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 conditions, the schist formed by local retrograde metamorphism along shear zones superimposed 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

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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 composition (100 Mg/Mg + Fe2 + Fe3 + Mn) of hornblende in the amphibolite and tremolite-actinolite in the schist was ob-

tained 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 narrow zones of fine-grained, highly silicified (and commonly epidotized) rock interpreted 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).

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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 nonopaque 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 characterized 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. Plagioclase two-thirds hornblende and one-third hornblende grains. and quartz appear locally (Table 2). plagioclase. Quartz is the only other quan- Plagioclase occurs typically as fresh, Titanomagnetite is the only opaque acces- titatively significant mineral present. Biotite equant, or lenticular grains. Twinning, both sory present. Sufficient quantities of it occur appears locally, and titanomagnetite is albite and albite combined with pericline, is in the northeasternmost schist zone (sample present in most thin sections. Other non- abundant. Enough untwinned grains are 157) to cause a deflection of the Brunton opaque minerals include sphene. apatite, present so that staining is essential for compass needle. Nonopaque accessories in- epidote, and allanite, in order of decreasing modal analysis. Some thin sections contain clude epidote and calcite. abundance. scattered larger, irregular relict plagioclase Hornblende occurs as strongly pleo- grains that are mostly sericitized and (or) Amphibole accounts for more than 50 chroic (yellow and green), equant, pris- saussuritized. The data for the fresh percent of the dominant minerals found in the schist zones. Based on habit and optical

TABLE 1. MODES, IN VOLUME PERCENT, FOR AMPHIBOLITE AND OPTICAL data, the main amphibole belongs to the DATA FOR HORNBLENDE IN AMPHIBOLITE actinolite-tremolite series. The N2 refractive

Sample no. 974 856 116 970 971 727 852 769 798 167 index indicates a range in the ratio 100 Mg/Mg + Fe2 + Fe3 + Mn of 67 to 100, Modes which suggests that both tremolite and Hornblende 64.7 63.4 76.6 79.9 61.8 63.8 84.0 75.1 54.2 70.2 magnesian actinolite are represented (Fig. Plagioclase 29.2 26.6 17.2 16.0 31.2 29.0 9.3 14.9 36.8 26.0 7). Because the refractive indices of Quartz 3.9 6.8 4.0 2.8 4.7 5.0 2.1 8.0 6.5 3.3 actinolite-tremolite series members are con- Biotite 0.5 0.3 tr. tr. 0.2 trolled by the extent of substitution of Si by Opaque accessories 0.7 1.2 1.6 0.9 0.1 1.2 1.4 0.1 A1 and of OH by F in addition to the rela- Nonopaque tive amounts of Mg and Fe present, the use accessories 1.0 2.0 0.3 0.4 2.2 1.0 3.0 2.0 2.4 0.5 of the Nz index to estimate the amounts of Optical data the Mg and Fe end-member contents may

Plagioclase be in error by as much as 15 mol percent composition (% An) 35 31 39 33 29 39 33 41 37 37 (Deer and others, 1966, p. 164). Neverthe- Hornblende 2V 59 62 70 58 68 72 64 73 76 66 less, the data indicate that the main am- V 22 18 16 18 16 24 23 24 25 24 phibole in the schist is at the magnesian end "z 1.673 1.1166 1.666 1.671 1.669 1.678 1.673 1.674 1.678 1.668 of the spectrum. Mg* 63 70 70 65 67 58 63 62 58 68 Tremolite-actinolite occurs as acicular or • 100 Mg/Mg + Fe2 + Fe3 + Mn (based on N ). z prismatic grains or as aggregates of several

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such grains. Most grains a::e subidioblastic Among the lesser minerals, rare calcite and characterized by ragged prism termina- forms as irregular, interstitial patches. Epi- tions; narrow reaction rims are present lo- dote occurs as equant, xenoblastic, or sub- cally (Fig. 6). Tremolite is typically non- idioblastic grains. Tiranomagnetite is pres- pleochroic or faintly pleochroic (colorless ent typically as equant or lenticular grains to pale green), whereas actinolite is dis- possessing highly ragged outlines. The more tinctly pleochroic (colorless to medium or elongate grains are invariably oriented dark green). Some of the larger actinolite parallel to the trace of foliation. prisms contain opaque oxide dust or larger granules of such oxides disseminated within CHEMICAL DATA the core of the grain. Twinning on (100) is For comparative purposes, chemical i PLAGIOCLASE relatively common. analyses of selected pairs of schist and A second amphibole occurs at two nearby (13 to 200 m away) amphibolite are localities. In addition to actinolite, sample given in Table 3. Samples were chosen 515 contains what is assumed to be an- primarily to span the geographic and thophyllite with distinct plecchroism (pale mineralogic spectra for the schist. Among brown to pale green). The habit and mode the major oxides, only silica and alumina of occurrence of the anthophyllite are iden- are relatively uniform in the amphibolite tical to those for the associated actinolite. from the localities sampled. Iron oxides, Sample 89 contains cross-fiber veins of as- magnesia, lime, soda, potash, and water bestiform anthophyllite (identified by x-ray exhibit a moderate amount of variation diffractometry). Although most veins are among the four amphibolite samples. Dif- less than 2 cm thick, several are as much as ferences in the amounts of these oxides are 12 cm thick. X-ray analysis of one of the not related to the proximity of the samples veins shows that it consists of a mixture of relative to the schist. anthophyllite asbestos and talc. The major oxides in the schist analyses Fiji^ire 5. Plot of major minerals from amphibolite Chlorite occurs as individual lathlike are much less conformaole compared with modi s (A) and schist modes (B). flakes and aggregates of flakes located those in the amphibolite analyses. variation among the ferromagnesian min- primarily between the larger amphibole Nevertheless, consistent compositional dif- erals of the schist, and the extent to which grains (Fig. 6). Locally, the flakes extend ferences exist between the analyzed talc and (or) chlorite dominate over into the amphibole prisms. The chlorite is amphibolite-schist pairs (Fig. 8). Each of tremolite-actinolite. Greater water and moderately pleochroic (colorless to medium the schist bodies contains less alumina, magnesia content is expressed in the schist green). Optic and extinction angles are zero lime, soda, and potash and more water and bodies by their magnesium-rich mineral in almost all cases. Refractive index meas- magnesia than the nearby amphibolite. The composition (tremolite and magnesian ac- urements indicate that both prochlorite (N„ loss of alumina, soda, and potash is related tinolite) and their high content of hydrous = 1.590 to 1.606) and clinochlore (N„ = primarily to the breakdown of plagioclase minerals (chlorite and talc) compared with 1.573 to 1.587) are present. during the formation of the schists. Al- the mineralogy of the amphibolite. Among Most schist bodies contain some talc, and though plagioclase occurs in the mode of all the major oxides, total iron oxide (as FeO) one contains almost 70 percent (Table 2). It amphibolite samples studied (Table 1), it is may be greater, less, or the same in the occurs as tiny interstitial patches or scaly either lacking or greatly diminished in the schist as compared with the amphibolite. aggregates and as large xenoblastic to sub- schist (Table 2). Loss of lime and variability idioblastic rectangular or square-shaped in iron oxide content relate to the presence CONCLUSIONS grains. Some talc occurs as secondary rims of tremolite-actinolite as the dominant fer- Although the origin of the schist may be on tremolite grains, but most occurs inter- romagnesian mineral in the schist — rather interpreted in several ways, the field and stitially intergrown with chlorite. than hornblende of the amphibolite, modal laboratory data accumulated indicate that

TABLE 2. MODES, IN VOLUME PERCENT, FOF SCHIST AND OPTICAL DATA FOR CHLORITE AND TREMOLITE-ACTINCLITE IN SCHIST

Sample no. 94 97 «55 123 119 117 17 969 972 973 39 750 851 515 169 157 7104

Modes

Tremolite-actinolite 4 0 84.6 22.3 33.6 82.7 61.9 25.2 57.3 69.3 40.5 81.5 5.5 86.7 83.7* 77.4 72.6 75.2 Chlorite 81 3 13.1 5.4 29.0 6.0 27.7 50.7 40.3 30.3 55.8 17.9 90.6 8.6 0.8+ 17.2 20.8 18.6 Talc 12 3 1.5 68.5 37.0 10.3 22.8 0.5 2.0 tr. 3.5 3.7 0.2 4.2 3.6 Plagioclase 2.5 8.7 0.5 12.5 Quartz 2.3 0.5 Opaque accessories 2 4 tr. 1.3 0.4 0.1 1.3 1.7 0.4 1.7 0.4 0.4 1.2 6.6 2.6 Nonopaque accessories 0.8 0.3 0.2 0.2 2.8

Optical data

Tremolite-actinolite 2V 79 83 81 78 78 77 78 81 77 82 78 76 79 79 76 79 zkc 18 18 18 16 16 17 17 18 15 20 17 16 16 21 18 21

"z 1.646 1.626 1.629 1.645 1.639 1.634 1.642 1.639 1.632 1.642 1.636 1.642 1.653 1.640 1.635 1.645 Mgi 75 100 97 77 84 91 80 84 93 80 87 80 67 83 89 77

Chlorite ny 1 573 1.605 1.506 1.595 1.647 1.606 1.585 1.601 1.603 1.587 1.601 1.590 1.598 1.598 1.590 1.600 Plagioclase composition {% An) 15 14 14

* Includes 7.7 percent anthophyllite. t Entirely biotite. 2 3 § 100 Mg/Mg + Fe + Fe + Mn (based on N2).

TREMOLITE| JCTINOLITE

li_ ° 4 CE

90 00 70 60 50 40 30 20 IO Co2Mg5Si8022(0H)2 CagFegS^O^ztOH^ lOOMg/Mg+Fe+Fe+Mn Figure 7. Histogram of tremolite-actinolite com- positions. amphibolite after the latter had formed. Additional critical field evidence for such an interpretation includes the local drag effects in the amphibolite adjacent to several of the schist zones, the gradational nature of schist-amphibolite contacts locally, and the presence of relict amphibolite blocks isolated within one of the schist zones. The contrast in metamorphic facies represented by the schist bodies and the associated amphibolite is readily explained if the former are interpreted as having been Figure 6. Photomicrograph of schist illustrating inequigranular texture (sample 973). Tr = tremolite, Chi = formed by retrograde metamorphism of the chlorite. latter. The mineralogy (hornblende and it formed by retrograde metamorph.sm of the amphibolite body (Uthe and Heimlich, andesine) of the amphibolite is indicative of the amphibolite with which it is intimately 1969). Another alternative is that the schist the amphibolite facies (Turner, 1968, p. associated. If the schist bodies represent represents former concordant and discor- 303, 307). The common minerals (ac- premetamorphic units that are lithologi- dant ultramafic dikes that intruded the am- tinolite-tremolite, chlorite, talc) of the cally distinct from the amphibolite phibolite parent. However, the schist and schist indicates formation under greenschist precursor, such as magnesian ultramafic the amphibolite represent different facies conditions (Turner, 1968, p. 270). segregates within a mafic sill, then their dis- metamorphic faciès, a fact that is difficult to The occurrence of some anthophyllite at tribution should make some stratigraphie reconcile with the above interpretations. two localities, associated with talc (sample sense, and all of the schist bodies should be The linear character of the schist zones, 89) and with actinolite-tremolite and trivial concordant relative to the amphibolite. But the fact that some of them lie at an angle to biotite and talc (sample 515), may represent such is not the case. Their interpretation as the long axis of the amphibolite body and a transitional stage in the conversion of highly dolomitic beds within ar. inter- transect the strike of foliation within it, and amphibolite to schist. The presence in the layered shale-carbonate sedimentary se- the fact that locally the schistosity dips at a schist of scattered relict plagioclase grains quence is ruled out because of the grossly different angle from the amphibolite folia- that possess a composition similar to that in discordant nature of a number of the schist tion suggest that the schist defines zones of the amphibolite suggests a genetic relation zones and the mafic igneous parentage of shearing that were superimposed on the between the two. The strong mineral paral- lelism and local flaser texture of the schist, TABLE 3. CHEMICAL ANALYSES, IN WEIGHT PERCENT, 01' AMPHIBOLITE-SCHIST PAIRS in contrast to the granoblastic, massive, or layered character of the amphibolite, sug- Rock type* Sch Am Sch gest deformation by shearing. They are thus

Sample no. 855+ 972 167 157 compatible with the concept of derivation of the schist from the amphibolite by Oxides retrograde metamorphism. 51.14 43.64 Sl02 50.41 53.76 50.13 49.30 51.40 45.24 Retrograde metamorphism is defined by Ti02 0.90 0.18 0.90 0.45 0.87 0.45 0.53 0.45

A120J 16.53 5.13 15.14 11.85 16.22 10.12 15.23 9.15 Turner (1948) as "the mineralogical ad-

Fe203 1.40 1.21 2.29 2.02 2.63 1.64 1.95 5.83 justment of relatively high-grade metamor- FeO 8.16 5.25 9.07 9.28 8.01 9.16 8.39 6.96 phic rocks to temperatures lower than those MnO 0.14 0.08 0.17 0.19 0.14 0.17 0.19 0.20 of their initial metamorphism." Spry (1969) MgO 7.34 27.30 9.28 14.54 6.65 20.69 8.31 23.56 noted that "retrograde metamorphism is CaO 10.10 1.06 8.95 8.51 9.71 8.48 11.50 5.54 normally a special (and common) case of 1.40 3.27 0.61 1.58 0.37 Na20 2.35 0.18 2.92 repeated regional metamorphism where a 0.03 0.24 0.18 0.26 0.05 0.12 0.05 K20 0.20 lower-grade episode is superimposed on a P20s 0.07 0.04 0.05 0.07 0.10 0.06 0.04 0.21

+ higher grade, or where dynamic meta- H20 2.37 5.13 1.00 1.97 0.30 2.34 0.82 4.57 morphism is superimposed on regional" M" 0.00 0.09 0.00 0.02 0.92 0.01 0.00 0.00 0.00 metamorphism. Becke (1909) and Knopf C02 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Cr203 0.03 0.61 (1931) emphasized that differential move-

Total 100.00 100.05 100.14 99.78 100.48 99.02 99.80 100.53 ment is the essential agent in facilitating the mineralogic changes accompanying retro- Note: Analyst! H. B. Wiik. * Am = amphibolite, Sch ' schist. grade metamorphism. On the other hand, t From Uthe (19701. Barth (1936) and Schwartz and Todd

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Heimlich, R. A., 1969, Reconnaissance petrology 856 855 16 I 19 971 972 167 157 30 of Precambrian rocks in the Bighorn Moun- h- tains, Wyoming: Contr. Geology, v. 8, p. z 25 47-61. Ld 20 O 15 1971, Chemical data for major Precambrian

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