Crustal Segmentation, Composite Looping Pressure-Temperature Paths, and Magma-Enhanced Metamorphic ﬁ Eld Gradients: Upper Granite Gorge, Grand Canyon, USA

Home , Pegmatite, Topaz Canyon

Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic ﬁ eld gradients: Upper Granite Gorge, Grand Canyon, USA

Gregory Dumond† Kevin H. Mahan‡ Michael L. Williams Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

ABSTRACT pressures recorded at peak temperatures, subhorizontal shortening and subvertical and intensity of late-stage thermal spikes due extension during crustal thickening. The Paleoproterozoic orogen of the south- to local dike emplacement. High-precision western United States is characterized by ΔPT “relative” thermobarometry confi rms Keywords: Grand Canyon, P-T-t-D path, conti- a segmented, block-type architecture con- lateral temperature variations on the order nental crust, rheology, channel fl ow. sisting of tens of kilometer-scale blocks of of 100–250 °C with little to no variation in relatively homogeneous deformation and pressure. The Upper Granite Gorge thus INTRODUCTION metamorphism bounded by subvertical high- represents a subhorizontal section of low- strain zones. New fi eld, microstructural, and ermost middle continental crust (~0.7 GPa). Proterozoic rocks of southwestern North petrologic observations combined with pre- Results imply that the entire ~70-km-long America are part of a >1000-km-wide orogenic viously published structural and geochrono- transect decompressed from ~0.7 to ~0.3– belt that records the progressive southward logical data are most consistent with a tec- 0.4 GPa levels as one large coherent block in growth of Laurentia (Karlstrom et al., 2001) tonometamorphic history characterized by the Paleoproterozoic. (Fig. 1). An enigmatic characteristic of the belt a clockwise, looping pressure-temperature The transect represents a 100% exposed is its block-type architecture, i.e., blocks with (P-T) path involving: (1) initial deposition fi eld laboratory for understanding the het- internally homogeneous deformational and met- of volcanogenic and turbiditic supracrustal erogeneity and rheologic behavior of lower- amorphic characteristics are separated by sub- rocks at ca. 1.75–1.74 Ga, (2) passage from most middle continental crust during oro- vertical high-strain zones from blocks with dif- <12 km (below pressures equivalent to the genesis. Hot blocks achieved partial melting ferent characteristics (Karlstrom and Bowring, aluminosilicate triple point) to ~25 km conditions during penetrative subvertical 1988). This distinctive architecture has been depths (~0.7 GPa) between ca. 1.70 and fabric development. Although these blocks investigated for over twenty-fi ve years, with 1.69 Ga, (3) decompression back to ~12 km were weak, large-scale horizontal channel particular focus on issues involving growth, sta- depths (0.3–0.4 GPa) by ca. 1.68 Ga, and fl ow was apparently inhibited by colder, bilization, reactivation, and the general character (4) a protracted period of near-isobaric cool- stronger blocks that reinforced and helped of middle continental crust (Karlstrom and Wil- ing (ca. 200–250 Ma). The general geometry preserve the block-type architecture. Devel- liams, 2006). Consequently, there is now great of this looping P-T path is similar for rocks opment of dramatic lateral thermal gradi- potential for use of this natural fi eld laboratory across the entire traverse; however, signifi - ents and discontinuities without breaks in to answer fi rst-order questions about middle to cant differences in peak temperatures are crustal level is attributed to: (1) spatially lower crustal processes. In particular, to what recorded (~500 to >750 °C). Notable varia- heterogeneous advective heat fl ow delivered extent does the block-type architecture refl ect tions along the transect are also primarily by dense granitic pegmatite dike complexes the original accretionary geometry of the oro- thermal in nature and include differences in and (2) local transcurrent displacements gen as opposed to the result of subsequent upper the temperature of the prograde history (i.e., along block-bounding high-strain zones over to middle crustal tectonism? How susceptible is early andalusite versus kyanite), equilibrium an ~15–20 m.y. time interval. Exhumation of the middle continental crust to fl ow as a channel the transect from 25 to 12 km depths is inter- during orogenesis (e.g., Beaumont et al., 2004)? preted to refl ect erosion synchronous with And, how does rheology of continental crust †Corresponding author e-mail: gdumond@geo. penetrative development of steeply dipping evolve through time (e.g., Klepeis et al., 2004)? umass.edu. ‡Current address: Mail Stop 100-23, Division of NE-striking foliations and steeply plung- The answers to these questions have important Geological and Planetary Sciences, California Institute ing stretching lineations, consistent with an implications for our understanding of the evolv- of Technology, Pasadena, California 91125, USA. orogen-scale strain fi eld involving NW-SE ing strength and behavior of continental crust

GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 202–220; doi: 10.1130/B25903.1; 12 ﬁ gures; Data Repository item 2007007.

and their high-strain zone boundaries with an emphasis on understanding the character, rheol- WYOMING ogy, and exhumation of orogenic middle con- PROVINCE tinental crust (Figs. 2 and 3). Previous work CBCB North SGSG established the general structural and geochro- Pacific America FM-LMFM-LM MMMM nologic framework for the region (Ilg et al., Ocean 1996; Hawkins et al., 1996; Ilg and Karlstrom, Atlantic FCFC IRIR Fig.1 Ocean MOJAVE 2000). This paper focuses on a suite of samples PROVINCE N HSHS with semipelitic to pelitic bulk compositions and utilizes detailed microstructural observa- 1000 km tions, high-resolution electron microprobe X-ray mapping, published petrogenetic grids, BCBC Proterozoic shear zones and quantitative absolute and relative thermo- with thrust sense barometry to construct detailed P-T-D histories YAVAPAI NENE Precambrian exposures PROVINCE for each tectonic block. Data from each of the blocks, along with published geochronologi- CRC SCSC R Grand FOFO cal data, demonstrate that a single, composite looping P-T-t-D path can be constructed for the GNGN Canyon PLPL RTRT Fig.2 PTPT entire Upper Granite Gorge. The results represent a new synthesis of the metamorphic evolu- BWBW o tion of Precambrian basement in the Grand Can- MSMS MBMB SHSH 35 N MGMG yon. New techniques for reducing uncertainty in MTM MAMA T SCSC thermobarometry are applied to evaluate P-T FPFP MAZATZAL differences between blocks with maximum CFCF PROVINCE precision (i.e., Worley and Powell, 2000). The present-day exposure represents a near-isobaric ~0.7 GPa level (~25 km paleodepths) of continental crust, exhumed to 0.3–0.4 GPa levels as N a single, relatively coherent tectonic block by ca. 1.68 Ga. 100 km 109o W GEOLOGIC BACKGROUND Figure 1. Map of exposed Proterozoic rocks in southwestern Unites States with major prov- inces and shear zones and location of Grand Canyon (modiﬁ ed after Karlstrom and Wil- The Upper Granite Gorge of the Grand Can- liams, 2006). CB—Cheyenne belt; FM-LM—Farwell Mountain–Lester Mountain shear yon is divided into six lithotectonic blocks, sepa- zone; SG—Skin Gulch shear zone; FC—Fish Creek–Soda Creek shear zone; MM—Moose rated by NE-striking, steeply dipping, high-strain Mountain shear zone; IR—Idaho Springs–Ralston shear zone; HS—Homestake shear zone; zones (Figs. 2 and 3) (Ilg et al., 1996; Hawkins BCG—Black Canyon; NE—Needle Mountains; BW—Big Wash shear zone; GN—Gneiss et al., 1996). From southeast to northwest, these Canyon shear zone; CR—Crystal shear zone; MS—Mountain Spring shear zone; MB— blocks have been named: Mineral Canyon, Mesa Butte fault; CF—Chaparral fault; SH—Shylock shear zone; MG—Moore Gulch Clear Creek, Trinity Creek, Topaz Canyon, fault; MA—Mazatzal thrust belt; SC—Slate Creek shear zone; FP—Four Peaks shear Tuna Creek, and Walthenberg Canyon (Figs. 2 zone; SP—Spring Creek shear zone; FO—Fowler Pass shear zone; PL—Plomo thrust; and 3) (Hawkins, 1996). The tectonic features RT—Rincon thrust belt; PT—Pecos thrust; MT—Manzano thrust belt. of the Upper Granite Gorge are attributed to an

early event of thrusting (D1), isoclinal folding, and penetrative NW-striking fabric development during orogenesis and the subsequent stabiliza- metamorphic thermal and baric gradients are (S1), followed by progressive subhorizontal con- tion and preservation of continental lithosphere. important prerequisites for exploiting these “ﬁ eld traction (D2) and production of a NE-striking,

Field transects of tilted crustal sections or laboratories.” The Upper Granite Gorge of the steeply dipping foliation (S2) associated with large exposures of exhumed middle to lower Grand Canyon is a 100% exposed cross section upright to overturned folds within blocks and continental crust represent important laboratories of six crustal blocks along an ~70-km-long tran- subvertical high-strain zones between blocks for understanding the character and evolution of sect. The cross section is interpreted to represent (Ilg et al., 1996; Ilg and Karlstrom, 2000). Four deformation, magmatism, and metamorphism at the mid-level of 30- to 40-km-thick continental granite pegmatite dike complexes occur within different crustal levels (e.g., Proterozoic rocks crust at the culmination of the ca. 1.72–1.68 Ga the six-block transect and locally intrude across of northern New Mexico: Grambling, 1986; the Yavapai orogeny (Ilg et al., 1996), interpreted as some of the high-strain zone boundaries. From Kapuskasing Uplift: Percival and West, 1994; the a consequence of accretionary tectonism involv- southeast to northwest, these are the Cotton- Fiordland-Westland orogen: Klepeis et al., 2003; ing juvenile, island-arc terrane assembly of thin wood, Cremation, Sapphire, and Garnet pegma- the East Athabasca granulite terrane: Mahan and (15- to 25-km-thick) crustal fragments (Bowring tite complexes (Fig. 3) (Ilg et al., 1996). Loca- Williams, 2005; Flowers et al., 2006). Establish- and Karlstrom, 1990). tions in the Grand Canyon mentioned herein are ing comprehensive P-T-t-D (pressure-tempera- This contribution explores the signiﬁ cance designated by the number of river miles (RM) ture-time-deformation) paths and quantifying of the tectonic blocks, their P-T-t-D evolution, downstream of Lee’s Ferry, Arizona.

Geological Society of America Bulletin, January/February 2007 203 Dumond et al.

78 067 1 90 2 S

A' S = S

t Vishnu r zone shear t u G

r Block

River mile n s in Grti GrtG i e S d d i Mineral Canyon x 69 Block t 228 Oxides O r 2 0.5 mm GrtG Clear Creek S 59 ate key microstructural observa- ate key microstructural d n 2 ection in plane-polarized light parallel to And A S B

Mile 84

ight oriented with respect to north on the map ight oriented with respect

Angel shear zone shear

2 mm Bright t r l i 82 Grt G S + 2 t S Bt+Sil B 240 A Mile 79 0.5 mm 1 t S

St S 2 = S

1 t i S Ms r S +Bt S

= S GrtG

i S

2 mm shear zone shear t 96-Mile Bt B 2 Sil S 2 And after S 85 210

Trinity Creek Block Trinity Bass E Qtz

vein shear zone shear Block Mile 98.3 C Tuna Creek Tuna Mile 95.5

t Crystal

r shear zone shear GrtG 1 S t

r = S

i 0.5 mm GrtG S foliation trajectories 074 2 t S 77 Bt B Block t t r r 25' W GrtG ° GrtG 2 2 S 83 S

117 Walthenberg Canyon Walthenberg = S D 263

i 2 mm Granite Granodiorite/ rocks Ultramafic and Brahma Rama schist schist Vishnu S Mile 96.7

0 ed geologic map of Upper Granite Gorge with foliation trajectories (adapted from Ilg et al., 1996). Insets A through F illustr F through A Ilg et al., 1996). Insets Granite Gorge with foliation trajectories (adapted from ed geologic map of Upper Explanation t Bt B eld-oriented hand samples collected in each lithotectonic block discussed in the text. Inset A is a kinematic view of thin s A eld-oriented hand samples collected in each lithotectonic block discussed the text. Inset N

A kilometers gure. Inset C is from a map view slab cut from a hand sample. Mineral abbreviations are after Kretz (1983). Kretz after are a hand sample. Mineral abbreviations a map view slab cut from Inset C is from gure. 02' N ° F 05

River Mile 12 36 Mile 108 Figure 2. Simplifi 2. Figure fi tions from map view thin sections in plane-polarized l from are to the foliation. Insets B and D–F the mineral lineation and perpendicular in the fi

204 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

Block-Bounding High-Strain Zones

Five high-strain zones characterized by

intense subvertical, NE-striking, S2-parallel foli- 5 0 -5 ation deﬁ ne boundaries between the six lithotec- Up

river tonic blocks. The Vishnu shear zone (RM 81:

99-78-1

W Fig. 3) marks the boundary between the Mineral

K3-78.9 3-79-1

Block Canyon and Clear Creek blocks. A penetrative Mineral W Canyon NE-striking foliation in the host schist is deﬁ ned

zone by sigmoidal pods of sillimanite and muscovite Vishnu shear Vishnu

3-80.6-3 W after sillimanite. The muscovite is texturally late

with respect to the peak assemblage but synkine-

83-1 matic with respect to deformation. New ﬁ eld 5-82.8-5

W work indicates sinistral sense of shear along a K3-83.8 moderately SW-plunging lineation in these rocks Cottonwood

Clear I3-84.0-3 adjacent to the eastern (up-river) margin of the pegmatite complex

Creek Block tens-of-meters-thick Grapevine Camp pluton

(1737 ± 1 Ma; Hawkins et al., 1996) (Fig. 2A).

shear zone shear Sinistral, NW-side-down fabrics in the shear SE Angel Bright zone are cut by granitic dikes similar to ones A' dated by Hawkins et al. (1996) in the Mineral Canyon block at 1.685–1.68 Ga. The bound- Cremation ary between the Clear Creek and Trinity Creek Figure 3. Geologic cross section depicting lithotectonic 3. Geologic cross Figure Ilg et al., blocks and high-strain zone boundaries (after thermobarometry 1996). Locations of samples used for reference. onto the section for projected are

pegmatite complex P1-90 blocks is deﬁ ned by the Bright Angel shear

Trinity zone (RM 88) (Figs. 2 and 3). The ~2-km-wide

B91-2 zone of penetrative migmatitic foliation experi- Creek Block

shear zone shear enced a component of dextral high-temperature 96-Mile 96-Mile movement inferred from microstructures and

a macroscopic transposition of NW-striking

K03-95.5-3

96-5 S fabrics (Ilg et al., 1996). The ~500-m-wide G03-95.6-2 1

W 96-Mile shear zone separates the Trinity Creek

Block

Topaz 7-97.4-1

Canyon

K6-96.7 block from the Topaz Canyon block. The locally zone B98-2

Colorado River mile ultramylonitic zone records greenschist-grade,

Crystal shear Crystal

7-98.3 W dextral (Karlstrom et al., 2003), and west-side- sample location projected orthogonally onto line of A-A' in Figure 2 section

down movement (Dumond et al., 2004) (Fig. 3). B91-2 The Topaz Canyon block is juxtaposed adjacent to the Tuna Creek block across the ~2-km-wide

Crystal shear zone (RM 98). Apparent dextral, K4-102.0-1 Sapphire west-side-up movement is inferred from asym-

metric folds and a map-scale deﬂ ection of S1 pegmatite complex foliation trajectories. Movement on the Crystal shear zone ceased by ca. 1.68 Ga based on the presence of crosscutting pegmatite dikes (Fig. 3)

Tuna Creek Block Tuna similar to those dated by Hawkins et al. (1996).

Finally, the Bass shear zone (RM 108) separates Brahma and Rama Schists Schist Vishnu

zone the Tuna Creek block from the Walthenberg Can-

Bass shear Bass yon block. The ~0.5-km-wide shear zone records

108 98 96108-1 88 81 W dextral, west-side-up motion in metasedimentary rocks adjacent to the northwestern margin of the

G03-108-1 ca. 1.717 Ga Ruby granodiorite pluton (Ilg et al.,

5-2 1996; Hawkins et al., 1996). Garnet P11

0 5 km Geochronologic Constraints on Timing

2.2-1,2 pegmatite complex

Granite Granodiorite/ Ultramafic rocks

Walthenberg 9-2

Canyon Block 6-11 of Magmatism, Deformation, and

B11 river W NW A

Down Metamorphism 5 0

Explanation Elevation (km) Elevation -5

Volcanogenic supracrustal rocks of the Upper Granite Gorge yield dates between 1750 ± 2 and 1741 ± 1 Ma (Hawkins et al., 1996). These rocks include the interlayered metavolcanic

Geological Society of America Bulletin, January/February 2007 205 Dumond et al.

Rama and Brahma schists and the metasedi- are presented in Table DR1 (see footnote 1). normally zoned plagioclase (i.e., decreasing XCa mentary (turbiditic) Vishnu schist. The age of All microprobe work was conducted using from core to rim) is common throughout these

D1 is constrained by foliated arc-related plutons the Cameca SX-50 facility at the University regions, where it defi nes the predominant S2 and crosscutting dikes to between 1730 ± 3 and of Massachusetts–Amherst. All pressures and foliation along with biotite and muscovite (see σ 1698 ± 1 Ma (Hawkins et al., 1996). D2 and the temperatures (reported with 2 uncertainties) zoning summary in Fig. 4). Muscovite and bio- peak of metamorphism are constrained by dates were calculated using THERMOCALC 3.21 tite crystals are generally unzoned, but in some on foliated granite plutons and crosscutting (average P-T approach of Powell and Holland, cases, biotite compositions may vary with prox- pegmatite dikes to between 1713 ± 2 and 1685 1994; http://www.earthsci.unimelb.edu.au/tpg/ imity to garnet (see Kohn and Spear, 2000). ± 1 Ma (Hawkins et al., 1996). Isotope dilu- thermocalc/) and the internally consistent ther- Biotite is variably altered to chlorite in most tion–thermal ionization mass spectrometry (ID- modynamic data set of Holland and Powell samples. Estimates of peak metamorphic condi- TIMS) U-Pb monazite and xenotime dates from (1998) with subsequent revisions. Activity-com- tions (summarized in the following sections and leucosomes in migmatites constrain the timing position relationships were modeled at 0.6 GPa in Fig. 4) were calculated using garnet rim and of lower granulite-grade metamorphism in the and 600 °C for all samples using the April 2000 plagioclase rim compositions with matrix bio- Mineral Canyon block to between ca. 1702 and version of AX (T. Holland, http://www.esc.cam. tite and muscovite. 1690 Ma (Fig. 3) (Hawkins and Bowring, 1999). ac.uk/astaff/holland/ax.html). Possible thermal High-T, i.e., upper amphibolite to lower Electron microprobe monazite ages from leuco- and baric gradients were evaluated using ΔPT granulite grade, conditions are preserved in the somes near the Elves Chasm orthogneiss in the “relative” thermobarometry software developed Mineral Canyon block (RM 77–81), the down- Walthenberg Canyon block (Fig. 3) support the by Worley and Powell (2000). Mineral abbre- river portion of the Clear Creek block (RM age of peak metamorphism between ca. 1700 viations are after Kretz (1983). 85–88), Trinity Creek block (RM 88–96), and and 1680 Ma (Williams and Jercinovic, 2002). the down-river portions of the Tuna Creek (RM Undeformed granitic dikes dated between General Observations Regarding Medium- 100–102) and Walthenberg Canyon blocks (RM 1685 ± 1 and 1680 ± 1 Ma crosscut leucosomes and High-Temperature Blocks 111–119). Typical peak mineral assemblages in migmatites that defi ne S2 in the Mineral Can- include Grt + Sil + Bt + Pl + Qtz ± Kfs ± Crd. yon block (Fig. 3) (Hawkins et al., 1996). All In general, fi eld and petrographic observa- Accessory phases include Mnz + Ap ± Xt ± Tur geochronologic data are consistent with burial tions, coupled with high-resolution X-ray maps, ± Ilm ± Zrn. Garnet porphyroblasts (up to 1 cm and peak metamorphism of middle continen- demonstrate that the blocks (or parts of blocks) in diameter) occur throughout these sections of tal crust during a protracted 20–30 m.y. period can be divided into medium- and high-T variet- the Upper Granite Gorge and are commonly (ca. 1.71–1.68 Ga) of subhorizontal shortening ies. All blocks display fi eld and microstructural resorbed and inclusion-poor (Fig. 2A). Most and granitic plutonism. evidence for syn-D2 growth of garnet in associa- crystals exhibit fl at zoning profi les across gar- tion with Bt + Pl + Qtz ± Ms (see also Ilg and net cores with abruptly decreasing Mg-ratio and

METAMORPHIC PETROLOGY AND Karlstrom, 2000). After summarizing general increasing XMn near the rims, which are typical MICROSTRUCTURE observations that pertain to medium- and high-T of retrograde zoning (e.g., Spear, 1993; Fig. 4). blocks along the transect, data unique to each tec- Anhedral plagioclase is reversely zoned in many Utilizing this tectonic and geochronologic tonic block are presented in separate sections. samples (Fig. 4), and the penetrative foliation is framework, we present petrologic and micro- Medium-T, i.e., upper greenschist to lower commonly deﬁ ned by biotite, laths of plagio- structural data pertaining to the tectonometa- amphibolite grade, regions along the transect clase, mats of ﬁ brolite, prismatic sillimanite, morphic evolution of the Upper Granite Gorge include the up-river portion of the Clear Creek and locally cordierite. and place each block in the context of a P-T-t-D block (RM 81–85), the Topaz Canyon block Except where noted, metamorphic condi- path. Of the over 700 samples available, 24 (RM 96–98), and up-river portions of the Tuna tions in high-T samples were calculated using oriented samples of pelitic to semipelitic bulk Creek (RM 98–100) and Walthenberg Canyon core compositions for garnet and plagioclase composition that span the length of the tran- blocks (RM 108–110). Typical peak mineral with matrix biotite and/or muscovite for the sect were chosen for detailed analysis (Figs. 3 assemblages include Grt + Bt + Ms + Pl + Qtz following reasons: (1) Cores of garnet crystals and 4; Table DR11). High-resolution compo- ± St. Accessory phases include Mnz + Ap ± Xt are least affected by high-T diffusion of Fe and sitional X-ray maps of garnet and plagioclase ± Tur ± Ilm ± Zrn. Poikiloblastic chlorite locally Mg or resorption of garnet during cooling and/ were collected from all samples to assess major- occurs as a postkinematic phase overgrowing or decompression (e.g., Kohn, 2003). These element zoning and to locate the best regions the S2 foliation. With few exceptions, garnet regions are typically the highest in Mg and low- from which to collect quantitative data (Fig. 4). porphyroblasts (up to 5 mm in diameter) contain est in Fe and Mn as observed in compositional Biotite compositions were collected both in the uniformly NW-striking, straight to sigmoidal X-ray maps. (2) Thermodynamic modeling in matrix and adjacent to garnet in order to evalu- inclusion trails that occur at a high angle to the S2 the system MnNCKFMASH for the assemblage ate the potential effects of retrograde net trans- matrix foliation (Figs. 2D and 2E; Ilg and Karl- Grt + Bt + Ms + Chl + Pl + Qtz demonstrates fer reactions and to constrain the most appropri- strom, 2000). At garnet rims, the internal fab- that in the absence of other Ca-bearing phases, ate choice for P-T calculations (i.e., Kohn and ric sweeps abruptly into the matrix orientation, growth of garnet necessarily leads to produc- Spear, 2000). Representative compositions used which is consistent with synkinematic growth tion of a more sodic plagioclase (Spear et al., for thermobarometry and analytical conditions of garnet during D2 (Fig. 2D). Locally, garnet 1991). The presence of distinct calcic rims on

crystals overgrow the steep S2 matrix foliation plagioclase, particularly near garnet, and the (Fig. 2F). Samples least affected by retrograde locally resorbed appearance of garnet in sam- 1GSA Data Repository item 2007007, silicate resorption or diffusion exhibit increasing Mg- ples throughout the high-T blocks are consistent compositions, analytical methods, and color versions ratio from core to rim with decreasing X and with breakdown of garnet and growth of anor- of Figures 5, 6, 8, 9, and 10, is available on the Web Mn at http://www.geosociety.org/pubs/ft2007.htm. Re- XCa, which are typical of prograde growth zon- thitic plagioclase during decompression (except quests may also be sent to [email protected]. ing (see Spear, 1993; Kohn, 2003). Lath-shaped, where noted), and, hence, the sodic plagioclase

206 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

o Sample GrtMg# GrtXCa GrtXMn PlXAn P (GPa)* T ( C) D1 D2 .13 .021 .20 .23 .22 .17 W99-78-1 .017 0.47 +/- 0.29 750 +/- 144 .08 Chl .07 .05 Bt K3-78.9 .17 .03 .32 0.53 +/- 0.20 752 +/- 133 .03 .30 Ms Grt .14 .05 .10 .33 Sil W3-79-1 .04 .07 .28 0.65 +/- 0.21 735 +/- 135 Kfs

Mineral Canyon Sp .10 .09 .03 .12 .23 W3-80.6-3 .12 0.53 +/- 0.14 600 +/- 118 .06 .09

.05 .14 W5-82.8-5 .11 .03 .08 .12 0.64 +/- 0.16 518 +/- 103 Chl .09 .21 Bt .12 .06 .07 W83-1 .10 .16 0.66 +/- 0.16 552 +/- 110 Ms Grt .10 .06 .08 .19 And K3-83.8 .05 .03 0.66 +/- 0.15 532 +/- 103 .08 .14 Clear Creek Sil St .11 .10 .10 .31 I3-84.0-3 .07 .07 .28 0.66 +/- 0.13 550 +/- 82

Chl .12 .30 P1-90 .11 .04 .23 0.67 +/- 0.26 738 +/- 184 Bt Ms .025 Grt .23 .015 .15 B91-2 .14 0.59 +/- 0.08 722 +/- 83 And .015 Sil St .23 .053 .32 .28 Creek Trinity G03-95.6-2 .23 0.65 +/- 0.13 644 +/- 111 Kfs .046 .29 Crd .095 .14 .085 .34 W96-5 .20 0.74 +/- 0.16 547 +/- 101 .065 .08 .005 Chl Bt .12 .039 .26 K6-96.7 .09 .07 .20 0.70 +/- 0.17 548 +/- 111 Ms .008 Grt .063 St .09 .20 W7-97.4-1 .11 .15 0.70 +/- 0.17 552 +/- 115 Canyon Topaz .049

.08 .12 .11 .36 B98-2 .23 0.69 +/- 0.16 533 +/- 109 Chl .07 .09 Bt Ms .11 .10 .10 .33 Grt W7-98.3 .08 .30 0.75 +/- 0.12 624 +/- 56 .05 .04 Sil

Tuna Creek Tuna St .17 .045 .025 .46 K4-102.0-1b .32 0.53 +/- 0.10 768 +/- 94 Crd .03 .01 .10 .17 .06 .07 .21 G03-108-1 0.66 +/- 0.17 654 +/- 132 Chl .04 .04 Bt .135 .05 .135 .27 Ms W6-112.2-1b .20 0.69 +/- 0.15 648 +/- 123 Grt .035 .05 Ky .105 .06 .065 .32 Sil B119-2 .24 0.64 +/- 0.38 622 +/- 306 .04 .045

o Canyon Walthenberg presence inferred in all samples GrtMg# GrtXCa GrtXMn PlXAn P (GPa)* T ( C) presence observed in all samples presence observed in some samples Figure 4. Summary of garnet (Grt) and plagioclase (Pl) zoning proﬁ les, average pressure-temperature (P-T) estimates calculated from THERMOCALC 3.21 (Powell and Holland, 1994) with 2σ uncertainties, and observed/inferred mineral paragenesis for each lithotectonic block in the Upper Granite Gorge. Mineral abbreviations are after Kretz (1983).

Geological Society of America Bulletin, January/February 2007 207 Dumond et al.

cores best approximate the compositions that portions and absent from the outer portions of H2O dissolved in the melt left the system upon may have been in equilibrium with garnet near the same garnet porphyroblasts, implying that crystallization (Spear et al., 1999). the metamorphic peak. spinel and quartz were not in equilibrium with one another. K-feldspar and plagioclase occur Clear Creek Block (RM 81–88) Mineral Canyon Block (RM 77–81) throughout the matrix in most samples. More calcic plagioclase also occurs as rims around Peak mineral assemblages in the Clear Creek The typical mineral assemblage in pelitic relict K-feldspar cores in some samples. Mus- block indicate middle amphibolite–facies condi- rocks of the Mineral Canyon block is Grt + Bt covite is rare, although Hawkins and Bowring tions from RM 81–85 grading to upper amphibo- + Sil + Kfs + Pl + Qtz ± Spl, which suggests (1999) reported the local preservation of early lite–facies conditions from RM 85–88. The char- upper amphibolite– to granulite-facies condi- muscovite largely replaced by sillimanite. acteristic assemblage in the medium-T portion tions. Migmatitic textures are common with of the block includes Grt + St + Bt + Ms + Pl centimeter- to decimeter-scale lenses of leuco- Phase Equilibria and P-T Path + Ilm + Qtz. The high-T portion of the block is some distributed in paleosomal biotite- and silli- Mineral assemblages and migmatitic textures characterized by Grt + Sil + Bt + Pl + Qtz (Ilg manite-rich schist (Ilg et al., 1996; Hawkins and indicate that rocks in the Mineral Canyon block et al., 1996). Large (centimeter-scale) andalusite Bowring, 1999). underwent anatexis. Using textural observations porphyroblasts, interpreted as postpeak and syn- Garnet porphyroblasts are commonly and the petrogenetic grid developed by Spear to postkinematic (see following), occur in rocks embayed and surrounded by plagioclase and/or et al. (1999) in the NKFMASH system, we near the mouth of Clear Creek Canyon (RM 84) K-feldspar, indicating signifi cant retrograde gar- infer a clockwise P-T path (Fig. 5C). Assuming (Figs. 2B and 6B). Dikes similar to the ones dated net resorption (Figs. 5A and 5B; see Fig. DR1 insignifi cant free H2O in the subsolidus system, by Hawkins et al. (1996) at ca. 1.68 Ga at RM 80 for color version [see footnote 1]). Garnet com- melting is interpreted to have proceeded above are exposed in Clear Creek Canyon. These dikes positional zoning varies depending on the size invariant point 1 (IP1) via the Ms-dehydration are locally transposed into S2 and boudinaged of the porphyroblasts. Although Mg-zoning melting reaction (Spear et al., 1999): along L2 (Ilg et al., 1996; this study). is essentially fl at for all garnet crystals, larger Garnet porphyroblasts characteristically porphyroblasts (>5 mm) commonly preserve Ms ± Pl + Qtz → Sil + Kfs + L. (1) have an inclusion-rich core (Qtz + Ilm + Ap) prograde zoning in Mn and Ca, whereas smaller and inclusion-poor rim (Fig. 6A; see Fig. DR2 porphyroblasts (<5 mm) display fl at compo- The absence of Ms and the abundant asso- for color version [see footnote 1]). The inclu- sitional profi les for all major elements. This is ciation of Grt + Bt + leucosome are interpreted sion-poor garnet rims locally appear to have interpreted to be a consequence of compositional to represent completion of reaction 1 and the preferentially grown on the sides of garnet clos- homogenization at high temperatures (Fig. 5B). onset of melting via the Bt-dehydration melt- est to staurolite (see following). Inclusion trails Such zoning and the presence of resorbed ing reaction: in garnet rims are parallel to the matrix fabric. textures are indicative of late Fe-Mg exchange Trails in garnet cores curve gradually toward → and net transfer garnet-consuming reactions Bt + Sil Grt 1 + Kfs + L. (2) the matrix/rim fabric (S2) orientation (Fig. 6A). (Kohn and Spear, 2000; Kohn, 2003). In some These observations are consistent with those of samples, a marked increase in Ca across a sharp The presence of extensively resorbed garnet Ilg and Karlstrom (2000), who interpreted garnet discontinuity occurs near the margins of garnet and clots of prismatic sillimanite and coarse to have grown during the development of S2. The porphyroblasts. These rims are interpreted as a biotite concentrated around garnet (Figs. 2A inclusion-poor overgrowths are characterized by second generation of garnet (Grt 2 in Fig. 5A). and 5A) may be attributed to reaction 2 operat- higher grossular content (Figs. 4 and 6A). They occur semicontinuously around embayed ing in reverse (e.g., path 1 of Spear et al., 1999). Staurolite porphyroblasts (mm to cm scale) Grt 1 margins, including the margins of small Such a reaction crossed on the cooling and/or contain abundant inclusions and well-defi ned isolated grains inferred to be relicts of the origi- decompression path is likely responsible for the curvilinear inclusion trails similar to those in nal Grt 1 porphyroblasts (e.g., Fig. 5A, upper calcic plagioclase that is commonly localized garnet (Fig. 6A). However, the trace of the right). These observations suggest that growth around resorbed garnet margins. We infer one innermost inclusion trail fabric commonly of Grt 2 occurred after signifi cant resorption of or more transient episodes of reheating follow- makes a smaller angle relative to the S2 matrix Grt 1 had already occurred. The most abundant ing initial retrogression to explain the growth of foliation than the trace of the internal folia- matrix plagioclase, commonly localized around Grt 2. These thermal spikes were then followed tion in garnet cores in the same thin section garnet (Fig. 4; Pl 1 in Fig. 5A), is of intermedi- by continued cooling and decompression result- (Fig. 6A). The margins of many staurolite por- ate anorthite content, and early sodic cores are ing in further retrogression of the high-T assem- phyroblasts contain an inclusion trail fabric locally present. Texturally latest and most calcic blage (Fig. 5C). continuous with the matrix S2 fabric (Fig. 6A). plagioclase (Pl 2 in Fig. 5A) is entirely restricted Peak metamorphic conditions were calcu- Partial resorption of staurolite is indicated by to the margins of garnet and specifi cally occurs lated from three samples using Grt + Bt + Sil + irregular margins and plagioclase-rich domains where Grt 2 rims are narrow or absent. This is Pl + Qtz equilibria and yielded temperatures of surrounding the porphyroblasts. Staurolite most consistent with a second stage of garnet 730–750 ± 144 °C and pressures of 0.47–0.65 composition does not vary signifi cantly across resorption, consuming both Grt 1 and Grt 2. ± 0.21 GPa (Fig. 4). In the sample illustrated in the grains. The microstructural observations Sillimanite occurs as dense mats of fi brolite Figure 5, peak conditions were calculated at 735 suggest that staurolite growth began relatively distributed throughout the biotite-rich matrix. ± 135 °C and 0.65 ± 0.21 GPa, whereas condi- later than garnet cores, having overgrown S1 Clots of sillimanite and coarse biotite are typi- tions calculated using Grt 2 and Pl 2 composi- domains that were more transposed. Garnet cally concentrated around resorbed garnet portions are 637 ± 115 °C and 0.50 ± 0.18 GPa. rims grew during S2 (i.e., concurrent with fi nal phyroblasts. In one sample, spinel is included in The absence of late muscovite (outside of the transposition of S1 domains) either after stau- the outermost portion of garnet porphyroblasts. Vishnu shear zone) implies that the retrograde rolite growth ceased or during breakdown of Quartz inclusions are abundant in the innermost path passed below IP1 (Fig. 5C), assuming that staurolite. The inferred paragenesis of garnet

208 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

GrtGrt2 1.0 PlPl2 NKFMASH A Ca Kα 1 PlPl2 GrtGrt1 Ms+Ab 2 GrtGrt2 As+Kfs+L Grt+Kfs+L IP4

Bt+Sil GrtGrt1 0.8

GrtGrt GrtGrt2 d+L 2 W3-79-1 Bt+Sil Grt Grt+Cr PlPl 1 2 0.6 PlPl1 1 mm Grt Bt+Grt Ky 2 Sil Opx+Crd+L

Pressure (GPa) 0.4 And IP1 High Mn Kα B IP2 Kfs+Ab+Grt+Cr 0.2 L

Ms+Ab d+Bt+V As+Kfs+V IP3

0 1 mm 500 600 700 800 Low C Temperature (°C)

Figure 5. Summary of data and pressure-temperature (P-T) path for Mineral Canyon block. (A–B) High-resolution X-ray maps of garnet (Grt) and matrix. Lighter colors correspond to relatively greater concentrations of the element, and dark colors represent low concentrations in all X-ray maps. Calcium map in A illustrates two generations of garnet and plagioclase (Pl). Arrows point to sharp boundary between Grt 1 and Grt 2. (B) Map of retrograde Mn zoning in garnet. (C) Partial NKFMASH petrogenetic grid after Spear et al. (1999) with location of reactions 1 and 2 discussed in text. See Figure DR1 for color version of ﬁ gure (see text footnote 1). Mineral abbreviations are after Kretz (1983). Note: IP1, IP2, and IP3 refer to invariant points in P-T space discussed by Spear et al. (1999).

growth, resorption, and regrowth in association The inclusion trail fabric is primarily deﬁ ned Grt + Chl ± Ms → St + Bt. (3) with staurolite is supported by observations of by ilmenite with rare muscovite, biotite, and relatively high-Y annuli in garnet coincident plagioclase. Garnet inclusions in the anda- This reaction implies that garnet growth with the edge of inclusion-rich garnet cores. lusite porphyroblasts contain inclusion-rich ceased and was possibly consumed with increas-

Resorption of early garnet left a zone in the cores deﬁ ning S1 and high-Ca inclusion-poor ing temperature. A period of garnet resorption is matrix enriched in Y that was subsequently rims, similar to matrix garnet (Fig. 2B). Fine- indicated by the occurrence of high-Y annuli out- incorporated into garnet rims during renewed grained mats of randomly oriented ﬁ brolite side the inclusion-poor garnet cores. Staurolite growth (i.e., Pyle and Spear, 1999). locally occur along grain boundaries between porphyroblasts show textural evidence for exten- Biotite occurs as a matrix phase aligned in the host andalusite and its inclusions, as well as sive resorption, while inclusion-poor garnet over-

S2 and as millimeter-scale porphyroblasts that within late shear bands. The presence and ori- growths imply a renewed period of garnet growth. have curvilinear inclusion trail fabrics similar to entation of these mats suggest a second period Renewed garnet growth and staurolite consump- garnet and staurolite. Muscovite and plagioclase of sillimanite growth after andalusite. tion may have occurred via the reaction: are abundant matrix phases with compositions → that do not vary signiﬁ cantly (Fig. 4). Phase Equilibria and P-T Path St + Bt Grt + Ms + H2O. (4) Andalusite porphyroblasts up to 10 cm in Mineral assemblages indicate that the up- diameter occur in outcrops in the vicinity of river portion of the Clear Creek block experi- The signiﬁ cantly higher Ca concentrations Clear Creek Canyon at RM 84. The porphyro- enced lower peak temperatures than the Min- in the garnet overgrowths (Fig. 6A) are inter- blasts completely overgrow the dominant eral Canyon block. However, we infer a similar preted to represent increasing pressure dur- matrix S2 fabric, suggesting post-D2 growth clockwise P-T path (Fig. 6C). Based on the ing this part of the path, based on contours (Fig. 2B). However, elongate andalusite por- textural evidence, growth of garnet cores was of grossular isopleths in the MnNCKFMASH phyroblasts are locally boudinaged and pulled followed by staurolite and porphyroblastic bio- system for similar bulk compositions (Spear et apart, with internal foliation that necks down tite growth. This likely occurred via a reaction al., 1991). into the matrix foliation, consistent with growth such as (i.e., the NKFMASH system modeled Thermobarometric calculations are remark-

of some andalusite late during D2 (Fig. 6B). by Spear et al., 1999): ably consistent among four samples studied

Geological Society of America Bulletin, January/February 2007 209 Dumond et al.

Ms+Ms+ 1.0 A GrtGrt GrtGrt NKFMASH

BtBt 3 St+Bt+V St+Ms

BtBt Grt+Chl+Ms Grt+Bt+Sil+L StSt 0.8 Syn-DSyn-D2 StSt I3-84.0-3 GrtGrt rimrim α BtBt Grt+Ms+H Ca K EarlyEarly il+V

syn-Dsyn-D St+Bt StSt 2 St+Ms Ms+Ms+ 0.6 GrtGrt

2 Grt+Bt+S BtBt GrtGrt corecore O GrtGrt BtBt 1 mm 4 GrtGrt 0.5 mm Sil Ky 2 Pressure (GPa) 0.4 1 3 B S2 228 And 81 AndA n 1 d 80 0.2 Late syn-D2 And(Fig. 6b) 2Sil after And Late syn-D2 QtzQtz 3 Andalusite, Post-D2 And(Fig. 2b) L2 Clear Creek Canyon 0 2 cm mile 84 C 500 600 Temperature (°C)

Figure 6. Summary of data and pressure-temperature (P-T) path for Clear Creek block. (A) Plane-polarized light photomicrograph from section cut perpendicular to foliation and parallel to lineation. Note trace of porphyroblast inclusion trails in staurolite (St) and garnet (Grt). Inset of Ca X-ray map depicts higher-Ca rim in Grt. (C) Partial NKFMASH petrogenetic grid after Spear et al. (1999) with location of reactions 3 and 4 discussed in text. See Figure DR2 for color version of ﬁ gure (see text footnote 1). Mineral abbreviations are after Kretz (1983).

from RM 82–84 (Figs. 4 and 6C). Peak condi- Millimeter- to centimeter-scale elongate lenses In one cordierite-bearing sample (B91–2), tions were estimated using Grt + Bt + Ms + Pl of leucosome deﬁ ne and are locally wrapped garnet porphyroblasts contain prominent high-

+ Qtz equilibria. Absolute temperatures range by the main S2 matrix foliation in some por- Ca annuli that occur just outside of inclusion- from 518 to 552 ± 110 °C. Calculated pres- tions of the block. The lenses consist of ran- rich cores (Fig. 4). Cordierite occurs as por- sures range from 0.63 to 0.66 ± 0.16 GPa. The domly oriented quartz, K-feldspar, plagioclase, phyroblasts in textural equilibrium with garnet presence of late andalusite overgrowing garnet and biotite. Late muscovite porphyroblasts are and matrix biotite (Fig. 7A). Muscovite is not (Fig. 2A) indicates decompression to the anda- commonly localized along the margins of the present in this sample, in contrast to the more lusite stability fi eld sometime after the pas- lenses. Poikiloblastic K-feldspar occurs locally typical cordierite-absent samples throughout the sage of reaction 4. However, textural evidence in the leucosome and as a matrix phase. block. Sillimanite occurs as inclusions in cordi- for a second period of sillimanite growth after Garnet porphyroblasts (<0.5 mm to 1.25 cm) erite and as a fabric-defi ning matrix phase. Fine andalusite suggests a brief period of reheating are typically inclusion-free (Fig. 7A), but in (<300 μm) euhedral staurolite and biotite grains at a pressure signifi cantly lower than calculated some samples (e.g., sample B91–2), larger occur along the margins of some cordierite por- pressures (Fig. 6C). garnet porphyroblasts have texturally distinct phyroblasts, and they are interpreted as a post- cores that contain irregular inclusions of quartz. kinematic retrograde assemblage. Trinity Creek Block (RM 88–96) From sample to sample, garnet morphology Sillimanite occurs as elongate prismatic laths

ranges from deeply embayed to euhedral, deﬁ ning the dominant S2 foliation in association The Trinity Creek block is characterized by demonstrating variable degrees of resorption. with biotite in most samples. Fibrolitic mats of upper amphibolite– to lower granulite–grade Mg-ratio is highest in the core and decreases sillimanite occur as pseudomorphs of early mil- assemblages (Grt + Sil + Pl + Bt + Qtz ± Crd dramatically at the margins. Mn zoning is limeter-scale andalusite porphyroblasts in some ± Kfs) with local presence of sillimanite pseu- generally ﬂ at for most garnet porphyroblasts, samples (Fig. 7B). Sillimanite also occupies domorphs after early andalusite, in addition to whereas some display a gradual decrease from strain shadows adjacent to garnet and rare early texturally late muscovite and staurolite. The core to rim. Grossular content is relatively low muscovite porphyroblasts. area contains abundant leucocratic quartz seg- in the core and commonly increases toward the Field and petrographic observations are con- regations, pegmatite pods, and granitic dikes. rim (Fig. 4). sistent with a single episode of early andalusite

210 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

A 1.0 Bt+SilBt+Sil CrdCrd NKFMASH 1

Ms+Ab CrdCrd As+Kfs+L 2 CrdCrd Grt+Kfs+L SilSil 0.8 Bt+Sil 5 BtBt CrdCrd Bt+SilBt+Sil Bt+Sil GrtGrt GrtGrt Grt+Crd+L 0.6 BtBt B91-2 leucosomeleucosome Syn-DSyn-D 1 mm 2 BtBt Ky Sil GrtGrt

Pressure (GPa) 0.4 And IP1 B TurTur EarlyEarly Bt+Grt S IP2 Opx+Crd+L 2 syn-Dsyn-D2 SilSil AndAnd Kfs+Ab+Grt+Cr +Qtz+Qtz SilSil 0.2 (Figs. 2c,7b) L

SilSil afterafter AndAnd SilSil Ms+Ab d+Bt+V SilSil As+Kfs+V IP3 +Qtz+Qtz 0 500 600 700 800 o 0.25 mm Temperature ( C) BtBt C Figure 7. Summary of data and pressure-temperature (P-T) path for Trinity Creek block. (A–B) Plane-polarized light photomicrographs. Section in A is in plan view; section in B is oriented parallel to foliation and parallel to lineation. (C) Partial NKFMASH petrogenetic grid after Spear et al. (1999) with location of reactions 1, 2, and 5 discussed in text. Mineral abbreviations are after Kretz (1983).

growth in the Trinity Creek block. Pseudo- et al., 1999) (Fig. 7C). High-Ca annuli in some system was not closed, and water was added at morphs of andalusite replaced by sillimanite samples, such as B91–2, may be explained by some later time. Textural evidence indicating occur in 94-Mile Canyon (Fig. 7B). Beards of renewed garnet growth after passage of reaction syn- to late-D2 muscovite growth suggests that sillimanite and interstitial quartz occur in strain 1 in the divariant ﬁ eld between reactions 1 and: muscovite was reintroduced shortly after peak shadows of the pseudomorphs. South of the metamorphism; this is most consistent with cool- river at RM 95.5 in Travertine Canyon (sample Bt + Sil + Qtz → Grt + Crd ± Kfs + L. (5) ing above IP1. However, the difference in pres- K03–95.5–3 in Fig. 3), large (~5 cm in diam- sure between the Mineral Canyon and Trinity eter) poikiloblastic andalusite porphyroblasts Because albite is consumed in reaction 1, Creek blocks at this stage in the cooling history overgrow a NW-striking, steeply dipping inclu- renewed garnet growth must occur with higher need not have been large (see path in Fig. 7C). sion trail (S1). The internal fabric sweeps into the grossular content in order to maintain equilib- matrix foliation at the margins of the andalusite rium with new higher-Ca plagioclase (Spear et Topaz Canyon Block (RM 96–98) porphyroblasts, suggesting early synkinematic al., 1999) (Fig. 4). The local occurrence of Grt

growth with respect to S2 (Fig. 2C). + Crd replacing sillimanite indicates that reac- The Topaz Canyon block experienced upper tion 5 was reached in rocks of appropriate bulk greenschist– to lower amphibolite–grade peak Phase Equilibria and P-T Path composition (Fig. 7C). metamorphic conditions. No pegmatite dikes are Textural evidence for early andalusite indi- Peak conditions were estimated using Grt + exposed in the block, but two weakly foliated pre- cates an early history of relatively low-pressure– Crd + Sil + Pl + Bt + Qtz and yield 722 ± 83 °C D2 granodiorite plutons are present: the 96-Mile high-temperature metamorphism (Fig. 7B). and 0.59 ± 0.08 GPa (sample B91–2: Fig. 4). pluton at RM 96.5 and the Crystal pluton at RM However, the relatively high-grade peak assem- Unlike rocks in the Mineral Canyon block, 97.5 (Hawkins, 1996). The predominant rock blage and leucosome textures in rocks from most rocks in the Trinity Creek block contain type is Grt + Bt + Ms + Pl + Qtz ± St ± Chl schist Trinity Creek block suggest that conditions of abundant texturally late muscovite. This implies with a penetrative E- to NE-striking, steeply dip- anatexis were attained. The absence of musco- either: (1) the rocks cooled above IP1, so that ping foliation (S2). Locally, the schist is associ- vite and abundance of sillimanite in the peak crystallization of the remaining melt across reac- ated with psammitic layering and graded bedding assemblage supports the passage of reaction 1 tion 1 provided the necessary H2O for muscovite (S0), which are interpreted as components of rel- (in the NKFMASH system modeled by Spear production (e.g., Spear et al., 1999), or (2) the ict turbidite sequences (Ilg et al., 1996).

Geological Society of America Bulletin, January/February 2007 211 Dumond et al.

44 271 Mg Kα 1.0 A Down-plunge B Fold Profile NKFMASH 3 S2 View D

s +Sil+L Bt S2 W96-5 t +M

l t+Ms +B Syn-D2 S Bt+V

+ 0.8 Grt rim Grt St

Grt+Ch

Mn Kα 0.6 1 mm Low High 1 mm S1

Ky Sil l+V +Si α α St+Ms Bt

Ca K Ca K F Pressure (GPa) C E 0.4 + 1 mm ~An21 And rt G Pl rim S2

0.2

~An31 core 0 0.25 mm 500 600 e) G Temperature (°C)

Figure 8. Summary of data and pressure-temperature (P-T) path for Topaz Canyon block. High-resolution X-ray maps for (A) Mn, (B) Mg, (C) Ca in garnet, and (E) Ca in matrix plagioclase. (D) Enlarged sketch of biotite porphyroblast from (B) highlighting preservation of subhorizontal inclusion trails. (F) Enlarged sketch of zoning in plagioclase in (E) highlighting alignment of sodic rims subparallel to S2. Images are from a thin section cut perpendicular to the fold-axis. (G) Partial NKFMASH petrogenetic grid after Spear et al. (1999). See text for discussion and Fig. DR3 for color version of ﬁ gure (see text footnote 1). Mineral abbreviations are after Kretz (1983).

Garnet porphyroblasts observed in oriented, with the S2 cleavage (Figs. 8E–8F). The align- 1714 ± 1 Ma in the 96-Mile pluton (Hawkins, map-view thin-sections contain uniformly ment is interpreted to reﬂ ect syn-S2 growth of 1996), in contrast to ca. 1.68 Ga titanite in the NW-striking straight to sigmoidal inclusion albitic plagioclase. high-T Mineral Canyon block (Hawkins and trails that occur at a high-angle to the S2 matrix Bowring, 1999) (closure temperature of ~550– foliation (Fig. 2D; Ilg and Karlstrom, 2000). At Phase Equilibria and P-T Path 660 °C; Cherniak, 1993). Together with the garnet rims, the internal fabric sweeps abruptly Peak metamorphic conditions were calculated apparent lack of signiﬁ cant retrograde re-equili- into the matrix fabric, which is consistent with the assemblage Grt + Bt + Ms + Pl + Qtz, uti- bration, these observations suggest that rocks with synkinematic growth of garnet during D2 lizing garnet rims, matrix biotite and muscovite, from the Topaz Canyon block cooled relatively (Fig. 2D). With few exceptions, most samples and plagioclase rims (all inferred to represent quickly after attaining peak pressure (e.g., P-T exhibit increasing Mg-ratio from core to rim, syn-D2 conditions based on our microstructural path for sample W96–5 in Fig. 8G). whereas XMn and XCa decrease from core to observations, e.g., Figs. 8A–8D). Calculated rim, typical of prograde zoning (Kohn, 2003; temperatures and pressures from four samples Tuna Creek Block (RM 98–108) Figs. 4 and 8A–8C; see Fig. DR3 for color ver- in the block range from 533 to 552 ± 115 °C sion [see footnote 1]). and 0.69–0.74 ± 0.16 GPa (Figs. 4 and 8G). The majority of the upper amphibolite–

Muscovite and biotite defi ne a penetrative S2 Some samples contain rare staurolite, implying grade Tuna Creek block consists of the Ruby foliation. Locally, poikiloblastic biotite defi nes that peak conditions locally approached ~550– granodiorite pluton (RM 102–107.5) (1716.6 the S2 cleavage and contains a subhorizontal 600 °C (i.e., the staurolite-in isograd as defi ned ± 0.5 Ma) and the Sapphire pegmatite complex inclusion trail, observed in fold-profi le sections, by Spear and Cheney [1989] in the KFMASH (RM 99–102) (Hawkins et al., 1996). Two dis- which is inferred to represent the trace of S1 system), within the uncertainty of calculated tinct assemblages occur in pelitic to semipelitic foliation (Fig. 8D). peak temperatures (Fig. 8g). schists between the Crystal shear zone at RM Plagioclase in the matrix is normally zoned Samples from the Topaz Canyon block argu- 98 and the southeastern margin of the pluton with lath-shaped calcic cores and sodic over- ably contain the best-preserved examples of pro- at mile 102. Nearest to the Crystal shear zone, growths (Fig. 4). In sections cut perpendicular grade syn-D2 garnet in the entire transect, and rocks consist of Grt + St + Bt + Ms + Pl + Qtz to the hinges of tight ENE-trending and moder- thus provide the best approximation for peak schist. Pelitic schists that host the Sapphire ately W-plunging F2 folds, sodic rims character- pressure. Peak temperatures (~550 °C) were not pegmatite complex at RM 102 contain Grt + istically occur as elongate overgrowths aligned high enough to reset primary titanite dated at Sil + Crd + Bt + Pl + Qtz.

212 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

Garnet porphyroblast textures and zoning in inclusion trails of quartz and plagioclase aligned respect to grossular content except for thin (up two-mica schists near the Crystal shear zone with S2 (Figs. 2E and 9A). Staurolite locally to 100 µm), high-Ca rims (Fig. 9E). These rims are similar to those observed in Topaz Canyon exhibits embayed margins and no compositional spatially coincide with a pronounced increase in (e.g., sample W7–98.3: Figs. 2E, 4, and 9A; see zoning (Table DR1, see footnote 1). Y. The high-Ca rims occur around most grains, Fig. DR4 for color version [see footnote 1]). Cordierite-biotite schists further down-river including grains that texturally appear to have Locally, however, garnet rims show marked (RM 102) contain subhedral to anhedral garnet, been part of once-larger garnet porphyroblasts, increase in XMn at the rim (50–100 µm wide) up to 5 mm in diameter, with abundant inclu- suggesting that they represent a second genera- (Fig. 4), which is typical of resorbed garnet zon- sions of quartz, apatite, biotite, oxides, mona- tion of garnet growth after initial garnet break- ing in the high-T blocks. zite, and abundant sigmoidal mats of ﬁ brolite down and resorption. These textures and zoning Plagioclase exhibits pronounced reverse zon- (Figs. 9C–9D). Garnet crystals appear to be patterns are similar to those observed in the sec- ing with lath-shaped sodic cores and distinct heavily resorbed, as indicated by relatively wide ond generation of garnet in migmatitic samples rim overgrowths of more anorthitic plagio- and irregularly shaped high-XMn rims that mimic of the Mineral Canyon block (RM 79, Fig. 5). clase (Fig. 9B). The schists contain abundant the geometry of low-XMg rims (Figs. 4 and 9C). Plagioclase is normally zoned and deﬁ nes staurolite (texturally after garnet) with straight Most garnet crystals are homogeneous with the foliation along with abundant elongate

High 1.0 NKFMASH W7-98.3 Pl 1 0.25 mm 3 Ms+Ab

As+Kfs+L AB St+Ms IP4 0.5 mm W7-98.3 Grt+Bt+Sil+L 0.8 St+Bt+V Syn-D2 Grt+Chl+Ms S2 GrtGrt Grt rim 5 Si = S1 + St BtBt Ca Kα Bt+Sil 0.6 Low Grt+Crd+L

K4-102.0 Ky Sil Grt+Sil+L St+Ms Crd+Kfs Grt G 1

S StSt Pressure (GPa) 0.4 IP1 rt C 2 And Grt+Bt+Sil+V +Kfs+L rd+Bt BtBt 6 IP2 K4-102.0 L C Kfs+Ab+Grt+Cr Grt rd+ 0.2 2 1 mm Mg Kα Bt+Grt L px+C O Ms+Ab d+Bt+V As+Kfs+V IP3

0 K4-102.0-1 500 600 700 800 Temperature (°C) F E D 1 mm BtBt CrdCrd Grt1 core GrtGrt ~Grs 3 BtBt SilSil Ca Kα Grt2 rim CrdCrd ~Grs 4.5 GrtGrt 0.5 mm E Pl+QtzPl+Qtz CrdCrd BtBt E

Figure 9. Summary of data and pressure-temperature (P-T) path for Tuna Creek block. (A, D) Plane-polarized light photomicrographs of sections cut perpendicular to foliation and parallel to lineation. (B) X-ray map of Ca in matrix plagioclase around garnet outlined in A. (C) Mg X-ray map keyed to part D. (E) Ca X-ray map of Grt in D with sketch depicting a low-Grs Grt core and slightly higher-Grs Grt rim separated by a sharp boundary. (F) Partial NKFMASH petrogenetic grid after Spear et al. (1999) with location of reactions 1, 3, 5, and 6 discussed in text. See Figure DR4 for color version of ﬁ gure (see text footnote 1). Mineral abbreviations are after Kretz (1983).

Geological Society of America Bulletin, January/February 2007 213 Dumond et al.

cordierite crystals in schists that host the Sap- Grt + Sil + Crd + Bt + Pl + Qtz equilibria with Plagioclase is zoned in all samples, with phire pegmatite complex (Figs. 4 and 9C–9D; garnet core, plagioclase rim, and matrix bio- lath-shaped albitic cores that are aligned with sample K4–102.0–1). Locally, cordierite tite compositions yield peak conditions of 0.53 S2 and discontinuous anorthitic rims. X-ray makes up as much as 60% of the mode. The ± 0.2 GPa and 769 ± 188 °C. As described ear- maps of plagioclase demonstrate that the Ca- cores of many cordierite grains contain abun- lier, garnet in K4–102.0–1 exhibits distinct high- rich rims have sharp boundaries with the albitic dant inclusions of prismatic sillimanite and Ca, high-Y rims attributed to a second phase of cores (Fig. 10B). mats of fi brolite. Rims are typically altered to garnet growth (Fig. 9E). Pairing garnet and pla- Kyanite is observed as poikiloblastic, pinnite. Sillimanite also occurs in the matrix gioclase rim compositions yields 590 ± 73 °C resorbed, and elongate blades in samples at as prismatic laths and needles that defi ne the and 0.36 ± 0.08 GPa (Fig. 4). mile 112 (Fig. 10A). Kyanite is locally mantled foliation along with biotite. Samples near and Taken together, the two-mica schists and Crd and replaced by muscovite, and elsewhere sil- within the pegmatite complex contain abundant + Bt schists provide evidence for a clockwise limanite occurs as pseudomorphs after kyanite leucosome (Grt + Pl + Qtz + Crd + Bt) textures P-T path to ~0.7 GPa followed by decompres- (Fig. 10A). Throughout the Walthenberg Can- mantled by Bt-rich selvages akin to paleosome, sion to ~0.3–0.4 GPa (Fig. 9F). Replacement yon block, in the vicinity of the Garnet pegma- suggesting that some partial melting occurred. of sillimanite and garnet by cordierite and the tite complex, sillimanite occurs as a foliation- Garnet in leucosome is locally replaced by cor- passage of reactions 5 and 6 imply decompres- defi ning matrix phase. dierite and biotite (Fig. 9C). K-feldspar and sion (i.e., Hollister, 1982; Spear et al., 1999). muscovite are absent from these samples. The second phase of garnet growth followed by Phase Equilibria and P-T Path resorption observed in K4–102.0–1 is attributed The presence of early kyanite implies load- Phase Equilibria and P-T Path to reheating below IP1 (Figs. 9E and 9F). ing followed by breakdown to sillimanite (via Samples W7–98.3 and K4–102.0–1 represent muscovite). Textures and assemblages at RM dissimilar bulk compositions, but are interpreted Walthenberg Canyon Block (RM 108–119) 112 and RM 119 suggest that temperatures did to preserve different parts of a similar P-T path not exceed those required for muscovite dehy- for the Tuna Creek block as a whole (Fig. 9F). The Walthenberg Canyon block hosts the dration melting. In those areas, the common The presence of distinct calcic rims on plagio- Elves Chasm orthogneiss (1840 ± 1 Ma) and peak assemblage of Grt + Sil + Bt + Ms + Qtz is clase, particularly near garnet, and the locally the Garnet pegmatite complex (1697 ± 1 Ma) divariant in the NKFMASH grid of Spear et al. resorbed appearance of garnet in W7–98.3 are (Hawkins et al., 1996). An apparent (1999). Thermobarometric estimates obtained consistent with breakdown of garnet and growth meta morphic fi eld gradient exists near the from garnet and plagioclase core compositions of anorthitic rims on plagioclase during decom- contact with the orthogneiss where Grt + Bt + in addition to matrix biotite yield conditions pression (Fig. 9A). Quantitative thermobarom- Ms schists (e.g., sample W108–1, Fig. 2F, and of 648 ± 123 °C and 0.69 ± 0.15 GPa (Figs. 4 etry using Grt + Bt + Ms + Pl + Qtz equilibria sample G03–108–1, Fig. 4) grade into coarse and 10C). However, abundant leucosomes in and highest Mg-ratio rim compositions in gar- migmatite (RM 115) with centimeter-scale Ms-absent samples at RM 115 demonstrate net and core plagioclase compositions in sample garnet porphyroblasts mantled by tails of leu- that partial-melting conditions were reached W7–98.3 yield peak conditions of 624 ± 56 °C cosome that contain K-feldspar, plagioclase, locally, pointing to the local passage of reac- and 0.75 ± 0.12 GPa, which are consistent with quartz, and biotite (sample P115–2 in Fig. 3; tion 1 (e.g., sample P115–2, Fig. 3; Williams the staurolite-in isograd (Spear and Cheney, Williams and Jercinovic, 2002). Kyanite and Jercinovic, 2002). Figure 10C illustrates 1989) (Figs. 4 and 9F). replaced by sillimanite occurs in an orthoam- the preferred path for these rocks. The abun- Reaction textures in sample K4–102.0–1 can phibole-rich layer at RM 112 (Fig. 10A; see dance of sharp, discontinuous anorthitic rims be interpreted using the same grid for anatec- Fig. DR5 for color version [see footnote 1]), on plagioclase and texturally resorbed garnet in tic pelites utilized for the Mineral Canyon and and is interpreted as an Al-rich weathering leucosome-absent samples (e.g., W6–112.2–1, Trinity Creek blocks (i.e., Spear et al., 1999; horizon (with ca. 1.75 Ga detrital zircon) that Fig. 10B) are interpreted as consequences of Fig. 9F). The presence of abundant inclusions developed on Elves Chasm basement (Karl- decompression soon after peak conditions of biotite and mats of sillimanite in garnet strom et al., 2003). Metarhyolite layers east of were reached (Fig. 10C). hosted by matrix cordierite support the pas- and above the contact with Elves Chasm gneiss sage of reaction 5. In the presence of a partial also contain kyanite replaced by sillimanite. DISCUSSION melt, the following reaction may account for Monazite dates from leucosomes at RM 115 the observation of garnet replaced by cordierite range from ca. 1.7 to 1.68 Ga, suggesting that Peak metamorphic conditions are commonly and biotite in the leucosomes: the fi eld gradient is only “apparent” and refl ects used to assess the magnitude of displacement Yavapai-age peak metamorphism, rather than across major structures as a step toward evalu- Grt + Kfs + L → Crd + Bt. (6) a contact aureole near the older Elves Chasm ating the tectonic signifi cance of metamorphic pluton (Williams and Jercinovic, 2002). fi eld gradients (e.g., Todd and Engi, 1997; Fra- The lack of matrix K-feldspar implies that Garnet in Bt + Ms schists near the Bass shear ser et al., 2000; Daniel et al., 2003; Tenczer these rocks were heated at pressures above zone (RM 108) and Bt + Ms + Ky/Sil schists and Stüwe, 2003). Next, we present a detailed IP1, whereas the absence of retrograde mus- (RM 112) that host the pegmatite complex comparison of thermobarometric results for covite implies the rocks cooled below IP1 contain inclusion trail relationships similar to the entire transect. We then describe how the (P < 0.38 GPa: Fig. 9F) (Spear et al., 1999). those observed in the Topaz Canyon and Clear P-T histories that we developed for each of Alternatively, removal of melt results in a K-poor Creek blocks and are interpreted to refl ect syn- the tectonic blocks (Figs. 5–10) can be used bulk composition; this explanation accounts for S2 growth of garnet (Figs. 2F and 10B). Cores to constrain a composite looping P-T-t-D path the K-feldspar–absent, restitic bulk composition of garnet locally display oscillatory zoning in for the entire Upper Granite Gorge. The results of K4–102.0–1 (e.g., see also path 4 of Spear et grossular content with distinct high-Ca rims provide important constraints for: (1) the block- al., 1999). P-T conditions calculated utilizing (G03–108–1, Fig. 4). type architecture of continental crust in the

214 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

A W6-112.2-2 BtBt 1.0 NKFMASH 1

Ms+Ab

KyKy As+Kfs+L W6-112.2-1 KyKy MsMs 0.8 Syn-D2 SilSil Grt + Sil KyKy Early 0.6 Ky BtBt SilSil 5

d+L 0.75 mm Bt+Sil SilSil Ky Sil Grt+Cr

Pressure (GPa) 0.4 IP1 And B W6-112.2-1 Mg Kα Ca Kα Na-rich Pl cores Ca-rich Pl rims 1 mm IP2 BtBt Kfs+Ab+Grt+Cr GrtGrt 0.2

1 mm L GrtGrt Ms+Ab d+Bt+V Si = S1 As+Kfs+V S2 LowLow HighHigh 0 PlPl 500 600 700 800 C Temperature (°C)

Figure 10. Summary of data and pressure-temperature (P-T) path for Walthenberg Canyon block. (A) Plane-polarized light photomicrograph illustrating reaction texture of Ky → Ms → Sil. (B) High-resolution X-ray maps of Mg in garnet and Ca in matrix plagioclase. Note trace of inclusion trail in garnet and distinct Ca-rich overgrowths on plagioclase. (C) Portion of NKFMASH petrogenetic grid after Spear et al. (1999). See text for discussion and Figure DR5 for color version of ﬁ gure (see text footnote 1). Mineral abbreviations are after Kretz (1983).

Paleoproterozoic, (2) the rheology of middle assemblage, activity-composition models, Powell, 2000). Results are illustrated at the 2σ continental crust during orogenesis, and (3) the and internally consistent thermodynamic data level of uncertainty (Figs. 11B and 11D). denudation of orogens and preservation of iso- set, much of the systematic error in thermo- baric levels of continental crust. barometry can be eliminated. One sample is Relative Thermometry (ΔT) chosen as a “baseline sample” against which Comparison and Evaluation of all other samples are compared. The approach The ΔT results show a signifi cant improve- Thermobarometric Data yields more precise estimates of differences in ment in the resolution of temperature differences pressure and temperature, providing a means from block to block (Fig. 11B). Peak tempera- Field and petrographic observations suggest to quantitatively evaluate thermal and baric tures of high-grade samples from the Mineral that peak temperatures varied signifi cantly gradients inferred from mineral assemblages Canyon, Trinity Creek, and Tuna Creek blocks along the transect and that thermal transi- and absolute thermobarometry (Hodges and all statistically exceed the baseline sample (W6– tions coincided, in some cases, with block McKenna, 1987). 112.2–1 = 639 °C) at the 95% confi dence level. boundaries. Elsewhere, the transitions more The bulk of the samples studied from the Most of the remaining samples are signifi cantly closely coincided with dense pegmatite dike Upper Granite Gorge contains one of the fol- below the baseline. Many of the high-T samples complexes (see also Ilg et al., 1996). However, lowing peak mineral assemblages or subassem- are spatially coincident with the Cottonwood traditional estimates of peak conditions and blages: Grt + Sil + Bt + Pl + Qtz or Grt + Bt + (RM 79–81), Cremation (RM 86–90), and Sap- uncertainties do not have suffi cient resolu- Ms + Pl + Qtz. Sample W6–112.2–1 was cho- phire pegmatite complexes (RM 98–102). Sam- tion for evaluating the tectonic signifi cance of sen as the baseline sample because all phases ples at RM 90–91 in the Trinity Creek block are block boundaries in the Upper Granite Gorge. in both assemblages are inferred to have been also spatially associated with centimeter-scale With few exceptions, differences in absolute in equilibrium at the peak of metamorphism; leucosomes, granitic veins, and pegmatite pods temperature and pressure are indistinguishable both subassemblages also yield nearly the same that are near the down-river extent of the Crema- at the 2σ level of uncertainty (Figs. 11A and calculated P-T conditions. Water is excluded, tion pegmatite complex. Thus, the results sup- 11C). A technique for comparing calculated and only fl uid-conserved equilibria are used port fi eld observations that entire high-T blocks P-T conditions from samples with the same for these calculations, which involve the fol- and portions of other blocks correspond to mineral assemblage or subassemblage has been lowing mineral end members as specifi ed in regions with a higher density of granitic pegma- developed recently (the ΔPT approach: Worley THERMOCALC: Ms, Cel, Phl, Ann, East, Py, tite pods and dike complexes, as suggested by and Powell, 2000). Utilizing the same mineral Alm, Gr, An, Sil, and Q (see also Worley and Ilg et al. (1996), e.g., the high-T Trinity Creek

Geological Society of America Bulletin, January/February 2007 215 Dumond et al.

on Block Topaz Clear on Block Topazon Block Clear on Block Trinity Mineral Trinity Mineral Walthenberg Block Walthenberg Block Cany Tuna Creek Canyon BlockCreek Block Creek Block Canyon Block Cany Tuna Creek Cany Creek Block Creek Block Cany

900

z 850 Bsz Vsz Bsz Vsz Csz 96sz BAs Csz 96sz BAsz 800 W6-112.2-1b

750 C) o 700

650 600

550 Temperature ( 500 450 400 116 112 108 104 100 96 92 88 84 80 76 116 112 108 104 100 96 92 88 84 80 76 A Colorado River MileAve TB Colorado River Mile T Grt + Bt + Ms + Pl + Qtz Grt + Bt + Ms + Pl + Qtz W6-112.2-1b Grt + Sill + Bt + Pl + Qtz Grt + Sill + Bt + Pl + Qtz Grt + St + Bt + Ms + Pl + Qtz Grt + Sill + Bt + Ms + Pl + Qtz = regions underlain by ~50% or more Grt + Crd + Sill + Bt + Pl + Qtz granite dikes (Hawkins et al., 1996)

1.0

Bsz Vsz W6-112.2-1b Bsz Vsz Csz 96sz BAs Csz 96sz BAsz 0.9

0.8

0.7

0.6

0.5 Pressure (GPa)

0.4

0.3 116 112 108 104 100 96 92 88 84 80 76 116 112 108 104 100 96 92 88 84 80 76 C Colorado River Mile Ave PD Colorado River Mile P W6-112.2-1b

Figure 11. Comparison of pressure-temperature (P-T) results across the Upper Granite Gorge transect. (A) Average T for all samples as reported in Figure 4 with full mineral assemblage. (B) ΔT results for samples containing the appropriate subassemblage discussed in the text. Baseline sample at mile 112 yields the same result for both mineral subassemblages in ΔT plot at right (i.e., 638 °C for Grt + Bt + Ms + Pl + Qtz versus 639 °C for Grt + Sil + Bt + Pl + Qtz). (C) Average P for all samples as reported in Figure 4 with full mineral assemblage. (D) ΔP results across the Upper Granite Gorge transect. Bsz—Bass shear zone; Csz—Crystal shear zone; 96sz—96-Mile shear zone; BAsz—Bright Angel shear zone; Vsz—Vishnu shear zone.

block versus the medium-T Topaz Canyon block entire transect. Most samples plot on or within P-T path, whereby the equilibrium pressure (compare Fig. 11B with Fig. 3). ~0.05–0.1 GPa of the baseline sample (W6– recorded at maximum temperature in a sample 112.2–1 = 0.71 GPa). Slightly lower pressures is commonly lower than the maximum pressure Relative Barometry (ΔP) are recorded in the highest temperature samples. achieved (Spear et al., 1984). Based on these This is interpreted to be a consequence of either results, we infer that samples from the high-T Relative pressures for the Upper Granite Gorge or both of the following: (1) re-equilibration blocks experienced similar pressures as samples are illustrated in Figure 11D. Even with the and/or growth of new garnet at postpeak condi- in the medium-T blocks, which implies that the increase in precision, the pressures are essentially tions, as documented in samples near RM 77–80, entire Upper Granite Gorge transect represents a indistinguishable at the 2σ level throughout the 90–95.6, 102, and 108; and/or (2) the clockwise near-isobaric, i.e., subhorizontal (0.7–0.65 GPa

216 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon

≈25 km paleodepth), level of continental crust. (compare Figs. 3 and 11B). Amphibolite-grade and Karlstrom, 1990), and (2) protracted subho- The lack of distinguishable pressure variation assemblages occur on either side of the Bright rizontal shortening accompanied by steep NE- precludes any amount of signifi cant syn- to post- Angel shear zone, and sillimanite-bearing rocks striking fabric development (D2) between 1713 peak vertical displacement along the high-strain occur in the Clear Creek block between RM 86 and 1685 Ma (Ilg et al., 1996). The presence of Δ zones. Mesoproterozoic to Phanerozoic tectonic and 88. Collectively, the PT results are most early syn-D2 andalusite (Fig. 2C) indicates that events (e.g., reactivation of the 96-Mile shear consistent with fl ux of a laterally varying source the transition from NW- to NE-striking fabrics zone [Dumond et al., 2004], Laramide mono- of heat in a subhorizontal section of middle occurred relatively early and at relatively low clines, and/or differential erosion) were insuf- continental crust that experienced minor verti- pressures in the overall tectonometamorphic his- fi cient to juxtapose different crustal levels, i.e., cal offset and local transcurrent displacement tory of the transect (#2 and #3 in Fig. 12B). relative vertical displacements were on the order along block-bounding high-strain zones. The The composite P-T-t-D path illustrates: (1) the of ~3 km or less (0.1 GPa ≈3–4 km). important role of heat advection by ascending similarities and differences between each block’s magma has been emphasized for several meta- history, (2) the unique combination of data that Implications of ΔPT Results morphic terranes, e.g., the Central Maine Belt, each block provides about the Upper Gran- USA (Johnson et al., 2003) and the Arunta Inlier ite Gorge in general, and (3) the series of ther- Apparent gradients and discontinuities of central Australia (Collins and Vernon, 1991). mal spikes attributed to fl ux of magma through revealed by the ΔT results can most likely be The absence of signifi cant differences in pressure dense pegmatite dike complexes between 1.7 and attributed to one or both of two possibilities: coupled with large differences in temperature 1.68 Ga. The prograde part of the path was differ- (1) postpeak strike-slip displacement along along the Upper Granite Gorge transect result in ent for various parts of the transect. For example, block-bounding shear zones juxtaposing sam- a stark contrast to other orogens that are charac- early kyanite was replaced by sillimanite in the ples in hotter tectonic blocks adjacent to sam- terized by a relatively large variation in pressure Walthenberg Canyon block (Fig. 10A), but silli- ples in colder tectonic blocks, or (2) proximity and little variation in peak temperature (e.g., the manite replaced andalusite in the Trinity Creek of a sample to locally elevated thermal regimes High Himalayan metamorphic sequence, Fraser block (Fig. 7B). The approach to peak conditions caused by intrusion of 1.698–1.680 Ga granite et al., 2000; the Lepontine–central Alps, Todd was controlled by both the degree of heating and dike complexes. As mentioned already, the ΔP and Engi, 1997; the Koralpe region–eastern the rate of loading along the prograde part of results preclude signifi cant dip-slip movement Alps, Tenczer and Stüwe, 2003). the path (e.g., the two example dashed paths at (>3–4 km) on the block-bounding shear zones. the origin of the P-T-t-D diagram in Fig. 12B). Abrupt thermal discontinuities exist across Composite Looping P-T-t-D Path for the Differences between initial P-T path trajectories the Vishnu shear zone (as noted by Ilg et al., Entire Upper Granite Gorge Transect from block to block can also be attributed to the

1996), which suggest a relatively signifi cant degree and style of thrust-imbrication during D1, amount of postpeak displacement. The shear Rocks from the Upper Granite Gorge have as described by Ilg et al. (1996) and Duebendor- zone coincides with an abrupt decrease in peg- been used to place constraints on the regional fer et al. (2001). matite density from SE to NW (Ilg et al., 1996), P-T-D history (Fig. 12A). Although no single The upper pressure limit on the prograde traversing down-river out of the Cottonwood tectonic block preserves a complete record of its portion of the clockwise P-T-t-D path is best pegmatite complex (Fig. 3). Field observations P-T-D history, there are a number of important constrained by near-peak pressure estimates of demonstrating sinistral sense of shear along a similarities among the blocks. Each block shows ~0.7 GPa from prograde syn-D2 garnet in the moderately SW-plunging lineation (Fig. 2A) evidence for a similar clockwise P-T path with Clear Creek and Topaz Canyon blocks (Figs. 6 and postpeak synkinematic muscovite are most at least some decompression after the metamor- and 8). The timing of peak metamorphic con- consistent with focused fl uid infl ux in the shear phic peak. We suggest that a single looping style ditions is best constrained from leucosomes zone concurrent with postpeak, oblique-slip of P-T path can explain the observations from dated in migmatitic rocks from opposite ends displacement. Thus, spatially heterogeneous all tectonic blocks (Fig. 12B). The path involves of the transect: 1702–1690 Ma based on isotope advective heat from the Cottonwood pegmatite an early stage of tectonic loading with pressure dilution–thermal ionization mass spectrometry complex and displacement across the shear zone increasing to ~0.7 GPa followed by decompres- (ID-TIMS) analysis of monazite and xenotime may have both contributed to development of a sion to ~0.3–0.4 GPa and a period of near-iso- in the Mineral Canyon block (Hawkins and thermal discontinuity across the Vishnu shear baric cooling. The looping geometry of the path Bowring, 1999) and ca. 1700–1680 Ma from zone. A second thermal discontinuity occurs is characteristic of much of the Paleoproterozoic electron microprobe dating of monazite in the across the 96-Mile shear zone (Fig. 11B). basement of the southwestern United States (Wil- Walthenberg Canyon block (Williams and Jerci- Sample G03–95.6–2 on the up-river side of the liams and Karlstrom, 1996). Pressures during the novic, 2002) (#4 in Fig. 12B). These conditions shear zone in the Trinity Creek block overlaps early stages of metamorphism varied from region were followed by a period of decompression as the baseline sample (Fig. 11B), in contrast to the to region, but essentially all regions resided at 10– inferred by garnet + sillimanite breakdown to three samples studied from the down-river side 15 km depths (i.e., ~0.3–0.4 GPa) from ca. 1.65 cordierite in the Tuna Creek block and evidence of the shear zone in the Topaz Canyon block, to 1.4 Ga (Williams and Karlstrom, 1996). throughout the Upper Granite Gorge for break- which all record lower temperatures. This is Following deposition of volcanogenic and down of garnet to plagioclase. Decompression consistent with postpeak displacement on the turbiditic supracrustal rocks between ca. 1750 to andalusite stability is best constrained in the

96-Mile shear zone inferred by Ilg et al. (1996) and 1740 Ma (#1 in Fig. 12B), rocks were trans- Clear Creek block, where late syn-D2 to post-D2 and conﬁ rmed by Dumond et al. (2004). ported to middle crustal depths during two over- andalusite occurs (Figs. 2B and 6B). Combined

Thermal gradients across other block bound- lapping periods of deformation: (1) penetrative with the presence of early-D2 andalusite in the aries are coincident with the location of dense NW-striking fabric development, thrusting, and Trinity Creek block, the observations imply that pegmatite complexes and boudinaged granitic isoclinal folding (D1) between 1730 and 1700 Ma relatively rapid burial from andalusite stability pods. Pegmatite complexes intrude into and/or (inferred here to represent the tectonic thicken- to peak pressure and decompression to andalu- across the Bright Angel and Crystal shear zones ing of originally thin, juvenile crust, i.e., Bowring site stability occurred entirely during D2. The

Geological Society of America Bulletin, January/February 2007 217 Dumond et al.

A Walthenberg CanyonTuna Creek Topaz Canyon Trinity CreekClear Creek Mineral Canyon 1.0

0.8

0.6

0.4 Pressure (GPa)

0.2

0 550 650750 550 650 750 450 550 650 550 650750 450 550 650 550 650 750 Temperature (°C)

Upper Granite Gorge, Grand Canyon, AZ

1.0 Metamorphic Peak ca.1700-1690 Ma 4 0.8 29.2

3 D2 ca.1713-1685 Ma

Grt1 0.6 21.9

2 D1 Ky ca.1730-1700 Ma Sil

Pressure (GPa) 0.4 And 14.6 Early Ky Grt2 Depth (Km) Approximate Early Late And And 5 6 Syn- to Post-D2 0.2 thermal spikes 7.3 Isobaric-cooling ca.1685-1660 Ma Deposition of and middle crustal volcanics residence 1 and sediments ca. 1685-1400 Ma 1750-1740 Ma 0 0 400 500 600 700 800 B Temperature (°C) Figure 12. (A) Summary of path geometries interpreted for each lithotectonic block. (B) Composite P-T-t-D (pressure-temperature-time- deformation) path for entire Upper Granite Gorge, Grand Canyon, Arizona, USA. See text for discussion. timing of decompression is best constrained by along the transect are due, in part, to variable divariant reactions during decompression to ca. 1.685–1.68 Ga undeformed dikes that cut D2 degrees of advective heating. Thermal “spikes” 0.3–0.4 GPa (see paths in Figs. 5C and 9F). fabrics in the Mineral Canyon block. along the retrograde path are interpreted to be Late ﬁ brolite and prismatic sillimanite in the

The stark differences in metamorphic grade the cause of the second generation of garnet Tuna Creek block plus sillimanite after post-D2 correspond spatially with proximity to dense documented in the Mineral Canyon and Tuna andalusite in the Clear Creek block are consis- pegmatite dike swarms and/or boudinaged gra- Creek blocks (#5 in Fig. 12B), i.e., ﬂ ux of tent with this interpretation. The thermal spikes nitic pods (Fig. 11B; Ilg et al., 1996); this obser- magma during dike emplacement provided were most likely transient and varied both spa- vation suggests that preserved thermal variations the heat necessary to drive garnet- producing tially and temporally with the distribution and

218 Geological Society of America Bulletin, January/February 2007 Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon timing of pegmatite dike intrusions. We suggest in a variety of crustal levels preserved in Fiord- Columbia (>2.0mm/yr; Hollister, 1982) and the that these “spikes” along the P-T-t-D path may land, New Zealand (e.g., Klepeis et al., 2004). ultrahigh-pressure Dora Maira Unit of the West- be a common phenomenon in orogenic belts ern Alps (16–34 mm/yr; Rubatto and Hermaan, with abundant syn- to postmetamorphic intru- Rheology of Middle Continental Crust 2001). We suggest that exhumation was driven sive rocks (e.g., Williams and Karlstrom, 1996; during Orogenesis by erosion during crustal shortening (e.g., Hol- Whitney and Dilek, 1998). lister, 1979), coupled with isostatic adjustments Recent contributions have emphasized the at the scale of the entire transect. Vertical fabric CONCLUSIONS potential importance of subhorizontal channel intensifi cation may refl ect strain hardening and fl ow in the middle crust during deformation strengthening of the middle continental crust, a Several important insights emerge from the in large, thermally weakened orogens (e.g., process that would enhance the preservation of data. (1) The entire Upper Granite Gorge tran- Beaumont et al., 2004). Such fl ow is thought near-isobaric sections of continental crust dur- sect followed a looping P-T-t-D path in which to result from development of an orogenic pla- ing exhumation. all blocks reached a pressure of ~0.7 GPa by teau with steep, marginal topographic gradients 1.70–1.69 Ga. All six tectonic blocks form (e.g., the Himalayas; Clark and Royden, 2000) ACKNOWLEDGMENTS a single, relatively coherent terrane of ~25- and subhorizontal fabric development at depth km-deep Paleoproterozoic middle continental (e.g., Royden, 1996). In contrast, the view from This study is an outgrowth of over a decade of col- laborative, multidisciplinary work on basement rocks crust. (2) Peak temperatures record lateral vari- the middle crust exposed in the Upper Granite in the Grand Canyon. We appreciate the many indi- ations in temperature of greater than 250 °C Gorge is one characterized by subvertical fabric viduals who have accompanied University of New (~520–770 °C) and were infl uenced by local development during the thermal peak, e.g., the Mexico research trips and have provided fi eld obser- advection of heat through pegmatite dike com- Mineral Canyon block. Although hot blocks in vations, mapping, and discussions. We acknowledge plexes along with local transcurrent movement the Upper Granite Gorge were at temperatures support from National Science Foundation (NSF) grants EAR-9508096, EAR- 9902955, and EAR- on block-bounding shear zones. (3) The entire on the order of 750 °C where partial melting 0003477, in addition to support and research permits subhorizontal section of continental crust was made the crust weak enough to fl ow (according received from Grand Canyon National Park. We are decompressed to ~0.3–0.4 GPa (exhumed to to thermal-mechanical numerical models; Beau- especially indebted to Sam Bowring, Becky Flowers, ~12 km depths) between ca. 1.7 and 1.68 Ga. mont et al., 2004), large-scale horizontal channel Philippe Goncalves, Matt Heizler, and Mike Tim- mons for numerous discussions regarding the Pro- (4) Microstructural data indicate that D2 short- fl ow was apparently inhibited by colder, stronger ening took place throughout the entire ~15– blocks (e.g., the Topaz Canyon and Clear Creek terozoic geology of the Southwest. Roger Powell is gratefully acknowledged for his advice regarding the 20 m.y. interval of peak metamorphism and blocks) that reinforced the block-type archi- application of the ΔPT technique. Our efforts would subsequent decompression. These conclusions tecture. Protracted development of subvertical not have been possible without the outstanding have important implications for our under- fabric throughout burial and exhumation of the microprobe analytical support of Mike Jercinovic at standing of the behavior of middle continental Upper Granite Gorge may be a record of strain- University of Massachusetts–Amherst. We appreci- crust during orogenesis. hardening in a heterogeneous medium undergo- ate the constructive reviews by John Fletcher, Kurt ing temporally and spatially varying degrees of Stüwe, and Associate Editor Ben van der Pluijm. Block-Type Architecture of Lowermost thermal weakening (see also Karlstrom and Wil- REFERENCES CITED Middle Continental Crust liams, 1998, 2006). Thus, the scale and style of heterogeneity of middle continental crust have Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Med- The Upper Granite Gorge preserves some a profound impact on the resulting rheological vedev, S., 2004, Crustal channel fl ows: 1. Numerical models with applications to the tectonics of the Himala- of the highest-pressure Paleoproterozoic rocks, behavior and susceptibility to fl ow that needs to yan-Tibetan orogen: Journal of Geophysical Research, and, thus, represents one of the deepest expo- be considered in future channel fl ow models. v. 109, B06406, doi: 10.1029/2003JB002809. sures of continental crust in the southwestern Bowring, S.A., and Karlstrom, K.E., 1990, Growth and stabiliza- tion of Proterozoic lithosphere in the southwestern United United States. This area experienced minimal Exhumation of Orogens, Strain-Hardening, States: Geology, v. 18, p. 1203–1206, doi: 10.1130/0091- reheating (~350–500 °C) during ca. 1.5–1.38 Ga and Preservation of Isobaric Levels of 7613(1990)018<1203:GSAROP>2.3.CO;2. tectonometamorphic events (Karlstrom et al., Continental Crust Cherniak, D.J., 1993, Lead diffusion in titanite and prelimi- nary results on the effects of radiation damage on Pb 1997). Consequently, the transect provides a transport: Chemical Geology, v. 110, p. 177–194, doi: rare window into ca. 1.7 Ga lowermost middle The P-T-t-D path that the middle crust fol- 10.1016/0009-2541(93)90253-F. Clark, M.K., and Royden, L.H., 2000, Topographic ooze: continental crust. The exposure records critical lows during orogeny is directly related to sur- Building the eastern margin of Tibet by lower crustal fl ow: information about the character of the crustal face erosion (e.g., Jamieson et al., 2004). This Geology, v. 28, no. 8, p. 703–706, doi: 10.1130/0091- column beneath the shallower levels that are has important implications for exhumation of 7613(2000)028<0703:TOBTEM>2.3.CO;2. Collins, W.J., and Vernon, R.H., 1991, Orogeny associ- now exposed throughout the rest of the Arizona the Upper Granite Gorge, where such a large ated with anticlockwise P-T-t paths: Evidence from transition zone (Karlstrom and Bowring, 1988; transect (>70 km long measured perpendicu- low-P, high-T metamorphic terranes in the Arunta Williams, 1991). The distinctive overprinting of lar to S ) records the same path. Exhumation Inlier, central Australia: Geology, v. 19, no. 8, 2 p. 835–838, doi: 10.1130/0091-7613(1991)019<0835: shallow S1 by steep S2 domains may be a com- took place synchronously with development OAWAPT>2.3.CO;2. mon style of middle and lower crustal fabric. It and intensifi cation of subvertical foliation and Daniel, C.G., Hollister, L.S., Parrish, R.R., and Grujic, D., 2003, Exhumation of the Main Central thrust from imparts a partitioned character to the crust with without evidence for differential uplift or tilt- lower crustal depths, eastern Bhutan Himalaya: Journal segmentation at all scales, from a single micro- ing. Decompression of the Upper Granite Gorge of Metamorphic Geology, v. 21, no. 4, p. 317–334. lithon (Fig. 8E) to the observed ten-kilometer- transect from ~0.7 to ~0.3–0.4 GPa (~11–14 km) Duebendorfer, E.M., Chamberlain, K.R., and Jones, C.S., 2001, Paleoproterozoic tectonic history of the Cerbat scale block-type architecture (Fig. 3). Similar between ca. 1.7 and 1.68 Ga indicates a time- Mountains, northwestern Arizona: Implications for fabric overprinting and the resulting partitioned averaged exhumation rate of ~0.55–0.7 mm/yr. crustal assembly in the southwestern United States: geometry are also seen in the lower crust of NW These rates are small relative to Phanerozoic oro- Geological Society of America Bulletin, v. 113, no. 5, p. 575–590, doi: 10.1130/0016-7606(2001)113<0575: Canada (e.g., Mahan and Williams, 2005) and genic belts like the Coast Mountains of British PTHOTC>2.0.CO;2.

Geological Society of America Bulletin, January/February 2007 219 Dumond et al.

Dumond, G., Williams, M.L., Mahan, K.H., Karlstrom, Johnson, T.E., Brown, M., and Solar, G.S., 2003, Low-pres- metamorphic reactions: Geological Materials Research, K.E., Heizler, M.T., and Jercinovic, M.J., 2004, Tec- sure subsolidus and suprasolidus phase equlibria in the v. 1, no. 6, p. 1–49. tonic heredity in the Grand Canyon and implications MnNCKFMASH system: Constraints on conditions of Royden, L.H., 1996, Coupling and decoupling of crust and for Tibetan-scale Mesoproterozoic intra-continental regional metamorphism in western Maine, northern Appa- mantle in convergent orogens: Implications for strain par- deformation in the southwestern United States: Geo- lachians: The American Mineralogist, v. 88, p. 624–638. titioning in the crust: Journal of Geophysical Research, logical Society of America Abstracts with Programs, Karlstrom, K.E., and Bowring, S.A., 1988, Early Protero- v. 101, p. 17,679–17,705, doi: 10.1029/96JB00951. v. 36, no. 5, p. 403. zoic assembly of tectonostratigraphic terranes in south- Rubatto, D., and Hermaan, J., 2001, Exhumation as fast as Flowers, R.M., Mahan, K.H., Bowring, S.A., Williams, western North America: The Journal of Geology, v. 96, subduction?: Geology, v. 29, no. 1, p. 3–6, doi: 10.1130/ M.L., Pringle, M.S., and Hodges, K.V., 2006, Multi- p. 561–576. 0091-7613(2001)029<0003:EAFAS>2.0.CO;2. stage exhumation and juxtaposition of lower continen- Karlstrom, K.E., and Williams, M.L., 1998, Heterogeneity of the Spear, F.S., 1993, Metamorphic Phase Equilibria and Pres- tal crust in the western Canadian Shield: Linking high- middle crust: Implications for strength of continental lith- sure-Temperature-Time Paths: Washington, D.C., Min- resolution U-Pb and 40Ar/39Ar thermochronometry with osphere: Geology, v. 26, p. 815–818, doi: 10.1130/0091- eralogical Society of America, 799 p. pressure-temperature-deformation paths: Tectonics, 7613(1998)026<0815:HOTMCI>2.3.CO;2. Spear, F.S., and Cheney, J.T., 1989, A petrogenetic grid for

v. 25, TC4003, doi: 10.1029/2005TC001912. Karlstrom, K.E., and Williams, M.L., 2006, Nature of the pelitic schists in the system SiO2- Al2O3- FeO-MgO- Fraser, G., Worley, B., and Sandiford, M., 2000, High-preci- middle crust-heterogeneity of structure and process K2O-H2O: Contributions to Mineralogy and Petrology, sion geothermobarometry across the High Himalayan due to pluton-enhanced tectonism: An example from v. 101, p. 149–164, doi: 10.1007/BF00375302. metamorphic sequence, Langtang Valley, Nepal: Jour- Proterozoic rocks of the North American Southwest, Spear, F.S., Selverstone, J., Hickmott, D., Crowley, P., and nal of Metamorphic Geology, v. 18, p. 665–681, doi: in Brown, M., and Rushmer, T., eds., Evolution and Hodges, K.V., 1984, P-T paths from garnet zoning: 10.1046/j.1525-1314.2000.00283.x. Differentiation of the Continental Crust: Cambridge, A new technique for deciphering tectonic processes Grambling, J.A., 1986, Crustal thickening during Protero- Cambridge University Press, p. 268–295. in crystalline terranes: Geology, v. 12, p. 87–90, doi: zoic metamorphism and deformation in New Mexico: Karlstrom, K.E., Heizler, M.T., and Williams, M.L., 1997, 10.1130/0091-7613(1984)12<87:PPFGZA>2.0.CO;2. Geology, v. 14, no. 2, p. 149–152, doi: 10.1130/0091- 40Ar-39Ar muscovite thermochronology within the Spear, F.S., Kohn, M.J., Florence, F.P., and Menard, T., 1991, 7613(1986)14<149:CTDPMA>2.0.CO;2. Upper Granite Gorge of the Grand Canyon: Eos, A model for garnet and plagioclase growth in pelitic Hawkins, D.P., 1996, U-Pb geochronological constraints on (Transactions, American Geophysical Union), Fall schists: Implications for thermobarometry and P-T the tectonic and thermal evolution of Paleoproterozoic Meeting Supplement, v. 78, no. 46, p. F784. path determinations: Journal of Metamorphic Geology, crust in the Grand Canyon, Arizona [Ph.D. thesis]: Cam- Karlstrom, K.E., Åhäll, K.I., Harlan, S.S., Williams, M.L., v. 8, p. 683–696. bridge, Massachusetts Institute of Technology, 320 p. McLelland, J., and Geissman, J.W., 2001, Long-lived Spear, F.S., Kohn, M.J., and Cheney, J.T., 1999, P-T paths from Hawkins, D.P., and Bowring, S.A., 1999, U-Pb monazite, (1.8–0.8 Ga) convergent orogen in southern Laurentia, anatectic pelites: Contributions to Mineralogy and Petrol- xenotime, and titanite geochronological constraints on its extensions to Australia and Baltica, and implications ogy, v. 134, p. 17–32, doi: 10.1007/s004100050466. the prograde to post-peak metamorphic thermal history for refi ning Rodinia: Precambrian Research, v. 111, Tenczer, V., and Stüwe, K., 2003, The metamorphic fi eld of Paleoproterozoic migmatites from the Grand Can- p. 5–30, doi: 10.1016/S0301-9268(01)00154-1. gradient in the eclogite type locality, Koralpe region, yon, Arizona: Contributions to Mineralogy and Petrol- Karlstrom, K.E., Ilg, B.R., Williams, M.L., Hawkins, D.P., eastern Alps: Journal of Metamorphic Geology, v. 21, ogy, v. 134, p. 150–169, doi: 10.1007/s004100050475. Bowring, S.A., and Seaman, S.J., 2003, Paleoprotero- no. 4, p. 377–394. Hawkins, D.P., Bowring, S.A., Ilg, B.R., zoic rocks of the Granite Gorges, in Beus, S.S., and Todd, C.S., and Engi, M., 1997, Metamorphic fi eld gra- Karlstrom, K.E., and Williams, M.L., 1996, U-Pb Morales, M., eds., Grand Canyon Geology (2nd ed.): dients in the central Alps: Journal of Metamorphic geochronological constraints on Paleoproterozoic Oxford, Oxford University Press, p. 9–38. Geology, v. 15, no. 4, p. 513–530, doi: 10.1111/j.1525- crustal evolution, Upper Granite Gorge, Grand Klepeis, K.A., Clarke, G.L., and Rushmer, T., 2003, 1314.1997.00038.x. Canyon, Arizona: Geological Society of America Magma transport and coupling between deforma- Whitney, D.L., and Dilek, Y., 1998, Characterization and Bulletin, v. 108, p. 1167–1181, doi: 10.1130/0016- tion and magmatism in the continental lithosphere: interpretation of P-T paths with multiple thermal 7606(1996)108<1167:UPGCOT>2.3.CO;2. GSA Today, v. 13, no. 1, p. 4–11, doi: 10.1130/1052- peaks, in Treloar, P.J., and O’Brien, P.J., eds., What Hodges, K.V., and McKenna, L.W., 1987, Realistic error 5173(2003)013<0004:MTACBD>2.0.CO;2. Drives Metamorphism and Metamorphic Reactions?: propagation of uncertainties in geologic thermobarom- Klepeis, K.A., Clarke, G.L., Gehrels, G., and Vervoort, J., 2004, Geological Society of London Special Publication 138, etry: The American Mineralogist, v. 72, p. 671–680. Processes controlling vertical coupling and decoupling p. 53–60. Holland, T.J.B., and Powell, R., 1998, An internally consis- between the upper and lower crust of orogens: Results Williams, M.L., 1991, Overview of Proterozoic metamor- tent thermodynamic data set for phases of petrologi- from Fiordland, New Zealand: Journal of Structural Geol- phism in Arizona, in Karlstrom, K.E., ed., Proterozoic cal interest: Journal of Metamorphic Geology, v. 16, ogy, v. 26, p. 765–791, doi: 10.1016/j.jsg.2003.08.012. Geology and Ore Deposits of Arizona: Arizona Geo- p. 309–343, doi: 10.1111/j.1525-1314.1998.00140.x. Kohn, M.J., 2003, Geochemical zoning in metamorphic logical Society Digest, v. 19, p. 11–26. Hollister, L.S., 1979, Metamorphism and crustal displace- minerals, in Rudnick, R.L., ed., Treatise on Geochem- Williams, M.L., and Jercinovic, M.J., 2002, Microprobe mon- ments: New insights: Episodes, v. 3, p. 3–8. istry: The Crust: Amsterdam, Elsevier, p. 229–261. azite geochronology: Putting absolute time into micro- Hollister, L.S., 1982, Metamorphic evidence for rapid Kohn, M.J., and Spear, F.S., 2000, Retrograde net transfer structural analysis: Journal of Structural Geology, v. 24, (2 mm/yr) uplift of a portion of the Central Gneiss reaction insurance for pressure- temperature estimates: p. 1013–1028, doi: 10.1016/S0191-8141(01)00088-8. Complex, Coast Mountains, B.C.: Canadian Mineralo- Geology, v. 28, no. 12, p. 1127–1130, doi: 10.1130/0091- Williams, M.L., and Karlstrom, K.E., 1996, Looping P-T gist, v. 20, p. 319–332. 7613(2000)028<1127:RNTRIF>2.3.CO;2. paths and high-T, low-P middle crustal metamorphism: Ilg, B.R., and Karlstrom, K.E., 2000, Porphyroblast inclusion Kretz, R., 1983, Symbols for rock-forming minerals: The Proterozoic evolution of the southwestern United States: trail geometries in the Grand Canyon: Evidence for non- American Mineralogist, v. 68, p. 277–279. Geology, v. 24, no. 12, p. 1119–1122, doi: 10.1130/0091- rotation and rotation?: Journal of Structural Geology, Mahan, K.H., and Williams, M.L., 2005, Reconstruction of 7613(1996)024<1119:LPTPAH>2.3.CO;2. v. 22, p. 231–243, doi: 10.1016/S0191-8141(99)00150-9. a large deep-crustal terrane: Implications for the Snow- Worley, B., and Powell, R., 2000, High-precision relative Ilg, B.R., Karlstrom, K.E., Williams, M.L., and Hawkins, bird tectonic zone and early growth of Laurentia: Geol- thermobarometry: Theory and worked example: Jour- D.P., 1996, Tectonic evolution of Paleoproterozoic ogy, v. 33, p. 385–388, doi: 10.1130/G21273.1. nal of Metamorphic Geology, v. 18, p. 91–101, doi: rocks in the Grand Canyon: Insights into middle crustal Percival, J.A., and West, G.F., 1994, The Kapuskasing 10.1046/j.1525-1314.2000.00239.x. processes: Geological Society of America Bulletin, v. Uplift—A geological and geophysical synthesis: Cana- 108, p. 1149–1166. dian Journal of Earth Sciences, v. 31, p. 1256–1286. Jamieson, R.A., Beaumont, C., Medvedev, S., and Nguyen, Powell, R., and Holland, T.J.B., 1994, Optimal geothermom- M.H., 2004, Crustal channel fl ows: 2. Numerical mod- etry and geobarometry: The American Mineralogist, MANUSCRIPT RECEIVED 25 SEPTEMBER 2005 REVISED MANUSCRIPT RECEIVED 25 JULY 2006 els with implications for metamorphism in the Himala- v. 79, p. 120–133. MANUSCRIPT ACCEPTED 2 AUGUST 2006 yan-Tibetan orogen: Journal of Geophysical Research, Pyle, J.M., and Spear, F.S., 1999, Yttrium zoning in gar- v. 109, B06407, doi: 10.1029/2003JB002811. net: Coupling of major and accessory phases during Printed in the USA

220 Geological Society of America Bulletin, January/February 2007