Journal of Petrology, Volume 39, Issue 7: July 1 1998.

Singular Equilibria in Paragonite Blueschists, Amphibolites and Eclogites

J. F. MOLINA¹ AND S. POLI²

¹DEPARTAMENTO DE MINERALOGA Y PETROLOGA, CAMPUS DE FUENTENUEVA S/N, UNIV. GRANADA, 18002 GRANADA, SPAIN ²DIPARTIMENTO SCIENZE DELLA TERRA, UNIVERSITÀ DI MILANO, VIA BOTTICELLI 23, 20133 MILANO, ITALY

RECEIVED JULY 20, 1997; REVISED TYPESCRIPT ACCEPTED FEBRUARY 2, 1998

Chemographic constraints from high-pressure mafic rocks of the Nevado-Filábride Complex (Betic-Rif mountain belts, SE Spain) and from worldwide high-pressure terrains have been used to unravel phase relationships between amphibole, garnet, clinopyroxene, and albite in the system Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O (NCFMASH) at quartz-, paragonite-, clinozoisite- and water-saturated conditions. Extensive analysis of element partitioning between amphibole and clinopyroxene suggests that the Fe-Mg partition coefficient is low at low-temperature and high-pressure conditions (`high-temperature' blueschists-low-temperature eclogites) and a progressive increase is achieved by an increase in temperature, or by a decrease in pressure, or by a decrease in the sodium content of the clinopyroxene. Mass-balance analysis of paragonite eclogites and amphibolites suggests that at least two singular reactions are possible in NCFMASH: clinopyroxene + paragonite + quartz = amphibole + albite + clinozoisite + water (garnet-absent), and amphibole + clinozoisite = garnet + clinopyroxene + paragonite + quartz + water (albite-absent). Albite-absent singular equilibrium is relevant for the blueschist-eclogite transition, whereas garnet-absent equilibrium may control the eclogite to amphibolite transition. A complex arrangement of divariant reactions, singular reactions and conjugate non-degenerate reactions arises in topologies at the blueschist-eclogite-amphibolite transitions. Schreinemakers' analysis in P-T diagrams, and mixed-variable T-X_M[prime]-X_C[prime] and P-X_M[prime]-X_C[prime] diagrams suggest that complex pressure-temperature paths are not mandatory if a recurrent appearance and disappearance of similar phase assemblages is observed in paragonite-bearing mafic rocks. The P-T location of the singular curve net is discussed in the context of Nevado-Filábride Complex mafic rocks.

Keywords: blueschist;eclogite;amphibolite; singular reactions; topology

Pages 1325-1346

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