Journal of Petrology | Volume 40 | Number 12 | Pages 1771-1802 | 1999
© Oxford University Press 1999
Petrogenesis of the 1.43 Ga Sherman Batholith, SE Wyoming, USA: a Reduced, Rapakivi-type Anorogenic Granite
Department of Geology and Geophysics, University of Wyoming Laramie, WY 82071, USA
Received October 13, 1998; Revised typescript accepted May 28, 1999
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ABSTRACT |
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 TOP ABSTRACT IntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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The 1.43 Ga Sherman batholith, southeastern Wyoming, USA, shows extreme A-type petrochemical characteristics compared with other Mid-Proterozoic granite batholiths of North America. It consists of: (1) the Sherman granite, a coarse-grained biotite hornblende granite that locally contains fayalite and pyroxenes; (2) the Lincoln granite, a medium-grained biotite granite; (3) a porphyritic biotite hornblende granite that probably formed by interaction of granitic and mafic magmas; and (4) iron-enriched mafic dikes and pods. The ilmenite-series, metaluminous Sherman granite exhibits extreme values of FeOt/(FeOt+MgO) and is rich in K, REE, Nb and Y. It crystallized at temperatures exceeding 900°C and a pressure of
2.5 kbar, with water activity of 0.7 and 
log fO2 of -0.1 to –0.5. The Lincoln granite, which is peraluminous and has less extreme A-type geochemical characteristics, crystallized at temperatures as low as 750°C and log fO2 of around 0.5 units above FMQ (fayalite–magnetite–quartz). The rocks of the Sherman batholith are chemically equivalent to lavas from the Yellowstone hotspot. Like the Yellowstone magmas, the Sherman batholith probably originated by partial melting of underplated, mantle-derived mafic rocks. KEY WORDS: A-type; anorogenic; granite; rapakivi; Proterozoic
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Introduction |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Granites and rhyolites with high K contents and extreme Fe enrichment are a distinctive rock type of problematic origin that are found throughout the Proterozoic ‘anorogenic’ granite provinces of the southwestern USA, Adirondacks, eastern Canada, southern Greenland and the Fennoscandian Shield (Anderson, 1983
). Phanerozoic examples include Paleozoic granites of southeastern Australia (Collins et al., 1982) and of Corsica (Poitrasson et al., 1995), and the Mesozoic Nigerian Younger granites (Jacobsen et al., 1958) and the White Mountain Magma Series (Foland & Allen, 1991). ‘Anorogenic granite’ refers to plutons that are not associated with compressional structures, and that were emplaced long after any known orogenic event. Anorogenic, or ‘A-type’ granites (Loiselle & Wones, 1979) are characterized by low H2O and O2 fugacities along with high FeOt/(FeOt + MgO), K2O/Na2O and K2O contents. A-type granites typically are rich in incompatible elements, including rare earth elements (REE), Zr, Nb and Ta, but poor in Co, Sc, Cr, Ni, Ba, Sr, and Eu. Minimum magma liquidus temperatures are 900–1000°C (Clemens et al., 1986; Creaser & White, 1991). Some workers suggest these rocks are derived from melting of tonalitic or more felsic crust (Anderson & Cullers, 1978; Collins et al., 1982; Clemens et al., 1986; Creaser et al., 1991), whereas others relate the granites to mantle-derived tholeiitic magmas (Jacobsen et al., 1958; Turner et al., 1992; Frost & Frost, 1997), or favor a combination of crustal and mantle sources (Barker et al., 1975; Foland & Allen, 1991).
The goal of this study is to understand the origin of the Sherman batholith of southeastern Wyoming. This intrusion, a ‘reduced, rapakivi-type granite’ (Frost & Frost, 1997), is composed of ilmenite-series metaluminous granites that commonly exhibit rapakivi texture. They have among the highest K2O and FeOt/(FeOt + MgO) contents and the lowest fO2 and fH2O of any Mid-Proterozoic, anorogenic granite in North America (Anderson, 1983; Frost & Frost, 1997), which limits possible source rocks, and restricts the tectonic environments in which it could be produced.
Geologic setting of the Sherman batholith
About 1300 km2 of Mid-Proterozoic Sherman batholith is exposed in the southern Laramie Mountains in southeastern Wyoming and the Front Range of northern Colorado, with smaller exposures in the southern Medicine Bow Mountains (Fig. 1). The batholith lies near the Cheyenne belt, a complexly deformed 1.76–1.78 Ga suture between Proterozoic island arc rocks and the Archean Wyoming province (Hills & Houston, 1979; Karlstrom & Houston, 1984; Duebendorfer & Houston, 1987; Chamberlain, 1998). The Cheyenne belt is exposed in the Medicine Bow Mountains, but in the Laramie Mountains its inferred trace has been obliterated by the 1.43 Ga Laramie anorthosite complex. Most of the Sherman batholith cuts Proterozoic igneous and metamorphic rocks that lie south of the inferred trace of the Cheyenne belt. Only the Mule Creek lobe, the northeasternmost exposure of the Sherman batholith, cuts Archean granitic gneiss (Fig. 1).
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The Laramie Mountains form an asymmetrical Laramide uplift in which Precambrian rocks have been thrust eastward over Phanerozoic sedimentary rocks and have been unconformably overlain in the west by Paleozoic rocks. The Sherman batholith cuts Early Proterozoic supracrustal rocks along its southern margins in the Laramie Mountains and the Colorado Front Range. Proterozoic country rocks are also found as a belt on the east side of the batholith that extends from Granite Village to Virginia Dale (Fig. 1). Most northern contacts cut the 1.43 Ga Laramie anorthosite complex (Scoates & Chamberlain, 1995
) or the 1.76 Ga Horse Creek anorthosite complex (Scoates & Chamberlain, 1997). |
Lithologic Units |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Our study was primarily of the Sherman Mountains area of the Sherman batholith, directly east of Laramie. This small area contains fresh exposures of the rock types throughout the batholith (Fig. 2). From our work in this area and reconnaissance mapping elsewhere we have identified four major units: (1) the Sherman granite, a coarse-grained biotite hornblende granite that locally contains fayalite and pyroxenes; (2) the Lincoln granite, a medium-grained biotite granite; (3) porphyritic biotite hornblende granite; (4) iron-rich mafic rocks. Rarer are sodic granitoid rocks, such as the Pole Mountain gneiss, which we interpret as the oldest unit of the batholith.
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Sherman granite
The dominant rock type of the Sherman batholith is coarse-grained, biotite hornblende granite. This reddish orange rock commonly weathers deeply to a thick grus. The Sherman granite is subporphyritic, with a seriate, hypidiomorphic granular texture. Locally it is an augen gneiss (Fig. 2), indicating late-stage deformation. Major phases are microcline, plagioclase, quartz, hornblende, biotite, and ilmenite. Accessory phases are zircon and apatite with rarer allanite and fluorite. Augite, pigeonite, fayalite, and magnetite are found in some samples. The more hydrous samples contain titanite, produced by Microcline is megacrystic, perthitic, and in places it is rimmed by plagioclase to create a rapakivi-textured mantle. The mafic minerals are locally glomerocrystic. The rock is a granite sensu stricto in the Sherman Mountains area (Fig. 3) but in the Virginia Dale area the Sherman granite consists of quartz syenite and quartz monzonite in addition to granite (Eggler, 1968
).
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The Sherman granite locally contains fayalitic olivine or its alteration products, clinopyroxene, and (in one sample) pigeonite. Pyroxenes are typically rimmed by hornblende. Olivine is rimmed by grunerite, which in turn is rimmed by hornblende; biotite is sparse. On fresh surfaces, olivine and/or pyroxene-bearing Sherman granite is green to black. Olivine-bearing samples contain both orthoclase and microcline; the order–disorder transition in K-feldspar evidently was sluggish in these relatively dry rocks (see Vorma, 1971). The fayalite granite does not crop out boldly. We were able to sample it only in blasted roadcuts along I-80. The contact between fayalite-bearing and fayalite-absent granite appears gradational, and apparently reflects variations in water activity. In many areas the Sherman granite weathers to dark grus, suggesting fayalite-bearing granite may be more abundant than we have documented.
Lincoln granite
This medium-grained, red–orange to orange–gray biotite granite was named after the monument that marks the summit of the old Lincoln Highway, US 30 (Edwards, 1993). The Lincoln granite occupies much of the area directly south of the summit of the Sherman Mountains, where it crops out as sub-horizontal sheets (Fig. 2). North of the Sherman Mountains, the Lincoln granite caps hills and knolls. The granite also occurs as dikes, in which it is locally commingled with monzodiorite. It is also found as inclusions in the Sherman granite. Lineated Lincoln granite occupies a small area at the northwestern end of the Sherman Mountains, suggesting that the unit was emplaced throughout the history of the batholith. Houston & Marlatt, (1997) equated medium-grained to porphyritic facies in the eastern portion of the batholith to the Lincoln granite of Edwards, (1993). Smith, (1977) described a medium-grained granite in the Mule Creek lobe of the Sherman batholith near Iron Mountain, which our reconnaissance work indicates is the Lincoln granite.
The Lincoln granite is composed of quartz, plagioclase, microcline, perthite, biotite, apatite, zircon, and locally traces of hornblende, ilmenite, and fluorite. It contains more modal quartz than does the Sherman granite (Table 1; Fig. 3). The rock is generally equigranular, with an allotriomorphic granular texture. Some samples display isolated alkali feldspar megacrysts that rarely make up more than 1% of the rock.
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Porphyritic granite
Orange–gray granite with 1–2 cm, orange–pink alkali feldspar phenocrysts is most abundant north of highway 210 in the Pole Mountain area. This unit was described by Harrison, (1951)
as the dominant constituent of the central part of the batholith, but also resembles the Inner and Outer Cap Rock quartz monzonite of Virginia Dale (Eggler, 1968). The porphyritic granite commonly contains oblate mafic enclaves and schlieren, and xenoliths of Sherman granite, and shows gradational contacts with monzodiorite and with Lincoln granite. However, most contacts between the porphyritic granite and the Sherman granite are sharp.
The major phases in the porphyritic granite are perthitic microcline, plagioclase, quartz, biotite, and hornblende. Titanite, ilmenite, apatite, and zircon are minor phases. Textures are porphyritic and hypidiomorphic granular, with megacrystic alkali feldspar. Modal compositions of porphyritic granite samples overlap those of the Sherman and Lincoln granites, but Eggler, (1968) showed that the porphyritic granite suite displays greater proportions of plagioclase (Table 1; Fig. 3).
Mafic rocks
Mafic rocks constitute less than 1% of the total area of the Sherman batholith. In the Virginia Dale area, the mafic rocks are gabbroic (Vasek & Kolker, 1999), whereas in the Sherman Mountains area ferrodiorite, monzonite and monzodiorite are present. Ferrodiorite sampled near the summit of I-80 contains plagioclase, pigeonite, magnetite and ilmenite, quartz, pigeonite and augite, biotite, and hornblende, thus resembling ferrodiorites in the Laramie anorthosite complex (Mitchell et al., 1996). Contacts are poorly exposed. Olivine- and pyroxene-bearing monzonite from the same locality occurs as a 100 m diameter, dark bluish purple enclave within Sherman granite. This rock contains fayalitic olivine, ferrohedenbergite, alkali feldspar, plagioclase, amphibole, biotite, magnetite and ilmenite, and is similar to monzonites of the Maloin Ranch and Red Mountain plutons, Laramie anorthosite complex (Kolker & Lindsley, 1989; Anderson, 1995). It is less siliceous yet has a higher FeOt/(FeOt + MgO) ratio than the Sherman granite.
The most common mafic rock type in the Sherman batholith is monzodiorite. Monzodiorite samples contain plagioclase, hornblende, biotite, quartz, and orthoclase, with minor titanite, magnetite and ilmenite, apatite, and zircon. Monzodioritic dikes and pods commonly contain irregularly distributed alkali feldspar megacrysts that display thin rims of plagioclase, along with plagioclase xenocrysts, and/or rounded quartz grains rimmed by hornblende. Contact relations vary. In some areas monzodiorite bodies sharply crosscut the Lincoln granite, whereas other contacts are gradational with porphyritic granites and lobate–cuspate with the Lincoln granite. Monzodiorite locally contains enclaves of Sherman granite. We have observed commingling and hybridization relations between monzodiorite and granite within the Sherman batholith immediately south of the Sherman Mountains. Similar relationships have been reported in Virginia Dale (Eggler, 1968; Vasek & Kolker, 1999) and near Granite Village (Houston & Marlatt, 1997).
Sodic rocks of the Sherman batholith
Gray to pink hornblende biotite gneiss (the Pole Mountain gneiss, Harrison, 1951) makes up most of the crest of the Sherman Mountains (Fig. 2). It contains plagioclase, quartz, biotite, sodic hornblende and microcline, and consists of granodiorite and quartz monzodiorite (Fig. 3). It is more sodic than the other units of the Sherman batholith (see geochemistry section below). Foliations of the gneiss vary in orientation, and feldspars show no evidence of subsolidus deformation; we therefore interpret the foliations to be magmatic. The gneiss encloses xenoliths of amphibolite. Both the Lincoln and porphyritic granites contain enclaves of the Pole Mountain gneiss.
Another member of the sodic series forms an isolated outcrop along I-80 (locality 91SMW27). This quartz monzodiorite looks like a whiter example of Sherman granite and is undeformed. However, it contains sodic hornblende and much less quartz and alkali feldspar than does the typical Sherman granite, and chemically appears to be closely related to the Pole Mountain gneiss. We interpret it as an enclave of the older, sodic rock series included in the Sherman granite.
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U–Pb Zircon Geochronology |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Previous U–Pb age determinations of zircon from the Sherman batholith exist only for porphyritic granite from a drillhole
5 km north of Buford. Analyses of multiple fractions of zircon yielded Pb/Pb ages of 1425–1377 Ma (Aleinikoff, 1983). Interpretation of the zircon data was complicated by inheritance and lead loss, but Aleinikoff, (1983) favored an age of 1415–1435 Ma. We selected four samples for U–Pb geochronology (Table 2): Sherman granite (91PH1), Lincoln granite (91PH6a), monzonite inclusion (90SMW4), and Pole Mountain granite gneiss (94SMW2 and 90SMW13). These samples appear to span the intrusive history of the Sherman batholith. U–Pb data for sample 91PH1 were obtained from five fractions of zircon, including two air-abraded fractions (Table 2). On a concordia diagram, these fractions range from 3 to 35% discordant, and yield an intercept age of 1433 ± 1.5 Ma (Fig. 4). Common lead was corrected based upon the Pb isotopic composition of coexisting feldspars. There is no evidence of inherited zircon or other complexities in the U–Pb systematics, which indicates that the intercept age is the crystallization age of the Sherman granite.
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Seven fractions of Lincoln granite sample 91PH6a were analyzed, including six air-abraded fractions (Table 2). The air-abraded fractions are nearly concordant, ranging from 0.6 to 6% discordant, whereas the unabraded fraction is 31% discordant. Together, the seven fractions yield an intercept age of 1430 ± 2.6 Ma (Fig. 4). The correction for common lead was made using the Pb isotopic composition of coexisting feldspar. Zircons from the Lincoln granite show higher U and Th/U ratios than zircons from the other samples, and contain the most common Pb of the samples. Common Pb is associated with the outer portions of the grains, because it was reduced by air-abrasion.
Monzonite inclusion sample 90SMW4 was dated to limit the age of the Sherman granite that engulfs this 10 m diameter enclave. U–Pb data for six fractions of zircon from this sample yield a non-linear array on a concordia diagram, with a small range in Pb/Pb ages of 1436–1440 Ma (Table 2; Fig. 4). All of the analyses are concordant or nearly concordant, with a maximum discordance of 1.4%. The range in Pb/Pb ages reflects small amounts of inheritance, and the youngest Pb/Pb age of 1436.3 ± 1.3 Ma is a maximum age for the monzonite.
Two samples of the Pole Mountain gneiss were analyzed. Sample 94SMW2 was collected from an outcrop free of enclaves of older country rock. The U–Pb systematics of this sample were reasonably simple. Four fractions of zircon are nearly concordant, ranging from 1 to 4% discordant on a U–Pb concordia diagram, and yield a linear array with an upper intercept age of 1437.8 ± 3.2 Ma (Table 2; Fig. 4). This is probably the crystallization age of the Pole Mountain gneiss. The second sample was collected from an outcrop with abundant enclaves of older rocks, including amphibolite and quartzite. Twelve fractions of zircon from this sample (90SMW13) yield Pb/Pb ages ranging from 1450 to 1689 Ma (Fig. 4). The range indicates varying proportions of inherited components in the zircon. The zircon fraction with the youngest Pb/Pb age of 1450 Ma has the least inheritance. Assuming that no fraction is completely free of inheritance, the results from this sample are consistent with those for sample 94SMW2, and imply that the Pole Mountain gneiss has a maximum age of 1438 Ma, and that the gneiss contains a component of assimilated older crust. In summary, the Pole Mountain gneiss is the oldest unit of the Sherman batholith, with a maximum age of 1439 Ma. The volumetrically dominant lithologies, the Sherman and Lincoln granites, were emplaced at 1433 ± 1.5 and 1431 ± 2.6 Ma, respectively.
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Geochemical Characteristics |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Major element geochemical data for the Sherman bathoith are available from several sources: the majority is from the Pole Mountain area (present study, Table 3 and Table 4), but additional analyses exist for rocks from Virginia Dale (Eggler, 1968
), the Granite Village area, and from Sheep Mountain in the Medicine Bow Mountains (Houston & Marlatt, 1997). Because published sample descriptions did not allow us to consistently identify the lithologic units analyzed, we plot only data from the Pole Mountain area in Fig. 5 Fig. 9.
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Most analyses form coherent arrays on major element Harker diagrams (Fig. 5). The mafic rocks and the Sherman, porphyritic and Lincoln granites decrease in TiO2, FeOt, MgO, CaO and P2O5 with increasing SiO2. Al2O3 and Na2O decrease and increase, respectively, in the most siliceous samples. K2O generally increases with silica except for Lincoln granite samples with the highest SiO2 contents. The Sherman batholith is subalkalic, and spans the metaluminous–peraluminous boundary (Fig. 6a). In contrast to calc-alkalic rocks, they exhibit extreme iron enrichment (Fig. 6b), and have large K2O values (Fig. 5). Rocks of the Sherman batholith contain abundant large ion lithophile elements (LILE; Rb, Ba, REE) and high field strength elements (HFSE; Zr, Y, and Nb), which are typical of anorogenic granites. Analyses of Sherman batholith rocks lie mostly in the within-plate granite field of Pearce et al., (1984)
, although Lincoln granite samples lie between the within-plate and volcanic arc fields (Fig. 6c).
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The older, volumetrically minor sodic rocks and the monzonite enclave do not follow the geochemical trends of the rest of the Sherman batholith. The sodic rocks (crosses in Fig. 5) show total alkali contents identical to younger units, but contain higher Na2O and lower K2O. Sodic rocks are also distinguished by much higher Al2O3 contents and lower FeOt and TiO2 contents. Analyses of the monzonite enclave lie off both these trends.
Mafic rocks
The mafic rocks of the Sherman batholith display high Ti, P, Fe, Zr, Ba, Ga and Nb contents relative to calc-alkalic basalts (Table 3). An MgO-rich group (MgO > 3.0 wt %), which contains biotite as the main mafic mineral, is found in the western Sherman Mountains. An MgO-poor group (MgO < 3.0 wt %) consists of pyroxene-bearing ferrodiorite and monzodiorite with subequal amounts of hornblende and biotite. The MgO-poor monzodiorites occur as dikes and commingled bodies in the eastern portion of the Sherman Mountains area.
Sherman granite
The SiO2 contents of Sherman granite samples range from 64.4 to 71.3 wt %. The Sherman granite has the highest K2O content of any unit in the batholith, along with the highest Nb and Y contents (Fig. 6c), as well as Zr and Ga contents as high as those in the mafic rocks (Table 3). The light REE (LREE) contents of Sherman granite sample (Ce = 282 ppm) are the highest of any of the rocks analyzed from the Pole Mountain area, and its heavy REE (HREE) contents are exceeded only by those of the monzodiorite sample (91SMW20; Fig. 7). TiO2, MnO, P2O5, and Rb are low compared with porphyritic granites with the same SiO2 contents. For these elements, the Sherman granites lie slightly below the trends defined by the monzodiorites, porphyritic granites, and Lincoln granites (Fig. 5).
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In the Sherman Mountains area, all Sherman granite samples are metaluminous (Fig. 6a). However, those exposed in the Sheep Mountain area of the Medicine Bow Mountains are peraluminous (Houston & Marlatt, 1997
). Although there is a 4 wt % difference in SiO2 content of the least siliceous Sherman granite and the most siliceous monzodiorite, the two groups are geochemically similar. Both are metaluminous and rich in Zr, Nb and Y, and of low Rb/Ba (Fig. 8).
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Lincoln granite
Samples of Lincoln granite vary from 72.0 to 76.2 wt % SiO2, the highest for any rock type in the Sherman batholith. Fourteen of 16 samples are peraluminous. The normative Ab–An–Or values for Lincoln granite samples lie along cotectics calculated by Nekvasil & Lindsley, (1990)
for the system Ab–An–Or–H2O (Fig. 9), and trend to higher Na2O and lower K2O values in the most SiO2-rich samples. Rb/Ba increases dramatically with increasing SiO2 (Fig. 8), a feature consistent with crystallization of feldspar. Zr, Y and REE contents are lower than for the other granites in the batholith; these are probably controlled by the accessory phases zircon, allanite and apatite (Fig. 7 and 10). The low Zr, REE and Y values in the Lincoln granite samples may result because the Lincoln granite is (1) a partial melt in which the accessory minerals were left in the residua, or (2) a differentiate that separated from an assemblage that contained these accessory minerals (i.e. monzodiorite and Sherman granite). The lowest REE contents are also found in the most siliceous units of the zoned monzonite-to-granite Red Mountain pluton of the Laramie anorthosite complex, in which it was demonstrated that REEs were sequestered in allanite and zircon during differentiation of a monzonitic magma (Anderson, 1995).
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Porphyritic granite
Porphyritic granite samples range from 66.8 to 74.4 wt % SiO2, the largest variation in the granites of the Sherman batholith. They occupy intermediate major and trace element compositions between Lincoln, mafic rock and Sherman units. The REE pattern of porphyritic granite sample 91SMW2 lies between Sherman sample 91SMW28 and Lincoln sample 91PH6 (Fig. 7), but is also LREE enriched with a negative Eu anomaly. Porphyritic granite samples lie between the MgO-poor monzodiorite and the Lincoln granite (Fig. 5 and 10). These geochemical features, along with petrographic and field evidence, indicate that porphyritic granite is probably a product of magma mixing or interaction of magmas and feldspar-rich crystal mush.
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Mineral Chemistry |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Mineral compositions were determined on the JEOL Superprobe using natural and synthetic minerals for standards (Table 6 Table 9). Oxygen abundance in silicate minerals was based upon stoichiometry. Fe–Ti oxide minerals were analyzed as weight per cent elements, but oxygen was analyzed along with the cations. For these Fe–Ti oxide minerals, ferric iron was calculated assuming that all elements apart from Fe have fixed valence.
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Olivine
We analyzed fayalitic olivine from a fayalite monzonite inclusion in the Sherman granite (90SMW4) and two samples of the Sherman granite (90SMW5, 90SMW9; Table 5). The olivine in the fayalite monzonite sample is richer in iron (Fa95Tp3Fo2) than that from the Sherman granite (Fa92Tp2Fo6) (Table 6; Fig. 11). The olivine from the fayalite monzonite is similar in composition to that from the monzosyenitic plutons of the Laramie anorthosite complex (Fuhrman et al., 1988
; Kolker & Lindsley, 1989; Anderson, 1995).
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Pyroxene
We analyzed augite from a sample of ferrodiorite (90SMW30) and of fayalite monzonite (90SMW4), and two samples of the Sherman granite (90SMW4; 90SMW9) (Table 3; Fig. 11). Inverted pigeonite occurs in the ferrodiorite and in 90SWM9; the latter sample also contains primary orthopyroxene that displays few augite lamellae. The orthopyroxene composition was obtained directly from the microprobe analyses. In contrast, compositions of highly exsolved pigeonite and augite were reconstructed using image analysis.
Amphibole
There are two types of amphibole in the Sherman granite. All rocks, apart from most of the Lincoln granites, contain hornblende. Fayalite-bearing Sherman granite also contains minor grunerite that has replaced fayalite. Hornblende contains 1.8 atoms p.f.u. of total Al in all rock types of the Sherman granite (Table 7). Samples that contain grunerite (and also fayalite) display hornblende compositions that extend to lower values of total Al and (Na + K). This probably reflects reaction of fayalite with feldspar to produce hornblende, and suggests that, like the grunerite, at least some hornblende in the fayalite-bearing Sherman granite samples formed under subsolidus conditions.
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Application of the Al-in-hornblende barometer (Anderson & Smith, 1995
) to samples with the limiting assemblage yields 1.8 kbar at 850°C. Because the temperature uncertainty is ±50°C, the uncertainty in the pressure estimate is ±1.8 kbar. Although the hornblende composition does not constrain pressure, its composition resembles that of hornblendes that have crystallized from high temperature, and low-pressure magmas (see Anderson & Smith, 1995).
Biotite
Biotite from the Sherman batholith shows textures indicating both primary and secondary formation. The Ti contents of both generations of biotite are identical, suggesting that formation of biotite from olivine and ilmenite was a late-stage magmatic reaction. Both are rich in iron (XFe = 0.75–0.90; Table 8; Fig. 12). The iron contents of biotite correlate with the FeOt/(FeOt + MgO) ratio of whole rocks: the most iron-rich biotite is from the fayalite monzonite, whereas the most magnesian biotite is from porphyritic granite. The Al content of biotite also reflects the whole-rock composition. Biotite with low Al contents comes from the low-SiO2 Sherman granite samples that contain pigeonite and fayalite, and hence are saturated in annite by the equilibrium.
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The most aluminous biotite comes from the Lincoln and porphyritic granites, both of which show higher Al/(Ca + Na + K) than the Sherman granite.
Biotite compositions from Sherman batholith samples lie on the iron-rich end of a trend of biotite compositions from 1.4 Ga granites in the SW USA (Anderson & Bender, 1989; Fig. 12), although they are less iron rich than biotite from the 1.1 Ga Pikes Peak batholith. This indicates that the Sherman batholith is both the most iron rich and the most reduced of the 1.4 Ga anorogenic granite batholiths in the SW USA.
Fe–Ti oxide minerals
Back-scattered electron images show that most ilmenite grains are altered to a cryptocrystalline mixture of phases, probably magnetite and rutile. Only a few grains preserve the oxygen to cation ratio typical of ilmenite. These had hematite contents of 0.04–0.06 (Table 9), values typical of ilmenite from fayalite granites (Frost et al., 1988). Mn contents of ilmenite from the Sherman granite range from <1 wt % to >16%; much of this range may be displayed within a single sample. Because olivine in these rocks is not rich in MnO, the Mn contents of ilmenite probably result from the oxidation of ilmenite to magnetite + rutile, in which Mn released by this reaction was sequestered in residual ilmenite.
Magnetite shows typical oxyexsolution lamellae ofilmenite, as well as low-temperature oxidation (probably to maghemite). Using the ilmenite composition in the rock, assuming the magnetite was stoichiometric magnetite, and taking into account the abundance of the ilmenite lamella in magnetite, the primary titanomagnetite composition from sample 90SMW9 was calculated to be Usp45. Despite uncertainties in this calculation, this composition is consistent with the silicate assemblage of the rock. Usp45 titanomagnetite, fayalite and quartz equilibrated at 800°C, consistent with the other thermometers applied to this sample.
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Intensive Factors |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Pressure
The assemblage olivine–pigeonite–augite–quartz present in sample 90SMW9 (Table 5) is both a geothermometer, because temperature can be determined from the pyroxene solvus or from the low-T limit of pigeonite, and a geobarometer, because the iron contents of ferromagnesian silicates increase with pressure. The olivine and pyroxene in 90SMW9 (Table 6) equilibrated at 624 ± 15°C and 3130 ± 720 bar, as determined from the QUILF program (Andersen et al., 1993
). This temperature reflects subsolidus re-equilibration. The uncertainties given from the QUILF expression are the calculated precision of the various equilibria involved. Where only a single equilibrium is used there is no uncertainty given. The accuracy of the thermometer is probably ±30°C or less. In these samples, mineral compositions changed in four ways during cooling: (1) by exchange of Ca and (Fe + Mg) between augite and pigeonite; (2) by inversion of pigeonite to orthopyroxene; (3) by redistribution of Fe and Mg among olivine, orthopyroxene and augite, which probably continued after Ca ceased to exchange between pyroxenes; (4) during late hydration reactions by which hornblende, biotite, and grunerite were produced.
By analyzing the effect of exchange or hydration reactions on the XFe of minerals in a given rock, one can qualitatively estimate the changes in mineral compositions as the rock cools. Fe–Mg exchange should cause the phases with high XFe to become progressively richer in iron, and those with low XFe to become more magnesian because Fe–Mg distribution is more extreme at lower temperatures. As a hydration reaction progresses, the parent will be enriched in Fe if it has a higher XFe than the hydrated product; it will be enriched in Mg if it has a lower XFe (see Thompson, 1976).
In order of decreasing XFe, the minerals in 90SMW9 are: fayalite > orthopyroxene > biotite > hornblende > grunerite > augite (if a melt were present, it would have XFe > fayalite). During cooling, ion exchange reactions will make olivine and orthopyroxene richer in Fe, as augite and hornblende become richer in Mg. Hydration of olivine to grunerite and orthopyroxene to biotite will enrich the remaining olivine and orthopyroxene in iron. Because the weighted XFe of Opx + Cpx resembles that of hornblende, hydration of pyroxenes to hornblende only slightly changes XFe of the minerals.
Mineral compositions in the assemblage olivine-orthopyroxene–quartz are governed by two reactions: the Fe–Mg reaction between olivine and orthopyroxene, and the mass transfer reaction
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During cooling, the minerals in 90SMW9 followed reaction (1) to higher XFe (Lindsley & Frost, 1992). Thus, pressure estimates for sample 90SMW9 depend on the temperatures at which Fe–Mg exchange ceased. If the Fe/Mg of orthopyroxene was fixed when pigeonite inverted to orthopyroxene, the pressure estimate is 3000 bar. If cooling continued down to 665°C (when hydration ceased), then the estimate is 1700 bar (Fig. 13). Because Fe–Mg exchange probably ceased before the texturally latest hydration reactions, the Sherman granite probably was emplaced at 2500 bar ± 500 bar.
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Temperature and water activity
Geothermometers record a range of temperatures during the crystallization and cooling of the Sherman batholith. The highest temperature, 968°C, comes from the composition of reconstructed pigeonite in ferrodiorite sample 90SMW30 [as determined by the QUILF program of Andersen et al., (1993)
]. This temperature critically depends on how closely the reconstructed pigeonite composition represents the original pigeonite composition. However, the temperature at which pigeonite inverts is a robust thermometer that depends only on the XFe of the pyroxenes (Lindsley & Frost, 1992). The pigeonite in 90SMW30 inverted at 874°C, and Fe–Mg exchange between pigeonite and augite continued down to 789°C. The small ferrodiorite body from which 90SMW30 was sampled is surrounded by the far more voluminous Sherman granite, and therefore 874°C is also the minimum temperature of the granite.
Reconstructed pigeonite compositions from fayalite granite sample 90SMW9 record a T of 869°C; it inverted to orthopyroxene at 806°C (Fig. 14). These temperatures are based on exsolution textures of the pigeonite, which probably formed late in the crystallization history of the rock. Because orthopyroxene also occurs in 90SMW9, crystallization of this sample continued to temperatures at least as low as 800°C. Because the Ca content of orthopyroxene is nearly what would be predicted for orthopyroxene in equilibrium with pigeonite, Ca exchange among the pyroxenes must have ceased at temperatures of 800°C.
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The temperature of hydration from sample 90SMW9 was calculated from the equilibria
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, (1991, , 1992; version 2.02), the feldspar solution model of Elkins & Grove, (1990), the program SOLVCALC (Wen & Nekvasil, 1994), and the grunerite thermodynamic data of Evans & Ghiorso, (1995) and Ghiorso et al., (1995), we calculate a sanidine activity of 0.76, a water activity of 0.7, and that hydration occurred at 665°C.
Two other thermometers are available, the zircon-saturation temperature (Watson & Harrison, 1983) and the apatite-saturation temperature (Harrison & Watson, 1984). These thermometers yield temperatures at which a melt of a given composition would become saturated with each of these two accessory minerals, and are valid only if the rock represents a liquid composition. If zircon and apatite both crystallize from a magma early in its history but do not accumulate, their saturation temperatures should be nearly identical. This is the case with the Lincoln granite, but not for the rest of the Sherman batholith (Fig. 15). Samples of the Lincoln granite that have the lowest zircon and apatite saturation temperatures lie along the lowest temperature portions of the An–Ab–Or cotectic (Fig. 9). These zircon and apatite saturation temperatures, 750–875°C, are similar to the liquidus temperatures determined for haplogranite melts of the same composition at 5 kbar water pressure (Nekvasil & Lindsley, 1990). The lower crystallization temperature of the Lincoln granite compared with the Sherman granite and ferrodiorite reflects its more siliceous bulk composition (Nekvasil & Lindsley, 1990) and greater water activity than the Sherman granite.
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Oxygen fugacity
Because the Fe–Ti oxide minerals in the Sherman granite are altered, its oxygen fugacity is difficult to characterize. The oxygen fugacity of the two samples of Sherman granite that contain fayalite + quartz can be determined because these rocks must have crystallized along the QUILF surface (Frost et al., 1988
). Their final oxygen fugacities lay between 0.1 and 0.5 log units below those of the fayalite–magnetite–quartz (FMQ) buffer (Fig. 16). Oxygen fugacity also can be determined for sample 90SMW1 of Lincoln granite that contains biotite–orthoclase–plagioclase–magnetite–ilmenite. The ilmenite and magnetite in this rock are both altered, but because the magnetite is saturated in Ti, the location of reaction (3) can be calculated using the TWQ program:
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These calculations were made using the composition of biotite in 90SMW1 and assuming the activity of water as 1.0, which yields the highest oxygen fugacity likely for the assemblage, 0.5 log units above the FMQ buffer. The range of input temperatures varied from 750 to 665°C, which encompasses the likely range of solidus temperatures.
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Pb, Nd AND Sr ISOTOPIC COMPOSITIONS |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Initial Pb, Nd and Sr isotopic compositions of samples from each unit of the Sherman batholith constrain possible magma sources (Fig. 17). Initial Pb isotopic compositions were estimated by the least radiogenic fraction of stepwise dissolved feldspars (Table 9); Nd and Sr initial isotopic ratios were calculated from present-day whole-rock values, corrected for radiogenic growth since 1.43 Ga (Table 10).
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Except for the Pole Mountain gneiss, different rock units of the Sherman batholith cannot be distinguished by their initial Pb isotopic compositions (Fig. 17a). The only variation outside of error is in the 206Pb/204Pb ratio, which could reflect modest radiogenic growth after 1.43 Ga, rather than true variation in initial composition. The Pb isotopic data from Sherman batholith feldspars plot above the model mantle evolution curve but below the model upper-crust evolution curve of Zartman & Doe, (1981)
. The Sherman batholith feldspar compositions have much less radiogenic 207Pb/204Pb ratios than Archean rocks from the Wyoming province at 1.43 Ga (Laramie Mts, Verts et al., 1996; Wind River Range, Frost et al., 1998; Beartooth Mts, Wooden & Mueller, 1988). Initial Pb isotopic compositions of feldspars from the Sherman batholith also have distinctly less radiogenic 207Pb/204Pb ratios than the 1.76 Ga Horse Creek anorthosite complex immediately to the north (Chamberlain, 1998), and display slightly less radiogenic to similar 207Pb/204Pb ratios compared with the 1.78 Ga granitoids of the Colorado province that lie 25–100 km to the south (Aleinikoff et al., 1993; Fig. 17a).
In contrast to their initial Pb isotopic compositions, initial Nd and Sr isotopic ratios vary outside of error (Fig. 17b). 
Nd values for Sherman batholith samples at 1.43 Ga vary from 1.1 to -1.5; the range of initial 87Sr/86Sr is larger, from 0.701 to 0.731 (Table 11). Samples of the Pole Mountain gneiss, mafic rocks, and Sherman granite define the tightest cluster, with Nd -0.4 to 1.1 and initial Sr of 0.7024–0.7126. The variation in Sr and Nd initial compositions of the Lincoln and porphyritic granites is larger, and includes the most negative Nd values and the most radiogenic initial Sr isotope ratios (Fig. 17b).
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Discussion and Conclusions |
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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Petrogenesis of the Sherman batholith
The Sherman batholith (1430–1438 Ma) and the Laramie anorthosite complex (1431–1436 Ma; Scoates & Chamberlain, 1995
; Verts et al., 1996) are part of a widespread magmatic event that affected much of the southwestern USA. The Sherman batholith and Laramie anorthosite complex crystallized at similar levels in the crust: the apparent pressure at which the Sherman batholith was emplaced, 2.5 kbar, is close to pressures of 3–4 kbar determined for the Laramie anorthosite complex (Fuhrman et al., 1988; Kolker & Lindsley, 1989; Grant & Frost, 1990; Spicuzza, 1990). Crystallization temperatures for the Sherman batholith (>900°C to 750°C) are only slightly lower than those for the monzosyenitic plutons of the Laramie anorthosite complex of 950°C (Fuhrman et al., 1988; Kolker & Lindsley, 1989; Anderson, 1995). The range of oxygen fugacities determined for the Sherman batholith, from logfO2 of -0.5 to+0.5, also are similar to oxygen fugacities calculated for the Laramie anorthosite complex (logfO2 = 0 to -2; Frost et al., 1996). As is typical of reduced, rapakivi-type granite batholiths and coeval, proximal anorthosite intrusions worldwide, the initial Pb, Sr and Nd isotopic compositions of rocks from the Sherman batholith and the Laramie anorthosite complex are comparable. Both have initial Nd near zero, and initial 87Sr/86Sr ratios mostly between 0.703 and 0.710. Initial Pb isotopic compositions of the Sherman batholith and Laramie anorthosite complex range from 207Pb/204Pb ratios slightly above the model mantle curve (for the Sherman batholith and southern Laramie anorthosite complex) to ratios more like those of the Archean Wyoming province (for rocks of the northern Laramie anorthosite complex; this study; Geist et al., 1989, , 1990; Kolker et al., 1991; Mitchell et al., 1995, , 1996; Scoates & Frost, 1996).
The Pb, Nd and Sr isotopic compositions of theSherman batholith rule out some potential mantle and crustal sources. Archean crust, which is present at depth beneath northern portions of the Sherman batholith (Chamberlain, 1998), has Nd -15, 87Sr/86Sr = 0.74–0.80, and 207Pb/204Pb = 15.7 for 206Pb/204Pb = 16.5, and cannot be a major component of the Sherman magmas (see Fig. 17). Model depleted mantle, with Nd = +4 to +6 at 1433 Ma, 87Sr/86Sr of 0.701, and a less radiogenic Pb isotopic composition than Sherman granitoid rocks, cannot be the sole source of Sherman batholith magmas. Neither the Laramie anorthosite complex nor the Sherman batholith has these model mantle isotopic compositions, not even the high-alumina gabbros and anorthosites interpreted as mantle-derived melts (Mitchell et al., 1995). Perhaps the mantle beneath the Sherman batholith has less extreme isotopic compositions: a mantle with Nd = +2 and 87Sr/86Sr = 0.703 best describes the source of the Laramie anorthosite complex gabbros and anorthosites, and could also have been a source for the Sherman batholith. Although a slightly depleted mantle or mafic lower-crustal source is compatible with the isotopic data, the isotopic data also permit a Proterozoic-age felsic crustal source. The samples display less radiogenic 207Pb/204Pb ratios than the Horse Creek anorthosite complex immediately to the north and lie only slightly below a 1.78–1.43 Ga reference isochron that originates in the field of 1.78 Ga Colorado province granitoids (Fig. 17a). The samples also exhibit Nd and Sr isotopic compositions indistinguishable from Colorado province volcanic rocks (Fig. 17b).
However, the high liquidus temperatures of the magmas, the geochemical composition and mineralogy of the rocks, and, most importantly, oxygen fugacity near or below FMQ rule out both typical felsic calc-alkalic and pelitic rocks as sole sources of reduced, rapakivi-type granites (see Frost & Frost, 1997). These oxygen fugacities are distinctly lower than those of other A-type granites and calc-alkalic granites in general (Fig. 16; Frost & Lindsley, 1992). The relatively low oxygen fugacity is important because the fO2 of a magma probably reflects that of its source (Carmichael, 1991). The low fO2 of the Sherman granite and other reduced, rapakivi-type granites is evidence for their derivation from a tholeiitic source, because calc-alkalic sources are more oxidized and pelitic sources, although appropriately reduced, produce peraluminous, not metaluminous melts (Frost & Frost, 1997). Although partial melts of tonalite or granodiorite mimic the bulk compositions of reduced, rapakivi-type granites, such sources are not likely to have appropriately lowfO2: calc-alkalic rocks typically have logfO2 in the range of +1 to +3 (Frost & Lindsley, 1991). For example, the A-type granite melt produced by melting of tonalite in Skjerlie & Johnston's, (1993) experiments had fO2 > 1 log unit above FMQ (Fig. 16). Melting of magnetite-free tonalite could produce magmas of appropriately low oxygen fugacities, but such magmas appear to be uncommon. Only some of the tonalites from the Sierra Nevada are ilmenite-series (Ague & Brimhall, 1988), but those from the Japanese arc are magnetite-series (Ishihara, 1979).
Experimental results indicate that A-type granite can be produced by differentiation or by partial melting of ferrodiorite (Scoates et al., 1996). Although the Sherman granite could have been produced by extreme differentiation of a tholeiitic magma as has been proposed for the Sybille monzosyenite of the Laramie anorthosite complex (Scoates et al., 1996), this is not likely for several reasons. First, the volume of Sherman granite is large compared with the amount of mafic and monzonitic rocks in the Sherman batholith. The opposite is true in the Laramie anorthosite complex and other batholiths for which this model has been proposed. The batholith lacks the continuum of rock types observed in the Laramie anorthosite complex, and is instead bimodal. The vicinity of the Sherman batholith lacks gravity anomalies that might indicate the presence of a large mass of mafic cumulates (see Scoates et al., 1996). Instead, the Sherman batholith was probably derived from partial melting of pre-existing tholeiites or their differentiates. Tholeiite-series rocks have low fO2 and fH2O, and evolved compositions have iron and LILE enrichment that are typical of reduced rapakivi-type granites.
Sherman ferrodiorite and monzodiorite resemble dioritic rocks of the Laramie anorthosite complex (Mitchell et al., 1996) in their geochemistry (Fig. 10). They are typical of iron-rich diorites found in many anorogenic batholiths, often mingled with granite (Wiebe, 1980; Noblett & Staub, 1990; Eklund et al., 1994; Salonsaari & Haapala, 1994; Vasek & Kolker, 1999). The mafic rocks intruded the Sherman batholith while some granitic magmas were still molten. We suggest that, as with dioritic rocks of the Laramie anorthosite complex, the mafic rocks of the Sherman batholith represent samples of mantle-derived mafic magmas and their differentiates that were variably contaminated by continental crust during ascent (see Mitchell et al., 1996).
In the Sherman batholith, Lincoln granite records incorporation of felsic continental crust. Lincoln granite is more oxidized than Sherman granite, is peraluminous whereas the Sherman granite is metaluminous, and has biotite richer in Mg than that of the Sherman granite. In addition, Lincoln samples exhibit radiogenic initial Sr isotopic compositions (87Sr/86Sr > 0.710). The only known sources with such radiogenic Sr isotopic compositions are Proterozoic and Archean metasedimentary rocks, which had 87Sr/86Sr of 0.715 and higher at 1.43 Ga (Fig. 17b). Although their Sr isotopic compositions are uniformly high, the Nd of the metasedimentary rocks appears to correspond to distance from the edge of the Archean Wyoming province: pelitic rocks at the south edge of the Wyoming province have Nd of -16, metasedimentary rocks 25 km south of this boundary have Nd of -8, and metasedimentary rocks along the Colorado Front Range have Nd of 0 to +3, all at 1.43 Ga. Interpolating these data, metasedimentary rocks near the Sherman batholith probably had Nd of -3 or -4 at 1.43 Ga. A component with an isotopic composition similar to that of such metasedimentary rocks is present in the Lincoln granite. Lincoln granite is very similar to the 1.4 Ga Silver Plume and St Vrain batholiths of the Colorado Front Range in mineralogy, geochemistry, crystallization conditions and radiogenic Sr isotopic compositions (Anderson & Thomas, 1985).
Much of the porphyritic granite may have formed by the interaction of mafic and felsic magmas. Feldspar megacrysts are not in textural equilibrium with the host rock, as evidenced by plagioclase rims on K-feldspar megacrysts, and K-feldspar with euhedral cores and inclusion-rich rims. Clots and schlieren of mafic material are present in the groundmass of some porphyritic granites. On Harker diagrams, the porphyritic granites invariably lie between the mafic rocks and the Lincoln granite, and the Sr isotopic compositions of porphyritic granite samples vary from the least to the most radiogenic, both of which also suggest magma mixing. To search for isotopic evidence of disequilibrium, we obtained Rb–Sr isotopic data for three samples of porphyritic granite, and for feldspar megacrysts separated from these samples. In two samples the megacryst is more radiogenic Sr than the bulk rock, and in the third the megacryst is less radiogenic (Table 10). One megacryst for which we also obtained Sm-Nd isotopic data has an initial Nd that is slightly more than one unit lower than that of the bulk rock. These data corroborate the field, petrographic and geochemical evidence for magma mixing.
Petrogenetic model for A-type granitoids
Our petrogenetic model for the Sherman batholith begins with the emplacement of mantle-derived mafic magmas at or near the base of the crust. This basaltic melt underwent differentiation along a tholeiitic trend at low fO2, producing ferrodiorite with high FeOt/(FeOt + MgO) and rich in REE and Zr. These underplated gabbroic to ferrodioritic rocks represent newly formed mafic lower continental crust. The presence of such mafic material beneath southeastern Wyoming is suggested by the seismic wide-angle studies of Gohl & Smithson, (1994). Those workers interpreted alternating low- and high-velocity layers in the lower crust beneath the Sherman batholith as evidence of mafic and ultramafic rocks interlayered with more felsic crust. This mafic lower crust may have been formed during 2.0 Ga rifting (Cox et al., 1999), at 1.76 Ga in association with the Horse Creek anorthosite complex (Frost et al., 1999), or at 1.43 Ga when the Laramie anorthosite complex and Sherman batholith formed.
Partial melting of anhydrous underplated mafic material can produce granitic melt with extreme A-type compositions (Frost & Frost, 1997). We suggest that ferrodioritic portions of this underplated material are the most appropriate source rock: a ferrodiorite source will yield larger volumes of granitic melt than basalt, and ferrodiorite also has a lower melting point than its basalt parent. Such a melt can form the Sherman granite, the unit with the lowest oxygen and water activities, highest temperatures, and highest incompatible element contents in the batholith. The composition of this granitic magma may be altered by assimilation of crustal wall rocks. Assimilation of metasedimentary rocks could produce the peraluminous Lincoln granite, for example. Coeval mafic magmas that commingled with the granitic magmas could yield the hybrid porphyritic granites.
The Sherman batholith contains both potassic and sodic granitoids, as does the 1.1 Ga Pikes Peak batholith of Colorado (Barker et al., 1975), the classic rapakivi terrane of southern Finland and the Ragunda massif of Sweden (Rämö & Haapala, 1995). Alkalic basalt and its differentiates are potential sources for the sodic rocks (Barker et al., 1975). Like tholeiites, alkali basalts evolve residual melts that have low oxygen fugacity, but these magmas have higher Na/K ratios than potassic A-type granites (Frost & Lindsley, 1991).
The Sherman batholith is the most reduced of the 1.4 Ga A-type granitoids in the southwestern USA. Its magmas may have ascended via fundamental crustal structures related to the Cheyenne belt, which marks the suture between Archean and Proterozoic crust. Moreover, Archean crust traversed by Sherman magmas was probably more refractory than Proterozoic crust, so that A-type granitoids farther south were more contaminated with oxidized, felsic crust than were magmas of the Sherman batholith.
Phanerozoic analogs to Proterozoic anorogenic granites
Phanerozoic A-type granites and rhyolites occur in three different tectonic settings: (1) they are associated with mantle plumes, such as the fayalite rhyolites of the Yellowstone–Snake River Plain province; (2) they occur in rifted continental settings, such as granite complexes associated with the opening of the Atlantic Ocean in Africa, South America and New England; (3) they are found in areas of large-scale continental extension, such as the Basin and Range province.
(1) Mantle plumes: the Yellowstone–Snake River Plain province
The lavas of the Yellowstone–Snake River Plain province exhibit remarkable petrochemical similarities to reduced, rapakivi-type granites. The fayalite rhyolites of the main caldera show extreme high iron and K2O contents, low fO2 and high incompatible contents, just like the Sherman granite (Hildreth et al., 1991; Frost & Frost, 1997). The lavas of the Snake River plain include abundant tholeiitic basalts and small volumes of differentiated lavas—icelandites—the eruptive equivalents of ferrodiorites (Leeman et al., 1976). The tectonic setting of the Yellowstone–Snake River Plain is clearly extensional, related to a propagating mantle plume (Smith & Braile, 1993).
Hildreth et al., (1991) concluded that the Yellowstone lavas were produced by partial melting of slightly older Cenozoic basalt, which generated the large volumes of fayalite rhyolite that erupted from the main caldera. The isotopic composition of these rhyolites limited the role of Archean crust to <15 wt %. Outside the main caldera, the rhyolites are poorer in iron, and show greater degrees of assimilation of Archean crust. This model is very similar to the one we propose for the Sherman batholith and for other reduced, rapakivi-type granites (Frost & Frost, 1997).
(2) Continental rifting: the opening of the Atlantic Ocean
Anorogenic magmatism is associated with the break-up of Gondwanaland, during which bimodal magmatism was focused along older lineaments or other major zones of weakness. In Africa, such anorogenic magmatism took place throughout the Phanerozoic, peaking during the Mesozoic opening of the Atlantic Ocean (Kinnaird & Bowden, 1987; Bowden et al., 1990). The similarity of the Jurassic Nigerian–Niger province to Proterozoic anorogenic granites was noted by Kisvarsanyi, (1981) and Van Schmus et al., (1993). Both suites contain alkaline to subalkaline granitic and rhyolitic rocks, with or without fayalite and Fe-rich pyroxene. The 410 Ma Aïr complex of Niger consists of 30 ring complexes. In seven of these, granite is associated with anorthosite and ferrodioritic rocks (Demaiffe et al., 1991). Moreau et al., (1994) concluded that the emplacement of the Aïr ring complexes was controlled by pre-existing lineaments. Their tectonic model links a transtensional tectonic regime with anorogenic magmatism.
(3) Continental extension: Basin and Range province, western USA
In the Basin and Range province, broadly distributed extensional deformation began in earliest Oligocene time, and continues today (Eaton, 1982). Extension-related magmas include tholeiitic and alkali basalts, basaltic andesite, and high-silica rhyolite (Eaton, 1982). Many of the rhyolites are fayalite bearing and reduced (logfO2 near or below the FMQ buffer; Frost et al., 1988). These fayalite-bearing rhyolites typically contain K2O > Na2O and K2O > 5 wt %, and include, for example, the Kane Spring Wash rhyolite, Nevada (Novak & Mahood, 1986), the Twin Peaks rhyolite, Utah (Crecraft et al., 1981), lavas of the McDermitt Caldera, Nevada–Oregon (Conrad, 1984) and the Coso volcanic field (Bacon et al., 1981).
Tectonic environment for A-type granites
Three extant tectonic models for the generation of Mid-Proterozoic anorogenic granites are: (1) large-scale mantle upwelling beneath a Mid-Proterozoic supercontinent (Hoffman, 1989); (2) extension and fragmentation of a Mid-Proterozoic supercontinent (Windley, 1993); (3) synorogenic gravitational collapse of the hinterland of a contractional orogeny (Nyman et al., 1994).
Hoffman, (1989) concluded that bimodal intraplate magmatism could result from mantle upwelling, and that the resultant doming would explain the lack of syn-plutonic alluvial and lacustrine sediments. He suggested that a Middle Proterozoic supercontinent effectively insulated the underlying mantle, leading to a very large area of mantle upwelling and partial melting that he termed a 'superswell’. Although a mantle plume of such large scale may be a unique Proterozoic phenomenon, we maintain that a smaller-scale version of this upwelling is represented by the Yellowstone–Snake River Plain province, which is sourced by a mantle hotspot. The rhyolites and icelandites of this province are petrochemically equivalent to A-type granites. Mantle upwelling events such as the one Hoffman, (1989) described should also produce magmas with A-type characteristics.
Windley, (1993) postulated that Mid-Proterozoic anorogenic granites were generated during extensional collapse of thickened crust after the assembly of Laurentia. In this model, Laurentia split to form a southern continent, which was reattached to Laurentia by the Grenvillian orogeny. Such intracontinental rifting may lead to magmatism across a broad area (e.g. the Basin and Range) or in narrow belts (such as in Africa). A-type granite compositions are found in both environments. However, Hoffman, (1989) noted that stretched continental lithosphere subsides upon cooling and that thick sedimentary deposits, such as those in the Basin and Range, should be found in regions of extensional collapse. Such contemporaneous sediments are not found in the southwestern USA, but their absence may reflect current levels of exposure.
Several Mid-Proterozoic granite plutons in the southwestern USA record syn-intrusive or post-intrusive deformation (Nyman et al., 1994). Although some of this deformation may reflect local strain related to emplacement of the plutons, part has been interpreted as reflecting regional stress fields. Nyman et al., (1994) proposed that deformation associated with 1.4 Ga plutons resulted from intraplate strains associated with a distant contractional orogeny along the southern margin of Proterozoic North America. Their model is somewhat analogous to the tectonics of the Tibetan plateau, in which extension and crustal thinning are taking place at the same time as convergence (Inger, 1994). The Tibetan plateau is roughly similar in size to the Mid-Proterozoic anorogenic granite province of the southwestern USA. However, the magmas associated with extension in Tibet are a calc-alkaline continental margin series (Coulon et al., 1986; Arnaud et al., 1992).
Nyman et al., (1994) observed that most 1.4 Ga granitoids in the southwestern USA were emplaced along or near pre-existing shear zones. The Sherman batholith lies immediately south of the Cheyenne belt, which marks the suture between the Archean Wyoming province and Proterozoic arc terranes to the south. In a number of cases, Mid-Proterozoic plutonism is synkinematic, with reverse motion on these reactivated shear zones (e.g. the Beer Bottle Pass pluton, Duebendorfer & Christensen, 1995; the Lawler Peak and Signal batholiths, Nyman & Karlstrom, 1994; the Mt Evans batholith, Graubard, 1991). This observation led Nyman et al., (1994) to infer a regional transpressive tectonic regime. However, normal motions are also observed (Mummy Range, Moeglin & Plymate, 1992; Mt Ethel, T. Foster & K. Chamberlain, personal communication, 1998; Sandia, Kirby et al., 1995). Thus, it is not clear whether these shear zones reflect the overall stress field during pluton emplacement or represent a late structural response following emplacement.
An important feature associated with 1.4 Ga plutonism in the southwestern USA is a broad thermal anomaly. K–Ar and 40Ar/39Ar muscovite and hornblende ages of 1.4–1.3 Ga have been obtained from Early Proterozoic rocks in northern New Mexico (Karlstrom et al., 1997), in Arizona (Van Schmus et al., 1993; Wendlandt et al., 1996), and Colorado (Selverstone et al., 1995). In southern Wyoming, K–Ar and Rb–Sr mineral ages of Archean rocks were reset at 1.4–1.5 Ga (Peterman & Hildreth, 1978). These ages reflect a regional thermal event rather than local heating related to emplacement of 1.4 Ga granitoids, because the reset area is broader than that affected by plutonism. Indeed, the 1.4–1.5 Ga mineral ages in southern Wyoming extend more than 50 km north and 200 km northwest of the nearest 1.4 Ga intrusions, delineating an area in which temperatures reached at least 325°C (the blocking temperature for Ar in biotite). The regional scale of this reheating and isotopic resetting, the widespread and voluminous A-type magmatism that occurs throughout the southwestern and central USA south of the Wyoming craton, and the presence of an extensive 1.4 Ga mafic dike swarm in northern Colorado and southern Wyoming (Braddock & Peterman, 1989; Chamberlain & Frost, 1995) together suggest the heat was supplied by the mantle.
The broad thermal perturbation at 1.4 Ga in the western USA provides strong supporting evidence for a mantle origin of anorogenic magmatism. Reduced, rapakivi-type granites, such as those described here from the Sherman batholith, and their extrusive equivalents, such as the fayalite rhyolites and icelandites of the Yellowstone–Snake River Plain province, require a source of tholeiitic-series rocks emplaced at or near the base of the continental crust. Large-scale mantle upwelling can provide the heat for partial melting of tholeiitic lower crust. Ascent along pre-existing structures will bring these magmas to the level of emplacement with minimal crustal contamination. In other areas, reduced granitic magmas may interact with felsic continental crust, producing more oxidized or peraluminous members of the A-type granite suite.
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Acknowledgements |
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We thank J. L. Anderson and O. T. Rämö for helpful reviews, and editor S. Sorensen for detailed constructive suggestions. This research was supported by NSF Grant EAR9706237 to C. D. Frost and B. R. Frost.
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FOOTNOTES |
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Present address:Department of Geology,Grand Valley State University, Allendale,MI 49401, USA. 
* Corresponding author: Telephone: +1-307-766-6254. Fax: +1-307-766-6679. email: frost{at}uwyo.edu
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TOPABSTRACTIntroductionLithologic UnitsU-Pb Zircon GeochronologyGeochemical CharacteristicsMineral ChemistryIntensive FactorsPb, Nd AND Sr...Discussion and ConclusionsReferences |
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