Post-Taconic tilting and Acadian structural overprint of the classic Barrovian metamorphic gradient in Dutchess County, New York
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Abstract:
Field mapping and 40Ar/39Ar age spectrum dating of white mica across the classic metamorphic gradient in Dutchess County, New York indicate that post-peak metamorphic and structural events complicate the interpretation of this sequence as an intact Taconic metamorphic field gradient. Oriented samples were collected and fabrics were measured along two ∼7 km transects. These transects extend from west of the biotite zone (west of Clove Mountain) to the sillimanite zone (Swamp River Valley) in eastern Dutchess County. Field and petrographic analysis reveals three pervasive foliations (S1, S2 and S3). S1 is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher zones by a penetrative foliation, S2. S2 dips steeply to the SE and is axial planar to folds in S1. S2 is overgrown by porphyroblasts of biotite, biotite + chloritoid, biotite + garnet ± staurolite ± kyanite ± sillimanite, at respective metamorphic grades, that appear to approximate peak temperature conditions. S1, S2, and peak temperature conditions are constrained as late Taconic based on a depositional age of ∼460 Ma (Potter, 1972) for the Walloomsac Formation and a staurolite age of ∼454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S3 is a greenschist facies foliation composed of muscovite ± biotite ± chlorite that truncates or overprints S2 in garnet through sillimanite grade rocks in the eastern part of the study area. S3 dips gently to the S-SE and becomes more penetrative to the east, such that in the garnet zone, S2 is overprinted by a weakly developed spaced S3 cleavage, while in many samples in the staurolite and higher metamorphic zones S2 micas are significantly overprinted by S3 micas. S3 is observed wrapping around and truncating peak temperature porphyroblasts. Three samples of lepidoblastic white mica (with varying proportions of S1, S2, and S3) from the biotite through staurolite zone all produce 40Ar/39Ar age spectra suggesting closure ages of ∼385 Ma. These white mica ages are interpreted to represent the time of cooling through white mica closure (∼350 °C). The 40Ar/39Ar ages in conjunction with field mapping and petrographic analysis suggest that S3 developed in the Devonian Acadian orogeny. Furthermore, the 40Ar/39Ar data constrains the timing of tilting and uplift of the Ordovician Barrovian sequence from the Middle Silurian to Early Devonian. This study suggests mid-crustal wedging during the Salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the Taconic-aged Barrovian field gradient at the surface today. We conclude the low-grade Acadian metamorphic overprint of these rocks involved deformation that produced a new (third) cleavage that was coincident with regional cooling from peak Taconic metamorphic conditions and thus required strain but no reheating during the Acadian.Keywords:
Sillimanite
Staurolite
Isograd
Greenschist
Muscovite
Pelitic and calcareous rocks in the Whetstone Lake area have an unusually wide range of chemical composition. Metamorphic reactions have been deduced that represent the observed 'discontinuities' in compatible mineral assemblages, and by plotting the reactant and the product assemblage of each reaction on a map, metamorphic isograds have been delincated 'from both sides'. For the pelitic rocks, successively higher-grade isograds are based on the following reactions: (1)chlorite+muscovite+garnet⇌staurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz⇌ kyanite+biotite+water; (3) kyanite⇌sillimanite; (4)staurolite+museovite+quartz⇌sillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartz⇌Ca-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4) in such a manner as to indicate that the H2O/CO2 fugacity ratio was significantly higher in the vicinity of a granite pluton than in the metasedimentary rocks remote from the pluton. Chemical analyses of the coexisting minerals in reaction (5) indicate that the real reaction may involve plagioclase, epidote, sphene, and Fe-Ti oxides as well.
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A kyanite-sillimanite isograd is well-exposed in the Mica Creek area, British Columbia, and both pelitic and basic rocks occur within a few hundred meters of the isograd. This provides us with an opportunity to test geothermobarometry in pelitic and basic bulk compositions against an independent metamorphic equilibrium, kyanite-sillimanite. The kyanite-sillimanite isograd is modeled by a univariant P-T curve that passes through the following points (T °C, P kbar): 564/5; 610/6; 656/7; and 796/10. The quoted experimental uncertainty is 25 °C or about 350 J in ΔG. Samples that bracket the kyanite-sillimanite isograd in the Mica Creek area have the following mineral assemblages: quartzmuscovite- biotite-garnet-plagioclase-kyanite or sillimanite in pelitic rocks; and hornblende-plagioclase- clinopyroxene-garnet-quartz in basic rocks. Using the thermodynamic database of Berman (1988) and the INVEQ program of Gordon (1992), we have calculated metamorphic pressures and temperatures for 17 samples that bracket the kyanite-sillimanite isograd.
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Connemara pelites show progressive metamorphism from staurolite to upper sillimanite zones and possess low Mg/(Fe + Mg) values, typically 0.30 to 0.35 from about 100 analyses. As a consequence of their composition, many sillimanite zone pelites lack both muscovite and K-feldspar. Staurolite, garnet, biotite, muscovite, feldspars and iron ores have been microprobe analysed in 48 samples. Assemblages, textures and mineral compositions indicate that metamorphism followed a sequence of continuous and discontinuous reactions with systematic variations in mineral Mg/(Mg + Fe) as predicted by theory. Contrary to some common assumptions, most reaction takes place along divariant equilibria; univariant reactions are seldom reached because reactants such as chlorite or muscovite are first consumed along divariant curves. Pelite petrogenetic grids showing univariant curves can only indicate limits to natural assemblages; they typically do not show which reactions have actually taken place. Physical conditions of metamorphism have been calculated by a variety of means; temperatures range from 550° for the staurolite zone to 650° for the upper silimanite zone, with the first appearance of sillimanite near 580°. An early kyanite-staurolite metamorphism at pressures above about 5 kb was followed by a steepening of the thermal gradient leading to regional cordierite and andalusite. This was probably accompanied by uplift with pressures of around 4 kb for roeks near the sillimanite-in isograd.
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Staurolite
Sillimanite
Greenschist
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Metamorphic reactions related to isograds derived from aluminum silicate-bearing pelitic schists were studied in an area of Grenville Province adjacent to Southern Province rocks near Sudbury, Ontario. Progressing from the northwest to southeast of the area, the meaningful hand-drawn isograds are: (1) sillimanite first occurrence, (2) the last occurrence of staurolite when associated with the entire assemblage, (3) K-feldspar first occurrence, (4) staurolite last occurrence as inclusions in garnet, (5) muscovite last occurrence, and (6) kyanite last occurrence. Whole-rock chemical analysis of 14 representative pelitic schist hand specimens in the area were collected and used to show that metamorphic factors, and not chemical differences, were responsible for the metamorphic isograds. The entire area lies thermally above the melting of rocks of granitic composition. Breakdown curves of the minerals related to the isograds have been used to imply a gradient of 670 °C to 750 °C and 6.3 to 7.3 kilobars, across the area, but the equations for these breakdowns are not entirely substantiated by the modal abundance and textural data.To a first approximation, the rocks may be considered homochemical, but many deviations (due partly to metasomatic change) from this exist. The ionic breakdown of kyanite to muscovite has been shown and an explanation as to why muscovite selectively replaces kyanite and not sillimanite is given. The breakdown of muscovite at the higher grades has been inferred to form K-feldspar, but not sillimanite. Near the kyanite isograd, textures showing the thermal breakdown of kyanite (left over after the partial ionic breakdown of the mineral) to sillimanite are shown. The rocks must have had at least K and possibly Fe added metasomatically to account for the textures shown. From generalized modal abundance surfaces (trend surface analysis), general equations representing the difference in modal abundance of minerals across various isograds were determined and from these, specific equations explaining the breakdown of a particular mineral at its isograd were derived. The most significant of these reactions is the first staurolite isograd, where it is inferred to breakdown in the following way, in the area studied:[Formula: see text]The dissolved Al and Si forms the fibrolite (sillimanite) lenses common in adjacent pelitic rocks
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Main Central Thrust
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The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature, and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow’s type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical, and isotopic changes in the vein-proximal rock, indicative of fluid–rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid–rock interaction. Garnet-, staurolite-, and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades.
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Sillimanite
Geothermobarometry
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