CONICET | Buscador de Institutos y Recursos Humanos

The Asha pluton, the oldest unit of the San Blas intrusive complex (Early Carboniferous), exhibits impressiveexamples of magmatic structures formed by accumulation of K-feldspar megacrysts, enclaves, and schlieren.Almost all recognized structures are meter-scale, vertically elongate bodies of variable shapes defined as fingers,trails, drips, and blobs. They preferentially developed near the external margin of the Asha pluton and generallyare superimposed by chamber-wide magmatic fabrics. They mostly have circular or sub-circular transversesections with an internal fabric defined by margin-parallel, inward-dipping concentric foliation and steeplyplunging lineation at upper parts and flat foliation at lower parts. The concentration of megacrysts usually gradesfrom upper sections, where they appear in a proportion similar to the host granite, to highly packed accumulationsof K-feldspar along with grouped flattened enclaves at lower ends. These features suggest an origin bydownward localized multiphase magmatic flow, narrowing and ?log jamming?, and gravitational sinking ofgrouped crystals and enclaves, with compaction and filter pressing as main mechanisms of melt removal. Crystalsize distribution analysis supports field observations arguing for a mechanical origin of accumulations. Themagmatic structures of the Asha pluton represent mechanical instabilities generated by thermal and compositionalconvection, probably owing to cooling and crystallization near the pluton margins during early stages ofconstruction of the intrusive complex.

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