HR: 1330h
AN: V22D-0608    [PDF]
TI: Modelling retrogressive reaction microstructures: a case study concerning staurolite to sericite reaction and the control of matrix composition on it
AU: * Sassi, R
EM: raffaele.sassi@unipd.it
AF: Department of Mineralogy and Petrology, University of Padova, Corso Garibaldi 37, Padova, 35137 Italy
AU: Mazzoli, C
EM: claudio.mazzoli@unipd.it
AF: Department of Mineralogy and Petrology, University of Padova, Corso Garibaldi 37, Padova, 35137 Italy
AU: Lombardo, T
EM: lombardo.tom@tin.it
AF: Department of Mineralogy and Petrology, University of Padova, Corso Garibaldi 37, Padova, 35137 Italy
AB: Pseudomorphic sericite aggregates after staurolite have been observed in garnet + staurolite $\pm$ kyanite micaschists from the Austroalpine basement outcropping in the Quaira (Karbach) Valley (Eastern Alps, Bolzano, Italy). Microstructural analysis, mineral assemblages and geothermobarometry give constraints on the Variscan metamorphic peak conditions (T = 600-650$\deg$C; P = 0.7-0.8 GPa) and on the subsequent retrogressive P-T path. Reaction microstructures have been analysed by optical microscopy and SEM, and a developing model has been proposed. Such a model resulted to be satisfactory although some approximations have been introduced. These are related to inhomogeneous distribution of mineral phases in the matrix and to the exclusion of some mineral phases (garnet and plagioclase) believed to be not relevant to the pseudomorph formation. The following reaction is predicted by the model: 1.00 St + 2.42 Bt + 4.76 Qtz + 26.2 H2O = 2.73 Ms + 3.56 Chl + 0.24 Ilm. Congruently to observations, this reaction predicts that staurolite porphyroblasts decomposition to sericite is strictly connected to biotite decomposition to chlorite + ilmenite in the matrix. In fact, although channelised fluids probably drove the reaction, the extent of staurolite decomposition resulted to be controlled by the availability of biotite in the matrix.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting