Phase Equilibria and Mineral Parageneses of Metabasites in Low-Grade Metamorphism

J. G. Liou, S. Maruyama and M. Cho
Department of Geology, Stanford University, Stanford, California 94305, USA

Abstract: The system Na2O-CaO-MgO Al2O3-SiO2-H2O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe2O3 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The XFe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-XFe3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean-floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpellyite and epidote with increasing metamorphic grade are oversimplified.

Keywords: metamorphism • metabasites • subgreenschist facies • zeolites • prehnite • pumpellyite • actinolite • epidote

Mineralogical Magazine; June 1985 v. 49; no. 352; p. 321-333; DOI: 10.1180/minmag.1985.049.352.03
© 1985, The Mineralogical Society
Mineralogical Society (