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

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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

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kinds of coarse-grained segregations con- tained the same assemblages. Quartz and quartz-calcite veins occur in every large outcrop up to approximately the staurolite isograd. Quartz dominates in pelitic rocks. Carbonate rocks were not studied systematically, but they contain veins of quartz, quartz-calcite, and calcite. Pure calcite veins were nowhere seen in pelite. Vein calcite appears to contain significant amounts of iron in some areas; the action of concentrated HChwater in a 1:1 ratio produced a deep yellow solution. Dolomite is common locally in the matrix of some cf the pelitic units, but it was not observed in B the veins in field studies. (All vein carbonates reacted vigorously with acid, 1:1, and wen: considered on that basis to be calcite rather than dolomite.) Minor dolomite was de- tected, however, by microprobe analysis in tiny fissures (1 mm thick) cutting across one sample taken near the biotite isograd. It wan also present in the matrix of that sample. Quartz-albite and quartz-albite-calcite veins occur at eight localities in a belt ~5 km wide just below the biotite isograd. Electron microprobe studies show both the vein and matrix plagioclase to be nearly pure albite (Anotol). The only Ca-bearing phase i<; calcite. Downgrade from this belt there is; detrital plagioclase in the matrix and only quartz and calcite in the veins; above the belt, plagioclase of widely varying An con- 0 1 CM. tent appears in the matrix, and, again, only - L- quartz and calcite appear in the veins. Two albite or other of the eight localities also contain potassium | | quartz | orthoclase feldspar in the veins, in all combinations plagioclase matrix with the other minerals. The rock at these Figure 3. Vein textures traced from micrographs. A and B, Quartz-albite veins below biotite isograd. Vein in A is two localities is high in potassium, and symmetric; texture suggests two-stage opening process: early growth of central grains that are now partially potassium feldspar appears in the matrix. recrystallized, and later growth of tiny parallel grains at edges. Veins in B are crosscutting; texture suggests progressive recrystallization after vein formation. These textures are also common in quartz and quartz-calcite veins below biotite Many of the smaller veins in samples of isograd. C, Quartz-plagioclase vein boundary just above sillimanite isograd. Nearly severed inclusion of matrix metamorphic grade, up to about the biotite material extends perpendicularly out into vein. D, Quartz-plagiodase-orthoclase vein boundary just above isograd, consist of greatly elongated single sillimanite-orthoclase isograd. crystals of quartz, albite, potassium feldspar, or calcite that extend all the way adjacent matrix. Both the concentration of Above the sillimanite-orthoclase isograd, across the vein. Optical orientation seems to veins and the proportion of quartz and quartz and quartz-plagioclase veins are be random. Length to width ratios greater plagioclase in them vary greatly within each always present, and quartz-plagioclase- than 10:1 are very common. A and B of outcrop, but quartz veins seem to predomi- orthoclase veins are also found where Figure 3 show this texture in microfissures. nate. In addition, a few crosscutting quartz orthoclase appears in the matrix. Com- It has been observed in veins up to 4 cm veins and networks of tiny, intact, monly, more orthoclase seems to be in the thick. Similar textures have been described quartz-filled microfissures seem to mark a vein than in the matrix. C and D of Figure 3 by Chidester (1968, p. 351) for serpentine late stage of vein formation at some show typical quartz-plagioclase and veins in deformed ultramafic bodies. localities. quartz-plagioclase-orthoclase vein textures. From approximately the staurolite iso- The transition between quartz-calcite and Muscovite is rarely present (in perhaps 1 grad up to the sillimanite-orthoclase iso- quartz-plagioclase veins occurs over a percent of the veins) but may occur as one or grad, all large outcrops contain quartz and distance of ~1 km. Calcite seems to more large crystals. Biotite is even rarer, but quartz-plagioclase veins. A few localities disappear first, then a concentration of medium-sized crystals are found occasion- that are close to marble or calcite-bearing plagioclase is observed in the matrix ally. Significantly larger matrix minerals calc-silicates contain late, crosscutting adjacent to the quartz veins and then in the tend to form in the matrix at the edges of quartz-calcite veins. The An content of borders of the veins, and, finally, large veins and in matrix inclusions in veins. plagioclase in the matrix ranges from An20 to plagioclase grains appear within the veins. These bordering regions and inclusions at least An50. Microprobe and optical studies Only the third stage was mapped; however, contain aggregates of all matrix phases, with show vein plagioclase to be similar in the other stages were extremely useful in matrix textures, and are not considered part composition to matrix plagioclase (within locating the transition. Quartz, calcite, and of the vein assemblage. ±An5). More detailed microprobe studies plagioclase were observed together in this Pegmatite bodies >3 m thick were on two samples indicate that the vein transition in only three veins. The calcite examined at four localities. Two of these are plagioclase may be 2 to 3 percent lower in appears altered, but altered vein calcite is below the garnet isograd and contain only An content than is the plagioclase in the common throughout the region. quartz, with possible weathered-out calcite.

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A third, between the sillimanite and are more equart and protrusions of the ACKNOWLEDGMENTS sillimanite-orthoclase isograds, contains matrix and separated pieces of it commonly quartz, plagioclase, and potassium feldspar, extend perpend cularly out into the vein This work was supported by National in about equal proportions, and smaller (Fig. 3, C and D). These textures suggest Science Foundation Grant GA—30862. I amounts of muscovite, biotite, and garnet. A addition of matrix material to the edges of thank Yngvar Isachsen and Donald Fisher of fourth, on the sillimanite-orthoclase iso- the segregation as the vein slowly opened. the New York Geological Survey for grad, cuts an amphibolite body and con- No texture suggesting flow through a vein assistance in initiating field work; Chris tains quartz and plagioclase, in equal pro- has been observed. The ubiquitous presence Powell and Pat Dodd for field assistance; portions, and occasional masses of large of calcite in veins in pelites at low grades, Arden Albee, Alfred Chidester, Yngvar potassium feldspar crystals. The proximity however, and the absence of C02-bearing Isachsen, and Donald Fisher for suggesting of the amphibolite here may have strongly phases at higher grades require some significant improvements in this manu- influenced potassium feldspar distribution transport of fluid through the rock. script; and Frank Sedlak for preparing the (Vidale, 1969, p. 864-869). The potassium Possible modes of origin considered for thin sections. feldspar "veins" indicated in Figure 2 at one the veins are metamorphic modification of locality in about the middle of the Pawling sedimentary layers, introduction of vein quadrangle appear to be chunks broken material from outside the local system (as a REFERENCES CITED from a fifth pegmatite body in a highly melt, or transported by an aqueous Balk, R., 1936, Structural and petrologic studies deformed matrix. Thus, at least one solution), metasomatic transfer of locally in Dutchess County, New York: Part I: pegmatite body is not consistent in mineral- derived materie., or some combination of Geologic structure of sedimentary rocks;: ogy with other coarse-grained segregations. these. The close correlation of mineral Geol Soc. America Bull., v. 47; p. 685-774, A metamorphic differentiation layering assemblage with metamorphic grade, cou- Barth, T.F.W., 1936, Structural and petrologic on a scale of 2 to 10 mm appears in these pled with the textural evidence, strongly studies i:ti Dutchess County, New York: Part II: Petrology and metamorphism of the suggests metasomatic transfer from the pelitic rock units, from metamorphic grades Paleozoic rocks: Geol. Soc. America Bull., near the sillimanite isograd on up. The surrounding matrix. This is further sup- v. 47, p. 775-850. layering is gradational, whereas the veins are ported by Garlick and Epstein's (1967) Chidester, Alfred H., 1968, Evolution of the discreet; however, the layering can be oxygen isotope data, which shows isotopie ultramafic complexes of northwestern New described completely by changes in the equilibration between vein and matrix England; in Zen, E-an, White, W. S., proportion of minerals observed in veins to assemblages. Long's (1962) age dates for Hadley, J. B., and Thompson, J. B., eds., other matrix minerals. Thus, between meta- this area suggest, however, the imprint of Studies of Appalachian geology, northern morphic grades marked by the silliman- both Taconic (~450 m.y.) and Acadian and maritime: New York, Interscience ite and the sillimanite-orthoclase isograds, a (~350 m.y.) tietamorphic events, with Pubs., Inc., 475 p. change in the proportion of quartz and Taconic predominating to the west and Fisher, D., Isachsen, N. W., and Rickard, L. V., 1972, Geologic map of New York, 1970: plagioclase to other matrix minerals consti- Acadian to the east. Two distinct genera- New York State Mus. and Sei. Service Map tutes the layering. At metamorphic grades tions of biotite are common between biotite and Chart Ser., no. 15, scale 1:250,000. above the sillimanite-orthoclase isograd, a and sillimanite isograds and may reflect thè Garlick, G. D., and Epstein, S., 1967, Oxygen change in the proportion of quartz, dual metamorphism. It is quite possible, isotope ratios in coexisting minerals of plagioclase, and orthoclase to other matrix therefore, that the vein and matrix assem- regionally metamorphosed rocks: Geochim. minerals constitutes the layering. Although blages reported here were produced during et Cosmochim. Acta, v. 31, p. 181-214. this layering is gradational, its correlation at least two successive, geographically Guidotti, Charles V., 1963, Metamorphism of the with vein assemblages and with metamor- overlapping, metamorphic events. pelitic schists in the Bryant Pond quad- rangle, Maine: Am. Mineralogist, v. 48, p. phic grade suggests that a genetic relation The simplicity of the vein assemblages may exist (Vidale, 1974). 772-791. suggests that an unusually large number of Korzhinskii, D. S., 1936, Mobility and inertness The vein minerals reported here include chemical components were mobile in this of components in metasomatosis: Akad. all those found in pelitic rocks of this study space in the rock, in the sense that their Nauk SSR Izv. Ser. Geol., no. 1, p. 58-60. area. However, I have also observed vein chemical potentials were controlled outside 1959, Physiocochemical basis of the analysis magnetite and pyrite in the Michigamme the vein assemblages (Korzhinskii, 1936, of the paragenesis of minerals (English trans- Schist of northern Michigan, ilmenite and 1959, p. 14-19; Vidale and Hewitt, 1973). lation): New York, Consultants Bureau, andalusite in Vermont, kyanite in western Simple flow of pore fluid into the open space Inc., 142 p. Connecticut, and sillimanite in eastern as a fissure opened up would not transport Long, Leon, 1962, Isotopic age study in Dutchess Count)', New York: Geol. Soc. America Connecticut — all in rocks in the pelitic sufficient amounts of vein mineral con- Bull., v. 73, p. 997-1006. range. Quartz and feldspar seem, in general, stituents to fill the fissure unless a circulating Vidale, R. J., 1969, Metasomatism in a chemical to be the most common vein minerals in pore fluid was involved. There is no gradient and the formation of calc-silicate pelitic rocks, but other minerals may be evidence for such circulation in these rocks, bands: Am. Jour. Sei., v. 267, p. 857-874. present. but it must remain a possibility. Diffusion of 1974, The relationship of "metamorphic dif- components through a relatively static pore ferentiation" layering to vein mineral as- CONCLUSIONS fluid requires gradients in their chemical semblages in Dutchess County, New York, Veins in the pelitic rocks of the Dutchess potentials. These could be generated by in Hoffman, A. L., Yoder, H. S.,Giletti,B. J., County area contain simple mineral assem- gradients in temperature, total pressure, and Yund, R. A., eds., Symposium on geochemical transport and kinetics: blages that differ as a function of meta- fluid pressure, and chemical composition. A Washington, D.C., Carnegie Inst, (in press). morphic grade. Minerals in a vein are also likely possibility in a slowly opening fissure Vidale, R. J., and Hewitt, D. A., 1973, "Mobile" always present in the matrix, and segrega- is a local gradient in total pressure. An components in the formation of calc-silicate tions of all shapes and of all sizes up to experimental and theoretical study of bands: Am. Mineralogist (in press). several tens of centimeters contain the same pore-fluid compositions in pelitic rocks as a assortment of mineral assemblages. At function of pressure is now being done MANUSCRIPT RECEIVED BY THE SOCIETY MAY 7, grades up to a little above the biotite isograd, (Vidale, in prep.) to provide data on the 1973 grains commonly extend across the vein activity gradients that might cause diffusion REVISED MANUSCRIPT RECEIVED AUGUST 21, (Fig. 3, A and B). At higher grades, grains of material into an opening fissure. 1973 Printed in U.S.A.

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