Metamorphic petrology of the northeast Front Range, Colorado: The Pingree Park area
WILLIAM D. NESSE Department of Earth Sciences, University of Northern Colorado, Greeley, Colorado 80639
ABSTRACT vides an opportunity to examine the metamorphism in a relatively small area where the geology has been well mapped (W. D. Nesse and W. A. The Pingree Park area in the northeast Front Range, Colorado, is Braddock, unpub. data). underlain by Precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1.75 b.y. ago, forming a distinct zonal REGIONAL GEOLOGY pattern of minerals indicating an increase in grade of metamorphism to the west and north. Three isograds have been identified. The The northern Front Range is an anticlinal range bounded in pla ces by andalusite-sillimanite isograd is defined by the first occurrence of sil- high-angle faults and cored with Precambrian igneous and metamorphic limanite in the prograde direction and is interpreted to mark the reac- rocks (Fig. 1). The metamorphic rocks are predominantly schist and gneiss tion of andalusite to sillimanite. A band as much as ~1 km across units the mineralogy of which forms a distinct zonal pattern indicating a contains both andalusite and sillimanite and represents a divariant general increase in g rade of metamorphism to the west and north (Fig. 2). band or reaction overstepping of 20 °C or less. The K-feldspar- The chronology of Precambrian events has been discussed by Pet:rman sillimanite isograd separates microcline + sillimanite from prograde and others (1968), LaFountain (1975), Abbott (1972), Braddock (1970), muscovite + quaitz assemblages and is interpreted to represent the and Cole (1977). A thick sequence of sediment was deformed into large reaction (K, Na) [Muscovite + quartz = (K, Na) microcline + sillimanite isoclinal folds (F-l) with production of a slaty cleavage (S-l) and then + H20. Despite the presence of significant amounts of sodium in both weakly metamorphosed (M-l), all prior to 1.75 b.y. ago. At about 1.75 muscovite and microcline, the reaction shows no perceptible divariant b.y. ago, these rocks were intruded by Boulder Creek granodiorile and band. There has been extensive reaction of microcline + sillimanite to related rocks, subjected to 2 episodes of folding (F-2 and F-3), and retrograde muscovite + quartz. The migmatite-in isograd is defined by strongly metamorphosed (M-2), forming the zonal pattern of minerals the first occurrence of migmatitic rocks in the prograde direction and now preserved in the rocks. A retrograde period of metamorphism 'M-3) is interpreted to represent the inception of partial melting. The iso- followed M-2. At about 1.4 b.y. ago, Silver Plume and Sherman granites grads form an eastward-plunging set of synformal surface the south and related rocks were emplaced with associated F-4 deformation. The limb of which has a gentle dip and the north limb of which apparently range is cut by a number of shear zones that were developed in the has a much steeper dip. Precambrian and along which there has been more recent movement. Based on the chemical composition of the minerals and compari- Evidence for M-l has been obliterated by M-2 in all but the least meta- son with experimentally calibrated equilibria, the conditions of morphosed rocks along the east edge of the range. It is possible that M-l metamorphism at the K-feldspar-sillimanite isograd are estimated to and M-2 are part of a single prograde metamorphic episode that over- 12 lapped F-2 and F-3. M-3 probably represents the waning stages of the M-2 have been PH2o - Ptotai = 3 to 4 kbar, T = 650 ± 30 °C, f02 = 10 to 15 metamorphism. 10 bars, and fHF =1/4 to 1 bar. The geothermal gradient at the peak of metamorphism was ~60 °C/km. GEOLOGY OF THE PINGREE PARK AREA INTRODUCTION Most of the Pingree Park area (Fig. 3) is underlain by a variety of Metamorphosed and deformed Precambrian sediment is exposed in pelitic and semipelitic schist and gneiss units that have been separated into large areas of the northeastern part of the Front Range of Colorado (Fig. two map units: knotted mica schist and quartzofeldspathic mica schist. 1). The metamorphic rocks are relatively homogeneous over large areas, Both are composed of quartz, plagioclase, biotite, opaques, and, depending and they show a complete range in grade of metamorphism from biotite on grade of metamorphism, andalusite, sillimanite, staurolite, muscovite, zone to migmatitic rocks. The pattern of the mineral zones has not been microcline, garnet, and cordierite. Knotted mica schist is relatively alumi- disrupted to any significant degree by intrusions, faulting, or other defor- nous and contains abundant sillimanite that commonly occurs as clots or mation, and the Precambrian metamorphic record appears not to have knots of fibrolite. In less intensely metamorphosed terrane to the southeast, been complicated by more recent periods of metamorphism. Metamor- andalusite, staurolite, and cordierite are common as porphyroblasts that phism in the Pingree Park area is typical of the medium- to high-grade give outcrops a distinct knotted appearance. Rocks mapped as quartzo- metamorphism found throughout the northeastern Front Range and pro- feldspathic mica schist include biotite gneiss and schist units that have a
Additional tabular material for this article (Appendix) may be secured free of charge by requesting Supplementary Data 84-24 from the Documents Secretary.
Geological Society of America Bulletin, v. 95, p. 1158-1167, 5 figs., 3 tables, October 1984.
1158
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40° 45'
Figure 1. Generalized geo- logic map of the northeast Front Range. The Pingree Park area shown in Figure 3 is outlined with hachures. Mineral zone boundaries are explained in Fig- ure 2. Adapted from Braddock and Cole (1979).
contact
fault
' mineral zone K boundary
40° 15* 105° 00'
Precambrian X pelitic & semi- pelitic metamorphic rocks
Ftoleozoic & younger sedimentary rocks Xi ' 1.75 b.y. intrusive igneous rocks 1.4 b.y. intrusive /vY[r Xg J! Precambrian X granitic gneiss — I v igneous rocks
relatively high quartz + feldspar fraction. Although clots of sillimanite or developed near the Skin Gulch, Buckhorn, and other shear zones. Micas other aluminous minerals may be present, they are not as abundant as in are commonly slightly bent and locally may be kinked. Some feldspar knotted mica schist. The two lithologies grade into each other and are grains are bent. The cataclastic effects are more strongly developed near commonly interlayered. Locally, there are amphibolite and calc-silicate shear zones and are relatively minor away from them. gneiss layers. The structures found in the Pingree Park area are similar to the MINERALOGY structures to the east and southeast. The F-l, F-2, and F-3 deformation episodes are all represented, but evidence for F-4 was not found. Most More than 280 samples of quartzofeldspathic mica schist and knotted samples of knotted mica schist and quartzofeldspathic mica schist exhibit mica schist were examined petrographically. Eight samples with the least the effects of minor cataclasis. Quartz commonly shows undulatory extinc- retrograde and cataclastic effects arrayed across the K-feldspar + silliman- tion, has sutured contacts, and may be polygonized. Mortar texture is ite isograd (Fig. 3) were selected for microprobe analyses of the major
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minerals (Table 1). Analyses of the silicates were performed on the ARL with errors in the neighborhood of ± 50%. The analyzed fluorine contents, microprobe at the U.S. Geological Survey microprobe lab in Denver, with however, are typical of micas and are consistent with the way in which a combination of wavelength and energy dispersion systems using natural micas fractionate fluorine (Munoz and Ludington, 1977). minerals of similar composition as standards. Compositions were com- Quartz is present in all samples as subequant-polygonal to irregular- puted using the procedures described by Bence and Albee (1968). The ameboid grains. It also occurs in some samples as vermicular intergrowths opaque minerals were analyzed on the MAC-400 microprobe at the Uni- in porphyroblasts of muscovite. versity of Colorado Department of Geological Sciences, using natural Plagioclase is present in all samples as subequant polygonal grains minerals as standards and the ZAF computation scheme. Between three and in some sillimanite-bearing rocks as irregular aggregates intergrown and six grains of each mineral were analyzed per sample, and between one with blades of biotite. It is usually untwinned in andalusite-bearing tcrrane and seven points were analyzed on each grain, depending on size. No but in higher-grade rocks is typically twinned. Most composition.! esti- zoning was detected in any mineral, and the standard deviation for ele- mated with the Michael-Levy method fall in the range of An20 to An30, ments present in amounts greater than 1 wt % was consistently better than with no recognized areal variation. ± 5%. The standard deviation for elements present in amounts less than Muscovite occurs as prograde lepidoblastic muscovite and as retro- 1 wt % was consistently better than ± 10%, with the exception of fluorine. grade porphyroblastic and coarse, bladed muscovite. Lepidoblastic mus- The fluorine analyses in the micas involve considerably greater error due covite is found in all rocks below the K-feldspar-in line and is typically to difficulty in detection and should be considered only semiquantitative, interleaved with biotite of similar habit and aligned parallel to bedding or
TABLE 1. SUMMARY OF MINERAL COMPOSITION
Sample Plagioclase Microcline Lepidoblastic Bioute muscovite x X x,„ X X X X X X < .b o, ab o, xan XP8 ms ms(F) 23-3-9B .80 .01 .19 .08 .92 .06 .84 .43 .57 .18 23-3-18 .82 .01 .17 .11 .89 .03 .85 .39 .61 .13 23-3-40 .75 .01 .24 .09 .91 .01 .85 .39 .61 .18 23-3-45 .81 .01 .18 .08 .92 .04 .21 .40 .60 .04 23-3-46 .78 .02 .20 .09 .90 .01 .36 .14 .23 23-3-50 .78 .02 .20 .10 .89 .01 .32 .68 .18 23-3-86B .78 .01 .21 .07 .93 .04 .85 .36 .64 .13 EP-4N .79 .01 .20 .07 .92 .01 .40 .60 .19 Note: complete chemical analyses have been placed in the GSA Data Repository. These may be secured by requesting Supplementary Data 84-00 from the Documents Secretary. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 METAMORPHIC PETROLOGY, FRONT RANGE, COLORADO 1161 40 37' 30" Quaternary Alluvium & till Q : Precambrian Y Ys Silver Plume Granite Yh Granite of Hagues Peak Precambrian X [xk'.j Knotted mica schist Quartzofeldspathic GD mica schist Xa | Amphibolite mineral zone boundary 10 12 miles i i i i i kilometers i_ contact ••' fault • 23-3 sample location 105 37' 30' EP-4N 105 Figure 3. Generalized geologic map of the Pingree Park area. Mineral zone boundaries are the same as in Figures 1 and 2. SKSZ, Skin Gulch shear zone; BSZ, Buckhorn shear zone. to the S-l slaty cleavage (Braddock, 1970). It may be shingled around the lepidoblastic muscovite. Although not found in the Pingree Park area, hinges of F-2 and F-3 crenulations and microfolds and is locally aligned helicitic microcline overprinting F-2 and F-3 folds is common in adjacent parallel to a steep northwest-striking cleavage related to F-3, indicating areas (Punongbayan, 1972; Bucknam, 1969), indicating post-F-3 growth. that it probably was present prior to the F-2 and F-3 deformations but Biotite is present in all samples of quartzofeldspathic mica schist and recrystallized during or after those episodes of folding. knotted mica schist. Below the K-feldspar-in line, it is usually strongly Randomly oriented porphyroblastic and coarse-grained muscovite is lepidoblastic, parallel to associated lepidoblastic muscovite. Above the found infrequently in lepidoblastic muscovite-bearing rocks but is found K-feldspar-in line, it is generally coarse grained and somewhat less everywhere above the K-feldspar-in line. Its abundance generally corre- lepidoblastic. lates with a paucity of K-feldspar but does not vary as a function of Within the Pingree Park area, staurolite occurs only as irregular distance from the line. Characteristically, porphyroblastic muscovite con- grains or groups of optically continuous grains within and partially re- tains fibrous to fine prismatic sillimanite. The fibrolite generally traces out placed by andalusite porphyroblasts. In the area to the east and southeast, the swirled pattern characteristic of fibrolite clots, indicating that it has the amount of staurolite decreases in the prograde direction due to re- been partially replaced by the muscovite. This muscovite commonly also placement by andalusite and locally cordierite (Punongbayan, 1972). contains vermicular intergrowths of quartz, indicating the simultaneous Sillimanite occurs as fibrolite and as somewhat coarser prisms, with a growth of both minerals (Spry, 1969, p. 102). These observations are complete gradation between the two in places. Fibrolite is found every- interpreted to indicate that the porphyroblastic muscovite is a retrograde where above the sillimanite-in boundary, whereas fine prismatic silliman- (M-3) phase. ite is generally restricted to K-feldspar-bearing terrane and is relatively Microcline occurs in habits ranging from small interstitial grains to common in migmatitic rocks. Sillimanite is more abundant above the large, irregular poikiloblasts that contain inclusions of biotite, quartz, plagi- K-feldspar-in line than in lower-grade rocks. oclase, and occasionally sillimanite. It sometimes exhibits minor flame or The fibrolite occurs as clots, as fine arrays of needles, and as small ribbon perthitic exsolution, and myrmekite may be present at microcline- fibers included in biotite. The clots are usually composed of swirled ellip- plagioclase boundaries. Microcline is not present in samples that contain soidal aggregates a few millimetres to several centimetres across, sur- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 1162 W. D. NESSE rounded by a halo of quartz. The clots usually contain ragged remnants of an unidentified high-Ti phase. Although there probably have been some biotite grains, indicating that the fibrolite has locally grown at the expense retrograde oxidation and exsolution of ilmenite from magnetite, the origi- of the biotite, probably as part of a mosaic equilibrium (Chinner, 1961). nal magnetite composition probably was very low in Ti, because the The swirled nature of the clots is probably controlled to some degree by composition of magnetite with or without ilmenite lamellae is the same in the orientation and spatial distribution of the biotite that has been replaced a single sample, and the widely scattered ilmenite grains show no particu- (Punongbayan, 1972). Fibrolite clots may be located in the limbs of F-2 lar relation to the location of magnetite. and F-3 folds and crenulations and are elongate parallel to the fold axes. The textural evidence indicates that staurolite, cordierite, garnet, an- The shape of the clots probably was controlled by the fabric of the rock dalusite, sillimanite, lepidoblastic muscovite, and microcline, which pro- developed during folding, and it is likely that growth was in the waning duce the distinctive zonal pattern (Figs. 2 and 3), all crystallized or stage of, or after, F-3 folding. recrystallized as part of a single prograde metamorphic event (M-2), the Fine prismatic sillimanite appears to have recrystallized from fibrolite peak of which probably occurred during the waning stages of F-3. Retro- and is relatively common in the microcline-bearing and migmatitic ter- grade metamorphism (M-3) produced porphyroblastic muscovite, minor ranes. It is found studded through or adjacent to fibrolite clots or may be alteration of mafic and other minerals to chlorite, and local retrograde distributed through biotite blades. In some cases, there is a complete conversion of sillimanite to andalusite. gradation in size between prisms and adjacent fibrolite. The prisms typi- cally show alignment approximately parallel to the steeply plunging axes ISOGRADS of associated F-2 and F-3 folds, suggesting that some nonhydrostatic stress associated with folding may still have been present when the prisms grew. The mineral 2:one boundaries shown in Figures 1, 2, and 3 indicate Andalusite occurs as relatively solid porphyroblasts that commonly the areas within which the various minerals can be found. In a strict sense, include and partially replace staurolite, and as moderately to strongly they are distinct from isograds (reaction-isograd of Winkler, 1979), be- sieved poikiloblasts with quartz, scattered fine opaque grains, and biotite cause they are not defined on the basis of recognizing the product and inclusions. Both may helicitically overprint F-2 and F-3 crenulations, indi- reactant assemblages of mineral reactions on opposite sides of a line or cating post-F-3 growth. Andalusite usually occupies the continuation of zone. It is clear, however, that three isograds (andalusite-sillimanite, K- relatively micaceous layers. feldspar-sillimanite, and migmatite-in) can be defined. Andalusite and sillimanite occur together over a band that in map view is as much as — 1 km across (Fig. 3). Within this zone, andalusite may Andalusite-Sillimsinite Isograd be separate from sillimanite or may contain or be juxtaposed to sprays or clots of fibrolite. Andalusite may also be included as a number of optically Within less than 1 km on the prograde side of the sillimanite-in line, continuous grains within aggregates of plagioclase, biotite, and a little andalusite and stau rolite are lost from the system. The coincidence of loss muscovite. Withi n the Pingree Park area, there was no distinctive textural of andalusite and introduction of sillimanite strongly suggests that silliman- evidence to indicate the relative age of andalusite and sillimanite, but to the ite grew at the expense of andalusite by the net reaction: southeast, Punongbayan (1972) described andalusite that is both older and younger than sillimanite, indicating both prograde (M-2) and retrograde andalusite = sillimanite. (I) (M-3) growth of andalusite. Cordierite was recognized in only a few samples, where it occurs as The andalusite-sillimanite isograd is placed coincident v/ith the poikiloblasts, subequant grains, or coarse aggregates of grains with quartz sillimanite-in line. The loss of staurolite is probably coincidental and due and biotite inclusions. Included biotite may be oriented in a steep, to the loss of andalusite, with which it was armored. The growth of northwest-striking cleavage related to F-3, indicating post-F-3 growth. sillimanite probably was part of a mosaic reaction (Carmichael, 1969; Andalusite occurs with cordierite in several samples but without textural Chinner, 1961) involving growth of sillimanite at the expense of biotite relations that would allow interpretation of their relative ages. Punong- and some plagioclase and muscovite at one location and replacement of bayan (1972) and LaFountain (1973) reported that cordierite encloses and andalusite by biotite and some plagioclase and muscovite at others. The apparently replaces andalusite in several locations to the east of the Pingree persistence of andalusite beyond the first appearance of sillimanite may be Park area. due to divariance of the reaction (Althaus, 1969; Okrusch anc Evans, Garnet occurs in very few samples, predominantly in andalusite- 1970), or to metastable persistence of andalusite to temperatures higher bearing terrane. It forms anhedral to subhedral grains, some of which have than required for equilibrium with sillimanite, due to the small free-energy been partially to completely replaced by aggregates of randomly oriented difference between the two phases even at substantial distances from equi- biotite, muscovite, plagioclase, and quartz. librium (Holdaway, 1971). Based on the temperature estimates provided Chlorite wi:hin the Pingree Park area occurs as a retrograde (M-3) below, the temperature difference across the andalusite + sillimanite zone alteration product and is found in all mineral zones. Biotite is locally is probably no more than ~20 °C. altered along grain margins to green chlorite with high first-order interfer- ence colors. In a few samples, small blades, aggregates, or rosettes of pale K-Feldspar-Sillimanite Isograd green to nearly colorless chlorite with anomalous blue first-order interfer- ence colors appear to replace andalusite, cordierite, or biotite. To the The introduction of microcline and abrupt increase in the abundance southeast of the Pingree Park area, prograde chlorite is found in rocks of sillimanite at the K-feldspar-in line is coincident with the loss of pro- below the sillimanite-in line (Fig. 2). grade muscovite and defines the K-feldspar-sillimanite isograd, based on The opaque minerals are magnetite and ilmenite. Magnetite is essen- the reaction: tially Ti-free and occurs as subequant subhedral to anhedral grains. Some (K, Na) muscovite + quartz = (II) contain lamellae of ilmenite, but most do not. Minor oxidation to hematite (K, Na) microcline + sillimanite + H2O. along cracks and grain margins may be present. Ilmenite occurs as lamel- lae in magnetite and as widely scattered isolated grains. In contrast to the At two locations, it was possible to bracket the location of the isograd to relatively unaltered magnetite, ilmenite has been unmixed to hematite and within 50 m. The horizontal temperature gradient across the southern part Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 METAMORPHIC PETROLOGY, FRONT RANGE, COLORADO 1163 of the area appears to have been between 15 and 30 °C/km (see below), porphyroblastic muscovite has replaced sillimanite and grew synchronous and the isograds are regular and continuous over long distances, indicating with intergrown vermicular quartz. that the abrupt nature of the K-feldspar-sillimanite isograd is not the result These observations indicate that porphyroblastic muscovite was pro- of an unusually steep temperature gradient. The reaction shows no signifi- duced by the retrograde breakdown of microcline + sillimanite to musco- cant divariance, despite the fact that both muscovite and microcline con- vite + quartz. The water needed to allow the retrograde reaction to proceed tain substantial amounts of sodium. is probably the same water that was released during the prograde reaction Reaction II can be univarient in a Na-bearing system only if the K and "stored" in the melt fraction of migmatitic rocks immediately upgrade. and Na activities are externally buffered, or if the K:Na ratios of muscovite The water would be released from the melt during crystallization in M-3, and microcline in equilibrium are the same. Given that there does not as the rocks cooled after the peak of M-2. appear to be any potential external buffering agent such as nearby syn- metamorphic intrusions, and the K:Na ratios in muscovite and microcline Migmatite-in Isograd on opposite sides of the isograd are essentially the same (Table 1), it is evident that the reaction was either univariant or divariant over a very The leucosomes in the migmatitic rocks are interpreted to be meta- small temperature range. Other field studies of the K-feldspar-sillimanite morphic segregations derived in response to partial melting, for the follow- isograd have also suggested that the presence of Na in the muscovite and ing reasons. K-feldspar does not lead to the development of significant divariance, 1. The migmatite-in isograd is concordant with and on the high- because the Na:K ratio of neither phase appears to vary systematically temperature side of the other isograds in the area. This indicates that across divariant bands produced by variable fluid composition (Tyler and migmatization was related to the metamorphism that produced the other Ashworth, 1982; Evans and Guidotti, 1966; Ashworth, 1975). mineral zones and was produced in rocks that were at the highest tempera- Chatterjee and Froese (1975) and Thompson (1974) indicated that ture in the area and under conditions appropriate for melting (Thompson there should be a —20 °C divariant band over which reaction II occurs. and Algor, 1977; Winkler, 1979). Their conclusions are based on the mixing properties of alkali feldspar and 2. The migmatitic rocks show no spatial relation to bodies of igneous muscovite-paragonite solid solutions. As these authors, Eugster and others rock that could have served either as a source of granitic material to inject (1972), Blencoe (1977), and Hovis and Waldbaum (1977) indicated, these into the metamorphic rocks or as a source of metasomatizing fluids. data contain significant uncertainty. As such, the phase relations derived 3. There is commonly a concentration of biotite in the melanosome from them should be used with caution. There is also the additional adjacent to the leucosome. This was interpreted by Mehnert (1968), complication of applying the theoretical phase relations to natural musco- Brown (1973), Olsen (1977), Dougan (1979), and others to represent the vite and alkali feldspar, which have compositions that are significantly residue left after segregating some portion of the quartz-feldspar fraction different from the ideal Na-K end members. As the field evidence indicates into the leucosome. that the divariant band for reaction II in the Pingree Park area is very small 4. Cole (1977) concluded that the reintroduction of cordierite and or absent, the theoretically derived relations do not appear to apply di- garnet into migmatitic pelitic and semipelitic rocks substantially above the rectly to the conditions found in the northeast Front Range. migmatite-in isograd was caused by the initiation of incongruent melting Tyler and Ashworth (1982) determined that the average K:Na ratios of biotite. of muscovite and K-feldspar on opposite sides of completed reaction II were 0.92 and 0.85, respectively, and Evans and Guidotti (1966) inferred values of 0.94 and 0.86, respectively. This requires that some of the albite component of plagioclase be added on the left side of reaction II to balance it. In the Pingree Park area, the average muscovite and microcline K:Na ratios are 0.91 and 0.90, respectively, which is consistent, but the differ- ence is not statistically significant. Although the factors controlling this aspect of the reaction are not understood, it does not appear necessary to include albite to balance reaction II in the Pingree Park area. Porphyroblastic muscovite is interpreted to have been produced dur- ing retrograde metamorphism (M-3) (Punongbayan, 1972; Braddock, 1969; Abbott, 1970). Braddock (1969) and Abbott (1970) suggested that it may have been produced by potassium-bearing fluids introduced into the rocks from Silver Plume and Sherman granites. A metasomatic origin is unlikely, however. The Silver Plume and Sherman granites appear to have been relatively dry magmas (Cole, 1977), the great majority of porphyroblastic muscovite occurs in rocks above the K-feldspar- sillimanite isograd, and its distribution is not related to bodies of Silver Plume or Sherman age intrusives. Moreover, porphyroblastic muscovite has been deformed by F-4, which was associated with emplacement of Silver Plume and related intrusives (Cole, 1977). Porphyroblastic muscovite also is not recrystallized lepidoblastic muscovite involved in the prograde reaction. An abundance of porphyro- blastic muscovite generally correlates with a paucity of microcline, yet T(°C) there is no systematic distribution of porphyroblastic muscovite within the K-feldspar-bearing terrane. If it were part of prograde reaction II, its Figure 4. Experimentally calibrated reactions relevant to the Pin- abundance should decrease systematically above the K-feldspar-in line as gree Park area. 1, Holdaway (1971); 2, Chatteijee and Johannes part of a divariant band. In addition, the textural relations indicate that (1974); 3, von Platten and Holler (1966). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 1164 W. D. NESSE 5. Shaver (1980) recognized a migmatite-in boundary in amphibo- TABLE 2. MUSCOVITE-PLAGIOCLASE TEMPERATURES lite in higher-grade rocks to the north (Figs. 1 and 2) that he interpreted to be produced by anatexis of amphibolite at temperatures significantly Sample T°C ±50° above the solidus for pelitic and semipelitic rocks. 3.0 kbar 3.5 kbar 4.0 (bar 6. Olsen (1982) found that similar migmatites located 100 km south 23-3-9B 665 689 7 2 were formed in response to anatexis. 23-3-18 639 662 6114 23-2-40 645 668 6'U The outcrop traces of the isograds and mineral zone boundaries 23-3-45 664 688 7 1 23-3-86B 672 696 7:10 suggest that they form a set of nested, eastward-plunging synformal sur- faces. The wide spacing in the southern part of the area suggests a relatively gentle northerly dip, whereas the close spacing in the northern part of the area suggests a much steeper dip. Unfortunately, it was not 15 °C divariant band in the Pingree Park area would be -250 m thick if a possible to actually determine the dip at any location. geothermal gradient of 60 °C/km is assumed on the basis of the tempera- ture and pressure estimates provided here. Such a divariant band, the CONDITIONS OF METAMORPHISM outcrop width of which would be substantially greater due to the law dip of the isograds in the southern part of the area, would have beer easily Pressure recognized. It is probable that a pressure gradient for water extended at least from Pressure is estimated to have been between 3 and 4 kbar (Fig. 4). The the K-feldspar-sillimanite isograd to the migmatite-in isograd, because the upper limit of pressure is -4 kbar, because no kyanite is present and melt in migmatitic rocks would provide a sink for the water released by because the breakdown of muscovite and quartz to K-feldspar and silli- reaction II. The presence of a gradient, however, could not serve to buffer manite occurred at a lower temperature than required for the onset of the fluid composition, and so a divariant band still should have been anatexis. A minimum limit of pressure is the intersection of the muscovite produced if the fluid contained substantial amounts of C02. + quartz breakdown curve with Holdaway's (1971) andalusite = silliman- ite curve at ~2 kbar. The K-feldspar-sillimanite isograd is closer to the Temperature migmatite-in line than to the andalusite-sillimanite isograd; hence, it is likely that the pressure was closer to the high-pressure end of the possible The composition of coexisting muscovite and plagioclase iri equi- range. librium with an aluminosilicate can be used to estimate temperature (Cheney and Guidotti, 1979) from the equation: Fluid Composition AG° + AV°S (P - 1) _ RT It is indicated by two lines of evidence that the metamorphic fluid was composed predominantly of water. In Xmipg - In XP' - In f + In [Xm' ]2 + 1. There are no carbon-bearing phases such as graphite in any of the ab H2Q AI(M2) pelitic or semipelitic rocks that might indicate the presence of significant mi 2 (X ms) r Wm + 2Wm Wm 1 amounts of COz or other carbon-bearing species in the fluid. Calc-silicate RT L 'pg 'pg ( 'ms " W™pg) J • gneiss units that presumably were originally carbonate units are volumetri- cally insignificant. Even if the fluid originally in the rocks at lower temper- Temperature values in Table 2 were calculated using margules parameters atures contained substantial amounts of CO2, the dehydration reactions (W) from Chatterjee and Froese (1975), f^o data from Burnham and that occurred to remove prograde chlorite from the rocks would have others (1969), and molar volume data for solids (AV°S) and standard free produced a very water-rich fluid, because there are no minerals present energy (AG°) from Cheney and Guidotti (1979). Limits of error due to that by devolatilizing could add CO2 to the fluid (Rice and Ferry, 1982; analytic uncertainty are ±50 °C. These temperatures indicate a horizontal Greenwood, 1975; Walther and Orville, 1980). temperature gradien t of —20 °C/km across the southern part of the area. 2. The K-feklspar-sillimanite isograd is abrupt, indicating a lack of In Figure 5B, the same temperature data are plotted against distance significant divariance. Either the fluid was essentially all H2O or its com- normal to the K-feldspar-sillimanite isograd surface, which is assumed to position was externally buffered. The only potential agent to provide have a 20° northerly dip. The collinearity of the data probably involves external buffering appears to be Boulder Creek and related intrusions some element of luck, and more data undoubtedly would show more emplaced at about the same time as metamorphism. The few small intru- scatter, but it does imply that the thermal gradient normal to the iiiograd sions of these rocks found in the area are volumetrically insignificant surfaces was -65 °C/km, which, given the low dip of the isograd surfaces, (W. D. Nesse and W. A. Braddock, unpub. data) and could not have is an approximation of the geothermal gradient. significantly influenced fluid composition, except immediately adjacent to Temperatures at the migmatite-in isograd probably were no lower their contacts. If the fluid was not externally buffered and was not essen- than 670 or 680 °C, on the basis of the solidus curve shown in Figure 4. tially all water, th;n the composition of the fluid would change as the The temperature at the andalusite-sillimanite isograd probably was -575 reaction progressed and water was added from the dehydration of musco- °C, on the basis of Holdaway's (1971) diagram. This implies a horizontal vite. The result wo aid be a divariant band the width of which depends on temperature gradieni of between 15 and 30 °C/km across the southern the initial volume of fluid, the initial fluid composition, temperature gra- part of the area, which is in agreement with the muscovite-plagiDclase dient, and the amount of muscovite in the rock (Rice and Ferry, 1982; data. Greenwood, 1975). Tyler and Ashworth (1982) described a 15 °C divar- Provided that the fluid composition is known, the experimental data iant band for the K-feldspar-sillimanite isograd produced by an initial for reaction II can provide information about the temperature. The K:Na = fluid composition of XH2Q 0.69 in graphite-bearing pelites. A similar ratios in muscovite and microcline are the same on opposite sides of what Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 METAMORPHIC PETROLOGY, FRONT RANGE, COLORADO 1165 mi mi 700 - X ms[AG°ln] +(1-X ms) [AG°IV] = a ms qz T(°C) 680 (l-X^tRTlnC )]. d pg d qz sil af mi mi Substituting a sil = = 1, aH2o =fH2o> a kfs = a ms, a Pg = aP'ab, and including the pressure term to account for standard states, this becomes: 660 m X™ms [AG°[H] + (1 - X 'ms) [AG°iv] = m -RTlnfH2o - X 'ms [AV°s(III) (P - 1)] - map distance from isograd mi (km) (1-X ms) [AV°s(iv) (P - 1) ] • Terms for AG°(III) and AG°(IV) from Chatterjee (1972), Chatterjee and Johannes (1974), and Chatterjee and Froese (1975) are corrected to 700 - microcline, low albite, and sillimanite from sanidine, high albite, and andalusite based on data in Helgeson and others (1978), which gives: logf, : 8.037 - 1?32 + 0.0255 (P-l) + H2O (V) T(°C) 680 00008 P 1 X%s[-0.589 + f + T( - >] This term ignores the solvi that exist at intermediate compositions and 660 applies only if the reaction is actually isobarically univariant. If it is not, then for a given value of fH20, it will give a temperature at about the center of the divariant band. No significant error will be added as a consequence, 200 400 600 800 because any divariant band that exists in the Pingree Park area is very = m distance normal to isograd small. For conditions of Ph2o Ptotab X 'ms = 0.91, and total pressures of (m) 3.0, 3.5, and 4.0 kbar, this term gives temperatures of 632, 651, and 666 °C, respectively, at the K-feldspar-sillimanite isograd. Limits of error Figure 5. Temperature gradients from muscovite-plagioclase based on the uncertainty of mineral analyses are ± 10 °C. These tempera- temperatures at 3.5 kbar. A. Map distance from K-feldspar-sillimanite tures are lower than the muscovite-plagioclase estimates, but the limits of isograd versus temperature. The horizontal temperature gradient is error overlap. It is probable that there are systematic errors leading to high ~20 °C/km. B. Distance normal to the K-feldspar-sillimanite isograd temperature estimates, because the muscovite-plagioclase data imply surface versus temperature. The isograd surface is assumed to dip 20° temperatures appropriate for anatexis in rocks substantially below the to the north. The temperature gradient normal to the isograd is migmatite-in isograd. For the calculations that follow, temperatures ob- -65 °C/km. tained from equation V will be used. The geothermal gradient implied by the temperature-pressure esti- mates at the K-feldspar-sillimanite isograd are ~ 60 ± 15 °C/km, depend- appears to be an essentially univariant reaction. The activity coefficients ing on what pressure and average density for overlying rocks are assumed. for mica and feldspar must be the same, because the chemical potential of This agrees quite well with the gradient implied by the muscovite- the constituent species must be identical in mica and feldspar at the iso- plagioclase temperatures (Fig. 5B). grad. Provided reaction II is univariant, it can be expressed as the sum of: Oxygen Fugacity m X 'ms [muscovite + quartz] = (III) The amount of Ti that can substitute for Fe in magnetite in equilib- m X 'ms [K-feldspar + sillimanite + H20] rium with ilmenite decreases with increasing oxygen fugacity at a given and temperature (Spencer and Lindsley, 1981). At temperatures of — 650 °C, 1 (1 - X" '^) [paragonite + quartz] = (IV) the highest fg2 for which it has been possible to calibrate the composition of coexisting magnetite and ilmenite is —10~15 bars, where magnetite m (1 - X 'ms) [albite + sillimanite + H20], contains - 5% ulvospinel. At more-oxidizing conditions, ilmenite tends to unmix to hematite and ilmenite, and magnetite should have progressively Equilibrium constants for III and IV can be added in appropriate propor- less Ti. The presence of very low-Ti magnetite with unmixed ilmenite 15 tions (Skippen, 1971) to give a term for the net reaction: therefore implies fo2 greater than ~ 10~ bars. The upper limit of oxygen Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 1166 W. D. NESSE fugacity is the magnetite-hematite boundary, which for the temperatures The biotite in the Pingree Park area is very aluminous; hence, the composi- found here is between 10"12 and 10"13 bars. These are relatively oxidizing tions are treated as intermediate between phlogopite and siderophyllite. conditions and are consistent with the observation that the rocks contain Values of fHF based on biotite analyses are shown in Table 3. Errors no graphite. associated with uncertainty of the chemical analyses (particularly F) rep- resent somewhat more than one-half of the reported value of ÌHF. Within Fluorine these limits, the values of ÌHF computed from biotite and muscovite agree quite well and indicate that fHF was probably in the neighborhood of 1/4 Details of procedures used in applying F-OH exchange data to to 1 bar. natural micas are described by Ludington and Munoz (1975) and Munoz and Ludington (1975, 1977) and will not be covered in detail here. For CONCLUSION muscovite, the exchange reaction is: The well-defined isograds found in the northeast Front Range ind the estimated conditions of metamorphism (3 to 4 kbar and —650 ± 30 °C) at muscovite(OH) + HF = muscovite(F) + HzO (VI) the K-feldspar-sillimanite isograd are similar to those found ir many 2100 Xm f pelitic metamorphic terranes. It is typically found in other areas, however, , r m. I W)- H2o log KV| = — - 0.11 = log — —-. 1 A ms(OH) 'HF that the metamorphic fluid contained significant amounts of CO2 or other carbon-bearing species, as indicated by the presence of graphite or by the This omits a pressure correction term, because the molar volume of divariance of isobarically univariant dehydration reactions (Ferry and fluormuscovite is not known, and assumes that the substitution of Na for K Burt, 1982). The northeast Front Range provides an example where the and of Fe and Mg for A1 does not greatly affect the equilibrium constant. metamorphic fluid was apparently almost entirely water and, perhaps Values of log (f^o^HF) an TABLE 3. FUGACITY OF HF (BARS) DETERMINED FROM MICAS FOR Whether this observation can be extended to other areas remains to be > 3,500 BARS* *H2O seen. However, neither muscovite nor K-feldspar compositions appear to vary systematically across divariant bands produced by variable fluid Sample T(°C)t Lepidoblastic Biotite muscovite composition (Evans and Guidotti, 1966; Tyler and Ashworth, 1982), sug- gesting that it may rot be uncommon. 23-3-9B 644 0.9 0.4 23-3-18 612 0.3 0.3 23-3-40 623 0.1 0.4 23-3-45 643 0.6 0.1 ACKNOWLEDGMENTS 23-3-86B 651 0.5 0.4 EP-4N 684 0.7 23-3-46 676 1.0 This research was supported directly or indirectly by the University of 23-3-50 659 0.7 Northern Colorado, the Warren Thompson and Walker Van Ripper 'Water fugacity data from Burnham and others (1969). Funds at the University of Colorado, and the U.S. Geological Survey. I ^Temperatures are assig led on the basis of the thermal gradient shown in Figure 5B. and temperatures at the would like to thank W. Braddock, J. Munoz, and E. Larson for thoughtful K-feldspar-sillimanite isogradi are calculated from equation V. discussion of various parts of the paper and P. A. Nielsen and J. A. Grambling for their reviews of the manuscript. Biotite also provides a basis for determining the ratio f^o^HF by the exchange reaction (Munoz and Ludington, 1977, 1975): NOTATION J biotite(OH) + HF = biotite(F) + H20. (VII) a j activity of component i in phase j f fugacity (bars) Different equilibrium constants have been obtained for siderophyllite, an- AG° standard Gibbs free energy of reaction (cal/mole) nite, and phlogopite compositions. Using the data presented by Ludington P pressure (bars) and Munoz (1975) and assuming a linear variation of log Ks between the R universal gas constant ( 1.98717 cal °K~1 mole"1 ) end members (J. L.. Munoz, personal commun.), an equilibrium constant T temperature (°K, except as noted) for intermediate compositions is obtained: AV°S molar volume change of solids at 298 °K and 1 bar (cal/bar) WJ j margules parameter of component i in phase j Xbi fHi 2O 2100 bi(OH) J log log (VIII) X | molar fraction of component i in phase j Xb, 'HF bi(F) ab albite 0.0093 (P - 1) af alkali feldspar bi X ph [1.523 + A1-M2 octahedral A1 in muscovite 0.0181 (P - 1) an anorthite Xbla„„ [0.416 + ] + and andalusite T ann annite 0.0107 (P bi 1) bi(F) fluorbiotite X sid [0.079 cd cordierite Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/10/1158/3444727/i0016-7606-95-10-1158.pdf by guest on 02 October 2021 METAMORPHIC PETROLOGY, FRONT RANGE, COLORADO 1167 garnet Dougan, T. W., 1979, Compositional and modal relationships and melting reactions in some migmatitic metapelites from grt New Hampshire and Maine: American Journal of Science, v. 279, p. 897-935. kfs potassium feldspar Eugster, H. P., Albee, A. L., Bence, A. E., Thompson, J. B., and Waldbaum, D. R., 1972, The two phase region and excess mixing properties of paragonite-muscovite crystalline solutions: Journal of Petrology, v. 13, p. 147-179. mi white mica Evans, B. W., and Guidotti, C. V., 1966, The sillimanite-potash feldspar isograd in western Maine, U.S.A.; Contributions ms to Mineralogy and Petrology, v. 12, p. 25-62. muscovite Ferry, J. M„ and Burt, D. M., 1982, Characterization of metamorphic fluid composition through mineral equilibria, in ms(F) fluormuscovite Ferry, J. M., ed.. Characterization of metamorphism through mineral equilibria: Reviews in Mineralogy, v. 10, p. 207-262. Pg paragonite Greenwood, H. J., 1975, Buffering of pore fluids by metamorphic reactions: American Journal of Science, v. 275, p. 573-593. ph phlogopite Helgeson, H. C., Delany, J. M., Nesbitt, H. W., and Bird, D. D., 1978, Summary and critique of the thermodynamic plagioclase properties of rock-forming minerals: American Journal of Science, v. 278A, 229 p. Pi Holdaway, M. J., 1971, Stability of andalusite and the aluminum silicate phase diagram: American Journal of Science, qz quartz v. 271, p. 97-131. Hovis, H. L., and Waldbaum, D. R., 1977, A solution calorimetric investigation of K-Na mixing in sanidine-analbite sii sillimanite ion-exchange series: American Mineralogist, v. 62, p. 680-686. sid siderophyllite LaFountain, L. 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Burnham, C. W„ Holloway, J. R., and Davis, N. F.. 1969, Thermodynamic properties of water to 1000 °C and 10,000 Spry, A., 1969, Metamorphic textures: Oxford, England, Pergamon Press, 350 p. bars: Geological Society of American Special Paper 132,96 p. Thompson, A. B., 1974, Calculation of muscovite-paragonite-alkaki feldspar phase relations: Contributions to Mineralogy Carmichael. D. M., 1969, On the mechanism of prograde metamorphic reactions in quartz-bearing pelitic rocks: Contribu- and Petrology, v. 44, p. 173-194. tions to Mineralogy and Petrology, v. 20, p. 244-267. Thompson, A. B., and Algor, J. R., 1977, Model systems for anatexis of pelitic rocks: Contributions to Mineralogy and Chatterjee, N. D., 1972, The upper stability limit of the assemblage paragonite + quartz and its natural occurrences: Petrology, v. 63, p. 247-269. Contributions to Mineralogy and Petrology, v. 34, p. 288-303. Tyler, I. M., and Ashworth, J. 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V., 1979, Muscovite-plagioclase equilibria in sillimanite quartz bearing metapelites. Puzzle metamorphism: Geological Society of America Abstracts with Program, v, 12, p. 544. Mountain area, northwest Maine: American Journal of Science, v. 279, p. 411-434. Winkler, H.G.F., 1979, Pedogenesis of metamorphic rocks (5th edition): New York, Springer Verlag, 348 p, Chinner, G. A., 1961, Origin of sillimanite in Glen Clova, Angus: Journal of Petrology, v. 2, p. 312-323. Cole, J. C., 1977, Geology of east-central Rocky Mountain National Park and vicinity, with emphasis on the emplacement MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 19, 1982 of the Precambrian Silver Plume Granite in the Longs Peak-St. Vrain Batholith [Ph.D. thesis]: Boulder, Colorado, REVISED MANUSCRIPT RECEIVED JANUARY 3, 1984 University of Colorado, 344 p. MANUSCRIPT ACCEPTED FEBRUARY 6, 1984 Printed in U.S.A. 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