K-feldspar megacryst accumulations formed by mechanical instabilities in chamber 2 margins, Asha pluton, NW Argentina

ROCHER, S.; ALASINO P.H.; MACCHIOLI GRANDE, M.; LARROVERE M.A.; PATERSON, S.

JOURNAL OF STRUCTURAL GEOLOGY

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2018

0191-8141

The Asha pluton, the oldest unit of the San Blas Intrusive Complex (Lower Carboniferous), exhibits impressive amples of magmatic structures formed by accumulations of K-feldspar megacrysts, enclaves, and schlieren. Almostall recognized structures are meter-scale, vertically elongated bodies of variable shapes, defined as fingers, trails, blobs, and drops. They preferentially developed near the external margin of the Asha pluton and generally occur superimposed by chamber-wide magmatic fabrics. They mostly have circular or sub-circular transversal sections with an internal fabric defined by margin-parallel, inward-dipping concentric foliation, with steeply plunging lineation at upper parts and flat foliation at lower parts. The concentration of megacrysts usually grades from the upper part, where they appear in a proportion similar to the host granite, to highly packed accumulations of K-feldspar along with grouped flattened enclaves at its lower end. These features suggest an origin by downward localized multiphase magmatic flow, narrowing and ?log jamming?, and gravitational sinking of grouped crystals and enclaves, withcompaction and filter pressing processes as main mechanisms of melt removal. Crystal size distribution analysis supports field observations, which argue for a mechanical origin of accumulations. The magmatic structures of theAsha pluton represent mechanical instabilities generated by thermal and compositional convection, probably owing to cooling and crystallization near the pluton margins during early stages of construction of the intrusive complex.