mean of 1793 ± 11 Ma (MSWD = 1.3 Ma, and the third is 1.3% discordant with a this population are distinctly low (1–3; Table 6). 4.2) and the younger group of 4 domains yields 207Pb/206Pb date of 1761 ± 1 Ma. Four domains range from 1890 ± 55 Ma to 1809 a weighted mean of 1748 ± 7 Ma (MSWD = The low-Ca, high-Al maﬁ c granulite ROB3 ± 29 Ma. The third population (50–270 μm) is 0.96). The high MSWD of the former group may contains abundant monazite in a variety of the most abundant and occurs both as inclusions indicate that it still comprises multiple monazite textural settings. At least three populations are in garnet and commonly in the matrix. This growth episodes that are indistinguishable with- recognized based on Th, U, and Y composi- population is characterized by generally high out additional work. tion as well as textural setting. First, low Th but variable Th concentrations (1–8 wt% Th). ROB5 also contains evidence for multiple (<5000 ppm), low U (<500 ppm), and low Y There are 17 domains that range from 1829 ± populations of monazite (25–200 μm). Similar (<600 ppm) grains (15 μm) typically occur in 15 Ma to 1738 ± 14 Ma. The large range of dates to ROB3, the ﬁ rst population characteristically early textural settings. Two grains in the popula- and the observation that some grains within this occurs in early textural settings such as inclu- tion are recognized; one is included in a spinel , population have distinct cores and rims with sions in garnet and yields imprecise 2.0–2.2 Ga which is in turn included in the core of a large older and younger dates suggest episodic mona- dates (Fig. 10H). However, the composition of garnet porphyroblast (Figs. 10C, 10D), and zite growth over an interval of several tens of these domains is highly variable and spans the the second is included in plagioclase. Three millions of years. For example, ROB3 m6 con- full range of observed compositions in this sam- domains from these grains yield imprecise dates tains inner and outer core domains with dates of ple. Two grains that occur in matrix quartz and

1420 Geosphere, December 2012

TABLE 6. MONAZITE TRACE ELEMENT COMPOSITIONS AND CALCULATED DATES (continued) Trace element concentrations Date (ppm) Date 2σ Sample analyzed* Grain Setting Domain Y1σ Th 1σ Pb 1σ U1σ Th/U N (Ma) (Ma) ROB 5 Matrix Grains ROB5 7/3/2009 m13 incl. in Qz whole 16973 45 39303 68 3869 8 1942 20 20 6(6) 1795 18 ROB5 7/3/2009 m9 incl. in Pla whole 853 16 15575 31 2109 7 2697 24 6 6(6) 1785 26 ROB 5 Grain m4 ROB5 7/3/2009 m4 Grt-Pl bdy core 1083 16 30983 54 2988 7 2177 21 14 6(6) 1662 18 ROB5 7/3/2009 m4 Grt-Pl bdy rim 848 17 47275 89 3733 9 114 4 415 5(6) 1691 15 ROB6 Population 1: texturally early domains ROB6 2/28/2011 m17 core 870 19 63838 133 6285 12 3220 25 20 4(5) 1790 17 ROB6 2/28/2011 m20 core 28809 91 29216 73 10244 20 25844 72 1 3(3) 1781 14 ROB6 2/28/2011 m45 incl. in Grt whole 6121 26 2559 15 1494 6 4231 26 1 6(6) 1781 30 ROB6 3/2/2011 m28 core 907 39 69344 289 6977 26 4068 54 17 2(3) 1785 16 wtd mean 1715 10 MSWD 0.36

ROB6 Population 2: texturally youngest domains (grains that touch or occur within late reaction zones containing Opx2 + Spl2) ROB6 3/1/2011 m17 rim 8081 36 50217 105 4607 10 2227 24 23 4(4) 1706 18 ROB6 3/2/2011 m29 rim 4663 33 11247 35 1428 9 1775 32 6 3(4) 1738 47 ROB6 3/2/2011 m28 rim 4739 29 27112 60 2617 9 1540 25 18 4(5) 1726 26 wtd mean 1715 35 MSWD 1.3 ROB7 2/28/2011 m28 core 18287 57 63973 132 7290 13 6684 32 10 4(5) 1776 15 ROB7 2/28/2011 m28 rim 2081 18 106103 180 9218 13 1987 15 53 6(6) 1754 12 ROB7 2/28/2011 m46 rim 2851 24 85273 174 7445 14 1924 20 44 4(4) 1740 15 Consistency standard Standard§ 7/29/2008 11515 39 62155 116 1452 6 639 11 97 5(5) 503 7 Standard§ 6/29/2009 11104 35 62790 107 1468 6 653 11 96 6(6) 503 7 Standard§ 7/2/2009 10986 42 62907 131 1484 7 617 13 102 4(4) 508 8 Standard§ 7/3/2009 10910 37 62866 117 1465 6 688 12 91 5(5) 501 7 Standard§ 7/4/2009 10945 38 62875 117 1460 6 712 12 88 5(5) 498 7 Standard§ 2/28/2011 11233 35 63872 109 1526 6 907 12 70 6(6) 508 7 Standard§ 3/1/2011 11188 28 63936 89 1502 5 890 10 72 9(9) 500 5 Standard§ 3/2/2011 11250 38 63848 119 1502 6 895 13 71 5(5) 501 7 wtd mean 502 2.3 MSWD 1.06 Note: Mineral abbreviations after Whitney and Evans (2010). MSWD—mean square of weighted deviates; wtd—weighted. Analyses shown in italics are duplicate session analysis of same grains—not included in weighted means. For groups of ages fewer than three, simple averages are reported rather than weighted means. Setting abbreviations: incl—included; Bdy—boundary between listed phases; alt—alteration product; w/—with. *Dates are in month/day/year. †Compositional domains were determined based on X-ray maps of U, Th, Nd, and Ca. §Consistency standard has the following isotope dilution–thermal ionization mass spectrometry ages (W.J. Davis, personal commun., 2007): 506.7 ± 0.8 Ma 207Pb/235U, 515.2 ±0.6 Ma 206Pb/238U, 506.4 ±1.0 Ma 208Pb/232Th.

feldspar yield similar dates of 1795 ± 18 Ma restricted to grains that touch or occur within (<3000 ppm and <2000 ppm, respectively) and

and 1785 ± 26 Ma (Fig. 10J). One grain, which late reaction zones containing Opx2 + Sp2 the highest Th concentrations (to 10.6 wt%). occurs along a garnet-plagioclase grain bound- (Figs. 7B and 10G), suggesting that they grew Two of these rim domains give dates of 1754 ± ary, has a sharply zoned core and rim that yielded in association with this reaction. These grains 12 Ma and 1740 ± 15 Ma. Several monazite distinctly younger dates of 1662 ± 18 Ma and have small overgrowths (10–20 μm) that occur grains occur within late veins of Kfs, where they 1691 ± 15 Ma, respectively. outside of the high Y overgrowths described are resorbed and partially overgrown by apatite. ROB6 and ROB7 both contain abundant here (Figs. 10I, 10K). They have similarly monazite (grain sizes from 10 to 250 μm), com- high Y (5000–8000 ppm) but distinctly lower DISCUSSION monly with complex zoning patterns suggesting U (<2200 ppm). All three analyzed domains multiple episodes of dissolution and reprecipi- give a weighted mean date of 1715 ± 35 Ma Heterogeneous High-Velocity Lower Crust tation (Figs. 10G and 10I–10L). In ROB6, a (MSWD = 1.3). common pattern is for monazite grains to have The felsic granulite ROB7 contains three Heterogeneity is a notable characteristic of high-Th (~6–7 wt% Th), low-Y (<1000 ppm) compositional and textural populations (Fig. the entire suite of xenoliths studied from central resorbed cores with distinctly higher Y (>5000 10L), all of which occur in the matrix but are Montana. The suite displays a diverse range of ppm) overgrowths. Such grains occur both in also locally included in garnet. The oldest popu- modal mineralogy and textural characteristics, the matrix and as inclusions in orthopyroxene. lation occurs as innermost resorbed cores with bulk major element chemical compositions Other rare texturally early domains (e.g., cores the highest Y (~1.8 wt%) and U (>6000 ppm) (although the deepest xenoliths have generally or grains included in garnet) contain either very and the lowest Th concentrations (~6 wt%). basaltic compositions), and bulk seismic veloci- low Th (<3000 ppm) or very high U and Y One domain from this population was dated as ties, even within the seismically deﬁ ned 7.x (both >2.5 wt%). All texturally early domains 1776 ± 15 Ma; the second undated population layer. In addition, there is signiﬁ cant variation described here yielded dates between 1.79 and with lower U and Y concentrations overgrows in metamorphic histories and geochronological 1.78 Ga. A weighted mean of all three analyzed the former but is itself resorbed. The third and signatures from the xenoliths. Multiple samples domains is 1784 ± 10 Ma (MSWD = 0.36). texturally youngest population occurs as rims preserve evidence for prograde burial with at The texturally youngest monazite population is on most grains and has the lowest Y and U least one recording evidence for an early mid-

Geosphere, December 2012 1421

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A B m6 Th Cm28 Y core Cpx 1776 ±15 Ma rim 4(5) Grt 1754 ±12 Ma 6(6)

ROB1m6 ROB1 Pl ROB7 zrn inclusion

1312±99 9(9) μ BEI 250 m 20 μm 20 μm

m2 Th m6 Th D Grt E F core inner core 2042±130 1798±16 rim Zrn 5(5) 6(6) 1747±40 Spl 5(5) ROB3m2 inner core

ROB3 core rim core Bt 2170±128 4(5) 1794±20 3(3) μ BEI 100 m 10 μm 50 μm

G H m2 Th I m13 Th core 1795±18 6(6) 2156±70 Pl 6(6)

ROB5m2

ROB5 Grt rim Qz 2191±94 μ 6(6) BEI 500 m 20 μm 20 μm

J K m17 Y L m17 U Opx core 1 1790 ±17 Ma 4(5)

ROB6m17 Opx2+ Spl ROB6 rim rim Grt 1706 ±18 Ma Bt BEI 100 μm 4(4) 20 μm 20 μm

Figure 10. Key examples of the setting and zoning of monazite grains from samples ROB1, ROB3, ROB5, ROB6, and ROB7. Mineral abbreviations after Whitney and Evans (2010). A, D, G, and J are backscatter electron images (BEI) of the setting of key monazite grains presented in B, E, H, and K. X-ray maps show compositional zoning in Th, Y, or U (element noted in the upper right of each X-ray map). A and B are from sample ROB1; D, E, and F are from sample ROB3; G, H, and I are from sample ROB5; J, K, and L are from sample ROB6; and C is from sample ROB7. E shows the location of the domains in ROB3m2 analyzed during the 2008 session. The locations of background spots (squares) and analysis points (circles), the date of each domain, the total number of analysis points, and the number of accepted analyses used to compute the date are shown for each grain. The location of D is shown in the photomicrograph in Figure 6A.

crustal metamorphic event, and others record 1.5–1.3 Ga. Collectively, these data suggest that vated by the heterogeneity in composition and evidence for retrograde tectonic decompression the 7.x layer beneath Montana is a composite chronology observed in xenolith samples (Fig. and/or later reheating events. Along with mona- feature. 12). We acknowledge that any model that speci- zite U–Th–total Pb geochronologic data from fi es amounts or mechanisms for material added this study, published U-Pb data from zircon from Incremental Assembly of the Central to the lower crust based on currently available xenoliths in this locality and others in the region Montana Lower Crustal 7.x Layer data would be highly speculative. Instead, our (Davis et al., 1995; Scherer et al., 2000; Bolhar intent here is to provide a discussion of what et al., 2007; Blackburn et al., 2011) indicate We propose a history of incremental assem- events and processes that may have resulted in igneous, metamorphic, and/or fl uid fl ow events bly of the high-velocity lower crust in this region the addition of material to the 7.x layer in this before 2.6 Ga, and at 2.1 Ga, 1.83–1.68 Ga, and from Archean to Mesoproterozoic time moti- region through time. We present a compilation

1422 Geosphere, December 2012

Ma Ma (Bolhar et al., 2007). We suggest that this meta- 1600 1600 morphism may have been associated with the ROB3 ROB5 ROB6 ROB7 passage of rift-related mafi c magmas through 1662 ±18 Ma the crust and likely some degree of magmatic 1700 1748 ±7 Ma 1691 ±15 Ma 1715 ±35 Ma 1700 (n=4) MSWD =0.96 (n=3) MSWD =1.3 1740±15 Ma emplacement in the lower crust. 1785 ±26 Ma 1754 ±12 Ma 1784 ±10 Ma 1800 1793 ±11 Ma 1776 ±15 Ma 1800 Possible Addition ca. 1.8–1.7 Ga During (n=13) 1795 ±18 Ma (n=3) MSWD =0.36 MSWD =4.2 Great Falls Tectonic Zone Activity A ca. 1.8–1.7 Ga high-velocity magmatic 1900 4 grains in early 1900 textural settings: underplate beneath the Wyoming craton and 1890 ±55 Ma to Medicine Hat block was proposed (Gorman 1809 ±29 Ma Ma Ma 2000 1100 400 et al., 2002; Clowes et al., 2002; Eaton, 2006), 4 grains in early ROB1 primarily based on limited reports of geo- textural settings 502.3 ±2.3 Ma ca. 2.0–2.2 Ga (n=8) MSWD =1.06 chronology on lower crustal xenoliths from 2100 1200 500 the Sweet Grass Hills of southern Alberta and Consistency northern Montana (Davis et al., 1995). These 2 grains in early 3 grains textural settings: Standard data indicate multiple zircon growth events 2200 1300 ca. 1300 Ma 600 2170 ±128 Ma between 1.85 and 1.69 Ga. However, Davis 2039 ±154 Ma ca. 1300 Ma et al. (1995) interpretation is primarily one of 2300 1400 ca. 1750 Ma granulite facies reworking of lower crust during ca. 1780 Ma this interval rather than favoring a clear signal ca. 1830 Ma of new magmatic addition to the lower crust. 2400 1500 all horizontal scales are the same ca. 2100 Ma This time frame also coincides with available constraints for the age of convergent tectonic Figure 11. Summary of monazite geochronology from samples ROB1, ROB3, ROB5, ROB6, activity along the Great Falls tectonic zone. and ROB7, as well as a consistency standard. Curves indicate probability distribution func- Arc-related plutonism was ongoing at 1.86 Ga tions of each monazite domain analyzed. Gray boxes represent range of ages from mean in central Montana (Mueller et al., 2002), and square of weighted deviates (MSWD) calculations on groups of samples. Shading of the area several ranges in southwestern Montana con- under the probability density function corresponds to the general age range of each domain. tain 1.82–1.71 Ga records of high-grade tectonism that are associated with the Great Falls tectonic zone (Roberts et al., 2002; Big Sky based upon regional studies of surface geology, xenoliths is provided, and their model dates are orogeny of Harms et al., 2004). geophysical observations, xenolith studies, and not confi rmed with zircon data. While no direct The dominant monazite populations in four new data presented here. geochronological evidence for an Archean ori- of the fi ve xenoliths in this study also refl ect gin to components of the 7.x layer is yet pub- metamorphism during the interval 1.83– Archean Components lished, it seems likely that some of the layer is 1.66 Ga. This is also the interval within which Several mechanisms for Archean-aged com- Archean. deformation-related foliation and gneissic layer- ponents of the 7.x layer in Montana and Wyo- ing in some xenoliths are likely to have devel- ming have been proposed. The generally small Rift-Related Underplating and Intraplating oped. The 1.89–1.81 Ga population of monazite volume of exposed post-Archean magmatic at 2.2–2.0 Ga in ROB3 may coincide with arc-related mag- rocks in the Wyoming craton has led some to Major episodes of continental rifting during matism described by Mueller et al. (2002). The suggest that the high-velocity lower crust has the 2.2–2.0 Ga interval are suggested for both most prominent populations in this and several a signifi cant if not an entirely Archean ori- the northern and southern margins of the Wyo- other xenoliths are 1.79–1.78 Ga or 1.75 Ga and gin (Snelson et al., 1998; Chamberlain et al., ming craton based on surface geology. In south- likely represent the timing of peak metamorphic 2003). Proposed mechanisms include a restite western Montana, these include ca. 2.06 Ga events. Increases and late-stage decreases in associated with widespread magmatism at mafi c dikes (Mueller et al., 2004) and 2.1 Ga pressure recorded by some xenoliths in this time 2.9–2.75 Ga in the Bighorn subprovince or an contact metamorphism associated with ultra- frame, such as ROB3 and ROB6, respectively, underplate during the formation of the 2.7 Ga mafi c intrusions (Alcock and Muller, 2010). refl ect crustal thickening and thinning, pos- Stillwater layered mafi c intrusion (Chamber- Mafi c dikes and sills of similar age also occur sibly associated with Great Falls tectonic zone lain et al., 2003, and references therein). Zircon farther south in central (Harlan et al., 2003) and collision. Similar processes may also explain data from felsic xenoliths, broadly granitic or southern Wyoming (Premo and Van Schmus, the record of up-pressure destabilization and tonalitic in composition, from the Sweet Grass 1989; Cox et al., 2000). post-peak stability of plagioclase in the horn- Hills indicate Archean protolith ages, although Montana xenoliths also record deep crustal blende eclogite, although no geochronological they have pressures suggesting derivation from thermal events that we associate with this rifting data are yet available for this sample. Addi- above the 7.x layer in that region (Davis et al., episode. Two of the xenoliths from this study tional published data from Bearpaw Mountains 1995; Blackburn et al., 2011). Bolhar et al. contain texturally early 2.2–2.0 Ga populations crustal xenoliths include 1.76–1.68 Ga zircon (2007) used whole-rock and feldspar Pb iso- of metamorphic monazite, whereas a crustal from mafi c granulite ROB1 (same sample as topic compositions from Bearpaw Mountains xenolith from nearby Biebinger Ranch in the in this study; Blackburn et al., 2012b), 1.85– crustal xenoliths to argue for 2.8–4.0 Ga proto- Bearpaw Mountains contains zircon rims with 1.75 Ga zircon from a felsic granulite (Bolhar lith ages, although no depth information for the a concordant 207Pb/206Pb date of 2167 ± 26 Ma et al., 2007), and 1.71 Ga zircon from a garnet

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SE NW Depth pre-2.1 Ga (km) 0 Archean High Velocity Lower Crust -2.7 Ga Stillwater Complex WYOMING CRATON -Restite from 2.9–2.75 Ga Magmatism 20 - Imbricated Ocean Crust ? Archean high velocity lower crust ? 40 Moho

60 Probable components of composite 7.x layer Late Archean ca. 1.7 Ga ca. 2.1 Ga ca. 1.4 Ga

SE NW Depth ca 2.1 Ga Dikes in Tobacco Roots and (km) Dikes in SE Laramie Range Madison Range (?) Rifting 0 Rifting 2.1 Ga dikes WYOMING CRATON 20

40 ?? ? Moho lithospheric thinning? 2.1 Ga under and intra-plates lithospheric thinning? 60

Figure 12. Schematic evolution of the Wyo- SE Arc Magmatism NW ming craton based on geologic constraints Depth ca 1.86–1.8 Ga Exposed in the and chronology presented here and in the (km) Little Belt Mountains 0 literature. Each panel is a cross section, from modern southeast to northwest, of the WYOMING CRATON MEDICINE HAT BLOCK 20 crustal material in the northern Wyoming craton and the adjacent terranes from a 40 time period of signiﬁ cant addition to the Moho Subduction high-velocity lower crustal layer. Material 60 Polarity? added to the crust at each time step is shown in a different color.

SE NW Depth ca 1.8–1.7 Ga (km) Great Falls Tectonic Zone 0 MEDICINE HAT BLOCK 20 WYOMING CRATON

40 Moho 60 1.7–1.8 Ga under and intra-plates

SE NW Depth ca 1.5–1.3 Ga (km) Belt Basin Rifting U.S. 0 1.4 Ga dikes Canada WYOMING CRATON MEDICINE HAT BLOCK intermediate to felsic intermediate to felsic 20 middle & upper crust middle & upper crust

Moho 60 lithospheric mantle

1424 Geosphere, December 2012

pyroxenite (Scherer et al., 2000). Carlson and xenolith exhumed in the northern Bearpaw reasonable way to achieve the modern crustal Irving (1994) reported 1.78 Ga monazite from Mountain fi eld yielded Sm-Nd whole-rock and confi guration considering the current state of mantle xenoliths from the Highwood Moun- multimineral dates of 1.36–1.35 Ga, and zircon knowledge of the regional geologic history. tains, interpreted as evidence for fl uid or melt U-Pb intercepts of 1707 ± 15 and 1268 ± 34 Ma interactions with the lithospheric mantle. (MSWD = 0.49) (Scherer et al., 2000). The CONCLUSIONS Multiple mechanisms for 1.8–1.7 Ga addition Sm-Nd dates indicate either the time the sample of high-velocity and high-density lower crust in cooled through closure, which would not be Petrological, geochronological, and bulk the Wyoming craton and Medicine Hat block inconsistent with cooling from 1.47 to 1.45 Ga seismic velocity data for crustal xenoliths from have been proposed (e.g., Gorman et al., 2002; magmatism, or the most recent time at which central Montana provide new insight into the Clowes et al., 2002; Chamberlain et al., 2003; the Sm-Nd system was reset. nature and modes of formation of the lower crust Eaton, 2006); however, the xenolith record Magmatic addition to the crust at 1.47– in this region. Similar to some other past stud- points to granulite facies and high-temperature 1.45 Ga is clear from shallow geologic observa- ies of deep crust from xenoliths (Rudnick and eclogite facies metamorphism of lower crust. tion, and a similar magmatic contribution to the Taylor, 1987) and exhumed lower crust (e.g., It remains unclear whether a signifi cant vol- 7.x layer seems reasonable (Chamberlain et al., Williams and Hanmer, 2006), we emphasize the ume of new magmatic material was added at 2003). The ca. 1.3 Ga chemical and isotopic hetero geneity in geologic history and physical that time. Postorogenic collapse may have been signals may have developed in response to the properties of the deep crust from this xenolith accompanied by extension-related mafi c mag- addition of heat and/or fl uids from a magmatic record. We suggest that the modes of forma- mas that could have added some new material underplate near the end of Belt Basin rifting, tion of the high seismic velocity lower crustal to the lower crust and provided local sources for but the lack of evidence for near-surface mag- layer were also likely heterogeneous. From our reheating of older host material. Chamberlain matism at that time makes this possibility less xenolith data, earlier suggestions for the origins et al. (2003) suggested that a component might compelling. Although the nature of this distur- of the lower crust in this region, and considering refl ect mechanically imbricated oceanic mate- bance is unclear, the vein- and fracture-fi lling the regional tectonic history, we present a model rial added during suturing. All of these ideas, morphology of the ROB1 monazite suggests a for incremental assembly of the 7.x layer involv- and likely others unstated, are speculative until fl uid circulation event. ing episodic magmatic and possibly mechanical future work provides additional information. underplating and intraplating (and accompa- Simplifi ed Consideration of nying fractionation processes) associated with Rift-Related Addition ca. 1.5–1.3 Ga Incremental Assembly multiple regional tectonic events from Archean A previously proposed Mesoproterozoic to Mesoproterozoic time. origin for the 7.x layer is based on the wide- The high-velocity lower crust at the northern The thickness of the high-velocity lower spread occurrence of 1.47–1.45 Ga mafi c dikes margin of the Wyoming craton makes up almost crustal layer in this region is significantly across the Wyoming craton that are interpreted half of the ~55-km-thick crust (Gorman et al., greater than most similar layers observed else- to refl ect an eastern extension of the Belt Basin 2002). This is a much greater proportion than where. However, the observed crustal structure rift system (Chamberlain et al., 2003). Depo- that seen in averaged worldwide compilations appears consistent with a history of incremen- sition of Belt Basin sediments, which extend of shields and platforms (Fig. 1; Christensen tal assembly based on the surface and xenolith in the subsurface to as far east as 107.5°W in and Mooney, 1995). Using a simple plane strain observations available. In this context, perhaps central Montana (Fig. 3), and associated mag- calculation, we tracked the thickness of a hypo- it is the processes that have allowed preservation matism were episodic, with major pulses of thetical column of initially 35-km-thick crust of the thickness of this lower crustal layer rather rifting at 1470–1400 Ma (Zartman et al., 1982; that has a generalized history similar to that than the thickness that is anomalous. This study Höy, 1989; Anderson and Davis, 1995; Sears described in the preceding discussion. During provides an example of how integration of xeno- et al., 1998; Evans et al., 2000) and ca. 1370 Ma two rift-related thinning and magmatic under- lith data, surface observations, and geophysical (Doughty and Chamberlain, 1996). Post depo- plating episodes (at 2.1 Ga and 1.4 Ga) the crust studies can elucidate the formation, evolution, sitional magmatism and associated contact is thinned by 10 km, and 10 km of mafi c lower and present-day structure of the continental meta mor phism of sediments that were probably crust is added. This is generally consistent with lithosphere. part of the rifting cycle are documented to be as observations from the Rio Grande Rift (West ACKNOWLEDGMENTS young as 1320 Ma (Schandl et al., 1993; Zart- et al., 2004; Wilson et al., 2005) and the main man and Smith, 1995). Ethiopian Rift (Cornwell et al., 2010). During We thank M.L. Williams and M.J. Jercinovic at Crustal xenoliths from central Montana one intervening 1.8 Ga collisional episode, the the University of Massachusetts Ultrachron facility, appear to also record cryptic deep crustal ther- crust is homogeneously thickened by 20 km. J. Drexler at the University of Colorado microprobe facility, and S. Appleby at the Colorado School of mal and/or fl uid-related processes that may be The result is that an initial normal thickness Mines QEMSCAN facility. We thank C. Gerbi and associated with the end of the Belt Basin rift for- (35 km) column of crust that starts with a 5 km D. Schutt for constructive reviews that improved the mation. Monazite grains from the mafi c granu- (Archean) mafi c lower crustal layer thickens to manuscript. The research presented here was funded lite xenolith ROB1 yield dates of ca. 1.3 Ga 55 km, the lower half of which is a mafi c layer in part by a Geological Society of America Graduate Student Research Grant to Barnhart and National Sci- and have textural settings and morphologies similar to that observed in the Wyoming craton. ence Foundation EarthScope grants EAR-07464246 that indicate growth after peak metamorphism. This calculation does not include the effects to Mahan and EAR-0746205 to Bowring. 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