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Feldspar megacrysts from the Santa Angélica composite pluton — Formation/transformation path revealed by combined CL, Raman and LA-ICP-MS data

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Type of Publication:
Feldspar geochemistry, Feldspar microtexture, Magma mixing, Crystallization/recrystallization, Composite pluton, Santa Angélica — Brazil
Słaby, Ewa; Campos, Cristina P. De; Majzner, Katarzyna; Simon, Klaus; Gros, Katarzyna; Moszumańska, Izabela; Jokubauskas, Petras
269 - 283
Eighth Hutton Symposium on Granites and Related Rocks
The studied feldspar megacrysts from the Santa Angélica hybrid rock unit feature complex growth morphologies and geochemical compositions. Early crystallization formed zoned K–Na alkali-feldspar and unzoned oligoclase–andesine. The chemical composition of the zoned alkali-feldspar reflects crystallization in contact with different magma batches, and the chemical composition of the plagioclase indicates growth from a homogeneous magma. Further feldspar development produced mineral chemistry and growth morphology patterns (in both alkali feldspar and plagioclase) that indicate a multi-stage process caused by the combination of chaotic mixing and replenishment. Alkali feldspar and plagioclase megacrysts show dissolution and regrowth textures. Their compositional fields are not separated. Different crystallized and recrystallized domains pass smoothly from one to another. A multiple reequilibration process limited the ordering of the feldspar domain structure. A younger generation of feldspar strongly enriched in K and Na grew over the dissolved margins of the alkali feldspar and plagioclase megacrysts, filling all embayments. Some domains of the alkali feldspar megacrysts recrystallized, resulting in a K- and Ba-rich composition. The trace element patterns of these domains define relatively consistent trends, which may indicate a new stage of equilibrium. In turn, the trace element patterns of previously crystallized and recrystallized domains are irregular, pointing to blurring of the original compositional relationship. Some voids in the megacrysts, formed due to dissolution, are cemented with quartz. The sequential early to late crystallization, recrystallization, and alteration processes are clearly revealed by Polytopic Vector Analysis (PVA). This analysis shows the changes in the alkali-feldspar and plagioclase compositions in two different directions: an initial shift from megacryst crystallization environment towards alkali-rich magma and a second shift towards silica-rich magma. The recognition of the whole sequence of the processes is possible due to the preserved relics of each phase in different feldspar domains.
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