Metamorphic Mineral Assemblages of Slightly Calcic Pelitic Rocks in and around tlie Taconic Allochthon, Southwestern Massachusetts arid Adjacent*» Connecticut and New York GEOLOGICAL StJRVEY PROFESSIONAL PAFER 111 Metamorphic Mineral Assemblages of Slightly Calcic Pelitic Rocks in and around the Taconic Allochthon, Southwestern Massachusetts and Adjacent Connecticut and New York By E-AN ZEN GEOLOGICAL SURVEY PROFESSIONAL PAPER 111 A study of progressive regional metamorphism of pelitic schists from the Taconic allochthon of southwestern Massachusetts and its bearing on the geologic history of the area IMTED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Library of Congress Cataloging in Publication Data Zen, E-an, 1928- Metamorphic mineral assemblages of slightly calcic pelitic rocks in and around the Taconic allochthon, south­ western Massachusetts and adjacent Connecticut and New York. (Geological Survey professional paper ; 1113) Bibliography: p. 1. Rocks, Metamorphic. 2. Petrology Taconic Mountains. I. Title. II. Series: United States. Geological Sur­ vey. Professional paper ; 1113. QE475.Z46 552'.4 78-32050 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract ______________ 1 Mineralogy and mineral assemblages—Continued Introduction _______________________ 2 Model mineral compositions—Continued Acknowledgments __________________ 4 Chloritoid _______———_______________ 22 Summary of regional geology _______ 4 Garnet _______—_—__________________ 22 Stratigraphy __________________ 4 Chlorite _______________——_——— 23 Structure ___________•.. 5 Biotite ________________————_ 23 Mineralogy and mineral assemblages 6 Muscovite _____-__—_—————— 23 Muscovite _____________________ 7 Epidote ___________—_____________ 23 Paragonite __________________ 9 Hornblende _________—_-—————— 23 Biotite _______________________ 9 Plagioclase ___________—___________ 23 Chlorite ______________________ 10 Kyanite _____________________________ 23 Chloritoid _____________________ 11 A model multisystem ________________________ 23 Staurolite _______________ 12 Problematic phase relations ___________________ 26 Garnet _______________________ 13 First appearance of chloritoid _————_———___ 26 Kyanite ______________________ 15 The chemography of garnet and the biotite-chlori- Amphiboles ___________________ 15 toid-garnet-chlorite relations _________________ 27 Tremolite _________________ 15 Chloritoid-staurolite relations _———————__ 30 Cummingtonite _______ 15 Kyanite-bearing assemblage _———____________ 33 Hornblende __________ 15 Phase relations involving epidote _ — ___ —— — ___ 34 Epidote ______________________ 16 Relation between epidote-bearing and hornblende- Plagioclase ___________________ 17 bearing assemblages _______————________ 36 Ilmenite _____________ 21 Cummingtonite-bearing assemblages ____________ 37 Magnetite _____________________ 21 Conditions of metamorphism _____ —— — _———_ —— _ 38 Tourmaline ___________________ 21 A retrograde isograd? _——_————_—————___ 42 Quartz _________________ 21 Geologic implications of overburden pressure during Model mineral compositions _____ 22 metamorphism _________——__——_————__ 42 Staurolite _________________ 22 References cited _____——_———_—————————— 42 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of the study area ____________________—_—_—______ 2 2,3. Triangular diagrams of: 2. Ar'-Al lv-(Fetotai+Mer) compositions of muscovite analyzed by use of the microprobe ____ 8 3. Atomic A(Al-alkalis)FM for biotite samples analyzed by use of the microprobe _-______ 10 4. Photomicrograph of sample 376—1, showing ilmenite sandwiched between chlorite plates _____—__ 11 5. Graphs showing composition profiles across garnet crystals in samples 356-1 and 487-2-4 __________ 14 6. Photomicrographs of cummingtonite assemblages for samples 170-1 and 487-2 ___________ 16 7. Graphs showing composition profiles across a hornblende crystal in sample 102-1 _____—______ 17 8,9. Photomicrographs of: 8. Coexisting Staurolite and epidote in samples 356-1 and 590-1 _______________—_ 17 9. Triply zoned plagioclase from samples 3-3 and 360-1 _________________——_—— 18 10. Graphs showing composition profiles across zoned plagioclase of samples 3-3, 360-1, and 14-1 ______ 19 11. Photomicrographs showing extreme habit adaptation of metamorphic plagioclase to the foliation sur­ face of the rock, sample 423-1, and late plagioclase that includes folded foliation defined by dust trains, sample 340-1 _____________________________________________________________ 20 12. Graph showing manganese content of coexisting garnet and ilmenite (as inclusions in garnet) in sample 356-1 _____________________________________________ 21 13. Photomicrographs of zoned tourmaline, sample 509-1, and of a mantle of tourmaline, sample 102-2_ 22 III IV CONTENTS Page FIGURES 14,15. Diagrams showing: 14. A closed net for the quinary multisystem AWFeO-MgO-CaO-K^O, for the phases epidote- garnet-plagioclase-chlorite-chloritoid-staurolite-biotite-muscovite in the presence of quartz and having H20 and 02 as boundary-value components ———————————————————— 24 15. A part of the multisystematk net of figure 14 containing the four invariant points (St), (Ep), (Bt),and (Cd) _____________________—————————————————————— 26 16. Photomicrographs of chloritoid-biotite assemblages _———————————————————————————— 28 17. ACFM diagram wherein paragonite is projected to the A apex, for phases in coexistence with epidote 35 18. Graph showing experimental pressure-temperature curves pertinent to the mineral assemblages of the study area _____________________——————-—————————————————————— 39 19. Photomicrograph of sample 191-1, showing chlorite rimming magnetite ————————————————— 42 TABLES [Tables follow References Cited] Page TABLE 1. Mineral assemblages in samples of rocks from within and around the Taconic allochthon, south­ western Massachusetts and adjacent parts of Connecticut and New York —————————————— 49 2. Localities from which samples were obtained for mineral-assemblage information ———————————— 54 3. Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon: A. Muscovite ___________________—________—_—————————————————— 57 B. Biotite ______________________________________________———————— 67 C. Chlorite __________________________________________—_——_————— 75 D. Chloritoid ______________________________________________—_————— 81 E. Staurolite ____________________________________________-_—__ 88 F. Garnet ________________________ __________________——————— 95 G. Kyanite ____________________________________________-__-__ 106 H. Hornblende and cummingtonite ______ — ————————————————————————————— 107 /. Epidote ______________________________-_________________________ 109 J. Plagioclase _________________——————_—_———_—_———————————— 110 K. Ilmenite ____________________________________________________ 117 L. Magnetite ____________________________________________________ 123 4. Conventional wet-chemical analyses and the number of atoms calculated on the basis of the anhydrous formulas for selected minerals ___________——________________—_________ 124 5. Comparison of wet-chemical and microprobe analyses and of the calculated chemical formulas of staurolite from sample 355-1 ____________________________________—__ 126 6. Molar volumes of minerals used to calculate slopes of the univariant curves shown in figure 15 ___ 126 7. Compositions of coexisting chlorite, biotite, chloritoid, and garnet in the presence of muscovite, plagioclase, and quartz ____________________________________________ 127 8. Partial mineral-assemblage data for samples of epidote-bearing assemblages ______________ 127 9. Compositions of coexisting minerals in three hornblende assemblages ___________________ 128 10. Estimates of minimum temperature of metamorphism based on the K/(K+Na) ratios of muscovite compositions and the 2.07-kbar solvus of Eugster and others (1972) and micrroprobe data ——— 128 METAMORPHIC MINERAL ASSEMBLAGES OF SLIGHTLY CALCIC PELITIC ROCKS IN AND AROUND THE TACONIC ALLOCHTHON, SOUTHWESTERN MASSACHUSETTS AND ADJACENT CONNECTICUT AND NEW YORK By E-AN ZEN ABSTRACT stable existence of this phase. Inasmuch as the first ap­ The mineral assemblages from metamorphosed slightly pearance of staurolite is always in chlorite-bearing as­ calcic pelitic rocks of the Taconic Range in southwestern semblages, I suggest that the mapped staurolite zone Massachusetts and adjacent areas of Connecticut and New marker corresponds to a reaction whereby staurolite- York were studied petrographically and chemically. These chlorite becomes stable. The probable lower grade chemi­ rocks vary in metamorphic grade from those below the cal equivalent, for example, chloritoid-aluminum sili­ chloritoid zone through the chloritoid and garnet zones into cate, however, has not been found in the area of study. the kyanite-staurolite zone. Microprobe data on the ferro- Several staurolite-forming reactions discussed in the magnesian minerals show that the sequence of increasing literature are ruled out because of the relative sidero- Fe/ (Fe+Mg) value is, from the lowest, chlorite, biotite, horn­ phility of the minerals. A second staurolite isograd blende, chloritoid, staurolite, garnet. Hornblende, epidote, involves the reaction, chloritoid+chlorite+muscovite= garnet, and plagioclase are the