Description, origin and significance of the layered texture in the Zebra pegmatite

GALLISKI, M.A.; MARTIN, ROBERT F.; MARQUEZ ZAVALÍA, M. F.

Córdoba

Congreso; XIII MinMet y IV PIMA; 2019

Asociación Mineralógica Argentina y Comisión de Petrología (COMPETRO) de la Asociación Geológica Argentina

Among the intragranitic pegmatites emplaced in the A-type Las Chacras- Potrerillos batholith, Sierra de San Luis, Argentina, the so-called Zebra pegmatite shows an atypical textural development and unexpected compositional attributes. Dozens of uniformly stacked centimetric layers consist of the association albite + K-feldspar - quartz - K-feldspar + albite, with accessory tourmaline in the quartz-rich layers. Crystallization began from the bottom upward, and initially produced a feldspathic layer 1-2 cm thick containing fine-grained albite followed by euhedral crystals of microcline one order of magnitude larger; this layer then gives way to an accumulation of quartz in larger domains that show optical continuity. Zoned prismatic crystals of tourmaline (schorl) are rooted in the layers of albite + K-feldspar; they are enclosed in quartz and are parallel to the feldspathic bands or exhibit a flaring habit. Scarce flakes of muscovite grew over some of the microcline crystals. The bulk composition of the layered rock is granitic and peraluminous. A normative composition gives 35.3 wt.% Or, 38.1% Ab and 21.3% Qz. If one assumes that such a bulk composition is that of a granitic magma, it could well be consistent with the location of a boron-enriched magma in the expanded haplogranitic system. Such a system can be expected to have a relatively low viscosity. The rhythmic layering can be attributed to repeated swings from a feldspar-saturated melt to a quartz-saturated melt. The magma became periodically saturated in schorl, which reduced the concentration of H2O and Na in the melt, and provoked the change to quartz as a liquidus phase in this small batch of magma, at which point H2O began to rise again. The presence of a small batch of corundum-normative granitic magma emplaced among NYF intragranitic pegmatites is unexpected. It could result from the generation of an anatectic magma generated at the expense of an enclave of wallrocks at depth, as the Las Chacras ? Potrerillos batholith was being emplaced.