Petrogenesis and tectonic context of the Harney Peak Granite, Black

Petrogenesis and tectonic context of the Harney Peak Granite, Black Hills, South Dakota Peter I. Nabelek, Mona Sirbescu, and M i a n Liu Department of Geological Sciences, University of Missouri-Columbia, Columbia, M O 65211, U.S.A.

ABSTRACT Intrusion and crystallization of the Harney Peak Granite and associated plutons and pegmatites were the culminating events of the Pans-Hudson orogeny as expressed in the Black Hills. The granite was emplaced as thousands of sills and dikes at 1715 Ma, following an approximately 45 m.y. period of regional metamorphism and deformation of now exposed Proterozoic metasedimentary rocks. Isotopic ratios of neodymium and lead indicate that parts of the granite were derived from source rocks with Archean model T , , extraction-ages, whereas other parts were derived from sources with only 100-300 m.y. crustal residence times. However, oxygen isotope ratios and trace element concentrations suggest that both sources were metapelites or metagraywackes analogous to the country rocks. Boron and TiO, concentrations suggest that the granites were generated either by muscovite or muscovite + biotite dehydration-melting reactions. Published thermobarometric and argon cooling-agesshow that the country rocks cooled and decompressed from conditions at which garnet and staurolite grew at N7 kbar to less than 500"C and pressures of 3.5-4 kbar at the time of granite emplacement. We conducted a numerical simulation of metamorphism and generation of the Harney Peak Granite that is constrained by available data. The model involves shear-heating along a thrust to produce temperatures sufficiently high for melting of thrusted sedimentary rocks at relatively shallow levels in a thickened crust that is undergoing unroofing. The model successfully reproduces the metamorphic and magmatic events that occurred in the Black Hills segment of the Pans-Hudson orogen. KEY WORDS: Black Hills, Harney Peak Granite, Pans-Hudson orogeny, leucogranite, crustal collision, P-T-t paths, shear-heating, radiogenic isotopes, stable isotopes, metapelites.

INTRODUCTION Granite magmatism during the Proterozoic contributed significantly to evolution of the North American continental crust. Much of the granite magmatism, as evident from papers in this issue, was anorogenic, resulting from underplating of the crust by basaltic magmas during extensional collapse of orogens. Extension-related magmatism is usually characterized by bimodal mafic-felsic igneous suites. In contrast, petrogenesis of the 1.7-Ga Harney Peak Granite (HPG), Black Hills, South Dakota, was related to collision of crustal blocks during the Pans-Hudson orogeny. The orogeny was responsible for major coalescence of the northern part of the North American craton. The peraluminous leucogranitehas purely crustal affinities (Nabelek and Bartlett, 1998) that, together with characteristicsof the surrounding Proterozoic meta-

morphic rocks, have implications for the geologic and thermotectonic events that led to its petrogenesis. In this paper we: (1) summarize the available petrologic and geochemical data for the HPG and its metamorphosed country rocks; ( 2 ) discuss implications of the data for the petrogenesis of the granite; and (3) propose a thermotectonic model that explains generation of the HPG magma in the crust as a consequence of concomitant shear-heating and unroofing during the orogeny. Because analogous granites occur in collisional orogens that are similar in scale to the mans-Hudson orogen, the model may have application to petrogenesis of leucogranite magmas elsewhere. Possible examples are in the Himalayas (Harris and Massey, 1994; Le Fort et al., 1987) and the Central Maine belt

Rockg Mountain Geology, v. 34, no. 2, p . 165-181, 13figs., 2 tables, Novembel: 1999

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P. I. NABELEK ET AL.

(lbmascak et al., 1996; Brown and Solar, 1998).

112"

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GEOLOGIC SETTING AND CHRONOLOGY Much of the southern Ti-ansHudson orogen is now buried under Phanerozoic sedimentary rocks. However, it is exposed in the Black Hills as the result of the Laramide orogeny, and these rocks provide a window on Proterozoic processes that occurred in the region (Redden et al., 1990; Fig. 1, Thble 1). In Canada, the Trans-Hudson terrane contains relatively juvenile lithologies (such as accreted island-arc terranes) that generally yield dates > 1800 Ma for the orogeny (Lewry and Collerson, 1990). In contrast, metamorphic rocks in the southern Black Hills are dominantly quartzites and quartz-muscovitebiotite metapelites and metagraywackes, most of which originated as turbidites (Fig. 2). Rarer amphibolites, iron formations, and marbles also occur. The metasedimentary sequenceswere intruded by the HPG, satellite plutons, and simple to complex pegmatites (Norton and Redden, 1990). The Black Hills also include exposures of Archean peraluminous granites and minor pelitic sequences near Bear Mountain along the western margin of the exposed Precambrian terrane (Fig. 2) and near Little Elk Creek along the eastern margin of the uplift (Gosselin et al., 1988). Precursors of the metasedimentary sequences were deposited between ~2100and 1880 Ma, according to ages of intercalated gabbro sills and felsic tuffs (Redden et al., 1990). The earliest deformation of the strata resulted in northeast-trending F, folds that show little lineation or foliation (Redden et al., 1990). Dahl et al. (1999) suggested that the F, folding may be related to 166

A

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,,,

Superior province

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figure 1. A, Map showing relationship of the Black Hills, South Dakota, to major cratonic blocks and the Pans-Hudson orogen (THO; after Hoffman, 1990).Inset shows location of the map within North America. The dates show model mantle-extraction ages, based mostly on Sm-Nd isotopic data, for Precambrian rocks within each tectonic province. B, Interpretation of the COCORP transect indicated in part A by Baird et al. (1996), showing the Wyoming province, Dakota block, and wedge of continental arc rocks. The depths shown are below the present surface. BH, Black Hills.

accretion of island arcs from the vicinity of the kyanite isograd in south, beginning at N 1780 Ma. western parts of the Proterozoic This northwest-directed accretion terrane (Dahl and Frei, 1998). In also is expressed in the Cheyenne addition, Dahl and Holm (1996) belt in southeastern Wyoming. obtained a 1745 5 Ma 40Ar/39Ar Metamorphism during F, folding date on D, hornblende from a lois not apparent. The dominant cality northwest of the area structures in the meta- shown in Figure 2. Dahl et al. sedimentary rocks are NNW- (1998) also obtained dates on gartrending F, folds with steeply dip- nets from the metapelites that Ma. ping foliation. These are thought range from 1760 to ~ 1 7 2 0 to be related to the Trans-Hudson This suggests that the regional M, orogeny. Major subvertical faults metamorphism may have perthat superimposed contrasting sisted for several tens of millions metasedimentary lithologies of years following initial garnet against each other have similar growth, although the youngest orientation. The earliest date for garnets may have grown during the F, folding is 1760 f 7 Ma, contact metamorphism (M,) by based on combined z07Pb/z0sPbthe HPG. The Trans-Hudson orogeny step-leach ages for F,-related garnet and staurolite (M,) from the traditionally has been ascribed to

*

Roc@ Mountain Geology,v. 34, no. 2, p . 165-181,13 figs., 2 tables, November, 1999

PETROGENESIS AND TECTONIC CONTEXT OF HARNEY PEAK GRANITE

Figure 2. Geologic map of ArcheanProterozoic terrane in the southern Black Hills. Thin lines, including dashed lines, are faults; heavy lines are isograds: St-staurolite;S-first sillimanite; SK-secondsillimanite; and Kkyanite. A tuffaceous shale unit (now schist) is shown to highlight the major fold structures.Heavy dashed line within the main body of the Harney Peak Granite marks the boundary between mostly high-60 (outside) and low-60 granites (inside; Nabelek et al., 1992b). Regions with abundant pegmatites are noted.

43O45'

mble 1. Srammaq of Proterozoic geologic events and conditions, Black Hills, South Dakota. P

deposition of sedimentary sequences northwest-directed folding (F,) earliest regional garnet growth (Fz,MI) latest garnet growth (M, or Mz) intrusion of HPG conditions of country rocks just before HPG intrusion ~

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collision of the Archean Wyoming and Superior cratons. However, a recent COCORP profile across the orogen just south of the U.S.-Canadian border, interpreted with the aid of dates from basement cores, suggests that there may have been a small Archean crustal block (the Dakota block) between the two cratons (Baird et al., 1996). The Dakota block, rather than the Superior craton, collided directly with the Wyoming craton (Baird et al., 1996; Fig. 1). The dominantly metasedimentary sequences and leucogranites in the Black Hills probably are within what Baird et al. (1996) identified as a wedge of continental arc plutonic rocks between the Wyoming province and the Dakota block. Redden et al. (1990) obtained a concordant 1715 f 3 Ma U-Pb age on a monazite taken from a thin granite sill in the metamorphic rocks. This is generally considered the best date for crystallization of the HPG. However, Redden et al. (1990) also obtained a 1728 f 4 Ma age based on two highly discordant zircons. Krogstad and Walker (1994) also obtained concordant 1704-1 700 Ma U-Pb ages on apatites from the Tin Mountain pegmatite, located west of the HPG. The range of ages may represent uncertainty due to inheritance, discordancy, and differences in blocking temperatures of the U-Pb system in different minerals. One the other hand, it may also indicate the duration of the leucogranitic magmatism. Whatever the case may be, the combined metamorphic and HPG ages show that the granite was emplaced at the end of protracted metamorphism and deformation of the metasedimentary rocks. Holm et al. (1997) suggested that by the time the granite was emplaced, the country rocks cooled from peak metamorphic conditions to < 5OOOC; hornblende within the HPG aureole gives ages between 1686 and 1691 Ma (Berry et al., 1994). The close correspondencebetween these ages and those from HPG implies that the granite must have cooled relatively rapidly through the 500' C blocking temperature of Ar in hornblende. In contrast, micas from the vicinity of the HPG give 1270-1500 Ma ages, indicating protracted cooling once temperatures dropped below 50OOC (Holm et al., 1997). This implies little unroofing following emplacement of the HPG. Emplacement of the post (late?)-D, HPG flattened the steeply-dippingregional foliation in close proximity to the main HPG pluton and some satellite intrusions. Extensional boudinage structures are common within the second sillimanite zone southwest of the main HPG pluton. They are attributed to granite emplacement below the outcrops. The

168

granites are undeformed except locally by later emplacement of dikes and sills into plutons.

CONDITIONS OF METAMORPHISM Minerals i n the metapelites and metagraywackes in the staurolite zone include staurolite, garnet,biotite, muscovite, quartz, plagioclase, and accessory phases, including monazite, zircon, apatite, xenotime, and graphite. There is consensus that the staurolite isograd is the result of chlorite breakdown during M, metamorphism,based on the pre-HPG ages on garnet and staurolite and no apparent relationship of the isograd to the HPG (Dahl and Frei, 1998; Helms and Labotka, 1991; Redden et al., 1990). However, chlorite + quartz pseudomorphs after garnet and staurolite in rocks that are cut by quartz veins occur in the vicinity of the staurolite isograd. The pseudomorphs suggest that fluid-present retrograde metamorphism was associated with HPG magmatism. The origin and timing of the first sillimanite isograd is more problematic. Helms and Labotka (1991) ascribed the sillimanite isograd to polymorphic inversion of andalusite. However, andalusite is rare in the metamorphic rocks. Generally it is found in pegmatitic veins, near fractures,or it crosscuts foliation in metapelites where it probably grew after sillimanite. Moreover, most of the metasedimentary rocks are not sufficiently aluminous or magnesian to contain coexisting staurolite, muscovite, biotite, and andalusite (Nabelek, 1997). Andalusite is best attributed to growth during decompression of the terrane that was associated with emplacement of the HPG. Instead of polymorphic inversion, the sillimanite isograd can be ascribed to the reaction st + ms + q + grt + bt + sil + H,O (1) because sillimanite appears when staurolite disappears. It has not been established whether the first sillimanite isograd is: (1) the result of regional metamorphism that produced the staurolite isograd; or (2) related to heat advection associated with emplacement of the HPG. In the former case, the isograd may have been bowed upward during growth of the HPG. 'Lb the west and within the main HPG pluton, the second sillimanite isograd corresponds to extensive migmatization of the schistsby partial melting (Shearer et al., 1987; Nabelek, 1997, 1999). Thermal conditions within the second sillimanite zone generally have been attributedto contact metamorphism. The migmatites include leucosomesthat have either granite mineralogy or are comprised of

Rocky Mountain Geology, v. 34, no. 2, p . 165-181, 13figs., 2 tables, November, 1999


mostly quartz and sillimanite. Melanosomes are dominated by biotite and sillimanite with minor quartz. Mesosomes contain various proportions of quartz, biotite, sillimanite, and plagioclase. As expected for conditions above the second sillimanite isograd, muscovite is lacking in the migmatites except for retrograde grains that altered sillimanite and grew randomly across foliation. Similar anhedral tourmaline replacements of sillimanite also are found. Garnet is absent from the second sillimanite zone except in a few localities. Mineralogy of the migmatites was used by Nabelek (1999) to determine that migmatization 0 700"C. occurred at c 4.5 kbar and between ~ 6 7 and The estimate is based on an invariant mineral assemblage in the KFMASH system in one garnetbearing sample that also contains (possibly retrograde) muscovite and application of the thermodynamic data base of Holland and Powell (1998) (Fig. 3). The muscovite dehydration-melting reaction ms + q + pl a liq + sil + kfs (k bt), (2) as experimentally determined by Patiiio-Douce and Harris (1998) on schists with compositions similar to those in the Black Hills, indicates the minimum thermal conditions of partial melting. This reaction passes through the invariant point in the system, indicating that melt- and plagioclase-absent reactions can be used to estimate the P-T conditions of migmatization. The general lack of garnet in the migmatites implies that pressures in the migmatites were below the nearly isobaric reaction grt + ms a bt + 2sil + q (3) and temperatures were below the plagioclase- and melt-present analog to reaction sil + bt + 2 q a g r t + kfs + H,O. (4) The maximum pressure conditions i n the migmatites, and therefore pressure of granite emplacement, are consistent with the 3.54 kbar pressure estimates of Helms and Labotka (1991) on the unmigmatized schists, using the garnet + sillimanite + plagioclase + quartz (GASP) and garnet + biotite + muscovite + plagioclase (BPMG) mineral barometers. The presence of andalusite, whether prograde or retrograde, also limits the pressure to c 4 kbar. All these pressure estimates suggest relatively low pressures in the country rocks at the time of granite emplacement. Nevertheless,GASP barometry using cores of garnets that grew during the regional M, metamorphism near the kyanite isograd indicates pressure of N7 kbar at the time of their growth ( T k n y and Friberg, 1990). This implies decompression of the metamorphic rocks between the times of regional garnet growth and emplacement of the HPG.

MINERALOGY AND GEOCHEMISTRY OF HARNEY PEAK GRANITE Geology of the granitic rocks in the southern Black Hills was described extensively by Redden et al. (1982) and Norton and Redden (1990). Most plutons have limited depth and have domal structures that grew laterally by continual emplacement of numerous sills and dikes (Duke et al., 1988,1990), indicating extraction of small batches of melts from their sources. Many of the sills separated into layered granite-pegmatite couplets that resulted in local mineralogical and chemical fractionation (Rockhold et al., 1987; Duke et al., 1992). Pegmatitic layers are more common along the flanks of the main pluton and in satellite intrusions. The dominant minerals in the granite are quartz, sodic plagioclase (albite to oligoclase), microcline, and muscovite. The occurrence of almandinespessartine garnet is sporadic. Biotite (Mg# 32-38) is the dominant ferromagnesian mineral in the core of the main pluton. lburmaline (Mg# 0.18-0.48) dominates along the perimeter and in the satellite intrusions (Nabelek et al., 1992a). There is virtually a complete overlap in the concentrations of major elements in the biotite- and tourmaline-containing granites with no apparent differentiation trends on Harker diagrams (Fig. 4; Nabelek et al., 1992a). The granite is depleted in CaO, MgO, and FeO relative to anorogenic or calc-alkaline granite suites, indicating derivation from pelitic source rocks. The large variation in Na,O/qO ratios results from local aplite-pegmatite differentiation (Fig. 4D). Surroundingthe HPG and its satellite intrusions is a large pegmatitic field that contains approximately 24,000 pegmatite sills and small plutons (Norton and Redden, 1990). The pegmatites occur in areas that experienced metamorphic conditions above the staurolite isograd, although most occur above the first sillimanite isograd in areas southwest of the main pluton. However, several prominent large pegmatites also occur just below the first sillimanite isograd northeast of the main pluton. The Black Hills pegmatites have been categorized into three types: layered, simple, and zoned (Norton and Redden, 1990). Most ( ~ 8 0 %are ) simple, and less than 3 percent are zoned. The layered pegmatites consist of coarse layers of perthite and finer layers of plagioclase and quartz. They may have formed from individual sills analogous to those that fed the HPG and the differentiated satellite intrusions. Muscovite and tourmaline are common, and biotite is rare. Simple pegmatites have mineralogy similar to the layered pegmatites, but gener-

R o c k Mountain Geology, v. 34, no. 2, 165-181,13 figs., 2 tables, November, 1999

169


ally they are coarser-grained and lack prominent layering. Zoned pegmatites, which differentiated from border zones to cores, contain minerals found in the simple and layered pegmatites, as well as spodumene, amblygonite,lepidolite, beryl, cassiterite, and other rare-element minerals. The similarities between the HPG and pegmatites suggest that they are genetically related (Norton and Redden, 1990; Shearer et al., 1992).

IMPLICA!TIONS OF INITIAL I S r n P I C RATIOS

13 12 11 10

9 n

Q‘ 8 2 3

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cn 3 Q

6

The distribution of biotiteand tourmaline-containing gran5 ites largely defines two isotopically distinct suites of rocks, each 4 with internal isotopic heterogeneity. Whole rock 6l80 values of 3 granites from the core of the pluton, where biotite is the dominant 2 ferromagnesian mineral, ere 10.8-12.8 per mil with an average 1 of 12.0 per mil. In contrast, 6l80 values of granites from the flanks of the main pluton and satellite 0 intrusions, where tourmaline is 600 650 700 750 800 850 the dominant ferromagnesian temperature (“C) phase, are 12.3-13.6 per mil with an average of 13.1 per mil (Fig. 5; Fignre 3. Petrogenetic grid showing reactions applicable to Black Hills Nabelek et al., 199213). Thus, for migmatites. Reaction labels refer to reaction numbers in text. Patterned field purposes of the following discus- indicates probable ranges of pressure and temperature for partial melting in sion, it is convenient to refer to the second sillimanite zone. Muscovite and biotite dehydration-meltingrethe two suites as the “10w-6~~0actions (Patifio-Douce and Harris, 1998; Le Breton and Thompson, 1988) apsuite” and the “high-6180suite”. propriate for compositions of Black Hills schists also are shown. Dashed part of the muscovite dehydration-meltingreaction is extrapolated. Reaction 7 is The average 6l80value of the highrequired by intersection of muscovite and biotite dehydration-meltingreac6l80suite is virtually the same as tions. the average for the metasedimentary country rocks (Fig. 5; Nabelek and Bartlett, has low values of 207Pb/204Pb for its Krogstad and Walker (1996) ob1998). z06Pb/zo4Pb ratios relative to the tained a range of &Na (1715 Ma) of Potassium feldspars from 10w-6~~0 granites. These ratios -6.4 to -9.9 for ten samples of the samples of the high- and 10w-6~~0indicate, prior to melting: (1) a 1 0 ~ 4 suite ~ ~ and 0 -2.0 to -7.7 for - relatively short, 100-300 m.y. ten samplesof the high-6180suite suites define distinct 207Pb/204Pb zosPb/zo4Pb arrays (Fig. 6; Krogstad crustal residence of the source for (Fig. 7). One sample of the highet al., 1993). K-feldspar from the the highSl80suite; and ( 2 ) crustal 6l80 suite has an unusually low high-6180 granites and several residence since the late Archean average value of -12.6, whereas the simple and complex pegmatites for the source of the 10wS’~o suite. higher values for that suite over170

R o c b Mountain Geology, v. 34, no. 2, p . 165-181, 13 figs., 2 tables, Novembm, 1999


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granites 72

74

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80

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72

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(wt. %)

Figure 4. Variations of concentrations of selected major elements in the two isotopically distinct suites of the Harney Peak Granite (HPG of subsequent figures; after Nabelek et al., 199213). Note logarithmic scale of ordinate in part D.

lap the range for the Proterozoic schists. The schists have model T, extraction ages ~ 2 3 0 0Ma. Thus, all three isotopic systems indicate that regional major and trace element variations in the HPG, satellite intrusions, and pegmatites more probably are the result of melting of isotopically heterogeneous Archean to Paleoproterozoic crustal sources than of regional-scale fractional crystallization. Moreover, the isotopic compositions of the high-6l80suite and the schists suggest that the high6l80 granite suite was derived from rocks equivalent to the schists. The mineralogy, chemical composition, and 6l80 values of the 10w-6~~0 granite suite also suggest that its source was a similar metasedimentary rock sequence. The Archean metapelites at Bear Mountain (Gosselin et al., 1988) may be representative of this source. Unfortunately, there are no isotopic data for these rocks. However, the peraluminous Archean Little Elk Granite, which

is compositionally similar to the HPG and has &,,(1715) value of -14.2, may have been generated from such a source.

CONDITIONS OF MAGMA CRySTALLIZATION Peraluminous granites, including the HPG, lack mineral assemblages that would be amenable to cation-exchange thermometry. Therefore, to determine temperatures of emplacement and crystallization, Nabelek et al. (199213) used oxygen isotope fractionation among coexisting minerals by employing the isotherm method of Javoy et al. (1970). In this method, coordinates of coexisting minerals in the A180Qi - BQi versus AQi space are regressed to obtain temperature estimates. A180Qi is the observed difference between the 6l80values of quartz and mineral i, including quartz itself. BQi and AQi are the

Roc7y Mountain Geologj, v. 34, no. 2, 165-181,13figs., 2 tables, November, 1999

171

P.I. NABELEK ET AL.

I

Granites O1

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1 0

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1 2

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coefficients of the isotopic fractionation function between quartz and mineral i: A180Qi = lo3lna = AQi/F + BQi (5) where a is the fractionation factor. For anhydrous phases coefficient B is usually zero. The error on the regression estimates the deviation from isotopic equilibrium. An example of the method for one HPG sample that gives one of the largest errors is shown in Fig. 8.Histograms of the isotopic equilibration temperatures for the granites are shown in Fig. 9.Pegmatites are excluded because minerals in pegmatites are generally far out of isotopic equilibrium. The histograms show that most samples in both granite suites crystallized above 7OO0C, with some above 800' C. Oxygen isotopes in only a few samples equilibrated below the usually assumed granite solidus temperature of ~650'C. On the one hand, this may indicate subsolidus equilibration. On the other hand, solidi of granites that locally differentiated into pegmatites may be below this temperature (Nabelek and Ternes, 1997;Sirbescu and Nabelek, 1998). The many aplitic layers and preservation of isotopic equilibrium set at crystallization conditions suggest rapid cooling of hot magma within relatively cold country rocks, consistent with conclusions of Holm et al. (1997). The high crystallization temperatures of the magma imply that it was relatively dry,which may seem at odds with the abundant pegmatite layers

(WR) Fignre 5, above left. Histograms of whole rock 6I8O values. A, Biotiteaominatea low-o'w H Y ~ ;suite, tne tourmaline-dominated high-6180 HPG suite. B, Schist country rocks. Note similarity of isotopic compositions of metasedimentary rocks and h i g h P 0 granites. 1

K-feldspars 15.5

.

Figore 6, left. Variation of 207Pb/ zo4Pbversus zosPb/zo4Pbin K-feldspars

15.0

14.5 13

14

15

16

=Pb/ 204Pb

172

17

from low and high-6I80granites and pegmatites (after Krogstad et al., 1993). Convex line is p = 8 Pb isotope-evolution curve for 500-2900 Ma.A line passing through the field of 10w-6~~0 granites and 1715 Ma curve indicates crustal history since the Archean of source rocks for these granites. Straight line passing through field of high-6W granites and pegmatites indicates a short crustal history of their source. The shaded fields highlight the isotopically-distinct granite suites.

Rocky Mountain Geology, v. 34, no. 2,p . 165-181, 13 figs., 2 tables, November, 1999


in the HPG. However,based on compositions of primary mixed H,O-(20,-salt fluid inclusions in tourmaline and quartz, Nabelek and Ternes (1997) estimated that the melt contained approximately 3.5 wt. % H,O and 1500 ppm CO,. That amount of water is substantially below what would be necessary to saturate a granitic melt in the melting region (see below), but it is sufficient to saturate the melt relatively early during crystallization at 3.5 kbar. Thus, the pegmatite layers in the granite may be due partly to crystal growth under water-saturated conditions that probably existed during much of the magma's crystallization history. High concentrations of fluxing elements, including B and Li, also probably contributed to formation of the pegmatite layers.

CONDITIONS OF MAGMA GENERATION Crystallization temperatures near 800' C indicate approximate temperatures of melt-forming reactions in the source rocks, assuming near-adiabatic ascent of magmas. That assumption is reasonable because there is no evidence that the magma ascended as a crystal mush. The types of source rocks and melt-producing reactions are indicated by minor and trace elements in the HPG. For example, biotite-containing granites have on average higher concentrations of TiO, and lower concentrations of B than the tourmaline-containing granites (Fig. 10). The contrasting B/TiO, ratios are thought to indicate that the high-6l80granites were generated by dehydration-melting of metapelites that involved only the breakdown of muscovite, the major site of B in the metapelites at melting conditions. Generation of most of the 10w-6~~0 granites, in contrast, also involved the breakdown of biotite, the major site for Ti. Possible melting reactions are shown in Figure 3. Nabelek et al. (1992a) suggestedthat production of the 10w-6~~0 suite involved the continuous biotite dehydration-melting reaction bt + sil + pl + q 3 liq + grt + kfs (6) (Le Breton and Thompson, 1988), which lies at temperatures higher than the muscovite dehydrationmelting reaction 2 (Patiiio-Douce and Harris, 1998) below ~ 1 kbar. 0 However, trace elements that are controlled by major minerals during melting, including Rb,Cs, Sr, Ba, Sc, Cr, Zn,and Th,have similar concentration ranges in both granite suites (Fig. 11). A model for the trace element concentrations, assuming melting of source rocks having composit i o n s of t h e country rock metapelites a n d

+4 0 -4

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W

-8

0 Little Elk Granite at 1715 Ma

-16

high-61*0 granites

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2.6

granites

3.0

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3.4

time (Ga) Figure 7. Values of G~and calculated time-dependent trajectories for HPG and schists (after Krogstad and Walker, 1996). The diagram indicatesderivation of granite from source rocks with crustal histories since the Archean and the Paleoproterozoic.Candidatesfor the latter source are analogs to schists exposed in the Black Hills, whereas the low eNIsource may have been similar to the source of the Archean Little Elk Granite.

3.0,

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3.5

A04

Fignre 8. Oxygen-isotope isotherm plot for a granite sample. Regression of observed fractionations in terms of experimentally determined fractionation coefficients gives a temperature. Error on the regression indicates deviation from isotopic equilibrium.

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metagraywackes, suggeststhat biotite was a significant residual mineral for both granite suites 5 (Nabelek and Bartlett, 1998). This implies that the 10w-6~~0 suite could not have been derived by a 4 reaction that involved biotite as the sole mineral contributing potassium to the melt. Therefore, it is more probable that production of the 10w-6~~0 0 3 E = suite involved the reaction 0 bt + ms + pl + q a l i q + grt + kfs, (7) 0 2 which must emanate from the intersection of reactions 2 and 6 to high pressures (Fig. 3). Thus 1 pressure, rather than temperature, may have controlled which melt-forming reaction produced 0 each of the granite suites.

L 0 w - 6 ~ ~Granites 0

400

500

600

700

800

900

temperature ("C)

NUMERICAL THERMmECTONIC MODEL

H igh-6l80 Granites

Introduction Knowledge of the following factors provides constraints on the granite's petrogenesis within the context of the collision of the Wyoming craton with the Dakota block: (1) current crustal structure across the mans-Hudson orogen as interpreted from seismic studies; (2) available dates for regional metamorphism, deformation, and HPG emplacement; (3) petrogenetic and emplacement conditions of the granite; and (4) character of its source rocks. We use numerical modeling to reconstruct crustal pressure-temperature-timepaths that led to genesis of the granite (Fig. 12). Salient constraints on the model include the following considerations. A, Isotopic compositions of the granitic rocks are heterogeneous, with radiogenic isotopes indicating mantle-extraction ages of both Archean and Proterozoic models. This implies conditions that permitted contemporaneous melting of both Archean and Proterozoic crustal sources in the same region of the crust. B, Metasedimentary rocks in the Black Hills have Proterozoic model mantle-extraction ages, suggesting derivation from young crustal rocks such as island arcs. The metasedimentary rocks were thrust up over the Wyoming craton as suggested by the COCORP cross section (Fig. 1). Some metasedimentary rocks at Bear Mountain and near Little Elk Creek are Archean (Gosselin et al., 1988), as may be some formations in the center of the HPG (Redden, personal communication). Their presence indicates a tectonic juxtaposition of Archean and Proterozoic sedimentary sequences. C,Major- and trace element compositions of the granites indicate muscovite and muscovite +

174

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IXgum 9. Histograms of apparent oxygen-isotopeequilibration temperatures in the HPG based on the isotherm method (Fig. 8 ) . Note that for many samples temperatures are nearly 8OO0C, implying that this is the minimum temperature of melt formation in the source region.

biotite dehydration-melting of metapelites and metagraywackes. These reactions are consistent with granite emplacement temperatures. D, The granites intruded metasedimentary rocks that experienced garnet growth between ~ 1 7 6 and 0 1720 Ma, beginning at pressure of N7 kbar. The rocks decompressed and cooled below 50OOC prior to emplacement of the granites at ~1715 Ma. Thus, there was a delay of several tens of millions of years between initial garnet growth and granite emplacement. E, The COCORP cross section suggests that current thickness of the crust in vicinity of the mans-

Rockg Mountain Geology,v. 34, no. 2, p . 165-181, 13figs., 2 tables, Novembev, 1999


.. .

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Figure 10, left. Concentrations of B and TiO, in two isotopically distinct suites of HPG. Note the logarithmic scale for B. High boron concentrations imply breakdown of muscovite during melting, and high TiO, concentrations imply breakdown of biotite.

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Rocky Mountain Geology, v. 34, no. 2, 165-181,13figs., 2 tables, November, 1999

175


Hudson orogen is N45 km. However, during initial stages of regional M, metamorphism, these rocks were at a depth of 20-25 km, indicating that the total thickness of the crust may have reached approximately 70 km during the collision. The essential component in our model is shear-heatingalong a shear zone in thickened crust that was undergoing unroofing (Fig. 13). Shear-heating is perhaps the most feasible way to produce sufficiently high temperatures for dehydrationmelting reactions in relatively shallow parts of the thickened Trans-Hudson orogen. MetaOsedimentary rocks with short crustal histories most probably were located i n such parts (Nabelek et al., in review). Another way to produce adequately high temperatures is to add heat through intrusion of mafic magmas into crust of normal thickness during thinning of the lithosphere following orogenic collapse (Holm et al., 1997). We consider this mechanism unlikely, however, for the following three reasons. First, thinning of the lithosphere generally yields bimodal magmatism and granitic magmas with significant mass contribution from the mantle. There is no evidence for mantle magmatism in the southern Black Hills at any time during, or following, MI metamorphism; all intrusions in the Black Hills have purely crustal signature. Second, it could be argued that mafic magmas are below the level of exposure. However, there is no evidence within the COCORp data for thinning of the lithosphere, upwelling of the asthenosphere, or intrusion of basaltic magmas. Third, at the time of granitic intrusion, the crust was at least 55 k m thick (45 km current crustal thickness plus > 10 km of eroded crust). This indicates lack of sig176

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Figure 12. Thermal model for metamorphism and granite generation in the Black Hills. Input parameters and constraints are discussed in text and listed in Tkble 2. The model includes thrusting between 0 and 60 m.y. and unroofing from 10 m.y. A, Diagram showing initial and evolving geothems in 10 m.y. intervals (times noted at bottom right). Thermal anomaly is produced in the shear zone until thrusting ceases. Depths and temperatures at the thrust fault and Moho with time are indicated. B, Corresponding pressure-temperature-time(P-Tt) paths for four sections of the thickened crust. Numbers at each path indicate initial model depths, and dots indicate 10 m.y. intervals. Garnet- and staurolite-in isograds (Spear and Cheney, 1989), relevant dehydration-melting solidi of metapelites (Le Breton and Thompson, 1988; Patiiio-Douce and Hams, 1998), and stability fields for aluminosilicate polymorphs are shown. Dashed parts of paths for initial depths of 25 and 30 k m indicate possible duration of garnet growth between the garnetin isograd and maximum temperatures. Granite production is assumed to occur when P-T-t paths reach metapelite solidi. In the model, granite production in sections of crust that initially were at 35 and 45 km occurs 4 5 m.y. after beginning of garnet growth in crust that initially was at 25 km.At the time of granite generation, the latter part of the crust is at N3 kbar. Rockg Mountain Geology, v. 34, no. 2, p . 165-181,13 figs., 2 tables, November, 1999


nificant thinning of the lithosphere prior to intrusion of the granites. Given the weight of evidence against intrusion of mafic magmas as the cause of HPG production, we suggest that our hypothesis of melting induced by shear-heating along thrust faults is more plausible. Shear-heating has been shown to be a major contributor to inverted metamorphic grades below thrust faults that bound stacked crustal sequences (England and Molnar, 1993).

Black Hills

Model Parameters We simulated evolving geotherms and P-T-t paths of the crust (Fig. 12) using numerical techniques outlined by Liu and Furlong (1993). The model parameters selected are listed in Thble 2. The total initial thickness of the model lithosphere is 125 km with total crust thickness of 70 km, although location of the Moho plays no role in the model thermal structure. Thermal boundary conditions are fixed at the surface (0' C) and the bottom of the lithosphere (13OOOC). The rate of radiogenic heating at the surface is 2 ~ 1 W 0 ~m3,based on average concentration of in radioactive elements metasedimentary rocks of the Black Hills (Nabelek and Bartlett, 1998). We assumed exponential decay of radioactive heat production (A) with depth (A = AoeZlD), where Z is depth and D is taken to be 10 km. Crustal thickening during orogeny is approximated by thrusting a 35 km crustal sheet onto the reference lithosphere, although in reality crustal thickening is often accommodated by a system of imbricate listric faults (Fig. 13). It is tempting to suggest that the north-northwest-striking faults in the Black Hills are subvertical expressions of a listric thrust system, although the tim-

Black Hills

Figure 13. A, Schematic drawing, based on COCORP cross section (Fig. l), showing presumed source region of HPG within a ductile shear zone at the interface located where Proterozoic metasedimentary rocks were thrust over the Archean Wyoming basement. Granitic melts are assumed to have migrated within the shear zone and other weak structural zones to the present erosion level, analogous to the model of Solar et al. (1998) for leucogranite melt generation, migration, and emplacement in the Central Maine belt. Principal folding of the metasedimentary rocks occurred prior to melting. B, Alternative schematic drawing (analogous to part A) that may be more consistent with evidence for Archean basement bounding the Black Hills to the east (DeWitt et al., 1986).

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ing of movement on the fault systems is not well understood. The initial geotherm i n the lower plate (Fig. 12A) is at steady state, characterized by parameters listed in Thble 2. In contrast, the initial temperature of the bottom 20 km of the upper plate is arbitrarily set at 250"C, so that initial h e a t i n g of relatively cold, thrusted, sedimentary sequences can be approximated. It is noted, however, that within 10 million years after beginning of thermal relaxation, the upper plate temperature profile becomes virtually independent of the choice of the initial geotherm. We assumed that thrusting of sedimentary sequences over the basement continued for 60 million years. The duration is given by the approximate time it takes for rocks at N7 kbar to reach the garnet isograd (15 m.y.; Fig. 12B) plus the time difference between the oldest dates for regional metamorphism and intrusion of the HPG (45 m.y.). The chosen rate of thrusting is 3 cm y', which is appropriate for rates of crustal convergence. The resulting integrated period of thrusting permits stacking of the thrusted material to thicken the upper plate along imbricate thrust faults. It is noted that our model does not include heat advection due to lateral movement of over-thrustedrocks. Shear-heating was assumed to be concentrated in a 3 km-wide shear zone at the thrust fault with shear-heating decreasing away from its center, following a Gaussian distribution. The applied shear stress in the center of the shear zone is 35 MPa. That value is based on experimentally determined rheologic parameters for mica schists and is appropriate for high temperatures below the solidus (Shea and Kronenberg, 1992). Heat of fusion is incorporated into calculations

178

mble 2. Lithospheric properties and model parameters. Lithospheric properties total thickness of lithosphere depth of thrust fault temperature at top temperature at bottom density specific heat radiogenic heat production at top (Ao) drop-off length for heat production thermal diffusivity thermal conductivity

125 km 35 km 0" c 300" C

2900 kg m-3 1000 J kg-' K-' 2 . 1 0 ' 6 ~m3

10 km

-

1 lo4 m2s-l 2.25 W m ' K-'

Model parameters beginning of unroofing rate of unroofing duration of thrusting shear stress at thrust rate of thrusting

when temperatures reached the metapelite solidus. 'Itventy-five percent melting was assumed to occur over a 20°C interval, consistent with the average fertility of the Black Hills schists (Nabelek and Bartlett, 1998) as well as the discontinuous nature of the muscovite dehydration-melting reaction (Patifio-Douce and Harris, 1998).No provision was made for heats of metamorphic reactions. Unroofing was chosen to start only 10 m.y. after thrusting to allow initial thickening of the upper plate. The average unroofing rate is given by t h e t i m e it took t h e currently exposed metasedimentary rocks to decompress from 25 km at 1760 Ma to 12 km at 1715 Ma. Modeling Resnlts The calculated evolving geotherms are shown in Fig. 12A. They show the enhancement of temperatures to nearly 800" C near the shear zone while thrust-

10 my 0.3 mm y' 60 my 35 MPa 3 cm y'

ing continued. They also show that it is possible to develop a n inverted sequence of isograds below the thrust fault, as is observed below the Main Central Thrust in the Himalayas (Hubbard, 1989). However, t h e temperature anomaly due to shear-heating quickly dissipates once thrusting ceases. The 70 a n d 8 0 m.y. geotherms show t h a t without shear-heating, melting would be possible only in the deep crust below the thrust fault. For the Black Hills, this would mean that only Archean sources of the Wyoming province could have produced granites. This is inconsistent with the Proterozoic provenance for most granitic plutons and pegmatites. Model P-T-tpaths for four initial depths, with 10 m.y. intervals marked, are shown in Fig. 12B. The figure also includes the staurolite-in and garnet-in (for garnet with Mn/(Mn + Fe + Mg) = 0.4; Friberg et al., 1996; Helms and Labotka, 1991) isograds (Spear

Rockg Mountain Geology, v. 34, no. 2,p . 165-181,13figs., 2 tables, November, 1999


and Cheney, 1989), stability fields for the aluminosilicates, and relevant dehydration-melting reactions. The most important feature reproduced by the combined shear-heating with unroofing is the observed delay between initial garnet growth in metamorphic rocks and emplacement of the HPG. In the model, rocks in the middle region of the upper plate (initial depths 25 and 30 km) already were cooling down from peak temperatures when rocks near the shear zone and below (initial depths 35 and 45 km) were still heating up. For example, rocks that started at 25 km go through the garnet-in isograd 15 m.y after initiation of thrusting and then cooled below 400' C 45 m.y later, at the same time that rocks in the shear zone and deeper reached muscovite and muscovite + biotite dehydrationmelting reactions. This time difference between initial garnet growth and magma generation predicted by the model fits the observed chronology in the Black Hills (Dahl et al., 1998). Moreover, the model reproduces garnet growth in the middle of the upper plate for several tens of million years while metamorphic rocks were heating up (dashed portions of the 25 and 30 km curves), as well as the later intrusion of granitic melts intruded into rocks that have cooled below 500' C from peak temperatures (Holm et al., 1997). Rapid extraction and ascent of granite magma from the source region to the level of emplacement is assumed.

DISCUSSION The proposed model (Figs. 12 and 13) explains the isotopically distinct suites of leucogranites observed in the Black Hills. Source rocks in the lower part of the shear zone, within the Archean crust of the Wyoming province, would have reached the muscovite + biotite dehydration melting reaction. The reaction would have led to relatively high TiO, concentrations, as observed in the 10w-6~~0 HPG suite that requires derivation primarily from source rocks with Archean T, ages. In contrast, the upper part of the shear zone, where Proterozoic rocks predominate, would have reached only the muscovite dehydration-melting reaction. This reaction would have led to the high B and low TiO, concentrations that are observed in the h i g h P o HPG suite that require source rocks with mostly Proterozoic T,, ages of source rocks. It is noted, however, that the two distinct melting reactions for the two granite suites in the model are possible in part by the arbitrary location of the shear zone at 35 km, which happens to be at the approximate pressure of the intersection of the melting reactions (Figs. 3 and 12).

In the model, we assumed that shear-heating occurred along a single thrust fault. It is probable, however, that thrusting during the Pans-Hudson collision, as in other collisions, occurred along several imbricate listric faults, which would have led to widening of the melting region. The NNW-striking faults in the Black Hills may be indicative of imbrication. The internal isotopic and major- and trace-element heterogeneity within both granite suites may be evidence of imbrication of Archean and Proterozoic sedimentary rocks in the source region. The schematic cross section in Fig. 13A is based in part on the COCORP study of Baird et al. (1996), which indicates a fairly wide wedge of Proterozoic rocks near the U.S.-Canada border. However, in the Black Hills geophysical evidence indicates that the foliated Archean Little Elk Creek Granite, exposed along the eastern margin of the Precambrian terrane, is quite a large, northwesterly-elongatedbody at least 7 km wide and 12 km long (DeWitt et al., 1986).This suggests that the Black Hills are bounded to the east by an Archean crust, perhaps a southern extension of the Dakota block. Thus, the wedge of Proterozoic metasedimentary rocks may be much narrower here than at the latitude of the COCORP cross section. Therefore, an alternative cross section consistent with the proposed shear-heating model for generation of the HPG and perhaps more in accord with the available geologic evidence is shown in Fig. 13B.

CONCLUSIONS Leucogranitic magmatism was the culminating event of the Pans-Hudson orogeny as exposed in the Black Hills. It occurred by melting of crustal rocks, probably at mid-crustal levels rather than in the deep crust, as evidenced by emplacement of the HPG and satellite plutons as dikes spread over a wide area in the upper crust. Our petrogenetic model incorporates thrusting as a significant heat source for HPG generation. Our model is analogous to that of Harrison et al. (1998) for generation of Himalayan granites, and takes into account petrologic, geochemical, structural, and age constraints. Apparently late-to post-collisionalleucogranites are common in collisional orogens. A seemingly convenient way to explain post-collisional magmatism has been presumed input of mafic magmas into the crust following orogenic collapse. However, in continental crust that remains thick and lacks evidence for input of mafic magmas (e.g., bimodal anorogenic magmatism), this mechanism remains speculative.

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Nevertheless, each proposed model should be testable. For example, in many areas where syn-to postcollisional leucogranitic magmatism has occurred, it is associated with large shear-zone systems (e.g., High Himalayas, Maine). If granite generation is related to shear-heating,then detailed thermometry should reveal thermal anomalies around thrust faults or their vertical expressions (e.g., Hubbard, 1989).

ACKNOWLEDGMENTS

Duke, E. F., Redden, J. A., and Papike, J. J., 1988,Calamity Peak layered granite-pegmatitecomplex, Black Hills, South Dakota: Part I. Structure and emplacement: Geological Society of America Bulletin, v. 100,p. 825-840. Duke, E. F., Shearer, C. K., Redden, J. A., and Papike, J. J., 1990, Proterozoic granite-pegmatite magmatism, Black Hills, South Dakota: Structure and geochemical zonation, in Lewry, J. F., and Stauffer, M. R., eds., The Early Proterozoic mans-Hudson orogen: 'Ibronto, Geological Association of Canada Special Paper 37,p. 253-269. Duke, E. F., Papike, J. J., and Lad, J. C., 1992,Geochemistryof a boron-richperaluminous granite pluton: The Calamity Peak layered granite-pegmatitecomplex, Black Hills, South Dakota: 'Ibronto, Canadian Mineralogist,v. 30,p. 811-834.

The ideas presented here are to a large extent the product of many discussions about the Black Hills with Jack Redden, Bob Bauer, Pete Dahl, Ed Duke, Eirik Krogstad, and Richard Walker. Constructive and careful reviews of the paper by Ed Duke, Sue Swapp, and Bob Bauer were very helpful. The study was supported by NSF grant EAR 94-17979.

England, P., and Molnar, P., 1993,The interpretation of inverted metamorphic isograds using simple physical calculations: lkctonics, v. 12,p. 145-157.

REFERENCES CITED

Gosselin, D. C., Papike, J. J., Zartman, R E., Peterman, Z. E., and Lad, J. C., 1988,Archean rocks of the Black Hills, South Dakota: Reworked basement from the southern extension of the mans-Hudson orogen: Geological Society of America Bulletin, v. 100,p. 1244-1259.

Friberg, L. M., Dahl, P. S.,andTerry, M. P., 1996,Thermometric evolution of Early Proterozoic metamorphic rocks from the southern Black Hills, South Dakota, in Paterson, C. J., and Kirchner, J. G., eds., Guidebook to the geology of the Black Hills, South Dakota: Rapid City, South Dakota School of Mines and Technology Bulletin 19,p. 191-199.

Baird, D. J., Nelson, K. D., Knapp, J. H., Walters, J. J., and Brown, L. D., 1996,Crustal structure and evolution of the mansHudson orogen: Results from seismic reflection profiling: lkctonics, v. 15,p. 416-426. Harris, N., and Massey, J., 1994,Decompression and anatexis of Berry, J. M., Duke, E. F., and Snee, L. W., 1994, 4QAr/39Ar Himalayan metapelites: Tectonics, v. 13,p. 1537-1546. thermochronology of Precambrian metamorphic rocks north of the Harney Peak Granite, Black Hills, South Da- Harrison, T. M., Grove, M., Lovera, 0. M., and Catlos, E. J., 1998, A model for the origin of Himalayan anatexis and inverted kota: Geological Society of America Abstracts with Prometamorphism: Journal of Geophysical Research, v. 103,p. grams, v. 26,no. 6,p. 4. 27,017-27,032. Brown, M., and Solar, G. S., 1998,Shear-zone systems and melts: Feedback relations and self-organizationin orogenic belts: Helms, T. S.,and Labotka, T. C., 1991,Petrogenesisof Early Proterozoic pelitic schists of the southern Black Hills, South Journal of Structural Geology, v. 20,p. 211-227. Dakota: Constraints on regional low-pressure metamorDahl, P. S., and Frei, R, 1998,Stepleach Pb-Pb dating of incluphism Geological Society of America Bulletin, v. 103,p. sion-bearing garnet and staurolite, with implications for 1324-1334. Early Proterozoic tectonism in the Black Hills collisional orogen, South Dakota, United States: Geology, v. 26,p. 111- Hoffman, P. F., 1990. Subdivision of the Churchill province and extent of the mans-Hudson orogen, in Lewry, J. F., and 114. Stauffer, M. R, eds., The Early Proterozoic ?tans-Hudson Dahl, P. S., and Holm, D. K., 1996,Implications of hornblende orogen: 'Ibronto, Geological Association of Canada Special and mica thermochronology on the 1800-1400 Ma Paper 37,p. 15-39. tectonothermal evolution of the Black Hills, South Dakota,

in Paterson, C. J., and Kirchner, J. G., eds., Guidebook to the geology of the Black Hills, South Dakota: Rapid City, South Dakota School of Mines Bulletin 19,p. 200-209.

Holland, T. J. B., and Powell, R, 1998,An internally consistent thermodynamic data set for phases of petrological interest: Journal of Metamorphic Geology, v. 16,p. 309-343.

Dahl, P. S., Frei, R, and Dorais, M. J., 1998,When did the Wyoming Province collide with Laurentia?: New clues fmm step leach PbPb dating of garnet independent of its inclusions: Geological Society of America Abstracts with Programs, v. 30,no. 7,p. 109.

Holm, D. K., Dahl, P. S.,and Lux, D. R, 1997,mAr/39Ar evidence for Middle Proterozoic (1300-1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America Implications for Early Proterozoic P-T evolution and posttectonic magmatism: lkctonics, v. 16,p. 609-622.

Dahl, I? S.,Holm, D. K., Gardner, E. T., Hubacher, E A., and Foland, K. A., 1999,New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia: Geological Society of America Bulletin, v. 111,p. 1335-1349.

Hubbard, M. S., 1989,Thermobarometricconstraints on the thermal history of the Main Central Thrust zone and Tibetan slab, eastern Nepal Himalaya Journal of Metamorphic Geology, V. 7,p. 19-30.

DeWitt, E., Redden, J. A., Burack-Wilson, A., and Buscher, D., 1986,Mineral resource potential and geology of the Black Hills National Forest, South Dakota and Wyoming: U.S. Geological Survey Bulletin 1580,135 p.

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Javoy, M., Fourcade, S.,and Allegre, C. J., 1970,Graphicalmethod fractionations in silicate rocks: for examination of 180/'60 Earth and Planetary Science Letters, v. 10,p. 12-16. Krogstad, E. J., and Walker, R J., 1994,High closure temperatures of the U-Pb system in large apatites from the Tim

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PETROGENESIS AND TECMNIC CONTEXT OF HARNEY PEAK GRANITE

Mountain pegmatite, Black Hills, South Dakota, USA: Geochimica et Cosmochimica Acta, v. 58, p. 3845-3853.

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Krogstad, E. J., Walker, R. J., Nabelek, P. I., and Russ-Nabelek, C., 1993, Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA Geochimica et Cosmochimica Acta, v. 57, p. 4677-4685. Le Breton, N., and Thompson, A. B., 1988, Fluid-absent (dehydration) melting ofbiotite in metapelites in the early stages of crustal anatexis: Contributions to Mineralogy and Petrology, V. 99, p. 226-237. Le Fort, P., and six others,, 1987, Crustal generation of the Himalayan leucogranites: Tectonophysics, v. 134, p. 39-57. Lewry, J. E, and Collerson, K. D., 1990, mans-Hudson orogen: Extent, subdivision, and problems, in Lewry, J. F., and Stauffer, M. R., eds., The Early Proterozoic Pans-Hudson orogen: 'Ibronto, Geological Association of Canada Special Paper 37, p. 1-14. Liu, M., and Furlong, K. I?, 1993, Crustal shortening and Eocene extension in the southeastern Canadian Cordillera: Some thermal and rheological considerations: Tectonics, v. 12, p. 776-786. Nabelek, P.I., 1997, Quartz-sillimanite leucosomes in high-grade schists, Black Hills, South Dakota: A perspective on the mobility of A1 in high-grade metamorphic rocks: Geology, V. 25, p. 995-998. -1999, Examination of trace element distribution among rock-forming minerals in Black Hills migmatites, South Dakota, using a reaction-progress method: A case for solidstate equilibrium: American Mineralogist, v. 84, P. 12561269. Nabelek, P. I., and Bartlett, C. D., 1998, Petrologic and geochemical links between the post-collisional Proterozoic Harney Peak leucogranite, South Dakota, USA, and its source rocks: Lithos, v. 45, p. 71-85. Nabelek, P. I., Russ-Nabelek, C., and Denison, J. R, 1992a, The generation and crystallization conditions of the Proterozoic Harney Peak leucogranite, Black Hills, South Dakota, USA Petrologic and geochemicalconstraints: Contributions to Mineralogy and Petrology, v. 110, p. 173-191. Nabelek, P. I., Russ-Nabelek, C., and Haeussler, G. T., 1992b, Stable isotope evidence for the petrogenesis and fluid evolution in the Proterozoic Harney Peak leucogranite, Black Hills, South Dakota: Geochimica et Cosmochimica Acta, v. 56, p. 403-417. Nabelek, P.I., and'krnes, K., 1997, Fluid inclusions in the Harney Peak Granite, Black Hills, South Dakota, USA: Implications for solubility and evolution of magmatic volatiles and crystallization of leucogranite magmas: Geochimica et Cosmochimica Acta, v. 61, p. 1447-1465.

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MANUSCRIPT SUBMIWED SEPTEMBER 11, 1998 REVISED MANUSCRIPT SUBMITTED FEBRUARY 8, 1999 MANUSCRIPT ACCEPTED FEBRUARY 15, 1999

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