PETROLOGY OF THE CERRO MORADO BASIC MAGMATISM IN THE ISCHIGUALASTO-VILLA UNIÓN TRIASSIC BASIN (NW ARGENTINE)

Abstract

Studies carried out in Cerro Morado, situated in the Ischigualasto - Vale de la Luna Provincial Park (northernmost San Juan Province - Argentina), have allowed the identification and characterization of silica undersaturated hypabyssal alkaline basic rocks. This magmatism is probably related to an extensional reactivation of Paleozoic lineaments due to the breakup of the Gondwana supercontinent. The Cerro Morado hypabyssal rocks occur as a conformable body intrusive in sedimentary rocks of the Chañares Formation (Middle Triassic) of the Ischigualasto - Villa Unión Triassic Basin. The basal portion of the sill is formed by olivinebearing, dominantly porphyritic diabase, where plagioclase, clinopyroxene, and rare olivine phenocrysts are set in a hypocrystalline, fine-grained matrix of the same composition, plus opaque minerals, apatite and glass. Trachytic, intergranular and ophitic textures are also common. The intermediate and upper portions comprise mainly porphyritic tephrytic rocks, with plagioclase, feldspathoid, and clino-pyroxene phenocrysts set in a fine-grained matrix of the same composition. Chemical classifica-tions of the Cerro Morado rocks must be applied with care, due to their high L.O.I values. The behavior of major and trace elements suggests that mineral fractionation was the main magmatic differentiation process. The LILE, HFSE and REE contents are very similar to those of intra-plate alkaline basalts. The high LILE and HFSE contents, associated to incompatible element-ratios suggest a heterogeneous and enriched mantle source. The enrichment in Sr, K, Rb, and Ba may result from modification of a mantle source by aqueous fluids driven off the subducted oceanic crust. Rb-Sr and Sm-Nd preliminary data suggest mantle sources, including a HIMU isotopic reservoir, and minor influence of an enriched-mantle component.