Re-Thinking the Laramide: Investigating the Role of Fluids In
RE-THINKING THE LARAMIDE: INVESTIGATING THE ROLE OF FLUIDS IN
PRODUCING SURFACE UPLIFT USING XENOLITH MINERALOGY AND
GEOCHRONOLOGY
By
Lesley Ann Butcher
B.A., Brown University, 2010
A thesis submitted to the Faculty of the Graduate School of the
University of Colorado in partial fulfillment of the requirement for the degree of
Master of Science Department of Geological Sciences 2013
This thesis entitled:
Re-thinking the Laramide: Investigating the role of fluids in producing surface uplift using xenolith mineralogy and geochronology written by Lesley Ann Butcher has been approved for the Department of Geological Sciences
Dr. Kevin H. Mahan
Dr. Craig H. Jones
Date
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline.
iii
Butcher, Lesley Ann (M.S., Geological Sciences)
Re-thinking the Laramide: Investigating the role of fluids in producing surface uplift using xenolith mineralogy and geochronology
Thesis directed by Dr. Kevin H. Mahan
ABSTRACT
High-temperature, high-pressure mineral assemblages preserved in much of the North
American lithosphere owe their origins to Archean and Proterozoic tectonic processes. Whether subsequent mechanical, thermal, or chemical modification of ancient lithosphere affects overlying crust and the extent to which such processes contribute to anomalous deformation and topography in the interior of continents is poorly understood. This study addresses the occurrence and effects of hydration on continental crust in producing regionally elevated topography in the Colorado Plateau since the Late Cretaceous.
Mineralogical characteristics of two deep crustal xenoliths (GR-11 and RM-21) from the
Four Corners Volcanic field record varying degrees of hydrous alteration including extensive replacement of garnet by hornblende, secondary albite and phengite growth at the expense of primary plagioclase, and secondary monazite growth in association with fluid-related allanite and plagioclase breakdown. Results from forward petrological modeling for both deep crustal xenoliths are consistent with hydration at 20 km depth prior to exhumation in the ~20 Ma volcanic host. In situ Th/Pb dating provides evidence for a finite period of fluid-related monazite crystallization in xenolith RM-21 from 91 ± 2.8 Ma to 58 ± 4 Ma, concurrent with timing estimates of low-angle subduction of the Farallon slab.
Hydration-related reactions at depth lead to a net density decrease as low-density hydrous phases (hbl±ab±phg) grow at the expense of high-density, anhydrous minerals (gt±pl) abundant iv in unaltered Proterozoic crust. If these reactions are sufficiently pervasive and widespread, reductions in lower crustal density would provide a significant and quantifiable source of lithospheric buoyancy. Calculations for density decreases associated with extensive hydration recorded in xenolith GR-11 for an ~25 km thick crustal layer yield uplift estimates on the order of hundreds of meters associated with phase changes at depth. The results of this study substantiate the hypothesis that chemical alteration of lower continental crust by slab-derived fluids played a role in producing Laramide-related surface uplift of the Colorado Plateau and establishes chemical modification of continental lithosphere as a credible possibility for producing elevated regional topography in continental interiors. v
ACKNOWLEDGEMENTS
I would first of all like to thank Kevin Mahan for giving me the opportunity and encouragement to complete this work, for his help in keeping me focused on the task at hand, for his constructive criticism along the way, and for his enthusiasm towards his students and the scientific process. Thanks also to my other committee members: Lang Farmer for his keen interest in science and Craig Jones for his ability to keep me an honest and always critical researcher.
I would like to thank Julien Allaz for his ongoing patience and assistance in negotiating the electron microprobe and for his role as an enthusiastic and encouraging independent study advisor and thanks to Paul Boni for his help and expertise in the rock lab. I’d also like to acknowledge Joanne Brunetti, Barbara Easter, Marcia Kelly and Susan Pryor for their enduring efforts to keep me on track, organized, and sane. Thank you!
Additionally, I would like to extend gratitude to Rita Economos and Axel K. Schmitt at the W.M. Keck Center for Isotope Geochemistry at the University of California, Los Angeles for their expertise and tireless patience in helping me navigate my way as a geochronologist on the ion microprobe.
I am deeply grateful for the my hugely intelligent and witty office mates Justin Ball,
Melissa Bernardino, Danny Feucht, Will Levandowski, Colin O’Rourke, Will Yeck, and Dan
Zietlow. Their presence, support, and constant banter made me smile and carry on even in the most frustrating of times. Lastly, thank you to my family, Molly, Brian, Della and Eugene
Butcher, and to Ellen Wilcox and David Thuline for your tireless help, support, and encouragement. vi
CONTENTS ABSTRACT ...... iii ACKNOWLEDGEMENTS ...... v INTRODUCTION ...... 1 GEOLOGIC HISTORY AND XENOLITH BACKGROUND ...... 4
Geologic history of western North America ...... 4 Geologic history of the Navajo Volcanic Field, Colorado Plateau ...... 7 Results of prior xenolith studies ...... 8
ANALYTICAL METHODS ...... 15
QEMSCAN mineralogical mapping ...... 15 Electron microprobe ...... 16 Petrological modeling ...... 16 Monazite geochronology ...... 17 SEM analysis ...... 18
RESULTS ...... 20 Sample descriptions ...... 20 Pressure-temperature histories...... 26 Monazite geochronology ...... 31 Elevation calculations ...... 36
DISCUSSION ...... 40 Mineralogy ...... 40 Pressure-temperature histories...... 42 Monazite geochronology ...... 43 The case for the Farallon slab ...... 47 Elevation and density changes ...... 52 Timing of uplift ...... 53
CONCLUSIONS...... 59 REFERENCES ...... 60
vii
TABLES
Table 1. Petrologic models used in pseudosection analysis ...... 17 Table 2. Mineral compositions ...... 21 Table 3. Bulk compositions ...... 22 Table 4. Monazite age data ...... 33 viii
FIGURES
Figure 1. Map of hydrated xenolith localities ...... 7 Figure 2. Simplified modal mineralogy maps of RM-21 and GR-11 ...... 15 Figure 3. X-ray maps of Garnet 1 (RM-21) ...... 23 Figure 4. X-ray maps of Garnet 2 (RM-21) ...... 24 Figure 5. Graphs of elemental zoning patterns in garnet (RM-21) ...... 25 Figure 6. Pseudosection for RM-21 ...... 27 Figure 7. Peak assemblage pseudosection for RM-21 ...... 29 Figure 8. Secondary assemblage pseudosection for RM-21 ...... 30 Figure 9. BSE images of analyzed monazites ...... 32 Figure 10. Probability distribution functions of monazite dates ...... 35 Figure 11. BSE image of monazites and corresponding dates in Area B...... 36 Figure 12. Photomicrograph of Grt and Hbl (GR-11) ...... 37 Figure 13. Elevation change as a function of garnet consumption for GR-11 ...... 38 Figure 14. Summary diagram showing monazite dates and major geologic events in the western U.S...... 45 1
INTRODUCTION
The conventional notion of plate tectonics restricts deformation to plate boundaries and does not account for crustal modification within the interior of continents. A relationship between alteration of continental lithosphere and tectonic activity in overlying crust has been invoked to explain intracratonic deformation and uplift in several locations across the planet.
Areas of dramatically thinned lithosphere beneath the North China Craton are thought to result from chemical modification by migrating fluids (Xu et al., 2008; Chen et al., 2009), piecemeal lithospheric delamination has been proposed to explain localized deformation at the Andean
Eastern Cordillera—Altiplano boundary (McQuarrie et al., 2005) and a link between thermal perturbation of continental lithosphere as observed by large igneous province activity and kimberlite magmatism in the south African Plateau is suggested to be compatible with elevation gain in that region (Flowers et al., 2010). Similarly, post-Proterozoic chemical, thermal and/or mechanical alteration of North American lithosphere is a process often invoked to explain magmatic patterns, seismic velocity structures (7.x layer) and enigmatic uplift of the western
U.S. (Humphreys, 1995; Humphreys et al., 2003; Barnhardt et al., 2012; Gilbert, 2012).
Physical and chemical studies of xenoliths are powerful tools to reconstruct both the alteration history and processes involved in the modification of Archean and Proterozoic crust and mantle lithosphere. Available xenolith data from the Rocky Mountain region documents spatially significant fluid alteration of the continental mantle interpreted to be coincident with
Proterozoic accretion events in North America (Carlson et al., 1994; Lester et al., 1998; Downes et al., 2004; Farmer et al., 2005; Facer et al., 2009) and a more limited data set from the
Colorado Plateau records a period of mantle metasomatism in the Late Cretaceous (Usui et al., 2
2003; Smith et al., 2004; Smith and Griffin, 2005). Less attention has been paid to constraining the timing and extent of deep crustal hydration.
Fluid-related phase changes at depth lead to a net density decrease as low-density hydrous phases (hbl±ab±phg) replace the high-density, anhydrous minerals (gt±pl) that comprise ancient lower crust (Peacock, 1993); all mineral abbreviations following Whitney and Evans
(2010). If these reactions were sufficiently intense and widespread in the Late Cretaceous, reductions in lower crustal density would result in regionally extensive isostatic adjustments and could account for production of surface uplift in the Colorado Plateau. The results of this study address three questions of critical importance to this hypothesis: was the lower crust hydrated, when did this hydration occur, and how much topography could have resulted?
Detailed mineralogical analysis of two lower crustal xenoliths (RM-21 and GR-11) reveals variable degrees of hydration of the deep crust beneath the Colorado Plateau. Xenolith
RM-21 exhibits two distinct mineral assemblages reflecting limited hydrous alteration. Gt, bt, ms, kfs, and Ca-rich pl comprise the primary assemblage that likely represents peak metamorphic conditions. A secondary mineral assemblage recording limited hydrous alteration is characterized by breakdown of pl to phg, ab, and cc as well as fluid-related growth of monazite in associate with aln and pl breakdown. Petrological models for peak metamorphic minerals and for the secondary, hydrated mineral assemblage establish a history of decreasing temperature at relatively constant pressure for RM-21. Sample GR-11 is, on the other hand, pervasively altered.
Primary versus secondary assemblages are difficult to distinguish in this sample due to extensive replacement of gt by hbl and near-complete replacement of pl by secondary phg, cc and/or ab.
In situ Th/Pb ion microprobe dating of secondary monazites associated with hydrous mineral phases (ab+cc) in RM-21 was used to constrain the timing of deep crustal hydration. 3
Geochronologic results document monazite crystallization from the Paleoproterozoic to the
Miocene with a 56% majority of dates recording a finite period of lower crustal metasomatism in the Late Cretaceous. The influence of hydration on topography was addressed using the equation
for an extensively altered deep crustal garnet amphibolite xenolith
(GR-11). Isostatically driven surface uplift associated with garnet consumption in xenolith GR-
11 yields estimates on the order of hundreds of meters in an ~25 5km thick layer of pervasively altered crust.
The results of mineralogical analysis, petrological modeling, in situ geochronology, and elevation calculations establish a role for fluid-related modification of continental lithosphere beneath the Colorado Plateau and quantify its effects on uplift. Here I argue that metasomatism of the lower continental crust was integral to the production of regionally elevated surface topography in the Colorado Plateau since the Late Cretaceous.
4
GEOLOGIC HISTORY AND XENOLITH BACKGROUND
Geologic history of western North America
The deformation history and lithospheric evolution of the western U.S. involves a series of diverse tectonic events beginning with protracted Proterozoic accretion and culminating in enigmatic Cenozoic uplift. Lateral accretion of island arc and oceanic terranes to the margin of the Archean Wyoming Craton between 1.8 and 1.6 Ga created much of North America’s continental crust and mantle lithosphere, including the lithosphere beneath the Colorado Plateau
(Bowring and Karlstrom, 1990). This period of prolonged convergence established a number of distinct, NE-trending crustal provinces in the southwestern U.S., a majority of which lie above mantle lithosphere of corresponding age at present (Karlstrom and Bowring, 1993; Whittmeyer and Karlstrom 2007; Livacarri and Perry, 1993; Humphreys et al., 2003). Nd isotope work by
Bennett and DePaolo (1987) constrains the timing of juvenile crust formation in the Four
Corners region to between 1.7 and 1.9 Ga.
Crustal deformation and granulite-facies metamorphism in Four Corners closely followed amalgamation during accretion of the Yavapai and Mazatzal provinces to the continent between
1.75 and 1.6 Ga (Karlstrom and Bowring, 1988). Subsequent subduction of oceanic crust from
1.75 to 1.7 Ga accommodated convergence between these two island-arc provinces (Karlstrom and Bowring, 1988; Selverstone et al., 1999).
Granitic magmatism related to fluid and magmatic advection during asthenospheric upwelling beneath southern Laurentia pervasively metamorphosed Proterozoic crust ca. 1.4 Ga
(Wendlandt et al., 1993; Karlstrom et al., 1997; Crowley et al., 2006). 40Ar/39Ar dates of hornblende and muscovite by Karlstrom et al. (1997) and U-Pb zircon dates of lower crustal 5 xenoliths (Crowley et al., 2006) document metamorphism as occurring in four episodes from
1.42 and 1.36 Ga; variability in isotopic compositions of lower crustal xenoliths establish a link between magmatism and lower crustal metamorphic modification (Crowley et al., 2006).
Neoproterozoic to early Cambrian rifting established the present-day edge of the North
American craton and caused crustal deformation in the continental interior (Karlstrom and
Bowring, 1993). Extensional basins and northwest-striking fault networks in the Grand Canyon
Supergroup record two periods of intracratonic deformation at 1.1 and 700-800 Ma coincident with the breakup of Rodinia and initiation of the Cordilleran rift margin (Timmons et al., 2001).
A period of relative inactivity characterized by sedimentation along a passive continental margin lasted until the Late Devonian and was followed by continental shortening from the
Mississippian to the Jurassic (Dickinson, 2006). Successive episodes of contraction, subduction, and accretion caused the Antler, Sonoman, Nevadan, and Ancestral Rocky Mountain orogenies that advanced over established Archean and Proterozoic terranes and caused faulting, magmatism and uplift in Nevada, New Mexico, Colorado, and Utah (Speed and Sleep, 1982;
Karlstrom and Bowring, 1993; Humphreys et al., 2003; Dickinson 2006; Gilbert, 2012). A
Cordilleran magmatic arc spanning Arizona, California, and western Nevada, developed in the
Triassic possibly due to tectonic truncation of the continental margin in the late Paleozoic
(Busby-Spera, 1988; Busby-Spera et al., 1990; Dickinson, 2006). Thick depositional sequences of sedimentary and volcaniclastic rocks record intra-arc extension in these areas (Busby-Spera et al., 1990; Dickinson, 2006).
The most recent event causing ubiquitous deformation of the western U.S. was the ~80 to
55 Ma Laramide orogeny, which is linked to regionally-extensive elevation increases from near sea level in the Cretaceous to between 1 and 3.3 km at present from southern Canada and 6 northern Mexico and up to 1500 km inboard of the continental margin (Sahagian et al. 2002;
Jones et al. 2011). Arc magmatism ceased in the Sierra Nevada and propagated eastward to western South Dakota prior to the Laramide orogeny (Humphreys et al., 2003; Jones et al.,
2011); syn-Laramide magmatism was sparse and spatially limited mostly to the northeast striking
Colorado Mineral Belt (Christiansen and Yeats, 1992; Humphreys et al. 2003; Humphreys, 2005;
Jones et al. 2011). Subsequent Tertiary volcanism and extension in the Basin and Range and Rio
Grande Rift (Humphreys et al. 2003; Humphreys, 2005) generally did not affect the Colorado
Plateau, Southern Rocky Mountains, Wyoming Craton, and High Plains; these regions are therefore ideal study areas to investigate the enigmatic tectonic processes responsible for
Laramide-related uplift.
Low-angle subduction of the Farallon slab is one mechanism often invoked to explain the timing and extent of Cenozoic uplift and magmatism (Bird, 1984; Humphreys et al., 2003; Liu et al., 2010; Jones et al. 2011). A combination of several factors are thought to have caused a segment of the Farallon plate to subduct at a shallow angle during the Laramide including increased plate convergence between 80 and 55 Ma, an absolute upper plate motion of 4 cm/yr, subduction of an oceanic plateau (the Shatsky Rise), and/or viscous coupling between the slab and Archean keel beneath the Wyoming Craton (Engebretson et al. 1984; Jarrard 1986;
Humphreys 2003; Currie and Beaumont, 2011; Jones et al., 2011). The manner by which a flat slab produced one of the broadest orogens on earth is heavily debated and depends upon the nature of its interaction with overlying continental lithosphere.
An unresolved question related to Laramide tectonics is the timing of widespread surface uplift relative to the timing of low-angle subduction. While numerous studies address whether generation of regionally elevated topography in the Colorado Plateau and western U.S. occurred 7 post-low angle subduction i.e. in the mid- to late- Cenozoic (Bird, 1979; Sahagian et al., 2002;
Heller et al., 2003; Roy et al., 2009; Levander et al., 2011; Karlstrom et al., 2013) or was contemporaneous with flat slab subduction during the Late Cretaceous (Gregory and Chase,
1994; Parsons et al., 1995; Dettman and Lohmann, 2000; Pederson et al., 2002; Flowers et al.,
2008; Flowers and Farley, 2012), the issue remains controversial.
Geologic history of the Navajo Volcanic Field, Colorado Plateau
The Navajo Volcanic Field contains more than 50 Oligocene to Miocene volcanic centers in a >30,0002 km area of northeastern Arizona, southeastern Utah and northwestern New Mexico
(Laughlin et al., 1986; Mattie et al., 1997; Selverstone et al., 1999). Abundant crustal and mantle xenoliths are found in diatremes of two types: ultrapotassic minettes and ultramafic breccias
(Roden, 1981). Minettes are widely distributed across the Navajo Volcanic Field and likely formed by small degrees (<1%) of partial melting of metasomatized mantle (Roden, 1981; 8
Delaney, 1987). Serpentinized ultramafic breccias, on the other hand, consist of comminuted mantle and crustal material generated during minette intrusion into cool, hydrated mantle and are mostly localized to the Defiance, Hogback, and Comb Ridge monoclines (Roden, 1981; Delaney
1987; Smith, 1995; Selverstone et al., 1999). The timing of diatreme eruption in the Navajo
Volcanic Field was initially estimated at 22.8 to 45.0 Ma, with most dates between 28 and 35
Ma, by apatite fission-track dating of minettes (Naeser, 1971). More recent K-Ar ages assign emplacement ages of lamprophyre dikes to between 28 and 19 Ma (Laughlin et al., 1986).
Results of prior xenolith studies
Mineral textures, mineral assemblages, and age constraints from xenoliths provide direct evidence for both regionally extensive mantle metasomatism coincident with Proterozoic accretion events in the Rocky Mountain Region and for a finite period of fluid introduction into the mantle long after Proterozoic subduction beneath Montana, the Colorado Plateau and High
Plains. Figure 1 illustrates the locations of hydrated mantle xenoliths and associated constraints on the timing of metasomatism. Limited attention has been paid to the extent and timing of metasomatism in the lower crust notwithstanding a study by Selverstone et al. (1999), which advocates for Proterozoic deep crustal hydration in the northwestern Colorado Plateau based upon variable degrees of fluid-related xenolith alteration across the Proterozoic Yavapai-
Mazatzal lithospheric boundary.
Montana and Wyoming, Northern Rockies
Hydration of Archean mantle lithosphere has been extensively documented in the northern Rocky Mountain region by mineralogical and trace element studies (Eggler, 1987;
Carlson and Irving, 1994; Lester and Farmer, 1998; Carlson et al., 2004; Downes et al., 2004; 9
Hearn, 2004; Farmer et al., 2005; Mirnejad et al., 2008; Facer et al., 2009). Elemental and isotopic compositions of mantle xenoliths from Montana and the Wyoming Craton are interpreted to reflect a prolonged, episodic history of melt removal and associated major-element depletion followed by multiple, more recent metasomatic enrichment events in the mid-
Proterozoic (Carlson and Irving, 1994; Downes et al., 2004; Hearn, 2004) and Mesozoic or later
(Carlson et al., 1994). Evidence for mantle metasomatism extends geographically into the southernmost Wyoming Craton and northernmost Colorado, although the timing of this metasomatism is not well constrained (Eggler, 1987; Lester and Farmer, 1998; Carlson et al.,
2004; Farmer et al., 2005; Mirnejad et al., 2008).
Mineralogical and trace element evidence from xenolith suites in Montana are interpreted to reflect recent (i.e. post-Proterozoic) metasomatism. Hearn (2004) interprets the presence of tectonized or undeformed orthopyroxenite, clinopyroxenite and websterite veins in mantle xenoliths from the Homestead kimberlite to reflect an enrichment event post-dating ancient major element depletion and suggests that phlogopite-bearing veins represent kimberlite related fluid-addition or an K-rich metasomatic event (Hearn, 2004).
Mineral characteristics of mantle xenoliths from the Bearpaw Mountains, Montana also indicate mantle-fluid interactions. Downes et al. (2004) suggest that the presence of undeformed phlogopite in tectonized peridotites reflects phlogopite introduction, recrystallization, or redistribution after Proterozoic tectonism but before incorporation into the host minette and further suggests that phlogopite introduction occurred in the early Tertiary (Downes et al., 2004).
Moderative to extensive in situ serpentinization, talc-chlorite mineral assemblages, and fibrous textures in orthopyroxene observed in mantle dunite xenoliths from the Bearpaw Mountains lead 10
Facer et al. (2009) to advocate for interaction between the mantle and a fluid component suggested to be derived from the Farallon plate as it collided with the cratonic keel at ~55 Ma.
Additional evidence for enrichment via mantle fluid interaction comes from xenolith trace element characteristics. Glimmerite veins in peridotite xenoliths from the Bearpaw
Mountains exhibit Sr depletion coupled with LREE, fluid-mobile element (Cs, Rb, U, Pb) and trace element enrichment and suggest metasomatism by a subduction-related fluid (Downes et al., 2004).
Direct constraints on the timing of metasomatism indicate fluid enrichment events in the mid-Proterozoic (Carlson and Irving, 1994; Downes et al., 2004) and during or post-dating the
Mesozoic (Carlson et al., 2004). Glimmerite veins in harzburgites from the Highwood
Mountains, Montana define mid-Proterozoic Pb-Pb and Sm-Nd isochron ages interpreted to reflect either timing of formation or incompatible element enrichment by interaction with migrating fluids and/or melts (Carlson and Irving, 1994). U/Pb monazite data from a glimmerite vein in one sample yields a date of 1.8 Ga, which may reflect Proterozoic metasomatic enrichment from Archean crustal materials (Carlson and Irving, 1994). Additional evidence for mid-Proterozoic enrichment comes from a high 87Sr/ 86Sr ratio in phlogopite from a websterite xenolith from the Bearpaw Mountains, Montana which requires ~1.2 Ga to evolve to its current value (Downes et al., 2004).
On the other hand, Sm-Nd tie lines of xenoliths from the Williams and Homestead kimberlites, Montana provide Mesozoic or younger ages (Carlson et al., 2004). Carlson et al.
(2004) suggest that these age constraints, when interpreted in conjunction with trace element characteristics and Sr and Nd isotopic compositions of Williams and Homestead xenoliths, may reflect either internal melting and melt migration of ancient metasomatized mantle or 11 introduction of fluids and hydrous melts by the Farallon slab in the Late Cretaceous to Early
Tertiary.
Trace element characteristics reflecting metasomatism persist geographically into the southernmost Wyoming Craton. Distinctive Nd and Sr isotopic signatures, negative Ti, Ta and positive Pb anomalies, and enrichment in LILE in ultramafic to felsic crustal xenoliths from 1.0
Ma ultrapotassic volcanic rocks at Leucite Hills suggest hydrous metamorphism of ~2.6 Ga igneous protoliths in an arc-like setting (Farmer et al., 2005; Mirnejad et al., 2008). An abundance of hydrous minerals, lack of garnet and LREE and HREE enrichment suggests that the mantle beneath the WYC was re-enriched by hydrous, LIL- LREE-enriched fluids; the timing of this event is not constrained (Lester and Farmer, 1998).
Xenoliths from the Devonian State Line kimberlites, northern Colorado, record zircon crystallization at 400-600 Ma, 1100-1800 Ma, 1340 to 1380 Ma which may indicate crustal growth and/or metamorphic overprinting (Farmer et al., 2005). LREE enrichment in peridotite mantle xenoliths from the same locality reflects cryptic metasomatism that may be related to arc- related hydration and/or melt extraction (Eggler, 1987; Lester et al., 1998; Farmer et al., 2005).
The timing of this metasomatism is also unconstrained.
Eastern Kansas, High Plains
Work by Brookins et al. (1970, 1971, and 1974) on the Stockdale kimberlites in Riley
County, western Kansas reveal hydrous alteration of both xenoliths and emplacement volcanics in the High Plains. Mineral assemblages in xenoliths from above (pyroxenites, vermiculitized chlorites and granulites) and below (symplectites and lherzolite eclogites) the Moho demonstrate extensive alteration by which serpentine (lizardite and antigorite) and carbonates replace anhydrous phases (Brookins and Meyer, 1974). The development of micas and magnetite along 12 cleavage planes and subsequent kinking/breakage led Brookins and Meyer (1974) to suggest that this alteration occurred at depth and during dynamic conditions. Altered phlogopite in several xenoliths yield pre-emplacement ages of 380 +/- 40 Ma for six of the seven samples indicative of pre-Laramide alteration (Brookins and Meyer, 1974).
The Stockdale ultramafic micaceous kimberlites are also extensively altered with over
90% of kimberlites containing abundant serpentine and calcite (Brookins, 1970). Brookins
(1970) suggests a maximum age of kimberlite emplacement at 100 +/- 20 Ma. This timing may be coincident with catalytic interactions between the lower crust and Farallon slab-derived fluids as suggested by Currie and Beaumont (2011).
Colorado Plateau, Southern Rockies
Mineral assemblages in mantle and lower crustal xenoliths from the Colorado Plateau preserve evidence of low temperature, high pressure hydration (Broadhurst et al., 1986;
Wendlandt et al., 1993; Selverstone et al., 1999; Smith and Griffin, 2005; Sommer et al., 2008).
Mantle xenoliths (protolith ages ~1.8 Ga) from the Garnet Ridge diatreme are comprised of > 90
% garnet and accessory amounts of rt, ilm, chl, cpx, and zrn. Smith and Griffin (2005) suggest that the bulk composition and mineral textures of these garnetite xenoliths record growth in high pressure, low temperature metasomatic reaction zones at the contact of serpentinized oceanic crust and continental mantle lithosphere. Metasomatism at similar pressure and temperature conditions is documented by thermobarometry of mantle xenoliths with anomalously high water contents (Lee, 2005). Two pyroxene thermometry and Al in opx barometry reveal source depths between 70 to 140 km and source temperatures between 950° and 1200°C, suggesting that mantle hydration beneath the Plateau is pervasive in vertical extent rather than localized to a narrow zone (Lee, 2005). 13
Lower crustal metasomatism has also been observed in the Colorado Plateau. Broadhurst et al. (1986) describe the hydration reaction pl = zo + pg + qz + ab in granulite xenoliths from the Moses Rock Diatreme, Utah. Selverstone et al. (1999) attribute varying degrees of lower crustal hydration to geographic distribution across the lithospheric Yavapai-Mazatzal boundary; xenoliths sourced from diatremes to the south of the inferred boundary are unaltered whereas those exhumed to the northwest of the boundary display minor to pervasive hydrous alteration
(Selverstone et al., 1999). Mineralogical characteristics consistent with metasomatism of altered xenoliths include hydrous secondary mineral assemblages, plagioclase zoning, and garnet breakdown to hornblende (Selverstone et al., 1999). In light of the striking differences in the degree of hydration across the inferred Yavapai-Mazatzal boundary, the authors suggest that lower crustal metasomatism was coincident with Proterozoic, rather than Cretaceous, subduction
(Selverstone et al., 1999).
Further mineralogical evidence for hydration comes from Sommer et al. (2008) who relate excess opx and chlorite to the percolation of silica-rich fluids and subsequent precipitation of silica-rich minerals at depth. Trace element systematics reflecting metasomatism of upper and lower crustal xenoliths, sourced from depth between 20 km and the interpreted seismic Moho
(~43 km), include variable shifts in Sm/Nd isotope ratios, LREE enrichment and high SiO2 and
Na2O contents (Wendlandt et al., 1993). Nd isotopes of these xenoliths yield model ages between
1.63 and 1.98 Ga (Wendlandt et al., 1993). While protolith ages of lower crustal xenoliths are constrained, no geochronologic constraints on the timing of lower crustal metasomatism have existed prior to this study. On the other hand, numerous lines of geochronologic evidence point to hydration of the lithospheric mantle beneath the Colorado Plateau in both the Proterozoic and
Late Cretaceous. 14
U/Pb zircon dating demonstrates pervasive mantle hydration beneath the Colorado
Plateau coincident with Proterozoic accretion and Cretaceous subduction (Usui et al., 2003;
Smith et al., 2004; Smith and Griffin, 2005). Zircon dates from eclogite xenoliths reported by
Smith et al. (2004) reveal a mid-Proterozoic zircon component for all samples in addition to nearly concordant dates from 35 to 41 Ma in a fraction of the eclogites (Smith et al., 2004). The authors attribute Late Cretaceous recrystallization ages to the catalytic effects of water introduced by the Farallon slab which they speculate remained in place long enough for mantle fractures extending tens of kilometers into the wedge to dehydrate (Smith et al., 2004).
Additional U/Pb zircon work by Smith and Griffin (2005) reveals concordant dates from 60 to 85
Ma in eclogites with two conspicuous clusters at 70 and 85 Ma (Smith and Griffin, 2005).
Another set of Cretaceous U/Pb zircon dates (81-33 Ma) were reported in eclogite xenoliths by
Usui et al. (2003). The authors describe core to rim dates of 54 and 34 Ma on a single micrometer scale zircon (Usui et at., 2003). This observation is interpreted to reflect both prolonged recrystallization over a finite period of 20 m.y. and a relationship between Farallon subduction and zircon growth (Usui et al., 2003).
The results of this study establish a finite period of post-Proterozoic lower crustal metasomatism beneath the Colorado Plateau. In situ Th/Pb ion microprobe dating of secondary monazites in association with a hydrated mineral assemblage in the deep crustal xenolith RM-21 from the Navajo Volcanic Field yields a 56% majority of monazite crystallization dates in the
Late Cretaceous, which is consistent with interactions with fluids sourced by a shallowly subducting Farallon slab. 15
ANALYTICAL METHODS
QEMSCAN mineralogical mapping
Modal mineralogy was determined using quantitative automated scanning electron microscope analysis (QEMSCAN) at the Advanced Mineralogy Research Center, Colorado
School of Mines. QEMSCAN is a scanning electron microscope (SEM) based analytical method that acquires mineralogical and textural data by collecting calibrated backscatter electron (BSE) intensity information and energy dispersive x-ray spectrometer (EDS) spectra at each point in a thin section to identify and map mineral phases based upon set elemental BSE and intensity values (Hoal et al, 2009). Full modal mineralogy maps of RM-21 and GR-11 at 20 um step size were created as well as two additional maps for RM-21: an accessory mineral map of baddelyite, Fig 4. QEMSCAN thin section maps showing simplified modal mineralogy and textural characteristics of samples RM-21 and GR-11
a) RM-21 b) GR-11
Quartz Muscovite Biotite Plagioclase K-feldspar Hornblende Garnet Apatite Zircon Sulphides Fe-/Ti- oxides Others
2000 µm
Figure 2. QEMSCAN thin section maps showing modal mineralogy and textures of samples RM-21 and GR-11. 16 zircon, and monazite at 4 um step size and a high-resolution map of an ~6 x 6 mm area (Area 1) at 4 um step size (Figure 2).
Electron microprobe analyses
Samples were analyzed with the JEOL 8600 electron microprobe at the University of
Colorado, Boulder using natural and synthetic standards. A focused beam of ~1 μm was used for the majority of mineral analyses with the exception of mica, feldspar, amphibole, and calcite where a beam spot size of 5-10 μm was used to reduce volatilization of Na, Ca, and K. The beam current was fixed at 20 nA with an accelerating voltage of 15kV. Numerous analyses were made in pl, gt, and biotite grains to quantify chemical zoning and to ensure determination of representative compositions. X-ray maps of gt and pl were obtained to further identify zoning patterns.
Petrological Modeling
Forward petrological modeling of mineral assemblages was performed using the program
Perple_X 6.6.8 (Connolly, 2005) and the internally consistent thermodynamic database of
Holland and Powell (2002). Pseudosections, or equilibrium phase assemblage diagrams, define the mineral assemblage, mineral composition and mineral proportions for a given bulk composition at any point in P-T space. Pseudosections were calculated using the
MnNCKFMASHT system for fluid (H2O-CO2)-saturated conditions for water activities of 0.99 and 0.95 for sample RM-21 and under fluid (H2O)-saturated conditions for sample GR-11.
Solution models and phases used are listed in Table 1. Bulk compositions employed in these 17 models were determined by combining chemical compositions of major and minor phases from electron microprobe analyses and modal mineralogy assessments by QEMSCAN.
Monazite geochronology
Monazite grains were analyzed in-situ on the CAMECA ims 1270 ion microprobe at the
W.M. Keck Foundation Center for Isotope Geochemistry at the University of California, Los
Angeles. Thin sections were cut with a high-precision diamond saw or drilled with a diamond core, cleaned, mounted in epoxy with a pre-polished block of standards, and gold-coated with
100 Å of Au. SEM images were taken prior to analysis to identify dateable grains free of visible inclusions, bubbles, or alteration. Moderate- (554) and high-Th (Trebilcock) standards were used to obtain a curve of Th02/Th versus Pb/Th relative sensitivity factor before analysis and were re- analyzed after every 5-6 unknowns during data collection to evaluate changes in instrument calibration. Standard operating procedures as outlined by Harrison et al. (1995) were followed including use of an O- primary beam focused to 10-20 μm, 15 eV offset for 232Th, and a mass resolving power of 4500. High mass resolving power is necessary to distinguish between Pb and
Th isotope peaks and significant molecular interferences, particularly the PrPO2 interference at mass 204 and NdPO2 interference at mass 208. The field aperture was adjusted to within 5 microns to ensure that the instrument was sampling the dominant U and Th signals from the 18 target grain, even if the pit encompassed adjacent minerals, and to minimize sampling of adjacent common Pb domains. The small size (<15 μm diameter) of monazites only permitted one analysis per grain. Instrument precision is limited to ~2% based upon the reproducibility of the calibration curve and is most often affected by bad polish, compositional variability within a standard, or environmental conditions (i.e. 208Pb surface contamination) (Harrison et al. 1995).
The secondary ion mass spectrometer is a high-sensitivity, high-resolution instrument ideally suited for in situ dating of small monazite grains. An ion microprobe takes advantage of the kinetic energy distribution of U+, Th+, and Pb+ ions sputtered off the sample surface using a primary ion beam and subsequently measured by mass spectrometry. Flooding by primary O- ions increases the ion efficiency of Pb isotopes, all of which ionize with a similar efficiency of
1%, and preferentially charges ions of interest (Pb+, Th+, U+) with the opposite polarity. A
208 232 linear relationship exists between Pb/ Th and ThO2/Th for monazites with a uniform Th/Pb ratio because Pb behaves like ThO. Ionization efficiencies calculated for a standard are used to determine a correction factor by dividing the measured 208Pb/232Th of a standard grain at a reference ThO2/Th value by its known daughter-to-parent ratio. This relative sensitivity factor is in turn applied to determine the 208Pb/232Th age of an unknown under identical experimental conditions (Harrison et al., 1995).
SEM analysis
A necessary step to ensure the integrity of monazite age data is qualitative SEM analysis before and after dating. SEM images were taken prior to analysis to identify dateable grains free of visible inclusions, bubbles, or alteration. Determining whether microprobe pits in individual monazite grains overlap with other Th-bearing phases (aln in particular) or adjacent Pb domains 19 is of critical importance to substantiate the accuracy of dates in the unknown. SEM images of microprobe spots were taken in Areas A, B, and C; the remaining Areas will be the subject of future evaluation. 20
RESULTS
Sample Descriptions
Samples RM-21 and GR-11 were collected by Selverstone et al. (1999) from serpentinized ultramafic microbreccia diatremes along the Comb Ridge and Defiance monoclines in the Navajo Volcanic Field, Four Corners Region.
RM-21: Garnet biotite gneiss
Sample RM-21 is a felsic garnet biotite gneiss with a primary metamorphic mineral assemblage of >80% qtz + pl + kfs in addition to mu + gt + bt and minor amounts of ap, zr, al, ilm, and rt. Preferential alignment of bt grains and, to a lesser extent, mu defines a plane of weak foliation. A secondary mineral assemblage of cc + ab + phg + minor amounts of clay minerals
(>20 oxide wt % H2O), mnz, and rt records limited hydrous alteration at high pressure. Minerals comprising the primary assemblage (qtz + kfs + bt + gt + mu + pl) have been affected to varying degrees by this hydrous alteration. Mineral compositions are listed in Table 2; bulk compositions are listed in Table 3.
Quartz in RM-21 is extremely well preserved and unaltered despite the effects of fluids, comprises >45 volume %, and occurs primarily as a matrix mineral but also as inclusions in gt.
Kfs makes up 26% of RM-21 and is internally unchanged by the effects of hydrous alteration.
Some kfs grains, however, do exhibit resorption along grain boundaries that are in contact with altered pl. Biotite grains display well-preserved cleavage and range in size from < 50 to ~
1000μm with an average dimension of ~400 by 100 μm. Biotite intergrown with pl and/or kfs often occurs in association with primary mu and the vast majority of bt grains are unaltered, with the exception of a single identified grain being replaced by secondary ilm. 21
Table 2. Representative Mineral Compositions for Colorado Plateau Xenoliths RM-21 Oxide wt% Gt I (core) Gt II (rim) Bt Mu I Mu II Kfs Pl I Pl II Pl III SiO2 35.57 36.07 33.60 45.02 44.40 63.50 63.76 65.41 66.48 TiO2 0.00 0.01 3.08 0.25 0.01 0.02 - - - Al2O3 20.33 19.45 15.94 32.93 37.37 18.54 23.45 20.91 19.73 FeO 34.18 32.83 30.59 4.24 1.51 0.00 0.02 0.01 0.00 MgO 1.29 0.63 4.14 0.77 0.15 0.00 0.00 0.00 0.00 MnO 2.36 3.46 0.01 0.00 0.00 0.01 0.00 0.00 0.04 CaO 6.69 5.92 0.02 0.07 0.04 0.02 4.37 1.94 0.68 Na2O <0.01 0.01 0.03 0.29 0.38 0.54 9.06 10.54 11.10 K20 - - 9.81 11.27 11.23 16.27 0.12 0.07 0.06 Total 100.42 98.67 97.33 94.89 95.09 98.90 100.78 98.88 98.31
End-members Mg# 0.06 0.03 0.19 0.24 0.15 - - - - Alm 0.72 0.73 ------Pyr 0.05 0.03 ------Sps 0.05 0.08 ------Grs 0.18 0.17 ------An - - - - 0.00 0.21 0.09 0.03 Ab - - - - 0.06 0.78 0.90 0.97 Kfs - - - - 0.94 0.01 0.00 0.00
GR-11 Oxide wt% Gt I Bt Mu Pl Ab Amph SiO2 38.27 37.94 46.90 64.19 67.67 44.75 TiO2 0.06 1.73 0.10 - - 0.27 Al2O3 20.69 16.01 31.02 22.23 19.67 10.73 FeO 27.23 19.60 3.57 0.13 0.13 19.08 MgO 5.83 10.78 1.90 <0.01 0.00 8.85 MnO 1.20 0.13 <0.01 0.05 <0.01 0.25 CaO 6.06 0.01 0.03 3.05 0.21 8.44 Na2O 0.02 0.24 0.52 9.89 11.59 3.46 K20 - 9.47 9.90 0.10 0.03 0.41 Total 99.36 95.92 93.93 99.63 99.30 96.24
End-members Mg# 0.28 0.50 0.49 - - - Alm 0.58 - - - - Pyr 0.22 - - - - Sps 0.03 - - - - Grs 0.05 - - - - An - - 0.14 0.01 - Ab - - 0.85 0.99 - Kfs - - 0.01 0.00 -
Garnet in RM-21 is subhedral and contains qtz inclusions. The largest garnet grain is
~1200 by 1600 μm, the smallest is ~400 by 200 μm. X-ray maps and quantitative transects performed on two garnets reveal minor zoning around un-zoned cores (Figures 3,4). Calcium and magnesium contents show a decrease from core to rim whereas manganese and iron record an increase. Grossular contents in Garnet 1 show a slight decrease from core to rim (Xgr = 0.18 and
.15) in Garnet 1 whereas no consistent calcium zonation occurs in Garnet 2. Mg contents are 22
Table 3. Major element compositions calculated from modal proportions and mineral compositions Sample RM-21* GR-11*! SiO2 78.70 45.65 TiO2 0.13 2.64 Al2O3 11.10 14.07 FeO 1.68 18.01 MgO 0.19 6.25 MnO 0.04 0.35 CaO 0.88 7.54 Na2O 2.34 3.28 K20 4.94 0.99 P2PO5 <0.00 1.22 Total 100.00 100.00
*Includes modes and compositions of alteration phases ! Includes Ca contribution from modal apatite
lower along garnet rims in both Garnets 1 and 2, with a similar core to rim decrease of Xpy = 0.06
– 0.03. Manganese zoning in both garnets show a distinct Mn “kick-up” at the rims (as described by Kohn and Spear (2000)) with Xsp core to rim increases of 0.04 – 0.07 in Garnet 1 and 0.05 –
0.06 in Garnet 2. Less coherent, mineral zoning of Fe is also observed: Xalm core to rim contents increase from 0.72 – 0.74 whereas Xalm in Gt 2 increases from 0.73 to 0.75 with two outlying core values at 0.80 and 0.79. Magnesium numbers in both garnets decrease moderately from core to rim. Mg# decreases from 0.08 to 0.05 in Garnet 1 and from 0.08 to 0.04 in Garnet 2.
Elemental zoning patterns are plotted in Figure 5. Selverstone et al. (1999) similarly describe unzoned garnet cores in RM-21 but do report almandine zoning with Xalm increases of 0.03-0.05 in rims adjacent to matrix biotite.
Primary muscovite grains adjacent to matrix qtz display partial cleavage and are least likely to have been replaced by other minerals. Primary mu grains in association with pl, on the other hand, display a patchy texture as a result of mineral replacement by qtz and/or pl. Primary mu is easily distinguished from mu associated with the secondary, hydrated assemblage by grain size and occurrence. Primary mu grains are generally ~100 μm in their longest dimension and 23 occur in association with matrix minerals. Secondary mu grains, on the other hand, occur almost exclusively in altered pl as small (< 20 μm) euhedral grains with obvious cleavage. These two generations of white mica are also chemically distinct. Si contents for primary muscovite in RM-
21 varies from 3.07 to 3.14 per formula unit. Secondary phengite (Phengite II) is characterized by a representative Si content of 3.29 per formula unit, with end composition outliers 2.98 to
3.42 per formula unit.
The effects of hydrous alteration on RM-21 are especially apparent and complicated in pl.
Phg, ab, and cc ubiquitously occur as secondary phases in altered pl grains and variable pl compositions reflect the effects of fluid alteration. Three generations of pl comprising 3.5, 16, and 4 volume % of RM-21, respectively, were recognized based upon an contents and occurrence. Un-zoned pl cores are characterized by the highest an contents (X = 15-22) and Figure 7 An
a) Mg b) Mn
4 1 5 2 6 3 7 8 9 10
c) Ca d) Al
Figure 3. a) Mg b) Mn c) Ca and d) Al x-ray maps showing zoning in Garnet 1 (RM-21) 24 likely represent Ca contents during peak metamorphic conditions. A generation of intermediate composition (XAn = 7-12) pl was observed in areas of concentrated alteration within unzoned cores. The lowest an contents occurred on rims (XAn = ≤1-3) of ~30% of pl grains. These observations are comparable to those of Selverstone et al. (1999) who identify the first two generations of pl described above --- unzoned cores (XAn = 15-17) and concentrated zones of patchy, more albitic pl (XAn = 0-5). Figure 8
a) Mg 1 b) Mn 2 3 4 5 6 7 89 10 11
300 300
c) Ca d) Al
300 300
Figure 4. a) Mg b) Mn c) Ca and d) Al x-ray maps showing zoning in Garnet 2 (RM-21)
Growth of new minerals, including cc and minor amounts of clay minerals (>20 oxide wt *"!!#
% H2O), rt, and mnz accompanied)"!!# hydrous alteration. Calcite in RM-21 comprises 0.35 volume
% and occurs both as small (< 10 μm), euhedral grains in altered pl or as larger (≤ 200 μm), anhedral grains adjacent to, or contained within, altered pl. Clay minerals occur along grain 25 boundaries between feldspar grains or in breaking down aln grains in close spatial association to qtz and/or cc. Rt is identified in two breaking down aln grains as small (~10 μm) grains in association with cc, mnz, clay, and qtz. Mnz occurs as a secondary phase in RM-21 in association with either aln or pl. The majority of mnz grains are small (~ 5 μm) and the largest grain is 20 μm in diameter. Mnz in association with pl or in association with a large (~200 μm) aln grain is the most euhedral, whereas grains in association with breaking down aln are consistently subhedral and display a patchy, matted texture that may represent multiple recrystallization events over time.
GR-11: Garnet amphibolite
Sample GR-11 is a gt amphibolite schist composed of hbl (>50%) + gt + mu + pl + ab +
Figure 5. Graphs showing a) pyrope, b) spessartine, c) grossular and d) almandine contents along transects in Garnets 1 and 2 a b Grt 1 Grt 1 0.08 0.07 Grt 2 Grt 2
0.06 0.07 0.05
0.04
)
y )
P 0.06
p
(
S X
0.03 ( X
0.02 0.05
0.01
0.00 0.04 0 2 4 6 8 10 0 2 4 6 8 10 Transect Point Number Transect Point Number
c Grt 1 d Grt 1
Grt 2 Grt 2 0.18 0.80 0.79 0.78 0.17 0.77
0.76
)
l
)
r
A
( G
( 0.75
0.16 X X 0.74 0.73 0.15 0.72 0.71
0.14 0.70 0 2 4 6 8 10 0 2 4 6 8 10 Transect Point Number Transect Point Number 26
Fe-/Ti-oxides + ap and minor amounts of bt, ilm, and rt. Ca x-ray mapping by electron microprobe reveals almost complete breakdown of pl to ab and png. Quantitative analysis reveals a range of compositions (XAn = 7 and 14) for less-altered, Ca-rich pl; XAn contents of ≤1 characterize secondary ab. All mu in GR-11 is retrograde and contained within altered pl as grains with distinct cleavage. Si contents span a wide range (contents from 3.04 to 3.19 are found in a single grain) with an average composition of 3.12 per formula unit. Gt is partially to extensively replaced by hbl and, with theexception of partial rims identified on two grains, displays no zoning. An Xpy decrease of 0.05 and 0.09 and Xalm increase of 0.02 and 0.01 are observed from core to rim in these two garnets.
Pressure-Temperature Histories
Petrological models generated by Perple_X 6.6.8 (Connolly, 2008) were used to determine pressure and temperature conditions of primary and secondary assemblages under fluid (H2O-CO2)-saturated conditions (water activity of 0.99) for RM-21 and for fluid (H2O)- saturated conditions in GR-11. A P-T history for RM-21 was previously constrained by
Selverstone et al. (1999) using typical thermobarometric techniques. Selverstone et al. (1999) report peak conditions of 675°C and 6.5 kbar using the Gt-Bt-Fe-Mg exchange reaction thermometer and the Gt-Pl-Bt-Mu net transfer barometer and conditions of 7-9 kbar at 400-
600°C for the secondary assemblage phg + ab + (c)zo + qtz using phg barometry as outlined by
Massone and Schreyer (1987).
27
RM-21: Primary assemblage
A combination of Si contents of primary mu and an compositions for first generation plagioclase were used to constrain equilibrium pressure and temperatures for the primary assemblage in RM-21. Si in phengite is a widely used geobarometer because a strong, almost
Figure 6. Pseudosection for RM-21 showing mineral stability fields and mineral in/out reactions for fluid (H2O-CO2)-saturated conditions with a water activity=0.99 28 linear increase of Si content per formula unit occurs with pressure whereas only a moderate decrease occurs with temperature (Massone and Schreyer, 1987). The majority of Si compositions in primary mu fall between 3.07 and 3.13 Si per formula unit. Averaging the Si content of these grains for analyses with totals > 94 oxide weight % yields a representative composition of 3.09 Si per formula unit. Selverstone et al. (1999) report a similar representative
Si content in primary mu of 3.13 Si per formula unit. Compositions of unzoned plagioclase cores record an contents at peak conditions and Xan isopleths were used in pseudosection analysis as a second indicator of primary pressure conditions. A representative Xan composition of 0.20 was determined by averaging contents of Pl I for analyses with totals between 98 and
102. Assuming fluid-saturated conditions (water activity = 0.99), peak pressure-temperature conditions of 690°C at 7 kbar and were defined by the intersection of Xan = 0.2 and Si pfu = 3.1 isopleths. This estimate closely approximates P-T calculations for the primary assemblage of
675°C and 6.5 kbar using typical thermobarometric techniques (Selverstone et al., 1999). A pseudosection showing mineral stability fields for RM-21 is showing in Figure 6; Figure 7 illustrates pressure and temperature constraints for the primary assemblage.
RM-21: Secondary assemblage
Definitive constraints on pressure conditions of the secondary, hydrated assemblage were determined using Si in phengite compositions and volume percent of kfs. A representative Si content for Phengite II of 3.28 pfu was established by averaging Si compositions between 3.26 and 3.29 pfu that made up the majority of analyses. Selverstone et al. (1999) report a similar Si composition for secondary phengite of 3.29 pfu. A volume percent of 26 for K-feldspar was determined using QEMSCAN mineralogical mapping in sample RM-21. Modal proportions of kfs decrease with increasing pressure under fluid-saturated conditions according to the reaction 29
Figure 7. Pseudosection for RM-21 showing isopleths of anorthite content (solid white lines), Si content per formula unit of muscovite (dashed black lines) and equilibrium pressure temperature conditions for the peak primary assemblage (box).
bt + kfs + qtz + H20 = phg (Proyer, 2002). The intersection of isopleths for 26 volume percent kfs and Si pfu of 3.3 was used to determine equilibration pressure and temperature for the secondary, hydrated assemblage. Equilibration of the secondary assemblage is seen to occur at
6.5 kbar and ~450°C for a water activity of 0.99 (Figure 8). These P-T conditions are similar to
Selverstone et al.’s estimate of 7-9 kbar at 400-600°C for equilibration of the secondary, hydrated assemblage (1999). 30
Figure 8. Pseudosection for RM-21 showing isopleths of volume percent Kfs (solid black lines), Si content per formula unit of muscovite (dashed black lines) and equilibrium pressure temperature conditions for the secondary assemblage (box).
Pseudosections utilizing the same bulk composition and solution models were created under fluid (H2O-CO2)-saturated conditions for a water activity of 0.95 to evaluate the effect of variable activities on pressure-temperature calculations for RM-21. Primary conditions occur at
630°C at 6.8 kbar and secondary conditions at 510°C at 8 kbar occur for a water activity of 0.95.
Despite slight decreases in peak pressures and temperatures and more noticeable increases in secondary pressures and temperatures, a trend of decreasing temperature at relatively constant pressure persists for both water activities. A water activity of 0.99 was chosen to constrain the metamorphic history of RM-21 for two reasons. Equilibria estimates for a water activity of 0.99 are more consistent with estimates of P-T histories made by Selverstone et al. (1999) using other 31 thermobarometric techniques. Additionally, although (clino)zoisite was not identified in our thin section, it is likely to occur during pl breakdown at high pressure; (clino)zoisite is a stable phase in pseudosections with a water activity of 0.99 but not in those with a water activity of 0.95.
GR-11: Secondary Assemblage
Equilibrium conditions of 6.5 kbar at ~475°C are reported for GR-11 using the
MnNCKFMASHT system under fluid (H2O)-saturated conditions. Pressure and temperature constraints were made by identifying overlap between the stability fields for 3.12 Si pfu in muscovite and 1.26 volume percent biotite in P-T space. Xan contents were not used because the limited number of plagioclase analyses (four in total) and wide range of plagioclase compositions inhibit reliable determination of a representative composition.
Monazite geochronology
Identification of monazites in RM-21 was facilitated by electron microprobe full section
Ce x-ray mapping and by QEMSCAN automated mineralogical analysis. Monazite in RM-21 occurs in four textural settings in association with either allanite or plagioclase. The majority of monazite grains associated with aln are contained within individual aln grains that are breaking down to cc ± qtz ± rt ± clay minerals (>20 ox wt % H2O) (Figure 9, Areas B, C, E, G, H, K).
Isolated, individual monazites (5-10 microns) occur in a single area (Figure 9, Area A) in a matrix texturally associated with fractures in a large (~200 micron) well-preserved aln grain.
Monazite grains in breaking-down aln exhibit irregular morphology, variable size (diameters <5 to ~20 μm) and patchy textures whereas those associated with the well-preserved aln grain are consistently subhedral and small (~10 μm diameter). 32
The largest monazite grains
are isolated, euhedral to anhedral,
and variably well preserved within a
matrix of pl and in areas associated
with variable amounts of ab and/or
clay minerals (Figure 9, Areas D and
F). The most abundant population of
monazite occurs in association with
altered pl; more than 20 monazites
were identified within one pl grain.
A total of 27 monazites in
RM-21 associated with breaking-
down aln, with the large well-
preserved aln grain, and isolated
grains in a pl matrix were selected
Figure 9. BSE images of for dating by ion microprobe. monazite-containing areas with dated monazite grains Secondary monazites occurring in and associated mineral phases labeled. association with altered pl were un- dateable because of their low Th and
Pb contents. Whereas high-Th contents in aln serve as an abundant local source of Th for monazite, Th contents in pl are considerably less. As a result, monazite contained within altered pl grains does not incorporate sufficient Th to be dateable. BSE images of analyzed monazites are displayed in Figure 9. 33
Th/Pb dates from RM-21 record monazite growth from the Proterozoic (1871 ± 68 Ma) to the Miocene (22 ± 3.5 Ma) with the majority of dates recording a finite interval of monazite crystallization in the Cretaceous to Early Tertiary. Monazite age data is displayed in Table 4; all errors are reported as 1σ. Monazite dates are divided into a total of four groups based upon relative occurrence in time and plausible relationships to tectonic events; probability distribution functions for these groups are shown in Figure 10.
A total of 5 Proterozoic dates are reported for Group I. Two Paleoproterozoic dates (1871
± 68 Ma and 1698 ± 54 Ma) are reported in addition to three Mesoproterozoic dates between
1548 ± 53 Ma and 1480 ± 72 Ma. The second group of 5 “intermediate” dates spans an age range
Table 4. Monazite age data 208 232 208 208 232 Grain name Pb/ Th (±1! ) ThO2/Th (±1! ) Pb % Pb/ Th age (Ma) ±1! Bm2 0.00111 (1.74E-04) 1.02 (2.01E-02) 20.84 22.3 ± 3.5 Em2 0.00286 (1.17E-04) 1.05 (1.34E-02) 47.00 57.7 ± 2.4 Km2 0.00305 (2.51E-04) 0.951 (2.29E-02) 36.85 61.6 ± 5.1 Em1 0.00308 (1.17E-04) 0.757 (1.49E-02) 37.08 62.2 ± 2.4 Km1 0.00323 (1.56E-04) 0.633 (1.04E-02) 49.69 65.1 ± 3.1 Dm1 0.00328 (6.45E-05) 1.05 (2.10E-02) 94.47 66.2± 1.3 Bm4 0.00337 (1.41E-04) 0.851 (2.15E-02) 50.63 67.9 ± 2.8 Dm2 0.00349 (1.28E-04) 0.755 (1.34E-02) 86.42 70.4 ± 2.6 Cm1 0.00351 (4.04E-04) 1.02 (2.72E-02) 65.19 70.9 ± 8.1 Am2 0.00353 (2.32E-04) 0.341 (2.47E-03) 91.83 71.1 ± 4.7 Cm2 0.00386 (1.12E-04) 1.18 (1.71E-02) 76.52 77.9 ± 2.3 Cm3 0.00387 (1.27E-04) 1.07 (1.79E-02) 78.13 78.1 ± 2.6 Bm1 0.004 (1.49E-04) 0.969 (1.17E-02) 50.43 80.7 ± 3 Km3 0.00448 (6.30E-04) 0.395 (2.35E-02) 50.98 90.4 ± 12.7 Em3 0.00452 (1.37E-04) 0.811 (1.62E-02) 59.71 91.1 ± 2.8 Bm3 0.0053 (1.82E-04) 0.837 (2.96E-02) 57.43 107 ± 4 Am3 0.00863 (6.09E-04) 0.882 (2.40E-02) 87.24 174 ± 12 Bm5 0.0133 (6.10E-04) 0.758 (2.00E-02) 82.17 267 ± 12 Hm1 0.0139 (5.36E-04) 0.672 (7.62E-03) 94.01 279 ± 11 Fm1 0.0186 (1.76E-03) 0.842 (4.78E-02) 48.88 373 ± 35 Am1 0.0429 (3.52E-03) 0.52 (2.59E-02) 92.70 850 ± 68 Em4 0.076 (3.85E-03) 0.45 (9.23E-03) 87.13 1480 ± 72 Am7 0.0778 (3.27E-03) 0.488 (7.36E-03) 97.88 1514 ± 61 Am6 0.0796 (2.82E-03) 0.564 (7.07E-03) 97.99 1548 ± 53 Am4 0.0876 (2.93E-03) 0.612 (9.06E-03) 99.40 1698 ± 54 Gm1 0.097 (3.66E-03) 0.539 (9.97E-03) 99.70 1871 ± 68 34 from the Neoproterozoic (a single date = 850 ± 68 Ma) to the Jurassic (174 ± 12 Ma) with a majority of dates (3 of 5) recording monazite growth from the Devonian (373 ± 35 Ma) to the
Permian (267 ±12 Ma) (Figure 5c). Group II dates span such a wide range because they are defined to reflect the period of tectonism between Proterozoic accretion of North American lithosphere and flat-slab subduction in the Cenozoic. Group III crystallization ages comprise a
56% majority of monazite dates. Fifteen dates ranging from 91.1 ± 2.8 to 57.7 ± 2.4 Ma. An
MSWD of 10.3 may suggest that Late Cretaceous monazite growth occurred over a finite period of ~33 Ma rather than being limited to a single crystallization event. A single Neogene date of
22.3 ± 3.5 Ma for monazite Bm2 comprises Group IV. A clear relationship can be drawn between monazite crystallization and fluid flux during Miocene emplacement in serpentinized
Navajo volcanics such that this date has geological meaning and is not interpreted to reflect analytical error. This is further supported by low analytical uncertainty in the calibration curve
(ThO2/Th equal to 1.02 ± 2.01E-02).
Monazite ages include one date of 107 ± 4 Ma (Bm1) that is not easily related to the groups described above. This date cannot be considered an outlier based upon analytic criteria:
208 percent radiogenic Pb of 57.43 and ThO2/Th ratios are comparable to dates of similar age and limited analytical uncertainty is reflected in a low 1σ error.
No clear relationship exists between monazite age and size, texture, or morphology; monazites in Area B (Figure 11) illustrate this lack of correlation. Grains of similar size (Bm1 and Bm5) yield disparate ages (80.7 ± 3 Ma and 267 ± 12 Ma, respectively), whereas texturally and morphologically distinct monazites yield similar Late Cretaceous ages: Bm4 (67.9 ± 2.8 Ma) is texturally uniform and subhedral whereas Bm1 (80.7 ± 3 Ma) is anhedral and displays a 35 patchy texture. On the other hand, a relationship between association and monazite dates is observed.
Monazite growth requires the breakdown of an LREE- and Th-rich reactive phase typically involving the silicate minerals gt, aln, or pl (Williams, 2007). Allanite breakdown is widely associated with monazite and/or ab growth at temperatures in excess of 500°C (Tomkins,
2004; Rasmussen, 2007; Williams, 2007). Monazite growth concurrent with aln breakdown should have therefore occurred early in the metamorphic sequence nearest peak conditions of
630°C. This relationship is observed: all monazites older than 850 ± 68 Ma are associated with aln breakdown. The formation of monazite also occurs during high-pressure breakdown of pl to ab (Williams, 2007). Albite-in temperatures of ~450°C are reported for RM-21, suggesting that this plagioclase-related monazite growth would occur at a later time along a retrograde P-T path.
Two out of three grains associated with pl, Dm1 and Dm2, are 66.2 ± 1.3 and 70.4 ± 2.6 Ma old, respectively. The relationship between mineral association and monazite crystallization age is
Figure 10. Probability distribution functions for a) all dates, b) Group III Laramide dates, c) Group II “intermediate dates”, and d) Proterozoic dates with weighted averages and MSWD where appropriate. 36 evident but not necessarily absolute: the youngest date of 22 ± 3.5 Ma is located within an aln grain. However, this monazite is also entirely surrounded by clay (>20 ox. wt% water) minerals, such that this date could reflect the timing of clay replacement and not the timing of allanite breakdown.
Topography Calculations
The effect of density change on topography was evaluated for the pervasively altered xenolith GR-11 using the equation .
Estimates of present-day crustal thickness in the Four Corners Region from seismic studies were combined with estimates of equilibrium depths from pseudosection analysis of GR-11 for fluid
(H2O)-saturated conditions in the MnNCKFMASHT system to constrain the thickness of the hydrated layer. A 25.5 ± 5 km thick hydrated layer is reported beneath the Colorado Plateau based upon the difference between the
Figure 11. BSE image of monazites and corresponding ages in Area B illustrating the lack of an obvious relationship between age and grain morphology, size, or texture. 37 source depth of 19.5 km (6.5 kbar) for GR-11 and current estimates of an ~45 km thick crust beneath the Four Corners Region from receiver functions and surface wave tomography (Bashir et al., 2011; Gilbert, 2012; Shen et al., 2013).
A whole rock density of 3.19 g/cm3 was determined for GR-11 using the physical properties database of Hacker and Abers (2004) with modal mineralogy assessments from
QEMSCAN and P-T conditions of 475°C at 6.5 kbar from petrological modeling as inputs.
Figure 12. Photomicrograph showing garnet replacement by hornblende in GR-11and illustrating the possible size of a primary garnet grain.
Densities ≥ 3.21
g/cm3 were
calculated under
the same pressure
and temperature
conditions for mineral assemblages intended to represent primary, unaltered Proterozoic crust. All calculations were made using an asthenospheric density of 3300 kg/m3. 38
Primary and secondary assemblages are difficult to distinguish in GR-11 as a result of pervasive alteration following peak metamorphism. In particular, the initial volume percent of gt is not known because of extensive replacement of gt by amphibole (Figure 12). Mineralogy inputs were constructed as best as possible to reflect peak assemblages in a two-step process.
Removing secondary ab (7.25 volume %) and retrograde mu (6.08 volume %) as inputs from the assemblage and subsequently increasing the amount of pl (Xan = 14) by 13 volume % negated the density-reducing effects of high-pressure pl breakdown. Second, the amount of gt was increased to between 110 and 150% of the initial 15.69 volume % and hornblende was removed in corresponding amounts. A garnet composition of Xpy= 22, Xalm=58 and Xgr =17 was used for all density calculations.
Elevation shifts associated with phase changes in GR-11 evaluated using the equation
for layer thicknesses of 10, 20, 25.5 and 30 km are plotted in 39
Figure 13. Uplift on the order of hundreds of meters is expected to result from the density- decreasing effects of lower crustal hydration beneath the Colorado Plateau assuming the hydrated crustal column is pervasively altered in lateral and vertical extent. 40
DISCUSSION
Mineralogy
Mineral compositions and zoning patterns of secondary phg and pl in RM-21 indicate chemical re-equilibration of these minerals at fluid-saturated conditions along a retrograde path.
Zoning patterns in gt, on the other hand, suggest partial to incomplete equilibration at secondary pressure and temperature conditions for cations with variable diffusion rates.
The muscovite-celadonite solid solution series defines the range of Si compositions in phengite (Velde, 1965). Mu (low Si) occurs in igneous and high-grade metamorphic environments, celadonite (high Si) forms in sedimentary and hydrothermal alteration environments and phengite is found in low temperature, high-pressure metamorphic rocks
(Velde, 1965). The difference in compositions of Si per formula unit in primary (3.12 pfu) versus secondary (3.29 pfu) white mica in RM-21 indicates equilibration to high-pressure conditions as described by several reactions: 1) bt + kfs + qz + H20 = ms, 2) bt + pl + H20 = ms + ab (XAn = 0-
5) + zo + chl + cal, and 3) an + kfs + H20 = grt + ms + qz (Selverstone et al., 1999; Proyer,
2002). Reactions 2 and 3 illustrate a robust relationship between phengite formation and plagioclase breakdown.
Compositions and zoning patterns of plagioclase indicate high-pressure breakdown of anorthite to albite under fluid-saturated conditions. The highest anorthite contents (XAn = 15-22) found in unaltered cores are interpreted to represent Ca contents during peak metamorphism. The presence of intermediate (XAn = 7-12) calcium contents in zones of concentrated alteration and of low (XAn = 1-3) calcium contents along grain boundaries is interpreted to reflect discontinuous plagioclase breakdown. 41
Calcium release due to a breakdown in Ca-rich metagranites typically forms
(clino)zoisite and/or grossular-rich garnet, neither of which are observed. A simple explanation for the lack of (clino)zoisite is differences in thin-section scale sampling of various mineral phases where Ca released by an breakdown with increasing pressure formed calcite (0.4 volume
%) rather than (clino)zoisite on the scale of our thin section. This explanation is further supported by pseudosection analysis for water activity = 0.99 wherein (clino)zoisite has a very narrow stability field directly adjacent to the stability field of calcite (Figure 6, Area 6). The lack of new growth of grossular rich-rims on garnet likely indicates incomplete equilibration of garnet at secondary P-T conditions and warrants further explanation.
At temperatures less than 525 ± 25° C, diffusion of major cations in garnet slows, such that the length scale of diffusion may be significantly shorter than the spatial resolution of the electron microprobe (Spear, 1995; Chernoff and Carlson, 1999). Chernoff and Carlson (1999) interpret minimal Ca zoning in pelitic garnets from central New Mexico to reflect especially slow diffusion of Ca relative to other cations. Whereas diffusion rates of Mn, Mg, and Fe in gt are sufficiently rapid to allow hand-sample-scale equilibration during metamorphism at temperatures above 525° ± 25° C, the slow diffusion rate of Ca is not. The lack of new Ca-rich gt in the form of grossular rims typically associated with pl breakdown (an + kfs + H20 = grs + ms
+ qz) may therefore reflect slow diffusion of Ca and incomplete elemental re-equilibration at calculated secondary temperature conditions of ~460°C (Proyer, 2002). Compositional zoning of
Mn, on the other hand, may reflect partial equilibration of rapidly diffusing cations on the retrograde path as a result of net transfer.
Mineral zoning in garnet occurs due to changes in pressure, temperature or fluid conditions via two reactions: retrograde exchange and net transfer. Retrograde exchange 42 reactions partition cations between minerals without significantly altering modal abundances.
Compositional zoning in garnet is thought to signify the importance of the second reaction --- retrograde net transfer --- in which modal proportions change as phases are consumed and produced (Kohn and Spear, 2000). Kohn and Spear (2000) interpret an increase in Mn along gt rims (the “Mn kick-up”) in Himalayan metapelites to reflect net transfer during retrograde gt dissolution. The Mn kick-up bserved in RM-21 likely reflects a component of net transfer and partial re-equlibration on the retrograde path between a calculated peak temperature of 690°C and temperatures at which diffusion greatly slows i.e. 525° ± 25° C (Chernoff and Carlson,
1999).
Pressure-temperature histories
Pseudosection analysis is a well-suited tool for predicting primary and secondary P-T conditions in RM-21. Incomplete diffusion of calcium, partial re-equilibration of Mn upon cooling and the role of retrograde net transfer reactions in gt inhibits accurate identification of minerals in equilibrium, which is a critical step in the application of traditional thermobarometric techniques for felsic rocks i.e. the Gt-Bt-Fe-Mg exchange reaction thermometer and Gt-Pl-Bt-
Mu net transfer barometer (Spear, 1995; Kohn and Spear, 2000).
Pseudosections reveal a trend of decreasing temperatures at relatively constant pressure along a retrograde path from primary to secondary P-T conditions. For a water activity of 0.99, temperatures decrease by 240°C over an increasing pressure interval of 0.5 kbar. This P-T path is consistent with relaxation of a perturbed geotherm subsequent to Proterozoic orogenesis and deformation. Additionally, it is worth noting that the metamorphic history of RM-21 is not at odds with the influence of a subducting plate whereby relatively constant pressures reflect the 43 negligible magnitude of horizontal stresses imposed by a subducting slab and decreasing temperatures along a retrograde path may partially record refrigeration of overlying crust and mantle lithosphere by a cold oceanic plate.
Monazite geochronology
Twenty-seven monazite grains associated with secondary hydrous phases in RM-21 were selected for Th/Pb dating to constrain the timing of lower crustal metasomatism beneath the
Colorado Plateau. The main sources of systematic error in Th/Pb monazite dating include Pb loss due to prolonged exposure above the closure temperature, analytical uncertainty in the calibration curve, and sampling of overlapping mineral domains (Harrison et al., 1995). Evidence for systematic error associated with Pb loss or analytical uncertainty associated with the calibration curve is negligible whereas sampling errors are not well constrained.
Pb loss due to prolonged exposure at high temperatures is an unlikely source of error because the thermal closure temperature of monazite (> 900° C) exceeds an estimated peak metamorphic temperature of 690°C for RM-21 (Cherniak et al., 2004). Systematic errors in
Th/Pb monazite dates cannot be attributed to error in the calibration curve for the RM-21 data set because no systematic variations are observed between monazite ages and ThO2/Th ratios.
ThO2/Th ratios are used for instrument calibration, and, when the instrument is properly calibrated, should be consistent in both the standard and unknown with successive cycles for a given Pb domain. While ThO2/Th ratios in the unknown invert as the cycle count increases, this behavior is also observed in the standard suggesting that variable ThO2/Th ratios values are not a reliable indicator of inaccurate dates and that uncertainty in the calibration curve is not responsible for the wide span of measured dates. 44
If the wide range in the 208Pb/232Th data set for RM-21 is the result of systematic error, inaccurate age calculations would most likely result from sampling of variable Th, or Pb domains. Fluid-present conditions dramatically increase the potential for monazite resetting and recrystallization because Th and LREEs that comprise monazite are fluid mobile (Kelly, 2012).
The sensitivity of monazite growth to fluid flux may result in complex zoning and/or textures as monazite dissolves and re-precipitates along a retrograde path (Harrison et al., 1995). Complex, patchy textures within individual monazites associated with allanite likely record complicated recrystallization histories. Monazite Bm1 (Figure 7), for instance, displays an intricate, matted texture that may represent overlapping monazite domains of different ages. Another possible source of error is sampling of adjacent Pb or Th-bearing domains, particularly allanite that occurs in close spatial association to monazite; the potential for sampling errors is exacerbated by extremely small grain sizes and by limited spatial precision of the ion beam. While this source of error was minimized by reducing the field aperture to eliminate sampling of ions from other host phases following the methods of Chamberlain et al. (2012), sampling of non-monazite domains adjacent to the target domain may still have occurred and may help to explain intermediate dates that are not as easily relatable to geologic events. However, the fact that a majority of intermediate dates cluster at two endmembers in the Mesoproterozoic and Mesozoic suggests that two groups do in fact have geological meaning. This conclusion is supported by the lack of systematic variations in PrPO2 counts in the unknown.
Low PrPO2 counts reflect the small grain sizes or contact with a non-P or Pr-bearing phase. Low count rates have a tendency to generate unreliable data because, as mass peaks become noisy at counts less than 100, uncertainty as to whether mass peaks were well centered prior to analysis increases. Because no systematic trend relates low PrPO2 counts, monazite 45 dates, or 1σ errors for RM-21, it appears unlikely that sampling of non-target domains contributed to the wide span of ages.
The reactive nature of monazite ([LREE,Th]PO4) establishes it as a powerful chronometer of crustal events including diagenesis, prograde metamorphism, magma crystallization and fluid flow such that relative timing of monazite crystallization can be related to the tectonic history of the western U.S. (Kelly 2012). Figure 14 illustrates the temporal relationship between monazite dates and major geologic events in western North America.
Group I dates reflect accretion, subduction, and metamorphism during the formation of North
American lithosphere.
Figure 14. Summary diagram showing Th/Pb dates and major geologic event in the western U.S. Inset shows the probability distributions of Laramide dates. References: 1) Selverstone et al. (1999), 2) Engebretson et al. (1984), 3) Dickinson (2006), 4) Busby- Spera et al. (1990), 5) Wyld (1991), 6) Speed and Sleep (1982), 7) Timmons et al. (2001), 8) Nyman et al. (1994), 9) Karlstrom and Bowring (1988), 10) Bennett and DePaolo (1987) 46
Dates between 1698 ± 54 to 1480 ± 72 most likely record prograde metamorphism and fluid flow associated with accretion and subduction during the Yavapai-Mazatzal orogeny and/or pervasive crustal metamorphism at ~1.4 Ga (Bennett and DePaolo, 1987; Karlstrom and Bowring, 1993;
Dickinson, 2006). Monazite crystallization at 850 ± 68 Ma corresponds to the timing of intracratonic deformation related to continental rifting (Timmons et al., 2001) and intermediate dates from 373 ± 35 to 174 ± 12 Ma are concurrent with Mesozoic metamorphism and magmatism associated with crustal deformation during the Antler, Sonoman, Nevadan, and
Ancestral Rocky Mountain orogenies.
Fifteen monazite recrystallization dates reported for RM-21 document Late Cretaceous hydration of the lower continental crust in North American. A 56% majority of analyzed monazites record fluid-related recrystallization from 91.1 ± 2.8 Ma to 57.7 ± 2.4 Ma and a reported MSWD of 10.3 may indicate that monazite crystallization occurred over a finite period rather than during a single fluid-related event. This group of monazite dates are interpreted to reflect recrystallization associated with fluid flux from a shallow Farallon slab.
Late Cretaceous dates in RM-21 overlap with U/Pb zircon crystallization ages of 85 to 33
Ma in hydrated mantle xenoliths from the Colorado Plateau which are similarly interpreted to reflect the catalytic effects of water introduced by the Farallon plate in a number of discrete intervals. Smith and Griffin (2005) report two conspicuous clusters at 70 and 85 Ma and Usui et al. (2003) document core to rim dates of 54 to 34 Ma in a single zircon crystal recording a 20 m.y. period of recrystallization.
47
The case for the Farallon slab
The retrograde P-T path, hydrous mineralogy, and Late Cretaceous monazite crystallization ages in RM-21 establish a role for fluid-related alteration of the lower crust beneath the Colorado Plateau. It has yet to be demonstrated, however, whether a shallowly subducting Farallon slab is a plausible source for these fluids.
Constraints on the stability of fluid reservoirs in subducting slabs and anomalously high water contents in mantle xenoliths establish that sufficient amounts of water would be retained in an oceanic plate to distances > 800 km from the trench, and trace element/isotope systematics from xenolith suites in the Sierra Nevada and Colorado Plateau provide evidence that lithospheric hydration was linked to the same subduction system (i.e. subduction of the Farallon slab) in these two locations.
Slab fluid reservoirs
The fluid budget of a slab is not well-constrained and there is no consensus as to how much chemically bound water is stored in the crust and mantle of a subducting oceanic plate
(Ranero et al., 2003). Estimates of slab fluid content by Peacock (1991) range from 0.1 to 1 kg/m2. It is understood, however, that three fluid reservoirs comprise oceanic lithosphere: 1) oceanic sediments, 2) amphibolitized crust, and 3) variably serpentinized lithospheric mantle.
These reservoirs undergo continuous devolatilization throughout the subduction sequence according to their relative stability fields during prograde metamorphism which increases with distance from the trench in the sequence outlined above (Peacock, 1993).
Oceanic sediments are expected to expel two-thirds of chemically bound water at depths
<50 km by diagenetic and low-grade metamorphic reactions (Peacock, 1990; Ranero et al.,
2003). Ranero et al. (2003) estimate complete devolatilization of oceanic sediments at 120 km in 48 the Middle America trench, at which depth hydrous minerals in amphibolitized oceanic crust are expected to begin dehydrating.
Peacock (1993) constrains the major dehydration reaction in oceanic crust to depths <100
-150 km at which time the slab passes through the eclogite-facies phase transition and extensive
H2O release accompanies the breakdown of Na-amphibole, lawsonite, chlorite and/or clinozoisite
(Peacock, 1993). A study by Currie and Beaumont (2011) for rapid, flat subduction of the
Farallon plate positions the basalt-eclogite transition at ≥300-350 km from the trench on account of extra time needed to heat thickened Farallon crust. Peacock (1993) and Currie and Beaumont
(2011) reason that, if the addition of H2O-rich fluids into overlying crust and mantle lithosphere is of sufficient magnitude to depress the solidus, dehydration reactions could trigger partial lithospheric melting and/or kimberlite magmatism.
Variably serpentinized oceanic mantle lithosphere is the primary source of fluids at distances away from the trench. Antigorite is stable to 660°C and 2 GPa and is thought to progressively dehydrate between 120 and 200 km depth, at which point dehydration would greatly slow or stop altogether (Ranero et al., 2003; Hacker, 2008). Related distances from the continental margin over which this dehydration would occur are disputed. Smith and Griffin
(2005) attribute garnetite formation in metasomatic reaction zones to hydration of the mantle wedge 700 km from the trench and Li et al. (2008) estimate delayed dehydration of serpentine to distances 800 km from the trench based upon thermal models describing dehydration of an old, cold Farallon slab. Thermal modeling by Currie and Beaumont (2011), on the other hand, allows for hydrous minerals to remain stable to distances >1500 km from the trench assuming rapid subduction --- 4 cm/yr westward and 11 cm/yr eastward as proposed by Engebretson et al.
(1984). --- of thickened oceanic crust along a low angle trajectory. The authors argue that a 15 49 km thick layer of serpentinized peridotite would remain stable within the cool interior of the subducting plate to 1200 km inboard from the margin and that dehydration would continue for an additional 300 km as material passed through the 10Å phase transition (Currie and Beaumont,
2011).
Numerous studies constrain the location and vertical extent of serpentinization in oceanic mantle lithosphere. Nominally hydrous minerals in mantle peridotite become unstable and transition to serpentine at temperatures >500 °C (Ranero et al., 2003). A ~40% increase in volume accompanies this phase transition promoting self-sealing closure of fluid conduits
(Ranero et al., 2003). It is therefore likely that structural features such as cracks, fractures, and/or bending-related fault planes act as primary controls on serpentine stability (Smith and Griffin,
2005). Pre-existing weaknesses promote extensive hydrous alteration of the slab prior to subduction as water is persistently pumped into the chemically reactive lithospheric mantle
(Schmidt and Poli, 2003; Ranero et al., 2003; Hacker, 2008). A study by Li and Lee (2006) argue for serpentinization extending to depths ≤40 km into relatively old and cold oceanic lithosphere.
Importantly, the Farallon slab is though to have been old (>50 Ma) when it reached the trench at
80 Ma and therefore in a thermal state sufficiently cool to allow for extensive serpentinization and associated hydration of the continental lithosphere (Engebretson et al., 1985; English et al.
2003).
It is possible that the lack of a uniform decrease in core to rim calcium contents in altered plagioclase in RM-21 reflects fluid release from structurally controlled fluid conduits.
Discontinuous plagioclase breakdown is more likely related to variations in fluid activity than to abrupt changes in pressure. Fluid release, and associated plagioclase breakdown, would be 50 sporadic beneath the Colorado Plateau and occur with successive devolatilization of fluid conduits.
Xenolith water contents
Water contents in nominally anhydrous minerals from peridotite xenoliths verify that the fluid reservoir beneath the Colorado Plateau was of sufficient magnitude to pervasively hydrate continental lithosphere. Li et al. (2008) report water contents of nominally anhydrous minerals from 17 xenoliths along a 500 km transect from the Sierra Nevada to the Four Corners Region.
Peridotite xenoliths from the Navajo Volcanic Field have the highest water contents (45 ppm in olivine, 402 ppm in opx, and 171-957 in cpx) that plot above the highest inferred water contents for a MORB source. Two-pyroxene thermometry for Navajo pyroxenites reveals equilibration temperatures between 950 and 1200° C and evidence for equilibration pressures between 3.7 and
<2 GPa comes from Al in opx barometry and from the presence of paragonite and omphacite in a
Navajo pyroxenite (Li et al., 2008). These equilibration temperatures and pressures establish xenolith source depths across a vertical range from 70 to 140 km and make the case that elevated water contents are not isolated to a narrow zone within the mantle lithosphere. (Li et al., 2008).
Calculated source depths of ≤20 km for xenoliths RM-21 and GR-11 suggests that hydration beneath the Colorado Plateau occurred over a considerable depth range (25.5 km) in the lower continental crust as well.
Trace elements signatures
Trace element and isotopic signatures preserved in hydrated mantle xenoliths corroborate serpentinized mantle lithosphere as the source of fluids beneath the Colorado Plateau and link metasomatism in the Sierran Arc and Four Corners Region to the same subduction system i.e. subduction of the Farallon slab. Isotopic and trace element characteristics evolve with distance 51 from the trench in a manner consistent with element partitioning between fluid reservoirs: dehydration occurs continuously with increasing pressure and temperature during the subduction sequence and, as a result, progressive decoupling of water-soluble versus water-insoluble elements concentrate fluid-immobile trace elements in the slab (Schmidt and Poli, 2003).
Trace element and isotopic characteristics of xenoliths from the Sierran Arc (Big Creek,
CA) indicate extensive interaction with fluids derived from oceanic sediments and/or seawater- altered lithologies (i.e. amphibolitized oceanic crust). Near-trench xenoliths are enriched in the fluid-mobile elements Sr, Pb, U, Ba, and Cs, and exhibit low U/Pb and high Sr/Nd ratios suggesting interaction with a sediment- or crust-derived metasomatic component that would rapidly expel water as temperatures increased (Lee, 2005). Lithium concentrations (26 ppm) and light isotopic compositions relative to MORB further advocate for a sediment-derived metasomatic component (Chan et al., 2007). Nb and Ta are depleted relative to fertile mantle in
Big Creek xenoliths suggesting that near-trench temperatures were not of sufficient magnitude to cause silicate melting (Lee, 2005). Additional depletion of elements mobilized by silicate melts
(REEs and HFSEs) further invokes fluids as the dominant component affecting trace element and isotope systematics (Lee, 2005).
Whereas depletion of fluid-immobile REEs and HFSEs in Sierran xenoliths negates the presence of a melt component, trace element characteristics of Colorado Plateau xenoliths require the introduction of both an aqueous phase and a silicate melt (Lee, 2005). High U/Pb ratios, low Sr/Nd ratios, Sr depletion, reduced enrichment in fluid-mobile elements, and significant enrichment in fluid-immobile REEs and HFSEs characterize peridotites from the
Navajo Volcanic Field (Lee, 2005). These signatures suggest simultaneously interaction with eclogite-derived melts and serpentinite-derived fluids. 52
Fluids released from a subducting slab are thought to cause partial melting at temperatures above the eclogite solidus (Peacock, 1993; Currie and Beaumont, 2011). The resulting hydrous melt would inherit the trace element characteristics of eclogitized oceanic basalt: a negative Sr anomaly, minimal enrichment of fluid-mobile elements, and high U/Pb ratios and low Sr/Nd ratios resulting from preferential liberation of Pb and Sr during dehydration of the seawater-altered protolith early in the subduction sequence (Lee, 2005; Li et al., 2008).
Elevation and density changes
Estimates of fluid stability in oceanic lithosphere, anomalous water contents in mantle xenoliths and trace element signatures relating xenolith hydration beneath the Colorado Plateau and Sierran Arc to different fluid reservoirs in the same subduction system substantiate the hypothesis that pervasive hydration of continental lithosphere in the Four Corners Region resulted from de-watering of the Farallon slab. The relationship between this hydration and the generation of regionally elevated topography is constrained by the equation
and hydration of a 25.5 km thick layer beneath the Colorado
Plateau yields between 200 and 800 m of surface uplift for 10 to 50% consumption of primary garnet by hornblende, respectively.
A number of issues must be considered when interpreting these estimates. First, no constraints exist on the timing of hydration in GR-11. The last major subduction event affecting the Colorado Plateau occurred during the ca. 1.6 Ga Yavapai-Mazatzal orogeny. While it is highly unlikely that the extensive hydration and relatively unperturbed geotherm recorded by
GR-11 reflects metasomatism associated Proterozoic orogenesis, this possibility cannot be definitively discounted without geochronological constraints. 53
Additionally, GR-11 is one of the most pervasively altered lower crustal xenoliths in the sample suite of Selverstone et al. (1999) such that calculations using phase changes in GR-11 would produce maximum estimates of uplift.
Elevation shifts associated with phase changes in GR-11 evaluated using the equation
for layer thicknesses of 10, 20, 25.5 and 30 km are plotted in
Figure 13. Uplift on the order of hundreds of meters is expected to result from the density- decreasing effects of lower crustal hydration beneath the Colorado Plateau assuming the hydrated crustal column is pervasively altered in lateral and vertical extent. While these elevation changes do not account for the full vertical extent of regionally elevated topography in the
Colorado Plateau (i.e. average elevations ~2 km), this study demonstrates that isostasy-driven uplift due to variable crustal chemistry has a noticeable, though limited, effect on topography in continental interiors.
Timing of uplift
The question remains as to the timing of widespread surface uplift and associated mechanisms responsible for regionally elevated topography in the western U.S. Specifically, did uplift occur contemporaneously and in association with flat-slab subduction or were other post-
Laramide processes responsible? The results of this study permits a relatively small contribution
(≤ 1 km) to uplift from changes in crustal chemistry beneath the Colorado Plateau. It is therefore prudent to consider other mechanisms that may be responsible for uplift and to relate the timing of this uplift to timing of the subducting flat-slab. A number of studies address this issue and arguments exist for both post- and syn-Laramide uplift.
Post-Laramide uplift 54
Petrologic and sedimentological investigations in the Colorado Plateau and High Plains suggest recent, post-Laramide uplift of the western U.S. Sahagian et al. (2002) argue for a period of recent rapid uplift (220 m/m.y.) of the Colorado Plateau since 5 Ma following a period of slow uplift between 23 and 5 Ma (~40 m/m.y.) using paleo-atmospheric pressure reconstructions based upon relative vesicle size at the top and bottom of basalt flows. The temporal extent of this elevation reconstruction method, however, is inherently limited by a lack of volcanism in the
Plateau prior to 23 Ma (Sahagian et al, 2002). GIS evaluation of erosional exhumation of
Colorado Plateau similarly suggests significant uplift since ca. 30 Ma (Pederson et al., 2002).
Pederson et al. (2002) estimate 843 m of erosion and a corresponding 639 m of isostatically- driven post-Laramide rock uplift since deposition of coastal sandstones in the Late Cretaceous.
Heller et al. (2003) invoke long-wavelength tilting in the Miocene and Pliocene to explain the presence of gravel deposits in western Nebraska and southern Wyoming. The authors argue that the large volume and transport distance of Ogallala group gravels cannot be directly generated by northward-propogating Rio Grande Rift related paleo-topography but rather must be associated with dynamic (i.e. mantle-driven) topographic processes not necessarily related to
Laramide subduction (Heller et al., 2003). Numerous geophysical studies similarly suggest thermal or mechanical mantle processes as the source of uplift in the Colorado Plateau and argue for thinning or warming of heterogeneous lithosphere, whether subduction-related or not, as the driving force behind recent epeirogenic uplift (Bird, 1979; Parsons and McCarthy, 1995; Moucha et al., 2009; Roy et al., 2009; Liu and Gurnis, 2010; Levander et al., 2011; Levandowski et al.,
2013)
One-dimensional thermal modeling by Bird (1979) predicts the formation of a new thermal boundary layer at a half-life of 3*107 years as a result of unstable mechanical 55 equilibrium in continental lithosphere. Continental delamination and subsequent loss of cold mantle, Bird (1979) therefore suggests, would cause uplift in the Colorado Plateau that would be consistent with a proposed delamination event at 30 m.y. and a subsequent event at 5 m.y.
Roy et al. (2009) and Levandowski et al. (2013, submitted) also suggest a thermal mechanism for uplift. Roy et al. (2009) argue that warming of thick, iron-depleted lithosphere following removal of the Farallon slab (estimated at 35-40 m.y.) drove ~2 km of rock uplift in the Colorado Plateau without causing significant internal deformation. Levandowski et al. (2013, submitted) similarly argue that 1 – 1.5 km elevation difference between the Colorado Plateau and
Great Plains is due entirely to differences in mantle thermal buoyancy but, unlike Roy et al.
(2009) suggest that thinning and subsequent thermal equilibration since 70 Ma is the dominant source of this buoyancy and associated uplift.
The role of subduction as it relates to mechanical thinning of mantle lithosphere is invoked by Parsons and McCarthy (1995) to explain recent uplift. Constraints on crustal thicknesses beneath the southwest margin of the Colorado Plateau made using seismic and gravity data were used to isolate the mantle contribution to uplift and led the authors to conclude that subduction-related thinning of dense mantle lithosphere was a primary source of uplift, which they argue occurred in the past 1 to 6 m.y.
Mantle convection has more recently been proposed to explain high elevations in the
Colorado Plateau. Using a combination of tomography and receiver functions Levander et al.
(2011) resolve a high-velocity seismic anomaly beneath the west-central plateau that they interpret reflects continual, regional foundering of lower crust and continental lithosphere. This downwelling is suggested to have been triggered by hydration and weakening of Proterozoic mantle during Cretaceous subduction and is assigned as the cause of Pliocene (2.6 to 5.3 M.y.) 56 uplift and magmatism (Levander et al. 2011). Moucha et al. (2009) use joint seismic-geodynamic modeling to argue for viscous flow and strong mantle upwelling coupled to a sinking Farallon slab. The authors argue that this mantle flow concentrates a dynamic topography high beneath the Colorado Plateau that may responsible for recent uplift as well as magmatic activity in the
Rio Grande Rift and Jemez lineament (Moucha, 2009). Inverse mantle convection modeling used by Liu and Gurnis (2010) to predict vertical motion of the Plateau since 100 Ma demonstrates a role for dynamic subsidence caused by a shallow Farallon slab, which is predicted to have been at a maximum at 86 Ma (Liu and Gurnis, 2010). The effects of subsidence resulted in two stages of uplift, first during the descent of the slab and subsequently as a result of buoyant upwelling during the Oligocene (Liu and Gurnis, 2010).
Syn-Laramide uplift
Thermochronological, paleobotanical, and isotopic data suggest that regionally elevated topography in the western U.S. occurred in the Late Cretaceous to Early Tertiary, contemporaneous with flat-slab subduction. Fission-track studies by Flowers et al. (2008) and
Flowers and Farley (2012) use (U-Th)/He dates of apatite as a proxy for unroofing of the
Colorado Plateau. Unroofing history reconstructions for the southwestern Colorado Plateau reveal a single phase of unroofing in the Late Cretaceous to Early Tertiary, multiphase unroofing from Early to Late Tertiary time, little unroofing between 50 and 30 Ma, and significant unroofing in the southwest plateau interior from 28 to 16 Ma (Flowers et al., 2008). This data, in conjunction with evidence for mid-Tertiary relief in northeast-flowing drainages, strongly suggests that the southwestern Plateau interior had achieved substantial elevations by the mid-
Tertiary (Flowers, 2008). While the results of this study do not preclude a component of post- 57
Eocene elevation gain, they do strongly suggest that syn-Laramide sources of buoyancy caused substantial surface uplift (Flowers, 2008).
A more recent (U-Th)/He study suggests that the Grand Canyon lay near its present elevation in Late Cretaceous (Flowers and Farley, 2012). Flowers and Farley (2012) used thermochronometry of apatite from Grand Canyon basement rock to constrain the near-surface cooling history of the region; calculated cooling ages indicate canyon excavation, and, correspondingly, Plateau uplift, to within a few hundred meters of its position by ~70 Ma
(Flowers and Farley, 2012).
Additional support for syn-Laramide uplift comes from paleontological and isotopic evidence. Characteristic leaf morphology of 40Ar/39Ar-dated fossil flora led Gregory and Chase
(1994) to conclude that much of the Western Cordillera was at high elevation since the Late
Cretaceous, during which time paleoelevations between 3 and 4 km were not uncommon
(Gregory and Chase, 1994). Dettman and Lohmann (2000) interpret highly variable δ18O values of river waters preserved in mollusk fossils from Late Cretaceous and Paleogene basins in
Alberta, Montana, Wyoming, and Colorado to indicate 18O sourced from rainfall in warm, low- elevation basins and from snowfall in surrounding mountains. The differences in δ18O values are suggested to reflect local relief of up 2.5 to 3 km throughout the Cenozoic (Dettman and
Lohmann, 2000). Estimate depositional temperatures of Tertiary lake sediments in the plateau interior and adjacent lowlands to analyze the timing of Colorado Plateau uplift using carbonate clumped paleothermometry. Compare modern and ancient samples deposited near sea level to quantify the influence of climate and isolate the effects of elevation on paleotemperatures.
Results suggest that all or most of lithospheric buoyancy was acquired from ~80 - 60 Ma and suggest little to no elevation change in the past 6 Ma (Huntington et al., 2010). 58
The results of this study advocate for a Laramide-age source of buoyancy as the cause of regionally elevated topography in the Colorado Plateau. Density decreases due to alteration of the lower crust and mantle lithosphere by slab-derived fluids cause isostatically-driven uplift and timescales of isostatic rebound are on the order of tens of thousands to millions of years. This therefore suggests that lower crustal hydration, uplift and Farallon subduction were essentially contemporaneous. While the magnitude of uplift associated with phase changes at depth reported in this study cannot account for the full extent of topography in the Plateau, this study demonstrates that isostatically-driven, density-related changes in the lower crust provide a quantifiable and noticeable source of elevation in continental interiors. 59
CONCLUSION
The relationship between chemical, thermal and/or mechanical alteration of lithosphere and deformation in overlying continental crust is key to understanding the tectonic evolution of continental interiors. The results of this study substantiate the hypothesis that chemical alteration of North American lithosphere by fluids derived from the Farallon slab played a role in the production of surface uplift in the western U.S. Mineralogical characteristics and geochronological constraints from xenoliths RM-21 and GR-11 are used to interpret the chemical evolution of the lower crust beneath the Colorado Plateau and evaluate its effects on the generation of regionally elevated topography.
Petrological modeling for RM-21 establishes a trend of decreasing temperature at relatively constant pressure in the lower crust consistent with both relaxation of a perturbed
Proterozoic geotherm and residence above a cold oceanic plate. In situ Th/Pb dating of secondary monazite from the same sample reveals a prolonged period of monazite crystallization in the Four Corners Region in the Late Cretaceous (91.1±2.8 to 57.7±2.4 Ma). This episode of monazite growth is concurrent with the timing of low-angle subduction of the Farallon slab; the case for the Farallon plate as the source of fluids is further supported by assessments of fluid reservoir stability in oceanic lithosphere and trace element signatures relating xenolith hydration beneath the Colorado Plateau and the Sierran Arc to the same subduction system. Density-related uplift on the order of hundreds of meters is estimated for the Colorado Plateau based upon varying amounts of garnet and plagioclase consumption for an extensively altered xenolith in a
25.5 ± 5 km thick hydrated layer. These results advocate for a role of chemical alteration of continental lithosphere in creating elevated topography in the interior of North America and encourage reinterpretation of previous ideas explaining Laramide-related surface uplift. 60
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