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Liou et al, 1985 relaciones de fase en metabasitas de bajo grado

Liou et al, 1985 relaciones de fase en metabasitas de bajo grado

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MINERALOGICAL MAGAZINE, JUNE 1985, VOL. 49, PP. 321 333 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 Na20-CaO MgO A1203 SiO2-H20 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 Fe203 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 quanti- tative effects on P-T displacement and phase composi- tions are discussed. The XFe 3 + isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T XFe~+ relations were established for continuous and discontinuous reactions relating such facies transi- tions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric A1-Ca-Fe 3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-A1 silicates in the presence of quartz, aibite, 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 pumpelly- ite and epidote with increasing metamorphic grade are oversimplified. KEYWORDS; metamorphism, metabasites, sub- greenschist facies, zeolites, prehnite, pumpellyite, actino- lite, epidote. VERY low-grade metamorphism and hydro- thermal alteration of basaltic rocks are universal processes; such processes have been increasingly recognized through detailed investigations of litho- logies from both divergent and convergent litho- spheric plate boundaries. Major portions of the oceanic crust appear to have been subjected to mid-oceanic, hydrothermal metamorphism im- mediately after their creation at a spreading ridge. When transported to the continental margin, the oceanic mafic rocks are again recrystallized at or near convergent plate junctions; the alteration and deformation trends depend on whether the oceanic <~ Copyright the Mineralogical Society crust participated in a collision with the continent or was subducted or overridden by a continental plate. The processes of low-grade metamorphism of basaltic crust, therefore, are a function of tectonic setting. Each setting has its own appropriate thermal gradients, fluid compositions and many other intensive parameters and hence metabasalt displays characteristic mineral parageneses and compositions.
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