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Metamorphic Petrology of the Northeast Front Range, Colorado: the Pingree Park Area

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Metamorphic Petrology of the Northeast Front Range, Colorado: the Pingree Park Area 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 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 1159 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 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 1160 W. D. NESSE <D Figure 2. Mineral zone boundar- N CO Major mineral Assemblages — T3 ies. The G, St, +A, and K boundaries CO c mark the introduction of garnet (Grt), a ÏO •o staurolite (St), andalusite (And), silli- .Û V) TD c £ o o w o < w o manite (Sil), and microcline (Kfs), re- spectively. The -A boundary marks the last occurrence of andalusite in the G prograde direction. The Ma 2nd M boundaries mark the first appearance St of migmatitic amphibolite and migma- 1 i titic pelitic and semipelitic roclcs, re- +A 1 spectively. Garnet and cordierite (Cd) 1 1 1 1 1 ms, ms and, ms and cd, reappear in the most intensely meta- 1« 1 1 morphosed pelitic and semipelitic 1 1 ms and st, ms and st cd, rocks, but mappable boundaries mark- 1 1 ms, ms and sii, ms sii, ing their reappearance have not been 1 * 1 1 1 • 1 ms and st sii, ms cc, defined (Cole, 1977). Ch(p) and Ch(r) 1 • 1 1 are prograde and retrograde chlorite, 1 1 > *1 ms and sii grt « • 1 1 respectively, and Ms(p) and Ms(r) are -A t ti * prograde and retrograde muscovite. ? ? * • ms, ms sii, ms sii grt All mineral assemblages contain quartz, i K plagioclase, biotite, and opaques. As- kfs sii, sii semblages outside the Pingree Park M area are not shown. Adapted from kfs sii, sii Braddock and Cole (1979), Shaver Ma»- ? ? (1980), and Nesse (1977). ii ii 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.
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