Na-metamorphic pyroxenes in low-grade metabasites from the External Zones of the Betic Cordilleras (southern Spain): influence of rock chemical composition on their formation.

Abstract

ABSTRACT. Abundant metabasite blocks are present in the argillaceous sediments of the 'Trias de Antequera’ unit, in the External Zones of the Betic Cordilleras (Southern Spain). Two different geochemical affinities have been idel1tified in these metabasites: one tholeiitic and the other transitional-alkaline. Na-rich metamorphic pyroxenes are present in both types of metabasites, although with different textural arrangement and chemical characteristics. In the tholeitic metabasites Napyroxene is always present in veinlets, associated with albite and quartz. Compared to the fresh augite from which they derive, these pyroxenes are richer in Al2O3, FeOtot and Na2O, and poorer in TiO2, MgO and CaO. In the transitional-alkaline metabasites, the metamorphic pyroxenes result from epitaxial replacement of primary Ti-rich augite. This replacement may be complete, but always preserving the original ophitic texture. These metamorphic pyroxenes are richer in SiO2, FeOtot, and Na2O, and poorer in TiO2, Al2O3, MgO and CaO than the primary ones. The jadeitic contents in both types of metamorphic pyroxenes are similar, with maximum values of ca. 15-19%. As the maximum P-T conditions of metamorphism estimated for the tholeiitic and transitional-alkaline metabasites are very close, the influence of primary rock composition is advocated to explain the chemical differences between the two types of metamorphic pyroxenes. RESUMEN. Piroxenos sódicos metamórficos en metabasitas de bajo grado de las Zonas Externas de las Cordilleras Béticas (sur de España): influencia de la composición química de la roca en su formación. Numerosos bloques de metabasitas aparecen entre los sedimentos arcillosos del 'Trias de Antequera', en las Zonas Externas de las Cordilleras Béticas (surde España). En estas metabasitas se han evidenciado dos tipos de afinidades geoquímicas diferentes: una toleítica y otra transicional-alcalina. En ambos tipos aparecen piroxenos metamórficos ricos en sodio, pero con diferencias en cuanto a sus disposiciones texturales y a sus características químicas. En las metabasitas.