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. In spite of extensive investigation of low-grade metabasites over the last two decades, characteriza-
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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

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