pattison 2002

pattison 2002 - J. metamorphic Geol., 2002, 20, 99118...

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Thermodynamic modelling of the reaction muscovite + cordierite p Al 2 SiO 5 + biotite + quartz + H 2 O: constraints from natural assemblages and implications for the metapelitic petrogenetic grid D. R. M. PATTISON, 1 F. S. SPEAR, 2 C. L. DEBUHR, 3 J. T. CHENEY 4 ANDC .V .GU IDOTT I 5 1 Department of Geology & Geophysics, University of Calgary, Calgary, AB, T2N 1 N4, Canada ( 2 Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA 3 GR Petrology Consultants Inc., 4605 12 Street N.E., Calgary, AB, T2E 4R3, Canada 4 Department of Geology, Amherst College, Amherst, MA, 01002, USA 5 Department of Geological Sciences, University of Maine, Orono, ME, 04469±5790, USA ABSTRACT The reaction muscovite + cordierite p biotite + Al 2 SiO 5 + quartz + H 2 O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd + And ¡ Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pres- sure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlock- wise P±T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide con¯icting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replace- ment of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K 2 O±FeO±MgO±Al 2 O 3 ±SiO 2 ±H 2 O (KFMASH) which satis®es most of the natural and experi- mental constraints. Contouring of the Fe±Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet-absent andalusite + cordierite-bearing schists and hornfelses. The revised topology of reaction 1 allows for improved analysis of P±T paths from mineral assemblage sequences and microtextures in the same rocks. Key words: cordierite; metapelite; petrogenetic grid; phase equilibria; thermodynamics. INTRODUCTION The reaction involving the phases muscovite, cordierite, biotite, Al 2 SiO 5 mineral (andalusite or sillimanite), quartz and hydrous ¯uid: 2Ms ± 3 Crd ² KAl 3 Si 3 O 10 ³ OH ´ 2 ³ Fe ; Mg ´ 2 Al 4 Si 5 O 18 : nH 2 O 2Bt ± 8 And = Sil ± K ³ Fe ; Mg ´ 3 AlSi 3 O 10 ³ OH ´ 2 Al 2 SiO 5 7 Qtz ± 3n H 2 O SiO 2 ³ 1 ´ (balanced using idealized end members) is of consider- able importance to metapelitic phase equilibria; (1) its operation is implied in the assemblage muscovite + cordierite + andalusite ¡
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pattison 2002 - J. metamorphic Geol., 2002, 20, 99118...

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