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Isochemical Contact Metamorphism Of American Mineralogist, Volume 74, pages 53U548, 1989 Isochemical contact metamorphismof mafic schist, Laramie Anorthosite Complex' Wyoming: Amphibole compositionsand reactions Clnor, Russ-N.lnnr-pr* Department of Earth and SpaceSciences, State University of New York at Stony Brook, Stony Brook, New York I 1794, U.S.A. Ansrnrcr Contact metamorphism by anorthosite and syenite of the Laramie Anorthosite Com- plex, Wyoming, resultedin a thermal overprint of the regionally metamorphosedamphib- olite-facies country rocks at approximately 3.0-kbar pressure.The resulting assemblages in the mafic schist are amp + plg + ilm + qtz t cpx at the lowest-gradehornblende hornfels facies,amp + plg + cpx + opx * ilm + t11 + qtz at the orthopyroxene hornfels facies boundary, and amp + plg + cpx + opx * olv * ilm * mt at the highest grade, the olivine hornfels subfacies.Clinopyroxene first appearedat temperaturesof >670 "C, form- ing concomitantly with quartz from reaction of hornblende with plagioclase.Orthopyrox- ene first resulted from reaction of quartz with hornblende between >680 and 820 "C and above 730 'C from dehydration of hornblende where quartz was not available. Quartz appearsto have persistedup to 890 "C in some bulk compositions. Olivine first appeared at907 { from the reaction ofhornblende with plagioclase.The highest grade ofcontact metamorphism preserved in the mafic schist at Morton Pass was the olivine hornfels subfacies,which recorded temperaturesup to 933 "C. The dominant effect of isochemical metamorphism of these schistswas the changein abundanceand composition of hornblende with concomitant modal increaseor decrease in the other phases.Discontinuous reactions break amphibole down to pyroxenes and plagioclase(and eventually olivine) at facies and subfaciesboundaries, while continuous reactions allow for the progressiveenrichment of K, N4 A| Ti, F, and Cl as the quadri- lateral amphibole component, (Ca,Mg,Fe)rSirOrr(OH)r,becomes increasingly unavailable. There appearsto be a linear relationship between AIt" and temperature, strggestingthat systematictrends of other elementswith AlI" also reflect changesin temperature.Fdenite (NaAtSi-,tr-,) and tschermakite (AlrMg-,Si-,) components best describe the enrichment of Na and Al,u in amphibole with increasingtemperature. There are no significantchanges in the compositions of the other phaseswith grade. It is shown that progressivemeta- morphism as exemplified by the various hornfels of this study, is indicated by progressive loss of water with increasingtemperature without lossof Cl or F. Becausean isochemical suite of rocks is available that spansa wide temperature range, reaction spaceis used to provide an effective analysisof the progressof metamorphism in the various hornfels. One effect of reaction-spacemodeling is the derivation of reactions specificto the formation of the faciesand subfaciesat Morton Pass.Although the following reactions correspond to particular samples,they exemplify the incremental changesfrom one grade to the next. The reaction 0.203amp,+ 0.193p1g:0.l55ampr + l.l35qtz + 0.063cpx+ 0.048H,O, describesthe initial development of clinoplroxene and was followed, at higher tempera- tures, by the reaction 0.646amp,+ 0.l0lqtz + 0.030p1g: 0.606ampo+ 0.227cpx+ 0.040H,O, which consumed quartz rather than produced it. This reaction operated over a wide tem- perature range in the hornblende hornfels facies. The initial orthopyroxene-forming re- action l.480ampo"+ 4qtz : 0.72}amgs"* l.465cpx+ 0.51lopx * l.703plg+ 0.760HrO, * Presentaddress: Department of Geology, University of Missouri, Columbia, Columbia, Missouri 65203' U.S.A. 0003-004x/89/05064530$02.00 530 RUSS-NABELEK:AMPHIBOLE REACTIONS IN MAFIC SCHIST 531 was the major quartz-consuming reaction. After quartz reacted out, or in hornfels that were not quartz-saturated,orthopyroxene formed by a similar reaction l.480ampo:0.720amp,+ l.465cpx+ 0.51lopx* 0.703p1g+ 0.760H,O. At the highest grade, olivine formed by the reaction 0.677amp,+ 0.249p19: 0.413amp,+ 0.985cpx+ 0.27lopx + 0.l32fo + 0.264H,O, which continued within the olivine hornfels subfaciesuntil the ultimate demise of am- phibole above 940 "C. INrnooucrroN areal extent of more than 1000 km2 in the Laramie Previousstudies (e.g., Engel and Engel, 1962a,1962b1, Mountains (Fowler, 1930;Newhouse and Hagner, 1957; Harte and Graham, 1975;Iaird, 1980;Laird and Albee, Klugman, 1966). 1981a,l98lb; Abbott, 1982;Robinson, 1982a, 1982b; A particularly good exposure of the high-grade rocks (Fig. Stoddard, 1985)that have dealt with metamorphismof of the cohtact aureoleis in the Morton Passarea l). amphiboles in mafic systemshave provided valuable in- Archean felsic gneissescrop out in the western and cen- formation on the history and role of intensive variables tral regions of this study area. The eastern edge of the during the metamorphism of specific terranes. For ex- gneiss sequenceis in concordant contact with a supra- ample, Laird (1980) described systematic amphibole crustal unit (Fig. l) whose relative ageis unclear but that gneisses(Graff compositional changesand variations in proportions of is inferred to be younger than the et al., group coexisting phasesduring low-temperature, high-pressure 1982).A metasedimentaryunit of the supracrustal metamorphism of mafic schistsin Vermont. is composed in the north of thin layers of impure lime- The Morton Pass area of the Laramie Anorthosite stone and calc-silicate that rapidly broaden southward q\artz- Complex, Wyoming, provides an excellent example of into intercalated layers of dolomite, calc-silicate, the effects of low-pressure,high-temperature metamor- ite, and pelite. A volcanoclasticunit forms a continuous phism of mafic schists and therefore complements the 100-m-wide belt and is interlayered with the metasedi- study of Laird (1980).The Morton Passarea is especially mentary unit. This volcanic sequenceis composed of suitable for evaluating mafic schist phase relations be- massive and banded hornfels of mafic and ultramafic causethe last metamorphism occurred in responseto an composrtlons. isobaric thermal overprint by the anorthosite intrusion; The Archean units have been discordantly cut on the therefore, variations in phases and their compositions east by the Laramie Anorthosite Complex, an intrusive primarily expresschanges in temperature. Becauseiso- sequencebeginning with anorthosite including gabbroic chemical suites of the mafic rocks are available, the re- and noritic varieties and concluding the monzonite and actions that occurredat low pressureand medium to very monzosyenite(Fuhrman et al., 1983; Fuhrman, 1986; high temperaturescan be identified. Of particular impor- Fuhrman et al., 1988).The monzonite-monzosyenitese- tance with regard to these rock types is the presenceof ries appearsto have intruded along the contact between one or two pyroxenes,which, through the application of the anorthosite and the Archean rocks, and anorthosite pyroxenethermometry, indicatethe temperaturesat which is suspectedto lie beneath the younger syenites.Grani- the continuous and discontinuous reactions occur. toid bodies are ubiquitous in the Morton Passarea, oc- The approach to charcterization of the Morton Pass curring perhapsas late-stagedifferentiates ofpossibly lo- hornfels is (l) to identify an isochemical suite of rocks cal partial melts. spanningthe temperature range of metamorphism, (2) to It is evident from relict texturesand assemblagesin the identify the changesin mode and mineral composition lowest-gradecountry rocks that before the anorthosite- from low to high grade, and (3) to derive reactions de- syenite intrusions, the rocks of this area have been de- scribing the formation of facies and subfacies.Reaction formed at least once and regionally metamorphosed to space,following the procedureof Thompsonet al. (1982), greenschistor amphibolite facies.The mafic schistsin the is used to characterizethe reactions. The result of this volcanoclastic unit at the contact were then metamor- approach is a better understanding of the amphibolite- phosed by the intrusives to the hornblende hornfels and to-granulite facies transition at low pressuresin mafic pyroxene hornfels facies.Those ofthe hornblende horn- rocks. fels facies are interlayered with massive to finely lami- nated leucocraticbands. Gradations between the two types Gnor,ocy oF THE coNTAcr AUREoLE AT occur locally. Laminations in the massive rocks are in- MonroN Pass terpretedto have formed by local shearingin the anaphib- The Laramie Anorthosite Complex cores the eastern- olites during deformation accompanying regional meta- most of the three prongs of the Rocky Mountain Front morphism and are thereforerelict features.These texturel Range that extend northward into southern Wyoming. were partially to completely overprinted with increasing The complex and its contact aureole crop out over an grade. Layering of the nonhomogeneousrocks is inter- 532 RUSS-NABELEK: AMPHIBOLE REACTIONS IN MAFIC SCHIST Fig. l. Geologic map of the Morton Passarea, Albany County, Wyoming. Sample numbers referring to the letter locations are thefollowing:A:MPll-1andMPll-2A,B:83-17A,C:MPl2,D:MPl5-4,E:MPl8-1,F:83-10,G:83-11,H:83- l9A,I:83-128,J:83-13,K:MPl-3AandMP1-3B,L:83-14,M:83-18,N:MP6-4,O:83-15,P:83-16,Q:MP3-4, R : MP3-5 and MP3-6. S : MP57-9, and T : MP57-2. preted as a primary sedimentary feature, suggstingthat were counted separately and their weighted averages these were originally volcanoclasticsediments. combined. The modes are given in Table I along with The finely
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