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Calcium Zoning in Almandine Garnet, Wissahickon

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Calcium Zoning in Almandine Garnet, Wissahickon Canadian Mineralogist Vol. 15, w. M3-249 (1971) CALCIUMZONING IN ALMANDINEGARNET, WISSAHICKONFORMATION, PHIIADEIPHIA, PENNSYTVANIA MARIA LUISA CRAWFORD Bryn Mawr College,Bryn Mawr, Pa. 19010 USA ABsrRAcr INTRoDUcTIoN Garnets with clearly developed Ca zoning gen- As our knowledge of the details of garnet erally are associated with zoned plagioclase in chemistry in rnetamorphic rocks continues to samples of interlayered pelites and calcareous sand- grow it is apparent that the calcium content of stones in the Wissahickon Formation northwest of almandine garnets may be used as a geothermo- Philadelphia, Pa. Systematic Ca zoning is absent in meter or a geobarometerin certain metamorphic metamorphio assemblages with hieh variance, al- (Ghent et al. 1975). though the garnet may be zoned in other elements. assemblages 1976; Hensen Apparently calcium zoning in metamorphic alman- In addition, the calcium zoning patterns in gar- dine-rich garnets develops (1) during metamorphic nets usually differ, often markedly, from the reactions involving plagioclase-garnet equilibria; (2) zoning trends shown by Fe, Mg and Mn (Craw- as a rsult of reactions between otler minerals in ford 1974; de B6thune et al. 1968; Linthout & low-vaxiance assemblages; and (3) in volumes of Westra 1968). Most of the detailed studies of rock subject to changes in calcium content due to garnet equilibria have focused on the Fe, Mg and diffusion during metamorlfiic recrystallization. Mn zoning and the chemical interactions be- producing Where several of these mechanisms for tween garnet and otler ferromagnesian phases. Ca zoning operate simultaneously, the zoning pro- 'Where described,but little attention files are complex. the zoning is relatively Ca zoning has been parageneses simple, the trends of the calcium zoniDg itr alman- has been paid to the mineral and dine, when considered in conjunction with the zon. possible reactions whioh govern the variatign of ing in the coexisting plagioclase, can be used in the the calcium in almandine-rich garnet. interpretation of the thermal history of the sample; An advantage to studying compositional va- tho calcium zoning is independent of the mechanisms riation within refractory minerals such as garDet produce which 26ning of Mn, Fe and Mg. is that zoning patterns, where'preserved,provide Souuenr information on the development of the mineral assemblagelrom the moment when those min- Les grenats contetrusdans les 6chantillons de p€- erals first nucleate in the rocks. The thermal lites et de ere.scalcaires interstratifi6s de la forma- tion Wissahickonau nord-ouestde Philadelphie,Pa., histories of most metamorphic rocks can be Etats-Uni$ et montrant une zonation Ca bien d6- placed in one of three getreral categories: rocks veloppde, sont habituellement associ€s au plagio- which were subjected to rapid heating with con- clase zon6. Une zonation syst6matiquedu calcium sideiable overstepping of mineral reactions; est absentedans les associationsm6tamorphiques i rocks in which the mineral assemblageshave variirnce.6lev6e,bien que le grenat puisse€tre zoo6 been affected by extensive retrograde reactions pour d'autre$ 6l6ments. Apparemment, la zonation as the rocks cooled, and those which recrystal- de calcium dans les grenats m6tamorphiquesriches lized under prograde, near-equilibrium condi- en (l) pendant almandin se d6veloppe les rEactions tions and do not show evidence of retrograde m6tamorphiques impliquant un dquilibre entre le effects. The types of calcium zoning in garnets plagioclaseet le grenat (2) comme r6sultat r€- des discussed by actions entre d'autres mineraux en associations! observed in the first case were faible varianie, et (3) dans des volumes de roches Crawford (1974) and in the second by de 86- soumisesi des variations du contenu en calcium en thune et al. {1975) and Hollister (1976). This, raison d€ Ia diffusion pendant la recristallisation paper deals with garnets in rocks of various m6tamoryhique. Les profils de zonation sont trds compositions from the staurolite zone of the complexes quand plusieurs de ogs m6canismescle amphibolite facies of a prograde metamorphic zonation du calcium agissentau m6me endroit. On terrain. utiliso la zonation du calcium dans l'almandin de The'samples are from the Wissahickon For- pa.ir avec la zonation du plagioclasecoexistant, lors, in gue celle-ci est assezsimple, afin d'6tablir l'histoire mhtion exposed along Wissahickon Creek thermique de cet 6chantillon. I-a zonation du Ca northwestern Philadelphi4 Pa. Most of the sec- est ind6pendantedes m€canismesproduisant la zo- tion consistsof alternating pelitic and moderate- nation du Ml, du Fe et du Mg. ly psammitic units several tens of meters thick Clraduit par la R6daction) in the ex,posedsection. Locally, thin bands (2- 2/+3 2M THE CANADIAN MINERALOGIST 10 cm) of calcareous sandstone are interlayered mole percent pistacite in the clinozoisite. How- in the pelitic units. The samples were collected evet, these substitutions will shift the equilibrium from two outcrops of pelite and calcareous sand- in opposite directions and probably will tend to stone with tle intention of comparing the gar- cancel each other. In higher grade rocks in this net compositions in rosks of different calcium area, staurolite breaks down in the stability contents. field of kyanite. This suggests a pressure of at Two sets of samples were studied in detail, least 7 kbar for the region, which is compatible one collected shghtly above the staurolite isograd with the observed stability of the clinozoisite* and the other midway between the first appear- muscovite* quartz assemblage. ance of staurolite and the isograd representing One fundamental observation which is of the reaction staurolite*quartz=garnet+kyanite. considerable importance in deciphering the cal- The sampleswere about 1 km apart, perpendicu- cium zoning in almandine is the common asso- lar to the strike of the isograds. In this area the ciation of zoned plagioclase with calcium-zoned exposedwidth of the staurolite zone is 5.5 km. garnet. Thus the calcium content of a metamor- No differences were found in mineral composi- phic almandine garnet is sontrolled by plagio- tion between the two localities. As the assem- clase-garnet equilibria according to reactions blages are in the staurolite stability field, meta- such as: grossular{ALSiOr*quartz=anorthite. morphic temperatures during recrystallization In addition, equilibria involving additional cal- are inferred to be between550" and 650'C (Fig. cium-bearing phases may be involved: zoisite+ 1). Within the calcareous layers muscovite oc- quartz=anorthite*grossular+Hr0, and anorthite *calcite*quartz-grossular*COo. To a first ap- proximation therefore, the grossular content of tho garnet will reflect the Na/Ca ratio of the rocks, and secondly the metamorphic history of the sample, which influences the reactions be- tween Ca-bearing phases. ANar-Yrrcer- PRocEDrtREs In each specimen the garnets were analyzed for Mg, Fe, Mn, Ca, Al and Si with an ARL EMX electron probe microanalyzer. The anal- yses were made along traverses through the center of the grains at 6 and 10 micron intervals 5O0 6q' 7@ fo6 TABLE1, CO}IPOSITIOTIOf GARftETS ILLUSTRATED.II,I FIGI'RES 2-4 Frc. 1. Pressure-temperature field for samples st02 . 4t203 FeOr Mno Mgo c!0 Total studied. Experimentally determined '| equilibrium 37.4 2l.l r-'" core ?7.3 10.0 2.4 3.2 0't.4 curves: (a) chlorite*muscowite=staurolite4bio- rln 37.4 21.3 32.5 3.9 3.8 2.6 101.5 tite+quartz+H0 Gloschek 1969); O) staurolite rh core 37.4 20.s 29.6 6.5 3.6 3.0 t00.9 '" 31.5 3.9 2.2 t0t.7 fguartz-gdrnet+kyadte+HrO and (c) stauro- rlE 37.5 21.2 5,4 ' o @ra 35.5 20.7 fi.7 .2 1.5 4.6 100.2 litefquartz-garnet*silliinanite+H2o (Ganguly ' rln 36.9 21.1 35.4 .2 3.5 3.0 100.2 1972\; (d) mrlscovitefzoisitefquartz-anorthite 2I. l 26.7 3.3 2.3 9.7 rQ0.5 '. core 37.3 *K-feldsparaH0 (Johannes and Orville 1972; rln 38.1 21.3 25.3 2.0 2.6 ll.l 100.4 Hewitt 1973); (e) muscovitelzoisitelquartz= , ore 37.6 21.5 25,5 5.8 9.2 101.2 ' rln 37.6 21.1 30.0 2.2 2.6 7.0 100.5 atrorthite+K-feldspar (Ackermand & Karl 1972). 't0l.7 r core 38.0 2l.l 33.0 2.5 1.8 5.3 . duminosilicate stability curyes from .Holdaway - rln 38.5 21.1 31.1 1.5 10t.0 . Q97r). ll|fi$€rs of lons on the basis of l2(0) si., At Fe iln I',l9 {a .vrith '1.82 curs clinozoisite, plagioclase and quartz; ,- ore 2.98 '1.991.99 .68 ,28 .27 'o rln 2.97 2.15 .26 .45 ,22 potassium feldspar was not seen. The break- '1.96 rL core 2.9a 1.98 .a .43 .26 dirrvn of clinozoisite*muscovite*quafiz approx- '" rln 2.98 1.99 2.09 .37 .40 .19 imately parallels the kyanite=sillimanite isograd , core 2.96 1.98 2.49 .0r .18 .40 ' rlo 2.96 1.99 2.37 .02 .41 ,26 (Ackermand & Karl L972). Data from Johannes '1.98 , core 2.97. 1.78 .22 .27 .83 & Orville (1972) and Hewitt (1973) place this " rlm 3.00 1.97 1.67 .14 .3I .94 re,action about I kbar higher than suggested by , core 2.96 1.99 1.68 .39 .25 .78 ' rln 2.n . 1.98 2.00 .15 .31 .60 (Fi& '1.97 Ackermand & Karl 1). The experimental - core 3.01 2.19 .17 .21 .45 data need to be adjusted to account for 1O-15 " rln 3.03 1.95 2.O5 .10 .38. .47 'Totrl molerpercent albite,in the plagioclase and 10-15 F" reported as Feo CALCIUM ZONING IN ALMANDINE GARNET (depending on the size of the grain). Similar tra- ()2).
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