Fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists (Tauern Window,Eastern Alps) |
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| Authors: | Jane Selverstone James L Munoz |
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| Institution: | (1) Department of Earth and Planetary Sciences, Harvard University, 24 Oxford Street, 02138 Cambridge, MA, USA;(2) Department of Geological Sciences, University of Colorado, 80309 Boulder, CO, USA |
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| Abstract: | The assemblage hornblende+white mica occurs in graphite-free schists at two localities in the southwest corner of the Tauern Window, Eastern Alps. In interbedded graphitic layers (1 mm to 1 m thick), however, hornblende is typically replaced by pseudomorphs of biotite+plagioclase +epidote±chlorite+staurolite in the presence of white mica. Garnets adjacent to these pseudomorphs have pronounced growth discontinuities near their rims, in contrast to the continuously zoned garnets in nongraphitic layers. These observations imply that reactions of the type hbl+white mica gar+bio+plag+epid±chl±staur +H2O occurred in the graphitic samples, but that hbl+white mica remained stable in graphite-free layers.Calculation of the equilibrium constants for solid phases in five dehydration equilibria at locality 1 indicates thata(H2O) in the nongraphitic layers was 6 to 11 times greater thana(H2O) in the graphitic layers. Similar calculations involving six dehydration equilibria at locality 2 show no difference ina(H2O) between layers at the conditions of final equilibration. Initial differences in fluid composition maintained between the graphitic and nongraphitic layers caused the hbl+white mica reaction to occur at differentP-T conditions in different horizons of the schists.These data indicate that systematic differences in fluid composition were generated during metamorphism of the interlayered graphitic and non-graphitic schists but were subsequently homogenized at locality 2. The heterogeneities could initially have been produced while the rocks were in theP-T field of CO2-H2O immiscibility. Development of a penetrative, layer-parallel shear foliation at this time would have prevented subsequent mixing of the fluids across layers after temperatures exceeded the consolute temperature in the CO2-H2O system. Late-stage homogenization of fluids at locality 2 is thought to reflect loss of the buffer capacity of the mineral assemblage in response to total consumption of hornblende. |
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