RESEARCH Late Cretaceous Crustal Hydration in the Colorado Plateau

RESEARCH

Late Cretaceous crustal hydration in the Colorado Plateau, USA, from xenolith petrology and monazite geochronology

L.A. Butcher1,2, K.H. Mahan1, and J.M. Allaz1 1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF COLORADO–BOULDER, 2200 COLORADO AVENUE, BOULDER, COLORADO 80309, USA 2U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025, USA

ABSTRACT

Formation and subsequent modification processes of lower crust play important roles in evolution and rheological models for continental lithosphere. In the last two decades, numerous xenolith studies have documented metasomatism of continental mantle in the Rocky Moun- tain region of North America. However, much less attention has been paid to whether and to what extent these processes may have affected the crust. We address the nature and timing of hydrous alteration of extant deep Proterozoic crust in the Colorado Plateau through a petrological and geochronological study of a xenolith from the Red Mesa diatreme in the 30–20 Ma Navajo volcanic field. Fluid-related alteration is widespread in sample RM-21, with the main features recorded by breakdown effects in feldspar, garnet, and allanite, and the production of secondary assemblages characterized by lower Ca plagioclase (or albite) + muscovite + biotite + calcite + monazite ± zoisite at estimated conditions of 0.8–0.7 GPa (or ~27 km depth) and 480 °C. Th-Pb dating by ion probe reveals a period of monazite crystallization at 70–65 Ma, interpreted to reflect late alteration of the high-temperature metamorphic assemblage with hydrous fluid introduced by the shallowly subducting Farallon slab. Similar to previous suggestions involving mantle hydration, the growth of low-density hydrous phases at the expense of high-density, anhydrous minerals, which are abundant in unaltered Proterozoic crust, if sufficiently widespread, could have contributed to elevated topography in the Colorado Plateau and regionally across the Rocky Mountains and High Plains.

LITHOSPHERE; v. 9; no. 4; p. 561–578; GSA Data Repository Item 2017180 | Published online 13 April 2017 doi:10.1130/L583.1

INTRODUCTION by a hydrous fluid (Usui et al., 2003; Smith et al., 2004; Smith and Grif- fin, 2005; Schulze et al., 2015). Several suites of deep crustal xenoliths Physical and chemical studies of xenoliths are powerful tools for recon- sourced from depths of 20 km or more reveal variable degrees of hydra- structing modification processes and alteration histories of crust and man- tion that occurred at depth prior to eruption (Broadhurst, 1986; Selver- tle lithosphere. In the last two decades, numerous xenolith studies have stone et al., 1999), but again the timing of this hydration has remained documented metasomatism of continental mantle in the Rocky Mountain elusive. This study provides new mineralogical and textural observations, region of North America. Interpretations range from coincidence with and geochronological data from one of the Red Mesa diatreme samples Paleoproterozoic accretionary events (Carlson and Irving, 1994; Lester studied by Selverstone et al. (1999). Results from in situ Th/Pb dating and Farmer, 1998; Downes et al., 2004; Farmer et al., 2005; Facer et al., of monazite associated with secondary alteration assemblages suggest 2009) to Late Cretaceous–Paleogene hydration of the lithospheric mantle that deep crustal hydration occurred at the end of the Late Cretaceous by dewatering of a shallow Farallon slab (e.g., Smith et al., 2004). The Epoch, and they indicate that fluids with a similar source to those that latter interpretation has a number of significant implications, including the permeated much of the lithospheric mantle also made it into the lower potential for mantle de-densification to produce subsequent uplift (Hess, ~20 km of the crust. If the hydration reactions were sufficiently pervasive 1954; McGetchin and Silver, 1972; Silver and McGetchin, 1978; Hum- and widespread, the associated de-densification could have significantly phreys et al., 2003) and the distance inboard at which subducting slabs influenced the surface uplift history of the Colorado Plateau. can retain their ocean-derived fluid (e.g., Lee, 2005; Currie and Beaumont, 2011; Sommer and Gauert, 2011). However, much less attention has been GEOLOGIC AND GEOPHYSICAL SETTING AND BACKGROUND paid to whether and to what extent this process may have affected the crust, in part, due to far fewer documented records of crustal hydration Colorado Plateau and because dating metasomatic processes is notoriously difficult. In the Colorado Plateau region, ultramafic mantle and crustal xenoliths The Colorado Plateau covers an area of ~3.4 × 105 km2 in Colorado, from the Navajo volcanic field (NVF) have all been long recognized as New Mexico, Utah, and Arizona (Fig. 1). Lateral accretion of island-arc recording evidence of extensive hydration (e.g., McGetchin and Silver, and oceanic terranes to the southern margin of the Archean Wyoming 1972; Smith and Levy, 1976). Several eclogite and peridotite xenolith craton between 1.8 and 1.6 Ga created much of the crystalline continental studies have suggested mantle hydration events ranging in age from the crust and mantle lithosphere in the southwestern United States, includ- Late Cretaceous to the Oligocene, based on U-(Th)-Pb dating of zircon ing the Colorado Plateau (Bowring and Karlstrom, 1990). This period or monazite interpreted to reflect growth or recrystallization catalyzed of prolonged convergence established two distinct, NE-trending crustal

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B Front Range A Medicine Hat Block N SH BM

GFTZ Hs

Wyoming ) Craton LH SL

BC OC Yavapai NVF Province LS Ci VT Bowring and Karlstrom (1990 DH Mazatzal Province HM AB KH Roller 1965

ME R UM Bennett & DePaolo (1987) MR RM LP CO UT CV S AZ NM GR CM CB A MB TT GK Condie (1982) BP LA Geology RISTRA Arizona Transition Zone SUM Minettes Laccoliths Proterozoic rocks S. edge pre-1.7 Ga basement Geophysics Karlstrom and Humphreys (1998) Active seismic line Passive seismic line EarthScope seismic 100 km stations

Figure 1. (A) Simplified map of Colorado Plateau region after Selverstone et al. (1999; where CO—Colorado, NM—New Mexico, UT—Utah, AZ—Arizona). Gray-filled regions are exposed Proterozoic basement rocks. Key minettes and serpentinized ultramafic breccia pipes (SUM) of the Navajo volcanic field are labeled in bold (RM—Red Mesa, ME—Mule Ear, MR—Moses Rock, CV—Cane Valley, GR—Garnet Ridge, A—Agathla Peak, CB—Chaistla Butte, BP— Buell Parks, GK—Green Knobs, TT—The Thumb, MB—Mitten Rock, S—Shiprock). The approximate locations of laccolith intrusions are shown labeled in italics and include CM—Carrizo Mountains, UM—Ute Mountain, LP—La Plata, R—Rico, HM—Henry Mountains, AB—Abajo Mountain, and LS—La Sal Mountains. The traces of two seismic lines through the area are shown (Roller, 1965; Wilson et al., 2005 for LA RISTRA) along with EarthScope USArray station locations (open circles). Proposed Yavapai-Mazatzal Province boundary traces from several studies are shown by thick dashed lines. (B) Inset showing region of western United States, major Proterozoic terrane boundaries from Whitmeyer and Karlstrom (2007), and key mantle and crustal xenolith localities (SH—Sweetgrass Hills, BM—Bearpaw Mountains, Hs—Homestead, LH—Leucite Hills, SL—Stateline, NVF—Navajo volcanic field, VT—Vulcans Throne, KH—Kilbourne Hole, DH—Dish Hill, Ci—Cima, OC—Oak Creek, BC—Big Creek, GFTZ—Great Falls Tectonic Zone).

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provinces (e.g., Yavapai and Mazatzal) of largely juvenile crust, with some Ehrenberg, 1982; Smith et al., 1991; Roden and Shimizu, 2000; Behr and older intermixed crustal and/or underlying mantle components (Bennett Smith, 2016); (2) the age and possible source material of eclogite xenoliths and DePaolo, 1987; Karlstrom and Bowring, 1988, 1993; Livaccari and (Helmstaedt and Doig, 1975; Smith and Zientek, 1979; Helmstaedt and Perry, 1993; Bickford et al., 2008; Schulze et al., 2008). Widespread gra- Schulze, 1988, 1991; Wendlandt et al., 1993; Usui et al., 2003; Smith et nitic magmatism that affected much of the region in the Mesoproterozoic al., 2004); and (3) the possible role of metasomatic effects recorded in was long considered to be anorogenic (Anderson, 1983). However, much mantle peridotite and other ultramafic xenoliths in Late Cretaceous to of the previously presumed Paleoproterozoic aluminosilicate “triple point” Tertiary subduction processes in the western United States (Hunter and metasedimentary domain in northern New Mexico is now known to have Smith, 1981; Smith, 1995; Smith et al., 1999; Lee, 2005; Smith and Grif- 1.49–1.46 Ga depositional ages, thus requiring ca. 1.45 Ga burial and fin, 2005; Li et al., 2008; Sommer et al., 2012). widespread regional metamorphism (e.g., “Picuris orogeny”; Daniel et al., A more limited number of studies focused on crustal xenoliths from 2013). Extensional basins and NW-striking fault networks in the Grand the NVF and yielded valuable information about the age, evolution, and Canyon Supergroup record two periods of intracratonic deformation at 1.1 chemical composition of the crust. These studies revealed details of the and 700–800 Ma, coincident with the far-field Grenville tectonic events Paleoproterozoic and Mesoproterozoic crustal growth processes and asso- for the former and the breakup of Rodinia and initiation of the Cordilleran ciated deep crustal metamorphism (Wendlandt et al., 1996; Selverstone rift margin for the latter (Timmons et al., 2001, 2005). et al., 1999; Crowley et al., 2006). Geochemical, isotopic, and petrologic Broad uplifts and basinal structures developed in the northern part of studies showed that most crustal xenoliths from the NVF are consistent the plateau during the late Paleozoic (Ancestral Rockies; e.g., Marshak with original magmatic or depositional formation in a 1.8–1.6 Ga juvenile et al., 2000), whereas relatively minor faults and monoclinal folds devel- island-arc setting (Wendlandt et al., 1993; Mattie et al., 1997; Selver- oped in the plateau during the Late Cretaceous to Early Tertiary Laramide stone et al., 1999). Distinct pressure-temperature (P-T) paths recorded orogeny (e.g., Davis, 1978). Syn-Laramide magmatism (75–45 Ma) was by crustal xenoliths from the northwest versus southeast portions of the almost exclusively limited to the Colorado Mineral Belt (Armstrong, 1969; NVF are interpreted to reflect different types of crustal formation during Mutschler et al., 1987), which extends from the northern Colorado Front northwest-dipping subduction along the proposed Yavapai-Mazatzal litho- Range southwest to the Four Corners region. These include the ca. 70 spheric boundary ca. 1.7 Ga (Selverstone et al., 1999). Xenoliths from the Ma granitoid laccoliths of the Rico and La Plata Mountains on the east- northwest diatremes record counterclockwise P-T paths, consistent with ern edge of the plateau and the Ute and Carrizo Mountains near the Four heating, burial, and subsequent reequilibration during subduction or with Corners area (Semken and McIntosh, 1997). Other small-volume granitoid magmatic heating of crust above the slab followed by accretion of the intrusions on the plateau include the 31–20 Ma La Sal, Henry, and Abajo arc. Clockwise P-T paths reported for southeast crustal xenoliths, on the Mountain laccoliths (Nelson et al., 1992). The extent of Laramide orogen- other hand, suggest that these rocks underwent only partial burial prior to esis on the plateau stands in stark contrast to that in adjacent provinces, unroofing and were therefore not part of the overriding plate. Both felsic i.e., the Basin and Range, Southern Rocky Mountains, and Rio Grande and mafic crustal xenoliths record 1.4 Ga as the most recent period of Rift, which experienced substantial deformation and magmatism during high-grade metamorphism (Wendlandt et al., 1996; Crowley et al., 2006). this time. Accumulation of marine and nonmarine sediments, and the U-Pb zircon dates for most felsic igneous xenoliths in the NVF record widespread occurrence of the Mancos Shale constrain the elevation of crystallization events between 1.75 and 1.69 Ga, consistent with proposed the plateau to near sea level for much of the Phanerozoic (e.g., Morgan Proterozoic subduction (Condie et al., 1999; Crowley et al., 2006), whereas and Swanberg, 1985). Present elevations in the plateau therefore require a subset of mafic xenoliths crystallized at 1.43 Ga (Crowley et al., 2006). ~2 km of surface uplift since the Late Cretaceous. As in mantle xenoliths, evidence of late-stage hydrous alteration exists in deep crustal xenoliths in the NVF. While most studies have argued that Xenoliths of the Navajo Volcanic Field the alteration occurred before the xenoliths were brought to the surface, based on observed high-pressure minerals like phengitic muscovite and The NVF encompasses a >30,000 km2 region in the central Colorado zoisite + kyanite, the absolute age and regional extent of the alteration Plateau that contains more than 50 volcanic centers of two principal types— have been debated (McGetchin and Silver, 1972; Ehrenberg and Griffin, minettes and serpentinized ultramafic microbreccias (Fig. 1). Minettes 1979; Padovani et al., 1982; Broadhurst, 1986; Selverstone et al., 1999). are thought to be products of small-percentage melting of volatile-rich The various models for crustal hydration hinge on its timing (see Discus- upper-mantle garnet lherzolite (e.g., Roden, 1981), whereas serpentinized sion), but no geochronological constraints have previously been available, ultramafic microbreccias are thought to represent subsolidus eruptions of other than the observation that hydration must have postdated 1.4 Ga deep gas-solid mixtures (e.g., Smith and Levy, 1976). The ages of diatreme for- crustal magmatism (Crowley et al., 2006). mation range from 33 to 19 Ma, based on U-Pb zircon dates in recrystallized eclogite xenoliths (33 Ma; Usui et al., 2003; Smith et al., 2004), Sm-Nd Modern Crustal Structure and Composition from Xenoliths and garnet-clinopyroxene dates in recrystallized eclogite xenoliths (32–21 Ma; Geophysics Wendlandt et al., 1996; Smith et al., 2004), and K-Ar dates of phengite in an eclogite xenolith (30 Ma; Smith et al., 2004) and of phlogopite in lam- The plethora of xenolith-based studies and both active and passive prophyre dikes (28–19 Ma; Laughlin et al., 1986; McDowell et al., 1986). source geophysical surveys have inspired several attempts to compile The Navajo minettes and ultramafic diatremes host abundant perido- crustal structure and lithologic properties for the central Colorado Pla- tite, eclogite, and crustal xenoliths (e.g., Watson, 1967), which have been teau (Fig. 2). Crustal thickness estimates in this region range from ~40 analyzed using a variety of petrological, geochemical, geochronologi- to 48 km, and most seismic studies appear to agree on the presence of an cal, and petrophysical methods. Most previous xenolith work focused ~10–12-km-thick lower-crustal layer with distinctly higher velocity than on peridotite, pyroxenite, garnetite, and eclogite xenoliths. Significant that of the middle and upper crust (e.g., Roller, 1965; Wolf and Cipar, contributions from these studies include (1) chemical, geophysical, and 1993; Wilson et al., 2005; Bashir et al., 2011). A relatively mafic com- rheological models of the lithospheric and sublithospheric column in the position for this deep layer has been suggested based on high absolute Colorado Plateau (McGetchin and Silver, 1972; Smith and Levy, 1976; velocities, high Vp/Vs values, and gravity models that are consistent with

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0 km 0 0 2 km solid line -Selverstone et al. (1999) A McGetchin and Silver (1972) B Bt-Sil C (6.2) dashed line -Wendlandt et al. (1993) -Moses Rock 5 schist felsic granitoids/mixed 1 Ctd-schist volcanic/metasedimentary Padovani et al. (1982) Grt-amphibolite 20 -Moses Rock/Mule Ear 20 12 km t 1 4 2 mafic Bashir et al. (2011) - RFs 3 2 granulite

e Grt- felsic e para 40 40 Grt-Bt 1 6 6.4 schist (6.4) RM21 RM21 (this study) (this study)

tonalite/diorit RM21 amphibolite Grt-amphibolit 60 60 granite/felsic Bt-schis 6.7(this study27) Depth [km] Depth [km] 6 27 km eclogite xenoliths exceed 60 km (1.8 GPa) Smith and Zientek (1979) (6.7) e Wolf & Cipar (1993) e e

e reinterp-Roller (1965) -refraction 80 80 3 eclogite >90 km (2.5 GPa) Usui et al. (2003) tonalite/diorit amphibolite Grt-amphibolit Px-granulit Grt-granulit eclogite >100 km (2.6-3.4 GPa) Smith et al. (2004) 5 Moho and >4.5 GPa (Schulze et al., 2015) 43 km 100 100 Relative proportions 6.06.5 7.07.5 8.0 2.52.753.0 3.25 3.5 P-wave velocity [km/s] Density [g/cm3] Figure 2. Crustal structure for the Colorado Plateau near the Four Corners from geophysics and xenolith compilations. (A) Crustal model from Condie and Selverstone (1999). Note this column extends to base of the crust, whereas the other two panels (B and C) extend to 100 km depth. Bt—biotite; Grt—garnet; Px—pyroxene. (B) P-wave velocity models from xenolith studies (McGetchin and Silver, 1972; Padovani et al., 1982; this study), seismic refraction (Wolf and Cipar, 1993; reinterpretation of data from Roller, 1965), and passive source seismic studies (receiver function analysis from USArray by Bashir et al., 2011). Receiver function study of Bashir et al. (2011) identified a prominent lower-crustal layer constrained between 30 ± 0.9 km and the Moho at 42 ± 0.8 km with Vp/Vs of >2.0, suggesting mafic composition. (C) Crustal and eclogite xenolith groups (light gray—felsic and intermediate; medium gray—mafic amphibolite and granulite; black—eclogite) plotted by approximated depth at time of Navajo volcanic field eruption and versus approximate density. Pressure estimates, which are converted to depth, for xenolith groups are from Selverstone et al. (1999), Wendlandt et al. (1993), Usui et al. (2003), Smith et al. (2004), and Schulze et al. (2015). Densities for crustal xenoliths are from approximations by Condie and Selverstone (1999). Eclogite xenolith density estimate was taken from McGetchin and Silver (1972). P-wave velocity and density for sample RM-21 (star in B and C, respectively) were calculated at 480 °C and 0.75 GPa using the Hacker and Abers (2004) physical properties spreadsheet. Ctd—chloritoid. The numbers (1-6) next to each rectangle in C are the number of xenolith samples in that group.

relatively high densities (e.g., Bashir et al., 2011). These suggestions are TABLE 1. MODAL MINERALOGY AND CALCULATED BULK all in general agreement with models of crustal composition based on COMPOSITION xenolith compilations where the more mafic lithologies like amphibolite, Mineral % mode OxideComposition garnet-amphibolite, and garnet–mafic granulite tend to record higher pres- Grt1.21 SiO2 78.64

sures (Fig. 2; McGetchin and Silver, 1972; Condie and Selverstone, 1999; Qz 44.11TiO2 0.11 Pl 3.38 Al O 11.21 Crowley et al., 2006). At least some of the mafic xenoliths crystallized at 1 2 3 Pl 14.33FeO 1. 61 1.43 Ga and thus appear to represent Mesoproterozoic magmatic under- or 2 Ab 4.68 MgO0.22 intraplated material (Karlstrom et al., 2005; Crowley et al., 2006). Ms1 0.77 MnO0.03

Ms2 2.11 CaO0.8

SAMPLE DESCRIPTION, PETROLOGY, AND GEOCHRONOLOGY Bt 3.64 Na2O2.51

Kfs 25.30K2O4.86

Cc 0.44 P2O5 0.01 The studied sample (RM-21) is from the Red Mesa ultramafic micro- Ilm+Rt 0.003Total 100.00 breccia diatreme (Fig. 1) in the northeastern NVF (~37.01°N, 109.5°W Ap 0.03 on the map of Smith and Levy, 1976). Selverstone et al. (1999) classi- Total100.00 fied the sample as belonging to a group of felsic garnet-biotite gneisses Note: Modal mineralogy calculated from quantitative automated with either granitoid or sedimentary arkose protoliths. Estimated modal scanning electron microscope analysis (QEMSCAN) analysis. proportions from quantitative automated scanning electron microscope Mineral abbreviations are after Whitney and Evans (2010). analysis (QEMSCAN) in this study are listed in Table 1 and Figure 3. The sample contains a primary mineral assemblage of Qz + Pl + Kfs + Bt + 1999). A weak gneissic foliation is defined by shape-preferred orientation Ms + Grt (mineral abbreviations after Whitney and Evans, 2010), with of quartz and feldspar aggregates and by aligned mica. easily recognizable accessory allanite, zircon, ilmenite, and rare pyrite. Static alteration is moderate but widespread and is most obviously mani- Major Mineral Textures and Compositions fest by replacement textures such as more sodic plagioclase, albite, and secondary muscovite after primary plagioclase (Fig. 3C and 4). Additional Modal mineralogy (Table 1) was determined using QEMSCAN (Hoal secondary minerals associated with the alteration assemblage include cal- et al., 2009) at the Colorado School of Mines. Microprobe analysis on cite, monazite, rutile, and zoisite (the latter reported by Selverstone et al., silicate minerals was conducted with a JEOL-8600 electron microprobe

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A Fig. 3B B

Fig. 3C Area C

Area A Area B Area K

Area G Area H

Area D

Area F Area E 5000 µm 5000 µm

C 1000 µm

QEMSCAN image legends Small section (3C) Full section (3B)

Qz Kfs

Pl1

Pl2 + Ab

Ms1 + 2

Bt1 + 2 Grt Cc Fig. 4 Accessories Fig. 6C,D,8A (Area A)

Figure 3. Images showing simplified modal mineralogy and textural characteristics of thin section from sample RM-21. (A) Full thin-section plane-polarized light photomicrograph. (B) Near full-section quantitative automated scanning electron microscope analysis (QEMSCAN) map. Area mapped in A is shown by black outline. Pixel resolution is 20 μm. Lettered regions correspond to geochronological target areas. (C) Higher-resolution QEMSCAN map of subarea shown in B. Pixel resolution is 4 μm. Mineral abbreviations are after Whitney and Evans (2010). The mineral subscripts refer to different generations of those minerals, as explained further in the text.

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K map Ca map ABFigure 4. Wavelength dispersive Mnz2 Ab rim X-ray maps showing major compositional variation and replacement features of an altered plagioclase domain in sample RM-21. (A) K X-ray map. (B) Ca X-ray map. Blue Pl1 Al1 dots show locations of secondary monazite. Not all individual grains Ms are shown by dots because several 2 Ab rim occur closely spaced. The allanite Cc grain partially visible on the right side of the map is the same grain Kfs1 shown in Figures 6C, 6D, and 7A. Mineral abbreviations are after Bt 1 200 µm 200 µm Whitney and Evans (2010).

equipped with a W-filament at the University of Colorado–Boulder using beam (~0.7 µm). At these conditions, the analytical volume in monazite natural and synthetic standards. A beam current of 100 nA, accelerating is roughly a 1-µm-diameter sphere. Details of the analytical setup on the voltage of 15 kV, focused beam (~1 µm), and count times of 50–100 ms Cameca SX-100 can be found in Condit et al. (2015). A similar setup were used during X-ray mapping of garnet and plagioclase to identify was used on the JEOL-8230, with the difference that a lower current was zoning patterns. For quantitative analysis, an accelerating voltage of 15 used (50 nA) to further reduce the beam diameter (~0.4 μm), and the peak kV, beam current 20 nA, a focused beam for garnet and a defocused intensities were background-corrected using the mean atomic number beam (spot size of 5–10 μm) for mica and feldspar, to reduce volatil- correction for all elements (Donovan and Tingle, 1996; Donovan et al., ization of Na, Ca, and K, were used. Additional plagioclase analyses at 2016). Additional details on the analyzed elements, standards, counting identical beam conditions were obtained on a JEOL-8230 microprobe times, detection limits, and interference corrections for analyses obtained at the University of Colorado to include Sr analysis. Quantitative analy- on the JEOL-8230 are listed in Data Repository Table DR1.1 ses of monazite were acquired with a Cameca SX-100 at the University The mineral compositions for RM-21 reported by Selverstone et al. of Massachusetts–Amherst and with a JEOL-8230 at the University of (1999) are supplemented by results from this study (Table 2). Garnet

Colorado. Both instruments are equipped with a LaB6 electron gun for occurs as locally resorbed anhedral porphyroblasts ranging from 300 to enhanced beam resolution, and with 4 (out of 5) wavelength-dispersive spectrometers equipped with large-area monochromators. The same cor- 1 GSA Data Repository Item 2017180, Table DR1: Analytical setup for monazite rection routine was used, and results from both instruments are identical analysis (JEOL-8230 University of Colorado–Boulder), is available at http://www within error. Analytical conditions were 15 keV, 200 nA, and a focused .geosociety.org/datarepository/2017, or on request from [email protected].

TABLE 2. REPRESENTATIVE SILICATE MINERAL COMPOSITIONS FOR SAMPLE RM-21

Oxide Grt averageBt1 Bt2 Ms1 Ms2 Pl1 Pl2 Ab Pl1 Pl2 Ab

SiO2 36.24 33.43 33.98 45.02 49.29 63.7665.41 66.3763.43 67.2768.99

TiO2 0.01 2.58 1.25 0.25 0.05 <0.04<0.04 <0.04 0.010.010.00

Al2O3 19.61 16.66 17.02 32.93 28.1023.45 20.9119.96 22.2320.1718.97 FeO 33.10 28.34 27.26 4.24 6.100.020.010.020.010.020.00 MgO 1.44 4.42 5.08 0.77 1.51 <0.02<0.02 <0.020.000.000.00 MnO 2.15 <0.06 <0.06 <0.06 <0.06 <0.06<0.06 0.010.000.000.00 CaO 6.00 <0.02 0.05 0.07 0.02 4.37 1. 94 0.63 4.16 1. 44 0.10 SrO n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.25 0.24 0.22

Na2O 0.02 0.05 0.05 0.29 0.159.0610.54 11.259.4011. 14 11.89

K2O <0.01 9.96 9.66 11.27 11.02 0.12 0.07 0.06 0.08 0.06 0.07 F n.a. <0.12 <0.12 <0.12 <0.12n.a.n.a.n.a.n.a.n.a.n.a. Cl n.a. 0.12 0.10 <0.01 <0.01 n.a. n.a. n.a. n.a. n.a. n.a. Total 98.57 95.56 94.45 94.85 96.25 100.78 98.8898.30 99.57100.35100.25 End members Mg# 0.07 0.22 0.25 –––––––– Alm 0.73 –––––––––– Pyr 0.06 –––––––––– Sps 0.05 –––––––––– Grs 0.17 –––––––––– An – ––––0.21 0.09 0.03 0.19 0.07 0.00 Ab –––––0.78 0.90 0.97 0.79 0.92 0.99 Kfs –––––0.010.000.000.000.000.00 Si (apfu)– ––3.08 3.33 –––––– Note: Mineral abbreviations are from Whitney and Evans (2010). The mineral subscripts refer to different generations of those minerals, as explained further in the text. n.a.—not analyzed. Mineral formula recalculations are based on 12 oxygens in Grt, 11 for Ms and Bt, and 8 for Pl.

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1200 μm in their largest dimension. X-ray maps and quantitative tran- of Bt1 is generally uniform, with Mg# = 0.22, whereas Bt2 has distinctly

sects indicate relatively uniform garnet interior compositions, wherein higher but more variable Mg# (0.24–0.30; Fig. 5B). TiO2 content in Bt2 is

the highest Mg # (0.08), highest Ca content (XGr = 0.18), and lowest Mn also distinctly lower than Bt1 but appears to range as high as 1.8 wt% in

content (XSp = 0.18) occur. Gradual decreases in Mg# toward the margins, some larger secondary grains (up to 10 or 15 μm). Given this trend, some

particularly where the garnet is in contact with biotite, and increases in of the larger Bt2 grains may be incompletely reequilibrated remnants of

Mn likely reflect exchange and net transfer reactions, respectively, during resorbed Bt1. Secondary calcite occurs as small (<10–200 μm) grains in retrogression. Fractures in garnet are locally lined with chlorite. and adjacent to altered plagioclase. Similar to Selverstone et al. (1999), multiple generations of plagioclase and mica (biotite and white mica) are recognized (Fig. 5). In this Allanite and Monazite Textures and Compositions study, two generations of plagioclase as well as albite are distinguished.

Pl1 is the most anorthite-rich generation (XAn = 0.22–0.15) and occurs as Primary allanite grains of interpreted igneous origin (Alig) are coarse locally preserved unzoned cores (Figs. 3C, 4, and 5A). The second gen- (50–200 μm in longest dimension), euhedral to subhedral (Fig. 6), and

eration (Pl2, XAn = 0.12–0.07) occurs in cracks, along twin contacts in Pl1, partially pseudomorphed by Mnz + Cc + Qz ± Ab ± Rt ± clay. The largest and as other patchy domains of intergrowth with secondary muscovite, and most well-preserved allanite grains show evidence of recrystalliza-

biotite, and calcite that are contained within Pl1 grains. The albite (XAn tion, with compositionally distinct secondary allanite locally replacing

= 0.03–0.01) occurs locally as clean overgrowths, particularly where in the original mineral (Fig. 6). This secondary allanite (Al1) is slightly more contact with resorbed K-feldspar and early biotite. Two texturally and calcic (Fig. 6D) and more depleted in light rare earth elements (LREEs)

chemically distinct generations of each muscovite and biotite are rec- than the original igneous allanite (Alig).

ognized (Figs. 5B and 5C). Early Ms1 and Bt1 are coarser (up to several Monazite occurs in two general textural settings, in direct association hundred micrometers wide), are scattered throughout the matrix, and with either allanite or plagioclase. Type 1 monazite occurs exclusively

help define a weak foliation. Secondary Ms2 occurs as fine (<10–100 as a replacement phase in and around allanite, but there are two morpho- μm), irregularly oriented lathes intergrown with the second-generation logical subvarieties. Some grains, particularly those associated with large,

plagioclase and albite. Bt2 has a smaller grain size than Bt1 and lacks moderately well-preserved allanite grains, are equant or tabular, singular

preferred orientation, similar to Ms2, and it is most commonly localized grains, most commonly less than 10 μm in average diameter. They occur

near resorbed Bt1. Ms2 has a higher phengite component (Fig. 5C). Silicon in fractures and along altered allanite margins where Al1 is common (Fig.

atoms per formula unit (apfu) range from 3.07 to 3.14 for Ms1 and up 6C), suggesting coeval growth with Al1. Thus, we refer to this population

to 3.34 for Ms2. Secondary ilmenite occurs locally in altered plagioclase as Mnz1. In the more extensively pseudomorphed allanite grains, a second

near and as exsolution lamellae within resorbed Bt1. The composition variety of type 1 monazite exhibits an irregular patchy morphology that

A K-feldspar 2% Kfs Pl1 Pl2Ab

Anorthite 20% 10% Albite

20% Fe-Phengite 40%

BCAl Fe+Mg 30% Sid East 30% AlIV Ann Phl Ms2 Fe Mg 40% Si ~3.27 50% Phengite 50% 40% Fe-Ms Al Bt2 Fe+Mg Ti ~0.2% 60% 10% 40% Ms1 40% Si ~3.07 Fe Fe+Mg Bt1 ? Ti ~2.8% IV VI 70% Al 40%80% Al 30% 60% Muscovite 80% 10% 20%30% AlVI Mg

Figure 5. Mineral composition diagrams. (A) XKfs-XAn-XAb ternary diagram distinguishing Pl1, Pl2, and Ab. (B) Al-Fe-Mg ternary diagram IV VI distinguishing Bt1 and Bt2. (C) (Fe + Mg)-Al -Al ternary diagram distinguishing Ms1 and Ms2. Mineral abbreviations are after Whitney and Evans (2010).

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Alig

Figure 6. (A–C) Backscatter electron images showing selected euhedral and only moderately Alig 80 μm Al altered allanite grains in sample 1 RM-21. The grain in A contains Al readily visible oscillatory (igne- 1 ous?) growth zoning. As a result 100 μm of adjusting the contrast in this image to bring out the oscillatory

zoning, the secondary Al1 domains

are rendered black. The Al1 domains CDCa Ka are shown in A with the red dotted Al Al1 lines, whereas they are visible in B 1 and C as dark gray. Monazite occurrence in allanite from C is shown Al Al1 in Figure 7A. (D) Ca X-ray map of 1 allanite grain in C showing distinctly higher Ca composition in

Al1 (yellow) compared to that in Al Al1 the igneous allanite (purple/red/ 1 Alig Alig orange). Mineral abbreviations are after Whitney and Evans (2010).

80 μm 80 μm

Am7: ABAl 1514 ±61 1 Kfs Ab Dm2: 70 ±3 Am2: 71 ±5 Ms2

Al Alig 1 Al Dm1: 66 ±1 1 Am4: Al Figure 7. Backscatter-electron ig 1698 ±54 images showing selected monazite textural settings in sample RM-21. Am3: Am1: Kfs Ion probe analyses and dates 174 850 ±68 Ab are labeled. (A) Relatively well- ±12 preserved euhedral allanite grain Ms 80 μm 10 μm 2 with altered margins and numerous Am6: 1548 ±53 small type 1 monazite grains (mix

of Mnz1 and Mnz2). (B) Type 2 monazite in altered plagioclase domain. CDBm4: Em3: 91 ±3 (C) Heavily pseudomorphed alla- Bm3: 68 ±3 Bm1: nite grain with multiple mats of Rt Em2: 58 ±2 107 ±4 81 ±3 polycrystalline type 2 monazite. (D) Al Euhedral pseudomorph of allanite Cc i with partial replacement. Mineral abbreviations are after Whitney and Evans (2010). Clay Rt Em1: 62 ±2 Ali Qz 40 μm Bm2: Bm5: Qz Em4: 1480 ±72 22 ±4 267 ±12 60 μm

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appears to be generated by mats of thin monazite flakes (Fig. 7C). These wt%), SrO (2.7 wt%–3.5 wt%), and CaO (4.0 wt%–5.7 wt%). These mats can be as large as 40 μm in their longest contiguous dimension, but include some of both type 1 (allanite association) and type 2 (plagioclase they are more commonly less than 20 μm. Due to their association with association) occurrences. Enrichments in the anhydrite-celestine (Ca,Sr)

the pseudomorphing assemblage, we refer to this variety as Mnz2. The SO4 component are uncommon but have been observed at or above these second textural type of monazite grains (type 2) is represented by small levels in several localities, including in monazite from kimberlites in east- (<5 μm and very rarely up to 25 μm in maximum dimension), anhedral ern Siberia (Chakhmouradian and Mitchell, 1999) and in hydrothermal monazite, texturally associated with altered plagioclase. This type most monazite from albite-muscovite schist and ore-bearing muscovite-quartz commonly occurs either as intergrowths with the secondary minerals Ab veins in the Tauern Window, Austrian Alps (Krenn et al., 2011). Sulfur

+ Ms2 + Cc or locally along fractures within or adjacent to secondary Pl2 is positively correlated with Ca and Sr and negatively correlated with P (Fig. 4). Most altered plagioclase domains contain abundant monazite and REEs (Fig. 8B), suggesting that the coupled substitution REE3+ + P5+ of this type. For example, the plagioclase domain in Figure 4 contains ó (Ca, Sr)2+ + S6+ is responsible for the incorporation of the anhydrite- over 20 individual monazite grains. No monazite was observed in unal- celestine component into the monazite (Chakhmouradian and Mitchell,

tered Pl1. Again, due to the association of this type of monazite with the 1999; Krenn et al., 2011). Deviation from the expected 1:1 correlation

secondary replacement assemblage, we also label this Mnz2. Thus, the in Figure 8B suggests the presence of excess Ca and Si, most likely due textural association with allanite that constitutes type 1 monazite appears to the presence of silicate (allanite, plagioclase) or carbonate (calcite) in

to encompass two generations (Mnz1 and Mnz2), whereas type 2 monazite the direct vicinity of, or included in, these micron-scale monazite grains.

appears to be exclusively represented by Mnz2. These compositions translate to 10–24 mol% of the anhydrite (Ca,Sr)

Due to small grain sizes and the matted nature of some monazite occur- SO4 component in the monazite solid solution (Fig. 8A). Other monazite rences, obtaining high-quality chemical compositions proved challenging grains, also occurring in both type 1 and type 2 settings, contain more due to contamination from surrounding phases (recognizable by elevated normal concentrations of these elements (<<1%). The second observation elements such as Al, Si, Fe, Ca, or K). Some representative analyses of is that all type 1 monazite, with or without elevated S or Sr, has moderate

good to acceptable quality are listed in Table 3 with some notable observa- to high ThO2, typically ~3 wt% and as high as ~6 wt%, whereas most tions. There are roughly three compositional trends recognized: rare earth (but not all) type 2 monazite is almost a pure REE-phosphate with low element (REE)–rich, (Th,U)-rich, and (Sr,Ca,S)-rich trends (Fig. 8A). The Sr and S contents and lower concentrations of Th, mostly near or below

latter is the most notable observation, with elevated SO3 (2.5 wt%–3.5 the microprobe detection limits, but locally up to 2 wt% ThO2.

TABLE 3. MONAZITE AND ALLANITE COMPOSITIONS FOR SAMPLE RM-21

Oxide Mnz type 1 Mnz type 1 Mnz type 1 Mnz type Mnz type Mnz type Mnz type 2 Mnz type 2 Mnz type 2 Aln1 Aln2 † undated undated Mnz1(Am4) 1 mixed 1 mixed 2 Mnz2 Mnz2 (Dm2) Mnz2 (Dm2) undated (Bm5) (Bm3) (Dm1)*

P2O5 27.44 27.80 26.43 26.33 27.5729.75 28.5928.45 24.37b.d.l. 0.03

SiO2 0.52 0.83 0.44 0.30 0.51 0.30 0.55 0.34 0.79 31.4131. 71

SO3 0.01 0.02 3.03 3.47 2.79 0.03 0.01 0.01 7. 82 <0.01<0.01

ThO2 3.70 5.97 3.011.69 2.122.172.592.010.030.630.78

UO2 0.21 0.16 <0.02 <0.02 <0.02 0.06 0.17 0.09 <0.02<0.02 <0.02

Y2O3 1.00 0.63 0.41 0.40 0.47 0.57 1. 50 0.92 0.47 0.07 0.06

As2O3 0.23 0.21 0.18 0.23 0.25 0.32 0.19 0.18 0.20 n.a. n.a.

La2O3 13.55 12.81 12.34 12.02 12.09 14.6013.03 14.4811. 59 4.39 4.39

Ce2O3 27.14 26.10 25.57 25.65 25.83 29.6925.62 27.6124.55 10.279.71

Pr2O3 3.45 3.20 3.03 3.23 2.97 3.22 3.24 3.26 2.78 1. 27 1. 12

Nd2O3 14.39 14.71 11.88 12.29 12.21 13.3915.20 14.7011. 44 4.14 3.74

Sm2O3 3.13 3.00 2.02 2.33 2.24 1. 25 3.33 3.02 2.09 0.81 0.63

Eu2O3 0.42 0.93 0.41 0.49 0.68 0.21 1. 00 0.60 0.87 0.180.17

Gd2O3 2.34 1.81 0.94 1. 13 1.29 1. 49 2.77 2.19 1. 06 0.36 0.22

Tb2O3 0.23 0.17 0.07 0.08 0.100.050.230.170.090.050.05

Dy2O3 0.56 0.39 0.23 0.24 0.28 0.24 0.79 0.50 0.26 0.150.13

Er2O3 0.05 0.03 <0.03 <0.03 <0.03 0.07 0.05 <0.030.030.050.05

Tm2O3 <0.03 <0.03 <0.03 <0.03 <0.03 0.12 <0.03<0.03 <0.03<0.03 <0.03

Yb2O3 0.02 0.02 <0.02 <0.02 <0.02 0.04 <0.02<0.02 <0.02<0.02 <0.02

Ho2O3 <0.05 <0.05 <0.05 <0.05 <0.05 0.12 0.05 <0.05<0.05 <0.05<0.05

Lu2O3 <0.03 b.d.l. <0.03 <0.03 <0.03 n.a. <0.03<0.03 <0.03n.a.n.a.

TiO2 0.01 0.02 <0.01 0.08 <0.01n.a.<0.01 0.04 <0.01n.a.n.a.

Al2O3 0.02 <0.01 0.03 0.07 0.09 n.a. 0.01 0.02 0.13 16.9616.84 MgO 0.02 0.02 0.07 0.09 0.16n.a.0.020.030.430.360.44

Na2O 0.02 0.01 0.10 0.07 0.10n.a.0.010.010.05n.a.n.a. SrO 0.31 0.16 3.20 3.37 3.00 0.10 0.04 0.14 4.76 n.a. n.a. CaO 0.74 0.62 4.44 4.21 4.36 0.25 0.20 0.37 7. 27 11.0911. 56

K2O <0.03 <0.03 <0.03 <0.03 <0.03 0.16 0.09 0.20 0.03 <0.03<0.03 MnO 0.03 0.05 0.04 0.03 0.04 n.a. 0.05 0.05 0.04 0.26 0.23 PbO <0.02 0.02 <0.02 0.04 0.04 <0.02<0.02 0.02 0.04 n.a. n.a. FeO 0.09 0.22 0.52 0.22 0.32 n.a. 0.06 0.04 0.07 14.4614.71 TOTAL 99.63 99.90 98.39 98.05 99.51 98.1899.41 99.45101.2496.89 96.54 Note: n.a.—not analyzed; b.d.l.—below detection limit; Mnz—monazite. *Analyzed at University of Massachusetts–Amherst. All others were analyzed at the University of Colorado. †Reanalysis of same grain.

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0.8 AB“Ca,Sr”

80% 20% (Ca,Sr)(S,Si)O

Central Excess Ca (±Si ) plagioclase region in 0.6 (Ca,Sr)SO4 Fig. 3C 4 (REE,Th)(Si,P)O “REE” “Th,U” Ca 0.5

,Si)** Th 0.5 PO 4 A 0.4 4 Fig. 8A

REE(PO4) S+(Ca,Sr

B 0.2 95% Fig. 8C 5% 1:1 Fig. 8B,D D,E 0.0 5% 25% 1.41.6 1.82.0 ThSiO4 + REE+P+Th+U+(Si,Ca)* UMass (Cameca SX-100) CaTh(PO4)2 CU Boulder (JEOL-8230) UMass (Cameca SX-100) A Mnz/Aln domain CU Boulder (JEOL-8230)

Figure 8. Variation in monazite composition. (A) Ternary diagram highlighting differences in monazite-xenotime (REEPO4), cheralite-huttonite (Ca,Th)

(P,Si)O4, and anhydrite-celestine (Ca,Sr)SO4 components. This latter was calculated based solely on the measured S content in order to eliminate the effect of excess Ca. Regions encompassing analyses from lettered geochronological target areas and from the central plagioclase domain shown in Figure 3C are indicated. Mnz—monazite; Aln—allanite. (B) Diagram illustrating the enrichment in anhydrite-celestine component. Asterisk indicates Ca and Si are integrated in cheralite and huttonite components. Double asterisk is without Ca and Si integrated in cheralite and huttonite components,

and with excess Ca and Si. REE—rare earth elements; UMass—University of Massachusetts–Amherst; CU Boulder—University of Colorado–Boulder.

1 1.4 2 late alteration Figure 9. Pressure-temperature diagram sum-

mafic granulite marizing the estimated peak and retrograde Ti-in-Bt 1.2 xenoliths [S99] Ti-in-Bt 43 alteration conditions for crustal xenoliths from Red Mesa and Garnet Ridge diatremes. Peak and retrograde metamorphic conditions for 1.0 GR24 peak [S99] 36 mafic xenoliths from Selverstone et al. (1999), abbreviated as S99 in the figure, are shown with mafic granulite Si=3.3 [S99] light-gray ovals. Data for felsic xenoliths (includ- xenoliths [S99] 0.8 29 ing RM-21) from Selverstone et al. (1999) include Ky the dark shaded polygons (peak conditions) as well as the large light-gray shaded region (range RM-21 peak 0.6 675 °C 0.65 GPa [S99] 22 of Grt-Bt temperatures from rim compositions) Si = 3.4 and the Si = 3.3 apfu for phengitic muscovite

3°C

Si = 3.3 (barometer source from Massonne and Shreyer,

Pressure (GPa ) 2 Apparent depth (km)

Grt-Bt [S99]

0.4 65 15 1987). Data shown in black are for sample RM-21 Si = 3. Sil from this study and include (1) temperature esti- 0.2 And mates based on the Ti-in-biotite thermometer from Henry et al. (2005) for Bt1 (higher tempera-

ture) and Bt2 (lower temperature) and (2) Si (apfu) in phengitic muscovite from pseudosection analysis. Mineral abbreviations are after Whitney and 400 600 800 1000 Evans (2010). Temperature (°C)

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0.07 ABRM-21 60-85 Ma Zrn: Smith & Griffin, 2005 0.06 31-45 Ma 20 Monazite 70 Ma Zrn: Smith et al., 2004 Figure 10. Monazite Th-Pb dates from sample RM-21. Th232-Pb208 0.05 33 Ma 47-81 Ma Zrn: Usui et al., 2003 (A) Histogram plot of full data set with probabil- 28-30 Ma Mnz: Schulze et al., 2015 ity curve. (B) Normalized probability plot showing dates 0.04 the distribution of dates younger than 100 Ma. Also shown for comparison are U/Pb zircon and mona- Diatreme emplacement Mnz this study 0.03 2 zite dates for mantle and eclogite xenoliths from 15 19-33 Ma 0.02 previous studies. The Smith et al. (2004) data are multigrain zircon (Zrn) fractions analyzed by ther- Mnz 3 mal ionization mass spectrometry (TIMS) from three 0.01 this study eclogite xenoliths from Moses Rock, Mule Ear, and 10 0 Garnet Ridge diatremes. The Usui et al. (2003) data 0 20 40 60 80 100120 140 are ion probe U-Pb zircon analyses from three eclog- Age(Ma) ite xenoliths from Garnet Ridge and Moses Rock. The Smith and Griffin (2005) data are U-Pb zircon analyzed by laser ablation–inductively coupled 5 plasma–mass spectrometry (LA-ICP-MS) from an Number of Analyses Mnz1 ultramafic garnetite xenolith from the Garnet Ridge diatreme. The Schulze et al. (2015) data are Th-Pb monazite dates by LA-ICP-MS from three eclogite- facies xenoliths from the Moses Rock diatreme. 0 Diatreme emplacement range is from Laughlin et al. (1986), McDowell et al. (1986), Wendlandt et al. 0 500 1000150020002500 (1996), Usui et al. (2003), and Smith et al. (2004). Date [Ma]

Thermobarometry Monazite Geochronology

Peak metamorphic conditions for crustal xenoliths from the northern Analytical Method group of diatremes in the NVF (Red Mesa, Garnet Ridge) range from Monazite identification was facilitated by Ce X-ray mapping of full 1.15 GPa and 780 °C to 0.65 GPa and 675 °C, with the latter condi- thin sections and by QEMSCAN mapping. In situ Th/Pb geochronological tions from sample RM-21 (Fig. 9; Selverstone et al., 1999). Retrograde analysis was conducted with a CAMECA IMS 1270 secondary ion mass conditions for the hydrous alteration of these xenoliths were also esti- spectrometer (SIMS) at the W.M. Keck Foundation Center for Isotope mated by Selverstone et al. (1999) as 0.7–0.9 GPa at 450–550 °C using Geochemistry at the University of California–Los Angeles. Thin sections the Fe-Mg exchange thermometer for rim compositions of garnet and were cut with a high-precision diamond saw or drilled with a diamond core, biotite and Si-in-phengite barometry as calibrated by Massonne and cleaned, mounted in epoxy with a prepolished block of standards, and Schreyer (1987). gold-coated with 100 Å of Au. Moderate- (554) and high-Th (Trebilcock)

We supplement these estimates further by evaluating the Ti-in-biotite monazite standards were used to obtain a curve of ThO2/Th versus Pb/Th thermometer (Henry et al., 2005) for both generations of biotite, and by for a relative sensitivity factor before analysis and were reanalyzed after calculating Si-in-phengite isopleths using forward petrological modeling every 5–6 unknowns during data collection to evaluate changes in instru- with Perple_X 6.6.8 (Connolly, 2005) and the thermodynamic database ment calibration. Standard operating procedures as outlined by Harrison of Holland and Powell (1998; with updates through 2004). The latter is et al. (1995) were followed, including use of an O– primary beam focused useful in light of calculations by Goncalves et al. (2012) showing the to 10–20 μm, 15 eV offset for 232Th, and a mass resolving power of 4500. strong dependence of the phengite component in white mica on bulk High mass resolving power is necessary to distinguish between Pb and composition and the buffering assemblage. The calculated Ti-in-biotite Th isotope peaks and significant molecular interferences, particularly, the

temperature for average Bt1 compositions is 654 ± 33 °C (average of 29 PrPO2 interference at mass 204 and NdPO2 interference at mass 208. The analyses; Fig. 9), which agrees well with peak temperature estimates field aperture was adjusted to within 5 μm to ensure that the instrument

from Selverstone et al. (1999). Unfortunately, most Bt2 compositions was sampling the dominant Th signal from the target grain, even if the pit have Ti contents below the range calibrated by Henry et al. (2005), but encompassed adjacent minerals, and to minimize sampling of adjacent several yielded temperatures at the lowest end of the calibration range common Pb domains (Schmitt et al., 2010; Chamberlain et al., 2010; Ault (480–800 °C), indicating a maximum temperature of alteration of 480 °C. et al., 2012). Instrument precision is limited to ~2% based upon the repro- The Si apfu values in white mica calculated from a bulk composition ducibility of the calibration curve and is most often affected by bad polish, estimated from the QEMSCAN modes and measured mineral composi- compositional variability within a standard, or environmental conditions tions are also shown in Figure 9. The Si isopleths have a slightly steeper such as 208Pb surface contamination (Harrison et al., 1995). Therefore, the slope than those from the Massonne and Schreyer (1987) calibration data were corrected for common Pb using a 208Pb/204Pb value of 38.34 but a similar location in P-T space for the interval encompassing the based on San Diego wastewater (Sañudo-Wilhelmy and Flegal, 1994).

highest measured Si contents for Ms2 in this study and by Selverstone et al. (1999, 3.3–3.4 apfu, p. 595). In summary, the new results (0.7–0.8 Results GPa at 480 °C) are consistent with those determined by Selverstone Twenty-six monazite grains were analyzed with one analysis per grain et al. (1999). (with one exception: Cm1 was analyzed twice) due to grain size limitations

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(<15 μm diameter). The Th/Pb dates range from 1871 ± 68 Ma to 22 ± (e.g., Proyer, 2003), as well as this and other felsic xenoliths from the 3.5 Ma (Figs. 7 and 10; Table 4). All errors are reported as 1σ. Of the 27 NVF (Broadhurst, 1986; Selverstone et al., 1999), and they reflect altera- total analyses, 22 are from type 1 monazite, in association with allanite. tion primarily via the following generalized reaction:

Representing a mixture of Mnz1 and Mnz2, they span the full range of dates,

but more than half (12) are between 107 ± 4 Ma and 57.7 ± 2.4 Ma. The Pl1 + Kfs + Fluid = Pl2 + Ab + Ms2 + Cc ± Zo.

remaining five analyses are from type 2 monazite grains (Mnz2), which

are texturally associated with altered Pl1. Four of these dates are between Garnet also locally exhibits textures and replacement assemblages 71 ± 8 Ma and 66 ± 1 Ma (Cm1-a, Cm1-b, Dm1, Dm2), and the fifth is indicative of fluid-related alteration. A minor increase in Mn at garnet 373 ± 35 Ma (Fm1). The majority of type 2 monazite grains were not margins and fractures in garnet that are locally lined with chlorite suggest viable for analysis because of either small size or low Th and Pb contents. late net-transfer reactions that consumed garnet at the expense of chlorite growth. However, the lack of other obviously secondary phases that are DISCUSSION characteristic of adjacent feldspar domains make it unclear whether the development of these alteration effects was coeval. Nature and Timing of Deep Crustal Alteration Allanite and monazite typically have a close association in metamorphic rocks. In prograde metamorphism of metapelites, monazite is com- General Evidence for Retrogression mon at greenschist-facies conditions but reacts to allanite at uppermost Fluid-related alteration is widespread in sample RM-21, with the main greenschist- to amphibolite-facies conditions before reappearing again at features recorded by breakdown effects in feldspar, garnet, and allanite and higher grades (e.g., Wing et al., 2003; Krenn and Finger, 2007; Janots et al.,

the production of secondary assemblages characterized by Pl2 (or Ab) + 2008; Spear, 2010). Sample RM-21 has a granitic rather than metapelitic

Ms2 + Bt2 + Cc + Mnz2 ± Zo. Three compositionally distinct generations bulk composition, but nevertheless the monazite-allanite relations are of plagioclase are recognized. The preserved Ca-rich plagioclase core consistent with these general trends. The distinct euhedral to subhedral

(Pl1) is interpreted as the stable composition during peak metamorphism. morphology and concentric zoning patterns of large well-preserved alla-

Lower-Ca plagioclase (Pl2 and Ab) reflects discontinuous breakdown of nite grains suggest that they are an original magmatic accessory phase

Pl1 and Kfs during retrogression. Primary potassium feldspar is resorbed from an igneous protolith. Two distinct features also suggest that allanite locally along grain boundaries where Ab is particularly common. Two experienced at least two episodes of reaction. First, well-preserved grains

generations of Ms are also observed, where Ms1 represents part of the and remnants of largely consumed grains display compositionally distinct

peak assemblage, and Ms2 is associated with the alteration assemblages in rim domains of a subsequent generation of allanite. The contacts between feldspar and allanite reaction zones. These observations are consistent with these two compositional domains are sharp but irregular, suggesting that a family of reactions commonly observed in metasomatized granitoids the original igneous allanite became unstable and was partially consumed

TABLE 4. MONAZITE Th-Pb ION PROBE DATA FOR SAMPLE RM-21

208 208 208 232 Spot Textural Pb ±1σ ThO2/Th ±1σ 207.976625 203.4689 Pbr % Pb/ Th ±1σ Long Short Ave. dia. Textural description type 232Th 208Pb† 204Pb§ age (Ma) (mm) (mm) (mm) Bm2 Type 1 0.00111 0.0001741.020 0.02011.15E+01 9.31E–0420.822.33.5 87 8Irregular, radiating, matted Em2 Type 1 0.00286 0.0001171.050 0.0134 8.32E+00 2.75E–0447. 057. 72.4 65 6Subequant, whole Km2 Type 1 0.00305 0.000251 0.951 0.0229 9.66E+00 0.00E+00 36.9 61.6 5.1184 11 Regular, tabular, whole Em1 Type 1 0.00308 0.000117 0.757 0.0149 9.07E+00 6.31E–0437. 162.22.4 20 613 Irregular, matted Km1 Type 1 0.00323 0.000156 0.633 0.01041.07E+01 2.85E–0349.765.13.1 12 810Subequant, whole Dm1 Type 2 0.00328 0.000065 1.050 0.02101.14E+01 3.17E–0294.566.21.3 10 26Regular, tabular, whole Bm4 Type 1 0.00337 0.000141 0.851 0.02151.02E+01 2.49E–0350.667. 92.8 97 8Subequant, subhedral, whole Dm2 Type 2 0.00349 0.000128 0.755 0.01341.69E+011.51E–0286.470.42.6 25 20 23 Subequant, subhedral, whole Cm1-a Type 2 0.00351 0.000404 1.020 0.0272 1.29E+01 0.00E+00 65.2 70.9 8.1127 10 Subrounded, whole Cm1-b* Type 2 0.00352 0.000467 0.924 0.0262 1.43E+01 0.00E+00 53.4 71 9.4127 10 Subrounded, whole Am2 Type 1 0.00353 0.000232 0.341 0.0025 1. 16E+01 4.23E–0391. 871. 14.7 83 6Whole, fractured Cm2 Type 1 0.00386 0.0001121.180 0.0171 9.05E+00 8.51E–0376.577. 92.3 18 512 Irregular, matted Cm3 Type 1 0.00387 0.0001271.070 0.01791.16E+017.36E–03 78.1 78.1 2.6208 14 Irregular, matted Bm1 Type 1 0.00400 0.000149 0.969 0.01171.07E+01 3.89E–0450.480.7316 10 13 Irregular, radiating, matted Km3 Type 1 0.00448 0.000630 0.395 0.0235 1. 17E+01 0.00E+00 51.0 90.4 12.7 12 59Irregular, matted, fractured Em3 Type 1 0.00452 0.000137 0.811 0.01621.07E+011.82E–0359.791. 12.8 15 611 Irregular, matted Bm3 Type 1 0.00530 0.000182 0.837 0.0296 1.20E+01 3.33E–0357. 4107 4387 23 Irregular, matted; flaky Am3 Type 1 0.00863 0.000609 0.882 0.0240 1.73E+01 2.24E–0387. 2174 12 10 58Regular, whole, central hole Bm5 Type 1 0.01330 0.000610 0.758 0.0200 1. 17E+01 2.06E–0382.2267 12 19 412Irregular, radiating, matted Hm1 Type 1 0.01390 0.000536 0.672 0.00761.35E+01 0.00E+00 94.0 279113 23Irregular, cuspate Fm1 Type 2 0.01860 0.001760 0.842 0.0478 1.28E+01 0.00E+00 48.9 373357 56Irregular, pitted Am1 Type 1 0.04290 0.003520 0.520 0.0259 1.37E+01 0.00E+00 92.7 850686 24Regular, whole, fractured Em4 Type 1 0.07600 0.003850 0.450 0.0092 1.03E+01 7.05E–03 87.1 1480 72 52 4Regular, tabular, whole Am7 Type 1 0.07780 0.003270 0.488 0.00741.30E+01 3.43E–0397. 9151461104 7 Irregular, tabular Am6 Type 1 0.07960 0.002820 0.564 0.0071 1.45E+01 2.31E–0298.01548536 45Subequant, whole Am4 Type 1 0.08760 0.002930 0.612 0.0091 1.21E+01 3.26E–0399.4169854103 7Irregular, tabular, fractured Gm1 Type 1 0.09700 0.003660 0.539 0.01001.27E+01 6.82E–0399.7187168123 8 Irregular, tabular 208 Note: Pbr—radiogenic Pb. Pb, Th, and ThO2 were analyzed at –30 eV offset. *Reanalysis. †Ratio of 208Pb counts without and with offset. § 143 ++ 204 Ratio of 203.4689 (= NdThO2 ) to Pb.

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and replaced with metamorphic Al1 + Mnz1. This may have developed dur- are interpreted to reflect early recrystallization of igneous allanite to meta-

ing peak metamorphism, which is estimated to have occurred at ~650–675 morphic Al1 ± Mnz1 during metamorphism associated with the Yavapai °C (this study; Selverstone et al., 1999). Second, all allanite grains and both and Mazatzal orogenies (Bowring and Karlstrom, 1990) and possibly

Alig and Al1 are resorbed and replaced to varying degrees by the second- subsequent ca. 1.4 Ga magmatic (Crowley et al., 2006) or Picuris oro-

ary assemblage of Ab + Ms2 + Rt + Cc + Qz + Mnz2. The similarity of genic (Daniel et al., 2013) activity. All of these events have previously this assemblage to that of adjacent feldspar domains suggests retrograde been recognized in Sm-Nd (whole rock, garnet, and clinopyroxene) and alteration in a similar and possibly coeval fluid-rich environment. Simi- U-Pb zircon studies of crustal xenoliths from the NVF, including xeno- lar retrograde monazite after allanite relationships have been observed liths from Red Mesa (Wendlandt et al., 1993, 1996; Mattie et al., 1997; by others (e.g., Krenn and Finger, 2007; Janots et al., 2008; Krenn et al., Condie et al., 1999; Crowley et al., 2006). However, the small number of 2011). From previous work and additional thermobarometric constraints analyses of this oldest suite of monazite coupled with the likelihood that presented here, the pressure and temperature conditions of alteration of it would have been the most susceptible to subsequent alteration-related this xenolith are estimated to between 400 and 480 °C and between 0.6 disturbance, if any occurred, preclude further interpretations at this time. and 0.9 GPa, with alteration pressures (at similar temperatures) for mafic A second small group of five dates scatters broadly between 850 and granulite xenoliths in the NVF suite as high as 1.1 GPa (Selverstone et 174 Ma, whereas a large group of 16 dates spans fairly evenly between 107 al., 1999). and 58 Ma. Whereas the former range of dates does not readily correlate The monazite chemistry in RM-21 is also notable for its retrograde with known tectonic, magmatic, or fluid-fluxing events in the region, the signature and appears to at least partly have been controlled by proximity latter range is largely coincident with the timing of shallow subduction to preexisting phases in the immediate surroundings. Some monazite in beneath western North America (e.g., Coney and Reynolds, 1977; Lipman, RM-21 has elevated sulfur and strontium. Krenn et al. (2011) suggested 1980) and the Laramide orogeny. The younger range of dates also overlaps that S-bearing monazite could be a sensitive monitor for S-rich fluids, with U-Pb zircon dates from 85 to 33 Ma reported for hydrated eclogite although this seems unlikely given that not all secondary monazite in (Usui et al., 2003; Smith et al., 2004) and mantle (Smith and Griffin, 2005) the sample is S-rich. An alternative local source could be preexisting xenoliths from the NVF (Fig. 10). However, several sources of potential sulfides, and altered pyrite was noted in proximity to several of these error need to be evaluated before further interpretation can be made. monazite grains. Similarly, plagioclase could have provided a local Sr Given the small grain sizes relative to the ion beam diameter, some of source, as has been suggested by Krenn and Finger (2004) and Krenn et the scattered dates could represent artifacts from sampling non-monazite al. (2011) for other Sr-bearing monazite occurrences, although no distinct matrix. However, this source of error was minimized by reducing the field trend in Sr content was noted among plagioclase generations or albite in aperture to avoid sampling of ions from other host phases (Chamberlain this sample (all have ~0.20–0.25 wt% SrO). Microprobe investigations et al., 2010; Schmitt et al., 2010) and by monitoring for systematic varia-

of other potentially local Sr sources like allanite or secondary calcite do tions in PrPO2 counts. Low PrPO2 counts may reflect contact with a non- not reveal Sr above trace-element levels. All type 1 monazite contains P- or non-Pr-bearing phase. Because no systematic trends were observed,

significant Th concentrations (up to 6 wt% ThO2), whereas only relatively it appears unlikely that sampling of non-monazite domains contributed rare occurrences of type 2 monazite contain Th above electron microprobe significantly to the observed wide span of dates. Furthermore, if this is detection limits. Since the allanite has significant Th, its recrystallization a real source of error for some analyses, the implications are unlikely

during Mnz1 growth and consumption during Mnz2 growth likely provided to significantly affect the dates, based on the assumption that the major a local Th source for incorporation into the monazite. This type of local source of common Pb is modern because 208Pb/204Pb changes relatively control on monazite chemistry has been noted in several previous studies slowly with time. For example, using an alternative correction employing (e.g., Mahan et al., 2006; Williams and Jercinovic, 2012). a 2 Ga model (Stacey and Kramers, 1975) common Pb composition (e.g., if sampled from surrounding feldspar or allanite) changes the calculated Age of Alteration from Monazite Geochronology dates by less than the stated analytical uncertainties even for analyses with Monazite is a particularly reactive radiogenic phosphate, which makes intermediate (65%) radiogenic Pb contents (A. Schmitt, 2016, personal it a powerful chronometer for crustal events, including diagenesis, pro- commun.). Instead, we suspect that the observed wide range of dates grade metamorphism, magma crystallization, and fluid flow (e.g., Parrish, largely reflects sampling of mixed monazite domains of Paleoproterozoic 1990; Williams et al., 2007; Janots et al., 2012; Villa and Williams, 2013). or Mesoproterozoic and Late Cretaceous to Paleocene ages. This is easily However, this same susceptibility to reactions warrants special care in envisioned when considering the commonly complex, matted appearance

studies where fluid-rock interactions have taken place because of the of Mnz2 in type 1 settings (good examples are Bm3 and Bm5 in Fig. 7C). potential for earlier monazite to be partially or completely recrystallized As stated already, the largest group of dates (59% of the data set) spans

or to incorporate common Pb (e.g., Harlov et al., 2011; Williams et al., from 107 to 58 Ma. These Mnz2 dates are from grains in both types of tex- 2011; Seydoux-Guillaume et al., 2012; Grand’Homme et al., 2016). In tural settings, but they include four of the five analyses from type 2 grains this study, all monazite occurrence is secondary, either in association with (71.0 ± 9.4 to 66.2 ± 1.3 Ma), which are directly associated with the most

altered allanite (all Mnz1 and some Mnz2) or altered feldspar (Mnz2). The abundant evidence for late-stage fluid-assisted alteration of the xenolith

association of Mnz2 with the secondary hydrated assemblage in both tex- assemblage (Fig. 4). The range of dates could indicate that Mnz2 growth, tural settings suggests a cogenetic link between monazite crystallization and thus hydrous alteration, occurred over a prolonged period from the and wholesale fluid alteration of the rock, which allows for geochronologi- Late Cretaceous to Paleocene rather than during a single punctuated event. cal constraints to be made on the timing of deep crustal hydration of the However, there are several reasons that lead us to interpret that the main

xenoliths. Type 1 monazite after allanite is the most viable for dating by in episode of Mnz2 crystallization, and thus alteration, occurred at 70–65 Ma. situ Th/Pb SIMS due to relatively high Th content. With four exceptions, First, the weighted mean of all 15 dates (not including replicate Cm1-b) is all investigated type 2 monazite had insufficient Th signal to be dateable. 71.1 ± 6.0 Ma (2σ error). The mean square of weighted deviates (MSWD) All five monazite grains with dates between 1871 and 1480 Ma are for this calculation is 16.3, and while its relatively large magnitude sup-

type 1 grains, three of which are Mnz1 from the largest and most well- ports the protracted monazite growth hypothesis, it may also simply mean preserved allanite grain (e.g., Am4, Am6, Am7 in Fig. 7A). These grains that additional uncertainties are not adequately accounted for. As stated

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earlier, the most substantial uncertainty that remains unaccounted for is support this argument. However, several mafic xenoliths that contain the sampling mixed domains of different ages, and the six oldest dates in characteristic high-pressure alteration features, including three of five this range are all from grains that have the matted texture indicative of studied by Crowley et al. (2006) from the Red Mesa diatreme, have 1.43 multiple intergrown flakes. Exclusion of analyses from grains with this Ga zircon U-Pb crystallization ages, indicating that significant alteration

texture yields a weighted mean of 65.7 ± 3.4 Ma and MSWD = 2.51 (n = must have occurred at or after 1.4 Ga. This is consistent with Mnz1 in 8). Further exclusion of all analyses with <50% radiogenic 208Pb, which RM-21 being a relatively minor constituent of the overall age population are exclusively restricted to the youngest end of the spectrum of dates, and therefore its restriction to relatively early textural settings, albeit yields a weighted mean of 67.4 ± 2.1 Ma and MSWD = 0.76 (n = 5). Thus, secondary with respect to the igneous allanite. the most reliable monazite dates point to a relatively punctuated event of We interpret monazite data from sample RM-21 to indicate that the hydrous alteration near the end of the Late Cretaceous. bulk of the observed hydrous alteration occurred at 70–65 Ma. This age A single monazite date of 22 ± 3.5 Ma is the youngest and is from a approximately coincides with emplacement of the Carrizo Mountains type 1 grain in allanite (Bm2 in Fig. 7C). This date coincides with the diorite laccolith (74–70 Ma; Semken and McIntosh, 1997), in northwest- 33–19 Ma age range of diatreme emplacement, which is based on a variety ern Arizona (Fig. 1), which is the southwesternmost intrusive complex in of isotopic constraints from volcanic host rocks and xenoliths (Laughlin the Colorado Mineral Belt trend, and which could conceivably have been et al., 1986; McDowell et al., 1986; Wendlandt et al., 1996; Usui et al., the source of fluids inducing the alteration. However, this seems unlikely 2003; Smith et al., 2004). This grain is also inside one of a subset of partial given the small volume and shallow level of emplacement of the intrusion allanite pseudomorphs that additionally contain mats of clay. Thus, this (into Permian to Cretaceous sedimentary rocks) relative to the mid-crustal

date may reflect very late near-surface alteration (Mnz3?). depths of xenolith alteration, and also considering that mineralization associated with the Late Cretaceous components of the Colorado Mineral Potential Fluid Sources for Late Cretaceous Hydration Beneath Belt was apparently extremely limited (Cunningham et al., 1994). A more the Colorado Plateau likely fluid source emerges when considering that the RM-21 xenolith is one of a large suite of crustal and mantle xenoliths in the NVF for which Potential sources of fluids that could have induced the observed crustal similar alteration has been described, and that the age of alteration is also alteration include those related to local granitoid laccoliths or mid-Tertiary coincident with the timing of shallow subduction beneath western North host diatreme emplacement (Ehrenberg and Griffin, 1979; Padovani et America (e.g., Coney and Reynolds, 1977; Lipman, 1980). al., 1982; Broadhurst, 1986), Proterozoic tectonic or magmatic processes Hydration at mantle depths beneath the Colorado Plateau during the (e.g., Selverstone et al., 1999), and Late Cretaceous to mid-Paleogene Late Cretaceous to Eocene is well documented from previous xenolith subduction-related fluids from the Farallon slab (e.g., Broadhurst, 1986; studies, but to our knowledge, the data presented here are the first to Humphreys et al., 2003; Smith et al., 2004; Lee, 2005). Ehrenberg and constrain crustal hydration to the same time interval. The earliest studies Griffin (1979) observed secondary alteration of xenoliths hosted by the invoking an important role for Laramide-aged fluids beneath the Colo- serpentinized ultramafic microbreccias but not in those hosted by the rado Plateau focused on eclogite xenoliths, where Helmstaedt and Doig minettes, and they suggested that the hydration effects therefore likely (1975) argued for them as direct samples from the shallowly subducted reflect local interaction with the host gas-fluid mixture just prior to erup- Farallon slab (see also Usui et al., 2003). Several geochemical and petro- tion (see also Padovani et al. [1982] based on seismic velocity proper- logical xenolith studies also documented significant hydration of mantle ties). Broadhurst (1986) similarly argued for what was interpreted as the lithologies from the Colorado Plateau, many of which either inferred or second of two episodes of hydration recorded by crustal xenoliths as a constrained a Laramide age for these processes (e.g., Smith, 1995; Smith reaction with the fluid component of the host microbreccia, but still before and Griffin, 2005; Lee, 2005; Li et al., 2008). incorporation into the upper part of the diatremes, because conodonts While subsequent chemical and isotopic studies have shown that most from limestone xenoliths do not display evidence for maturation to tem- of the eclogite xenoliths more likely originated from Proterozoic subduc- perature levels of even the lower-temperature “second” hydration (≤330 tion processes (Wendlandt et al., 1993; Smith et al., 2004), most recent °C). The timing of alteration by this type of source fluid should obviously studies have concluded that significant recrystallization in the presence of a coincide with diatreme emplacement, which is constrained from a variety hydrous fluid occurred during the Laramide. Concordant U-Pb zircon dates, of studies to have occurred in the 33–19 Ma interval. While this study interpreted to record fluid-induced recrystallization, are reported from includes one 22 Ma date, and several other studies reported emplacement- eclogite xenoliths in the ranges of 70–35 Ma (Smith et al., 2004) and 81–47 related secondary zircon and monazite in eclogite xenoliths with dates Ma (Usui et al., 2003), and from an ultramafic mantle garnetite xenolith between 35 and 28 Ma (Usui et al., 2003; Smith et al., 2004; Schulze et al., at 85–60 Ma (Smith and Griffin, 2005). The temporal overlap between 2015), all other dates from this study (96%) point to hydration well before zircon recrystallization in the mantle and monazite dates presented here emplacement of the host volcanics and regional laccolith intrusions like (Fig. 10) suggests that hydration extended from mantle depths into the those in the Abajo, Henry, and La Sal Mountains (30–20 Ma; Nelson et lower crust during the period of low-angle subduction of the Farallon al., 1992). Hydrous alteration features have now also been recognized in slab, as also hypothesized by Broadhurst (1986) and Smith et al. (2004). the microbreccia- and minette-hosted xenoliths (Selverstone et al., 1999), removing that basis of argument for very localized fluids. Potential Implications of a De-Densified Lower Crust The Yavapai-Mazatzal Province boundary is thought by many to pass through the Colorado Plateau, and Selverstone et al. (1999) suggested Hydrous fluids interacting with deep continental lithosphere can have that hydration of the xenoliths might be sourced from Paleoproterozoic profound influences on rheological and other physical properties, and mag- subduction, based upon systematic differences between the degree of matic patterns and compositions, as well as the dynamic evolution and ulti- alteration observed in xenoliths sourced from diatremes northwest and mate survivability of cratonic lithosphere. Those effects receiving the most southeast of their proposed trace of this tectonic boundary (Fig. 1). The attention with regard to Laramide-aged hydration of lithospheric mantle

recognition of Paleoproterozoic Mnz1 in the Red Mesa xenolith from this in the western United States have centered on the potential for increased study, notably associated with recrystallization of igneous allanite, might buoyancy to drive Cenozoic surface uplift and on how the process may

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have fertilized the mantle for subsequent large-volume ignimbrite erup- and pyroxene as well as plagioclase (Broadhurst, 1986; Selverstone et tions once the shallow slab was removed (e.g., Humphreys et al., 2003). al., 1999; Crowley et al., 2006; Butcher, 2013). For example, preliminary The full impact of these effects relies on the spatial extent of the hydra- estimates from another sample from the Selverstone et al. (1999) suite, tion process. Whereas this study focuses on only one xenolith locality, an extensively altered garnet amphibolite from Garnet Ridge, suggests and some have suggested that fluid alteration in the mantle beneath the that pre- and postalteration density differences may be as high as 0.07 Colorado Plateau was confined to serpentinized fracture zones (Smith et g/cm3. Depending on the thickness of the altered layer, these values for al., 2004), other considerations suggest that deep alteration was both verti- density change would predict isostatic surface uplift on the order of several cally and laterally extensive across much of the broader region. First, this hundred meters. A more comprehensive evaluation of a broader suite of and other studies of xenoliths from the NVF suggest that at least the lower crustal xenoliths from the Colorado Plateau awaits a future study. The de- 20 km of the crust and mantle to 140 km experienced hydrous alteration. densification mechanism for surface uplift is particularly attractive because Sample RM-21 is one of the shallower crustal xenoliths (alteration depth it provides an explanation for uplift of relatively undeformed regions of the at ~27 km from 0.75 GPa pressure calculation using the density profile of greater Rocky Mountains such as the Colorado Plateau and High Plains. Christensen and Mooney, 1995), and others record pressures as high as 1.2 GPa (Selverstone et al., 1999), corresponding to ~43 km depth, which is CONCLUSIONS near current estimates of crustal thickness (~45 km; Fig. 2B; Bashir et al., 2011; Gilbert, 2012; Shen et al., 2013). Xenolith petrology also requires Xenoliths from the Navajo volcanic field in the central Colorado Pla- that hydration of the mantle lithosphere occurred to depths as shallow as teau commonly exhibit distinct textures and secondary mineralogical 70 km, and the chemistry of the serpentinized ultramafic breccias requires assemblages reflecting high-pressure, low-temperature hydration of the a hydrated eruptive source, which means that hydration occurred to depths deep crust and lithospheric mantle sometime prior to the 30–20 Ma erup- greater than the ~140 km recorded by the deepest xenoliths (e.g., Smith, tion of the host volcanics. In line with several past studies of mantle xeno- 1995; Smith et al., 2004; Smith and Griffin, 2005; Li et al., 2008). liths from the field, a crustal xenolith from the Red Mesa diatreme reveals Second, xenolith studies along transects across the western United a period of monazite crystallization at 70–65 Ma. We interpret this Late States show predicted changes in metasomatizing fluids that are consis- Cretaceous monazite growth to reflect late alteration of the high-temper- tent with progressive west-to-east dehydration of a slab with increasing ature Proterozoic metamorphic assemblage with hydrous fluid introduced distance from the trench, and they link mantle metasomatism in the Sier- by the shallowly subducting Farallon slab. Similar to previous suggestions ran arc and Colorado Plateau to the same Farallon subduction system involving mantle hydration, the growth of low-density hydrous phases at (Lee, 2005; Li et al., 2008). Similarly, mantle xenoliths in the northern the expense of high-density, anhydrous minerals, which are abundant in Rocky Mountains of Montana show evidence for extensive metasomatism, unaltered Proterozoic crust, could have contributed to elevated topography some of which is thought to have occurred during the Laramide orogeny in the Colorado Plateau. If crustal hydration was sufficiently widespread, (Carlson et al., 2004; Downes et al., 2004; Facer et al., 2009). These data, this crustal de-densification mechanism could have contributed to regional supporting widespread mantle hydration, are also consistent with numeri- uplift across the Rocky Mountain region and High Plains. cal modeling studies that predict hydrous minerals in a subducting slab to persist to well over 1000 km inboard from the trench (e.g., Li et al., ACKNOWLEDGMENTS 2008; Currie and Beaumont, 2011). We are grateful to J. Selverstone for providing the sample RM-21 for this study, and to C. Jones (University of Colorado [CU]), G. Lang Farmer (CU), and D. Smith (University of Texas–Aus- The conversion of olivine to serpentine during hydration reduces the tin) for valuable discussions on these topics. Financial support for this project was provided density of peridotite (e.g., Hess, 1954), and this process has been proposed by the National Science Foundation grants EAR-1252295 and EAR-07464246 to Mahan, and EAR-1427626 to Mahan, Allaz, and Farmer for the acquisition of the new JEOL-8230 electron to have provided a regional source of buoyancy to drive surface uplift microprobe at CU–Boulder. Butcher also received several graduate student research grants of large tracts of the Rocky Mountain region during the Laramide orog- to support this work, including grants from the Colorado Scientific Society (Snyder Memorial eny (Humphreys et al., 2003; see also Schulze et al., 2015). Jones et al. Research Fund), Society of Exploration Geologists (SEG) (Jim and Ruth Harrison Scholarship, and Henry Bates Peacock Scholarship) and the CU Department of Geological Sciences (W.O. (2015) made a similar argument for crustal hydration as a potential buoy- Thompson Graduate Research Fund). M. Jercinovic and S. Regan (University of Massachusetts– ancy source, where hydration reactions consume denser crustal minerals Amherst) are thanked for early work on obtaining monazite analyses by electron microprobe. Axel Schmitt (formerly at the University of California–Los Angeles) is thanked greatly for help like garnet to produce lower-density secondary phases such as mica and in obtaining the ion probe data. The W.M. Keck Foundation Center for Isotope Geochemistry amphibole. That study emphasized that the lower-crustal seismic velocity at the University of California–Los Angeles (ion microprobe) is partly supported by a grant structure in the Rocky Mountains, from both active and passive sources, from the Instrumentation and Facilities Program, Division of Earth Sciences, National Science Foundation. Fritz Finger and an anonymous reviewer, as well as K. Stuewe as handling editor, and shear wave velocity gradients in the uppermost mantle are also consis- provided very helpful comments that significantly improved the manuscript. tent with regional and progressive northeastward hydration of crust. From variations in densities and density-velocity relationships in Montana and REFERENCES CITED Wyoming crustal xenoliths, this mechanism was shown to be capable of Anderson, J.L., 1983, Proterozoic anorogenic granite plutonism of North America, in Medaris, L.G., Jr., Byers, C.W., Mickelson, D.M., and Shanks, W.C., eds., Proterozoic Geology: producing more than the observed elevation differences between northern Selected Papers from an International Proterozoic Symposium: Geological Society of Montana and southern Wyoming. Similarly, three-dimensional (3-D) den- America Memoir 161, p. 133–154, doi:10.1130/MEM161-p133. sity models derived from regional seismic, heat-flow, and gravity data are Armstrong, R.L., 1969, K-Ar dating of laccolithic centers of the Colorado Plateau and vicinity: Geological Society of America Bulletin, v. 80, p. 2081–2086, doi:10 .1130/0016-7606 consistent with up to 900 m of surface uplift relative to the eastern Great (1969)80[2081:KDOLCO]2.0.CO;2. Plains due to variations in crustal density (Levandowski et al., 2014; W. Ault, A.K., Flowers, R.M., and Mahan, K.H., 2012, Quartz shielding of sub–10 μm zircons from radiation damage–enhanced Pb loss: An example from a metamorphosed mafic dike, Levandowski, 2016, personal commun.). northwestern Wyoming craton: Earth and Planetary Science Letters, v. 339–340, p. 57–66, The xenolith sample studied here records these types of hydration reac- doi:10.1016/j.epsl.2012.04.025. tions as having occurred during the Laramide orogeny. While this particular Bashir, L., Gao, S.S., Liu, K.H., and Mickus, K., 2011, Crustal structure and evolution beneath the Colorado Plateau and the southern Basin and Range: Results from receiver function xenolith is one of the least altered of the suite, and its calculated density and gravity studies: Geochemistry Geophysics Geosystems, v. 12, Q06008, doi:10 .1029 is not dramatically different before and after alteration (less than 0.01 g/ /2011GC003563. 3 Behr, W.M., and Smith, D., 2016, Deformation in the mantle wedge associated with Laramide cm ; Fig. 2C), many of the deeper and more mafic xenoliths record more flat-slab subduction: Geochemistry Geophysics Geosystems, v. 17, p. 2643–2660, doi: extensive hydration reactions, with textures indicating the loss of garnet 10.1002/2016GC006361.

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