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METAMORPHIC HYDRATION of AUGITE -ORTHOPYROXENE ~V10nzodiorite to HORNBLENDE GRANODIORITE GNEISS, BEL.:CHERTOWN BAT-HOLITH, WEST-CENTRAL MASSACHUSETTS

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METAMORPHIC HYDRATION of AUGITE -ORTHOPYROXENE ~V10nzodiorite to HORNBLENDE GRANODIORITE GNEISS, BEL.:CHERTOWN BAT-HOLITH, WEST-CENTRAL MASSACHUSETTS METAMORPHIC HYDRATION OF AUGITE -ORTHOPYROXENE ~v10NZODIORITE TO HORNBLENDE GRANODIORITE GNEISS, BEL.:CHERTOWN BAT-HOLITH, WEST-CENTRAL MASSACHUSETTS BY LEWIS D. ASHWAL \ . \ ' \ \ CONTRIBUTION NO. f8 GEOLOGY DEPARTMENT UNIVERSITY OF MASSACHUSETTS AMHERST, MASSACHUSETTS METAMORPHIC HYDRATION OF AUGITE-ORTHOPYROXENE MONZODIORITE TO HORNBLENDE GRANODIORITE GNEISS, BELCHERTOWN BATHOLITH, WEST-CENTRAL MASSACHUSETTS by Lewis D. Ashwal Contribution No. 18 Department of Geology University of Massachusetts Amherst, Massachusetts July, 1974 iii TABLE OF CONTENTS Page ABSTRACT .. 1 INTRODUCTION 3 Location and Geologic Setting . 3 Summary of Previous Work 8 Objectives of Study . 10 Field Methods 11 Laboratory Methods 11 Hicroscopy . .. 11 X-ray Powder Diffraction . 12 Purification of Mineral Separates for Wet Chemical Analysis . 13 Electron Microprobe Analyses of Minerals . 15 Wet Chemical Analyses of Rocks . 15 Acknowledgments . 16 STRUCTURAL GEOLOGY . 17 General Considerations . 17 Primary Igneous Features 20 Hetamorphic Features 21 Post-metamorphic Features 29 LITHOLOGY AND PETROGRAPHY . 34 Two-pyroxene Monzodiorite .... 34 Hornblende-biotite Granodiorite Gneiss 40 Transitional Rocks . 44 Cataclastic Rocks . 47 iv Dikes • . • so Inclusions 53 Post-metamorphic Veins 54 BULK CHEMISTRY OF MAIN ROCK TYPES 55 MINERALOGY • • . 60 Plagioclase 60 Potassic Feldspar . 63 Pyroxenes •. • 69 Hornblende 76 Biotite 83 Epidote • 87 Opaque Minerals • • • • • • 87 Magnesioriebeckite 97 PETROLOGY • • • • 99 Chemographic Relations • • • • 99 Conditions of Magmatic Crystallization 112 Conditions of Metamorphic Hydration . • . 115 Effect of Igneous and Metamorphic Mineralogy on Rock Magnetism 116 SUMMARY OF CONCLUSIONS 118 REFERENCES 120 APPENDIX ............................ 124 v ILLUSTRATIONS Figure Page 1. Generalized regional geologic map showin location and structural setting of the Belcher Intrusive Complex . • . • . • • 4 2. Geologic map of the Belchertown Complex d related rocks . • . • • 6 3. Abandoned railroad cut at Three Rivers, ss. 9 4. Schematic map showing distribution of maip rock types within the Belchertown batholith . • • • . • • • . 18 5. Scale view of the northern side of the Three Rivers railroad cut . • . • • • • 19 6. Lower hemisphere equal-area diagram of poles to primary igneous foliation in the Three Rivers railroad cut . • • • • . • • 20 7. Enlarged view of eastern shear zone. 22 8. Lower hemisphere equal-area diagrams of planar and linear elements from shear-zones in the! Three Rivers railroad cut . • . 23 9. Enlarged view of western shear zone • • . 25 10. Schematic block diagram showing three-dimensional interpretation of shear zones . • . • . • . 27 11. Felsic dike with associated metamorphic hydration zone • • . 28 12. Lower hemisphere equal-area diagrams of planar and linear elements from shear zones and of joints in the Three Rivers railroad cut • . 1 29 13. Post-metamorphic epidote vein with associa~ed alteration zone . • . • . • . • . 30 14. Plot of 7 point-counted modes of primary b~tholithic, fully recrystallized, and transitional rocks 35 15. Sketch of two-pyroxene monzodiorite showing textural relations . • • . 37 vi 16. Photomicrograph showing dust-like inclusions in plagioclase . 38 17. Photomicrograph illustrating zonation of orthoclase microperthite . • . • . • . • . 38 18. Sketch of hornblende-biotite granodiorite gneiss . 42 19. Photomicrograph of complex reaction rim between hypersthene and potassic feldspar . 46 20. Photomicrograph of mylonite showing porphyroclasts of plagioclase and blue-green hornblende . 46 21. Feldspar ternary showing compositions of microprobe analyses of plagioclase and potassic feldspar from the Three Rivers railroad cut • . • . • . • 61 22. b-e plot of alkali feldspar data . • . 65 23. Pyroxene quadrilateral showing compositions of coexisting pyroxenes from monzodiorite specimen A-21-2 . • 74 24. Plot of A-occupancy versus tetrahedral Al for amphiboles . 82 25. Titanohematite with two sets of ferri-ilmenite exsolution lamellae 90 26. Titanohematite - ferri-ilmenite intergrowth showing small subhedral titanomagnetite inclusion with ferri-ilmenite rim . • . • . 90 27. Titanohematite- ferri-ilmenite intergrowth showing small subhedral magnetite inclusion with ferri-ilmenite rim ...•........ 91 28. Augite with rod-like inclusions of magnetite 91 29. Tio -Fe0-Fe o ternary diagram showing opaque 2 2 3 mineral compositions . • . • . • . 95 30. Modified A-C-F diagrams showing mineral and rock compositions • . • . • 100 31. ACF diagrams showing sequence of metamorphic hydrations ... 105 32. Tetrahedron (A+Na),Ca,Mg,(Fe+Mn) illustrating plagioclase projection ~ •....• 106 33. Plagioclase projections showing mineral compositions 108 vii 34. Plagioclase projections showing topologic relations for transformation of monzodiorite to granodiorite gneiss . 110 35. Illustration of redox relations in system Mg0-FeO-Si0 -o . 114 2 Table 1. Estimated and point-counted modes for the main rock types of the Belchertown Complex . 32 2. Estimated modes of cataclastic rocks from the Belchertown Complex . 48 3. Estimated modes of dikes, inclusions, and post­ metamorphic veins from the Belchertown Complex 51 4. Bulk-rock wet chemical analyses and computer­ calculated norms for major rock types of the Belchertown Complex . 56 5. Electron microprobe analyses for plagioclase feldspars • . 58 6. Summary of potassic feldspar structural state data . 64 7. Electron microprobe analyses for potassic feldspars . 66 8. Optical properties and angles of exsolution lamellae in pyroxenes . • . • . • . 70 9. Chemical analyses of augites . • . • . 72 10. Chemical analyses of orthopyroxenes 73 11. Optical properties of igneous and metamorphic hornblendes . 77 12. Chemical analyses of igneous and metamorphic hornblendes . 79 13. Chemical analyses of igneous and metamorphic biotites . • . 84 14. Electron microprobe analyses of epidotes 88 15. Electron microprobe analyses of opaque minerals 94 16. X-ray powder diffraction data on magnesioriebeckite .• 98 1 ABSTRACT The Belchertown batholith is a major syntectonic Devonian intrusion which lies near the axis of the Bronson Hill Anticlinorium in central Massachusetts. The margins of the batholith have been hydrated and sheared during Acadian kyanite grade regional metamorphism of the country rocks, and contain a strong tectonite fabric and amphibolite facies mineralogy. Certain central areas have escaped metamorphism, and retain primary igenous textures and minerals having compositions that suggest crystallization in a temperature range 800-1000°C, and an oxygen fugacity between the nickel -nickel oxide and hematite -magnetite buffers. A large railroad cut in the core region of the batholith exposes meta­ morphic shear zones along which primary monzodiorite has been converted to granodiorite gneiss by a series of hydration rea~tions controlled by introducti~n of aqueous fluid. Whole rock wet chemical analyses indicate that the transformation is virtually isochemical with respect to all major components except H o. Petrographic observations of two 2 selected specimens from the railroad cut and electron microprobe and wet chemical analyses of constituent minerals yield the following: Monzodiorite 50% plagioclase (An _ ) 23 29 9% orthoclase microperthite (or _ ) 71 86 5% quartz 7% orthopyroxene (Wo En Fs ,Al o = 2.25%; Ti0 = .22%; 4 67 29 2 3 2 Fe o = 1.83%) 2 3 2 13% augite (Wo En Fs ,Al o = 2.83%; Ti0 = .43%; Fe o = 3.33%) 46 43 11 2 3 2 2 3 2+ 2+ 1% hornblende (Fe /Fe +Mg .20; Si . ,Al o = 9.96%; Ti0 6 56 2 3 2 = 1.30%) 2 2 14% brown biotite (Fe +/Fe ++Mg = .29; Ti0 = 4.00%; Fe o = 2.59%) 2 2 3 1% titanohematite (hematite. , ilmenite. ) 53 47 <1% magnetite Granodiorite Gneiss 40% plagioclase (An _ ) 18 27 12% orthoclase (Or _ ) 84 89 12% quartz 2 2 20% hornblende (Fe +/Fe ++Mg = .25; Si . ,Al o = 7.19%; Ti0 = 7 03 2 3 2 .39%; Fe 4.05%) 2o3 = 2 2 9% green biotite (Fe +/Fe ++Mg = .31; Ti0 = 1.57%; Fe o = 3.00%) 2 2 3 4% epidote 3% sphene The overall reaction may be summarized as augite + orthopyroxene + plagioclase + Ti-biotite + titanohematite + magnetite + H = 2o hornblende + quartz + epidote + low Ti-biotite + orthoclase + sphene, leading to the total elimination of pyroxenes, titanohematite and magnetite. In agreement with this, only primary monzodiorite has appreciable magnetic susceptibility so that the core region is marked by a broad aeromagnetic high surrounded by a low underlain by grana- diorite and granodiorite gneiss. 3 INTRODUCTION Location and Geologic Setting The Belchertown Intrusive Complex, of probable Devonian age, consists of a major syntectonic batholith and related stocks and dikes located in the uplands of west-central Massachusetts, approximately 60 miles west of Boston and 12 miles southeast of Amherst (Fig. 1). The main portions of the batholith occupy an area of marked low relief of roughly 60 square miles which extends from the Chicopee River, near Three Rivers, northward to Belchertown center and the Quabbin Reservoir. The Complex lies near the axis of the Bronson Hill Anticlinorium, an en-echelon series of gneiss domes (Billings, 1956; Thompson et al., 1968) that extends from Long Island Sound to northwestern Maine (Fig. 1). The batholith intrudes late Precambrian dome core gneis~es (Naylor~ al., 1973) and Lower and Middle Paleozoic mantling rocks of the Pelham dome to the north and mantling rocks of the Glastonbury dome to the south (Hall, 1973) (Fig.2). To the east, the batholith intrudes metamorphosed Middle Ordovician and Lower Devonian sedimentary
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