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Vein Assemblages and Metamorphism in Dutchess County, New York ABSTRACT Others, 1972) and Shows the Study Area

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Vein Assemblages and Metamorphism in Dutchess County, New York ABSTRACT Others, 1972) and Shows the Study Area ROSEMARY J. VIDALE Department of Geology and Geophysics, State University of New York at Binghamton, Binghamton, New York 13901 Vein Assemblages and Metamorphism in Dutchess County, New York ABSTRACT others, 1972) and shows the study area. The Stuyvesant Falls, Mount Merino, Indian major geologic mapping of this area was River, and Austin Glen). Carbonate-rich Mineral assemblages of fissure veins in done by Barth (1936) and by Balk (1936) layers occur locally in pelitic rocks of both pelitic rocks are a consistent function of with significant additions by Fisher and autochthon and allochthon and are espe- metamorphic grade within a 1,700 km2 area others (1972). cially abundant near the base of the in and near Dutchess County, New York. The principal geologic units are the Walloomsac. The following vein assemblages are ob- Precambrian gneisses of the Hudson and Figure 2 shows metamorphic isograds in served with increasing grade: quartz and Housatonic Highlands; autochthonous this region. They were first mapped by Barth quartz-calcite up to just above the staurolite lower Paleozoic units, including Cambrian (1936, p. 777-779 and Plate 1) but have isograd, with limited occurrence of quariz- quartzite (Poughquag), Cambrian- been remapped (Vidale, in prep.). Barth's albite below the biotite isograd; quartz and Ordovician carbonate rock (Wappinger, isograds correspond accurately to first quartz-plagioclase (An20 to An50) from tie Stockbridge, and In wood), and Ordovician megascopic appearance of index minerals. staurolite isograde up to the sillimanite- pelitic rock (Walloomsac); and allochtho- Detailed thin-section examination of rocks orthoclase isograd; and quartz, quartz- nous Cambrian-Ordovician pelitic units with appropriate bulk chemical composi- plagioclase, and quartz-plagioclase- (Everett, Nassau, Germantown, Elizaville, tion for the first occurrence of index orthoclase above the sillimanite-orthoclase isograd. Extreme deformation makes rela- 73°30 tive crosscutting relations of veins difficult to determine; however, late quartz veins are fairly common, and late quartz-calcite veins are seen in high-grade rocks near marble contacts. A tentative conclusion is that quartz and quartz-calcite, quartz-plagioclase, and quartz-plagioclase-orthoclase veins form in pelitic rocks at successively higher metamorphic grades from material derived from the surrounding matrix. This may happen during prograde and retrograde l°45' metamorphism and during successive metamorphic events. INTRODUCTION The purpose of this study was to discover the nature and origin of veins and other coarse-grained segregations in pelitic rocks in a block of twelve 7lh' quadrangles in Dutchess and Putnam Counties, New York, and in Litchfield and Fairfield Counties, Connecticut. The segregations might reflect original layering, magma or aqueous fluids injected from outside the pelitic unit, material derived from the surrounding matrix during metamorphism, or some combination of these. Original layering or injected material would not be expected to vary systematically in bulk composition as a function of metamorphic grade, whereas material derived locally from the metamor- phic matrix could vary with grade. The l°30' backbone of this investigation, therefore, was field and thin-section observation of mineral assemblages in coarse segregations Allochthonous Cambrian - Ordovician Cambrian quartzite and comparison of their spatial distribution pelitic rocks to that of the regional metamorphic Autochthonous Ordovician Precambrian metamorphic pelitic rocks rocks isograds. Autochthonous and Allochthonous E2 Cambrian-Ordovician pelitic rocks LOCATION AND GEOLOGY Cambrian-Ordivican carbonate Gneisses of probable OF THE AREA m rocks Paleozoic age Figure 1 is simplified from the new Figure 1. Geologic map of Dutchess County, New York, and adjoining region (modified from Fisher and others, geologic map of New York State (Fisher and 1972). Quadrangle names indicated by locality designations. Geological Society of America Bulletin, v. 85, p. 303-336, 3 figs., February 1974 303 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/2/303/3429232/i0016-7606-85-2-303.pdf by guest on 01 October 2021 304 R. J. VIDALE 73°45 73-30' relative to the position of mapped isograds. A. secondary objective was to determine relative ages and vein types on the basis of crosscutting relations. However, the pelitic rocks of this region have been so strongly kneaded tectonically that nearly all segrega- tions are now discontinuous deformed plates and lenses in a micaceous matrix. Very few crosscutting relations can be seen, and one cannot be certain about most of these because of the strong deformation. A 4I°45 statistically significant number of observa- tions are required, and they were not achieved in this study. Determination of vein density (proportion of vein volume to matrix volume) also requires large-scale statistical treatment as well as correlation with position :.n major structural features. Most of the segregations range from a few millimeters to ~ 10 cm thick. There are some larger segregations, as much as ~30 cm :hick, and a few pegmatite bodies >3 m thick. Most of the segregations appear to lave been crosscutting veins before defor- mation; however, all observed coarse- grained regions in fold noses, boudin necks, en echelon gash fractures, and rolled garnet shadows were examined and included in the data. Except for the large pegmatite bodies, all sizes and forms of segregations at any given sampling locality contain the same assortment of mineral assemblages. For 4I°30 convenience, all will be called veins. The most practical sampling procedure proved to be examination and staining of I I Paleozoic pelitic rocks Isograds veins in the field. Large outcrops or roadcuts were used wherever possible. All veins were Contacts of lithologie units from Fig. 1 examined for geometry and mineralogy; an average of six were chosen for detailed study Vein Assemblages: at each outcrop. These included veins typical O Q, Q - Ce • Q, Q - Plag of the locality and any that appeared in any • Q, 0 - Ce, Q-Ab, Q-Ab-Cc A Q, Q-Plag, Q-Plag-Ksp way atypical. All were stained for plagio- clase and potassium feldspar, because S Q, O-Ce, 0-Ab, Q-Ab-Ce, • Rare muscovite also present Q-Ab-Ksp, Q-Ab-Ksp-Cc weathering and granulation commonly Figure 2. Metamorphic isograds and vein assemblages. Q, quartz; Ce, calcite; Ab, albite; Ksp, potassium feldspar; made distinctions among quartz and Plag, plagioclase in An20 to An50 range. feldspars uncertain. Hydrofluoric acid and cobaltinitrite and amaranth solutions were minerals has resulted in a general westward nowhere completely lost from the assem- used. Where any doubt remained (for shift of Barth's isograds. blage. The reaction producing orthoclase example, amaranth is Ca-specific and would All isograds except that of biotite is, approximately, muscovite + quartz = not be expected to stain highly albitic represent the first occurrence of the mineral sillimanite + orthoclase + H20. A reaction plagioclase), thin sections were cut. or assemblage. The biotite isograd marks the of this type, wh xh is dependent on 1 H2O Ninety-five localities in the pelitic units were occurrence of biotite in all pelitic outcrops. well as on bulk chemistry, would be studied in this manner; a concentration of Patchy, partially altered brown mica is expected to occur over a range of both localities was studied wherever assemblages found for several kilometers farther west. temperature ard total pressure. Similar seemed to be changing. In addition, thin This appears to be somewhat altered biotite; transition zones have been observed in other sections selected for accurate location of it shows bird's-eye extinction and gives an areas (see, for example, Guidotti, 1963). isograds were routinely cut across small electron-microprobe analysis much closer to The isograds are smoothed curves based segregations, and the mineral assemblages in that of biotite than of stilpnomelane. It is not on field observation and on study of more these segregations were checked for consist- detrital, because it cuts across detrital than 450 microscope sections. Sampling ency with the segregation assemblages seen layering. Probable stilpnomelane is also density, comb ned with limitations on in the field studies. present locally below the biotite isograd. exposure and bulk chemistry, is insufficient The sillimanite isograd marks the first to delineate small irregularities. VEIN ASSEMBLAGES occurrence of fibrolitic sillimanite. Nearly AND RELATIONS all sillimanite in this region is fibrolitic, and OBJECTIVES AND METHODS Figure 2, shows the field sampling no "isograd" can be mapped for the first The major objective of this study was to localities and indicates the types of vein occurrence of the massive variety. characterize all types and compositions of mineral assemblages seen at each locality. Occurrence of coexisting sillimanite and coarse-grained segregations in the pelitic No distinctions could be drawn on the basis orthoclase is very spotty; muscovite is lithologies and to note their distribution of size, shape, or structural environment; all Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/2/303/3429232/i0016-7606-85-2-303.pdf by guest on 01 October 2021 VEIN ASSEMB LAGES AND METAMORPHISM IN DUTCHESS COUNTY, NEW YORK 305 kinds of coarse-grained segregations con- tained the same assemblages. Quartz and quartz-calcite veins occur in every large outcrop
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