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Bulletin of the Geological Society of America Vol. 63, Pp BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 63, PP. 26-68, 22 FIGS. JANUARY 1952 METAMORPHIC FACIES IN THE WISSAHICKON SCHIST NEAR PHILADELPHIA, PENNSYLVANIA BY DOROTHY WYCKOFF ABSTRACT The mica schists and mica gneisses of the Wissahickon formation in the Philadelphia region are de- scribed in terms of metamorphic facies. Representatives of the amphibolite facies (sillimanite-almandine and staurolite-kyanite subfacies) predominate, but show incipient alteration to minerals characteristic of the epidote-albite amphibolite facies. The status of rocks containing the assemblages kyanite-orthoclase, sillimanite-muscovite, and kyanite-almandine is also discussed. Metamorphism of the highest grade is found in the southwestern end of the schist belt; but evidence is presented to show that the most intense metamorphism took place, not at the time of highest temperatures, but during a succeeding period of declining temperatures, when mineral changes were facilitated by copious hydrotherrnal solutions and strong regional deformation. The decipherable history is therefore largely one of retrograde metamorphism; many of the higher grade rocks have been converted by granitization to micro- cline gneiss ("granodiorite"), especially in the southern part of the schist belt, while farther to the north and west, incipient chloritization has been favored by late crushing. The period of metamorphism is tenta- tively dated as Paleozoic. CONTENTS TEXT ILLUSTRATIONS Page Figure Pag« Introduction 26 1. AKF diagram: staurolite-k yanite subf acies... 27 Philadelphia region as a test case 28 2. AKF diagram: sillimanite-almandine sub- Acknowledgments 30 fades 27 Chemical character of the rocks 30 3. AKF diagrams: possible intermediate sub- Significant mineral associations 31 facies 28 Classification of specimens 31 4. Chemical analyses plotted on AKF diagram Descriptions of minerals 35 for staurolite-kyanite subfacies 31 Mineral associations as indices of metamorphic 5. Chemical analyses plotted on AKF diagram processes 37 for sillimanite-almandine subfacies 31 Indices of granitization 37 6. Triangular diagram showing relation be- Indices of chloritization 39 tween color of biotite and content of Relation between granitization and chloriti- TiOs, MgO and FeO 36 zation 39 7. Distribution of significant minerals, of hy- Indices of regional metamorphism 39 drothermal changes and of crushing 38 Metamorphic processes 40 8. Average composition of plagioclases 40 Effects of hydrothermal activity 40 9. Distribution of micas in granitized, chlo- Effects of crushing 41 ritized and unaltered rocks 41 Relation between hydrothermal alteration 10. Distribution of micas in crushed and un- and crushing 42 crushed rocks 42 Effects of "pure" regional metamorphism.... 43 11. Relation between crushing and hydro- Aluminosilicate index minerals 45 thermal changes 43 Descriptions of minerals 45 12. Distribution of micas in rocks without Mineral sequences 47 crushing or hydrothermal alteration 43 Field relations 48 Summary and conclusions 48 13. Range in composition of plagioclases in Subfacies and isograds 48 sillimanite-almandine and in staurolite- Chemical composition 49 kyanite subfacies 44 Overlapping of zones 50 14. Distribution of euhedral almandine crystals. 46 Retrograde metamorphism 50 15. Crystal habits of sillimanite and kyanite. 47 Metamorphic history of the region 52 16. Relation between granitization and chlo- References cited 56 ritization 52 25 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/63/1/25/3441300/i0016-7606-63-1-25.pdf by guest on 25 September 2021 26 DOROTHY WYCKOFF-WISSAHICKON SCHIST, PENNSYLVANIA Figure Page Page 17. Map: Index minerals: kyanite, staurolite, 2. Chemical analyses, 1-13 .................. 32 almandine, micas. .................... 53 3. Comparison between actual and theoretical 18. Map: Index minerals: sillimanite (kyanite), mineral assemblages. ................... 34 orthoclase, muscovite.. ................ 54 4. Grouping of specimens for petrographic 19. Map: Granitized rocks.. ................. 54 study.. ............................... 34 20. Map: Chloritized rocks. ................. 55 5. Regrouping of specimens to bring out effects 21. Map: Crushed rocks.. ................... 55 of hydrothermal alteration. ............. 40 22. Map: Isograds.. ........................ 56 6. Regrouping of specimens to bring out effects of crushing. ........................... 42 TABLES 7. Grouping of sillimanite and kyanite rocks. .. 46 Page 8. Field associations of sillimanite, kyanite and 1. Chemical analyses, A-J .................. 29 pegmatites. ........................... 48 sets of physical conditions: in regional mapping inter-facies boundaries would correspond to iso- Two of the most important concepts under- grads. Within a single facies, however, differ- lying modern interpretations of metamorphic ences in chemical composition may give rise to terranes are the concept of zoltes or grades of rocks of widely differing mineralogical make-up. regional metamorphism and the concept of For example, the amphibolite facies may in- metamorphic facies. Metamorphic zoning was clude rocks characterized by biotite, almandine, first demonstrated in the mapping of the Scot- staurolite, or kyanite-all formed simultane- tish Highlands (Barrow, 1893; 1912), and has ously under similar conditions of temperature, since been applied to other regions. Meta- pressure and shearing stress. Schists containing morphic facies were first discussed by Eskola staurolite or kyanite are not therefore inter- (1915; 1920) and the idea has been elaborated preted as rocks of "higher grade", but merely by many other workers. Attempts have also as rocks somewhat richer in A1203 and poorer in been made to correlate these two ways of group- KzO than schists which contain only micas. ing metamorphic rocks: a clear statement is This point is illustrated by the two facies given by Turner (1948, p. 76). diagrams (Figs. 1, 2) representing possible The two concepts are not mutually contradic- mineral assemblages within the amphibolite tory, but they do differ somewhat in emphasis. facies. I have adopted Turner's suggested names The concept of metamorphic zones or grades (Turner, 1948, p. 81-87) : staurolite-kyanite emphasizes the effect of varying physical condi- subfacies (Fig. 1) and sillimanite-almandine sub- tions upon material of constant chemical com- facies (Fig. 2). In these systems A = A1203 - position. The zonal boundaries-delimited by (CaO + Na2O + K2O); F = FeO + MnO + mapping the distribution of certain "index" MgO; K = K20. It is assumed that CaO and minerals such as biotite, almandine, staurolite, Nan0 are combined with A1203 in plagioclase, kyanite, and sillimanite in pelitic sediments- which may occur as an additional phase; and if are assumed to correspond to lines of equal Si02 is present in excess, quartz also appears, temperature and have been designated isograds making 5 main phases possible in each assem- (Tilley, 1924) to indicate that they are lines blage. along which metamorphism has reached the A rock with the initial composition repre- same grade. Thus, on the basic assumption sented by point would, under the physical that the rocks selected as a standard have the R conditions characteristic of the staurolite-ky- same initial composition, schists containing kyanite or staurolite are considered to have anite subfacies (Fig. I), be a muscovite-biotite reached a higher grade of metamorphism than schist. Such a rock cannot by "progressive those which contain only almandine or biotite metamorphism" develop in succession the index as an index mineral. minerals almandine, staurolite, kyanite, unless The concept of metamorphic facies, on the at the same time its chemical composition is other hand, provides for consideration of the progressively altered along some such course as effect of varying chemical composition as well is indicated by points S, T, U: either A1203 as varying physical conditions. The different must be added or KzO, FeO, and MgO must facies are distinguished as representing different be lost as metamorphism proceeds. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/63/1/25/3441300/i0016-7606-63-1-25.pdf by guest on 25 September 2021 INTRODUCTION 27 For comparison, a rock of the same composi- regularities encountered in the mapping of tion, R', is shown on Figure 2, which represents metamorphic zones. The failure or repetition of the assemblages stable under the physical con- certain zones may be due to the chemical com- ditions of the sillimanite-almandine subfacies. position of the sediments. This possibility has This rock would be a schist or gneiss containing long been recognized in the case of staurolite OR BI FIGURE 1.—AKF DIAGRAM: STAUROLITE-KYANITE FIGURE 2.—AKF DIAGRAM: SILLIMANITE- SUBFACIES ALMANDINE SCBPACIES KY—kyanite, MU—muscovite, ST-^-staurolite, SI—sillimanite, AL—almandine, OR—orthoclase, AL—almandine, MI—microcline, BI—biotite BI—biotite (Harker, 1939, p. 225); another case has re- sillimanite, almandine, and orthoclase. Its pro- cently been described by Freedman (1950), duction, under conditions of rising temperature, where the kyanite zone is lacking, staurolite ex- from the muscovite-biotite schist represented by tending to the border of the sillimanite zone. R of Figure 1 would be a case of truly isochem- The sillimanite isograd is generally accepted ical metamorphism, and R' would be a rock of as marking the transition to a higher grade of higher metamorphic
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