Retrograde Metamorphism of Amphibolite, Bighorn Mountains, Wyoming

Retrograde Metamorphism of Amphibolite, Bighorn Mountains, Wyoming R. A. HEIMLICH Department of Geology, Kent State University, Kent, Ohio 44242 ABSTRACT and minéralogie descriptions of an schist zones, and no chemical data are actinolite-plagioclase rock and chlorite- available. It is the purpose of this paper to Twenty well-defined zones of schistose talc-actinolite schist from several of the describe the petrographic, mineralogic, and rocks occur within a thick amphibolite zones north-northwest of Hazelton Peak. chemical changes that accompanied the body in the southern Bighorn Mountains, Williams (1962) also described the petrog- formation of the schist bodies. Wyoming. The zones are 2 to 12 m thick raphy of chlorite-actinolite-talc schist from and as much as 300 m long. Many of the a few of the zones. LABORATORY METHODS zones are parallel to the strike of the am- However, no over-all petrographic or Modes of the schist and the amphibolite phibolite body. However, a significant minéralogie study has been made of these were determined on the basis of 1,200 number are oriented across its trend and at an angle to the strike of its foliation. Lo- cally, the strike of amphibolite foliation is deflected near the schist zones, and blocks of amphibolite occur isolated within one of the zones. Adjacent to the schist, the amphibolite is typically a massive or layered, granoblastic aggregate of hornblende, andesine, some quartz, and minor accessories. The schist bodies consist of tremolite or magnesian actinolite, chlorite, talc, and local sodic oligoclase. Chemical analyses of schist and adjacent amphibolite indicate that the schist contains smaller amounts of TiC)2, A1203, CaO, Na20, K20, and greater amounts of MgO and H20. Representing greenschist facies condi- tions, the schist formed by local retrograde metamorphism along shear zones superim- posed on the amphibolite, which itself evolved during amphibolite-facies regional metamorphism. Retrograde metamorphism was facilitated by means of shearing stress and magnesium-rich aqueous fluids. Key words: metamorphic petrology, retrograde schist. INTRODUCTION Within the Hazelton Peak amphibolte body of the southern Bighorn Mountains, Wyoming (Fig. 1), are some 20 well-defined zones of schistose rocks. Beckwith (1939), in a discussion of asbestos deposits in Wyoming, was the first to make reference to them. According to him, the occurrence of brittle amphibole asbestos and talc in several of the zones prompted some pitting Ò Ò o o and the sinking of a shallow shaft in 1934. However, no deposit of economic value was discovered. Figure 1. Index map of Precambrian outcrop, Mountains, Wyoming, showing area investigated Osterwald (1959) provided petrographic (rectangle). Geological Society of America Bulletin, v. 85, p. 1449-1454, 8 figs., September 1974 1449 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/9/1449/3429141/i0016-7606-85-9-1449.pdf by guest on 28 September 2021 1450 R. A. HEIMLICH points per thin section usir.g measurement areas large enough to maintain analytical error at a maximum of 2 percent. Refrac- tive indices were measured on crushed grains from each sample using oils checked with an Abbé refractometer; indices are accurate to ±0.002. Optic angles (within 2°) and extinction angles were measured with a four-axis universal stage. The approximate partial chemical com- position (100 Mg/Mg + Fe2 + Fe3 + Mn) of hornblende in the amphibolite and tremolite-actinolite in the schist was obtained by use of the Nz refractive index and the appropriate determinative curves given • GNEISS ^.ATTITUDE OF FOLIATION by Deer and others (1966, Figs. 59, 63). I AMPHIBOLITE FAULT For two samples, plagioclase composi- H SCHIST 727 SAMPLE LOCATION tion was estimated by the Michel-Levy • SANDSTONE (CAN.BRIAN) 2km method using the universal stage. For all MAPPED BY R.A.HEIMLICH AND G.C.NELSON. other samples, it was obtained by use of the N„ refractive index and Smith's low- Figure 2. Geologic map of area investigated. temperature plagioclase curve in Hess (1960); values so obtained are accurate much of it exhibits layering defined by al- body and at an angle to the amphibolite within 2 percent An. ternating hornblende-rich and plagioclase- foliation. rich tabular units from 1 mm to 2 cm thick In almost every case, schist-amphibolite GEOLOGIC SETTING that are typically lenticular and laterally c ontacts are obscured. Where visible (as at The Hazelton Peak amphibolite body, discontinuous. the locality from which samples 851 and with its numerous well-defined schistose The schist bodies occur in well-defined, 852 were collected), the contacts are grada- zones, dominates the area represented by linear zones (Figs. 2, 3) wholly within the tional. At this locality, blocks of amphibo- Figure 2. This mass is one of a large number amphibolite body; they range in thickness lite, approximately 30 cm across, occur as of amphibolite bodies (all much thinner) from 2 to 12 m. Although many of the isolated remnants within the schist (Fig. 4). that occur interlayered with quartzo- zones are small, discontinuous schist out- Locally, as the schist zones are approached, feldspathic gneiss in the southern part of crops have been traced 300 m along strike. the strike of foliation in the amphibolite is the Bighorn Mountains (Osterwald, 1959; No measurements of dip were possible; deflected, which suggests a drag effect. Heimlich, 1969). The over-all mineralogy however, the dip of the schistosity, proba- of the gneiss and associated amphibolite is bly comparable to that of the schist- PETROGRAPHY AND MINERALOGY that of regional metamorphism (2.75 b.y. amphibolite contacts, ranges from 25° to ago) to low amphibolite fades (Heimlich, vertical, with most dips averaging 50° or Amphibolite 1969; Heimlich and others, 1972; Heimlich more. The foliation v/ithin these zones may The following discussion is based on a and Armstrong, 1972). dip the same as the adjacent amphibolite study of thin sections of amphibolite sam- Along with the gneiss with which it is foliation or it may differ by as much as 40°. pled near the schist zones. However, these interlayered, the Hazelton Peak amphibo- As Figure 2 shows, the strike of many schist samples are petrographically and minera- lite body defines a westerly plunging an- zones follows the str:ke of the amphibolite logically similar to samples from through- tiformal structure. Progressing from the body. However, a significant number of the out the body (Heimlich and Uthe, in prep.). flank to the nose of the fold, dips of folia- zones are oriented across the trend of the Texturally the amphibolite is granoblastic tion in the gneiss and the amphibolite steepen from 40° to 55° typically. Varia- tions in outcrop thickness along the length of the body are the result of tectonic thin- ning and local steepening or flattening of its dip. The gneiss that immediately surrounds the amphibolite consists oi: a monotonous assemblage of weakly to strongly layered, quartzofeldspathic rocks. Although biotite is the common ferromagnesian mineral in the gneiss, hornblende-rich layers are locally prominent. In a few areas, the strike of foliation in the gneiss is transected by nar- row zones of fine-grained, highly silicified (and commonly epidotized) rock inter- preted as "healed" shears. None of these zones in the gneiss can be traced up to the Hazelton Peak amphibolite body. The amphibolite has a maximum thickness of 750 m and a minimum length of 13 km (Uthe and Heimlich, 1969). Although large portions of the bocy are massive, Figure 3. Typical outcrop of • schist zone (sample 972). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/9/1449/3429141/i0016-7606-85-9-1449.pdf by guest on 28 September 2021 RETROGRADE METAMORPHISM OF AMPHIBOLITE, BIGHORN MOUNTAINS, WYOMING 1451 plagioclase (Table 1), indicate a composi- tion of An29_4i, andesine. Quartz occurs as clear, equant, or lenticular grains. Long axes of rounded elongate or subrectangular titanomagnetite grains are typically parallel to the foliation defined by banding or major mineral paral- lelism. Sphene, the most common non- opaque accessory mineral, possesses a distinctive rounded rhombic or lenticular habit. Trains of such grains occur in some thin sections. Larger equant sphene grains enclose cores of titanomagnetite. Like that of sphene, the hexagonal shape of apatite crystals is subdued by a distinctive round- ing effect. Epidote occurs as equant or elongate locally ragged grains or grain aggregates. Although it may lack contact with hornblende, some epidote embays and replaces hornblende. Schist The schist bodies are typically inequigranular lepidoblastic or nematoblastic rocks. Inequigranular texture is most obvi- ous in chlorite-rich schists that are charac- terized by aggregates of smaller chlorite flakes between the larger prismatic am- Figure 4. Exposure of schist zone (sample 851) containing isolated block (outlined in black) of amphibolite phibole grains (Fig. 6). Amphibole prisms (sample 852). range in length from 0.5 to 2 mm, whereas chlorite flakes are typically less than 0.5 or lepidoblastic and layering is common in matic, or lenticular grains that are unzoned mm long. Local lenticular grains and grain thin sections. The major minerals are typi- and locally poikiloblastic relative to quartz. aggregates suggest flaser texture in the cally xenoblastic or subidioblastic and un- 2 3 Values for 100 Mg/Mg + Fe + Fe + Mn schist. commonly, idioblastic. Grain size averages (Table 1) fall in the range 58 to 70. Al- Modal analyses of the schist (Fig. 5B) 5 mm but ranges from 4 to 7 mm. though typically unaltered, some show considerably more scatter than those Amphibolite modes are listed in Table 1 hornblende grains have been replaced by of the adjacent amphibolite (Fig. 5A). The and presented graphically in Figure 5A. The small, ragged laths of brown biotite, some major minerals in the schist are tremolite- typical amphibolite consists of roughly of which transect the boundaries of several actinolite, chlorite, and talc.

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