Evolução magmática do Sill de Limeira: petrografia e geoquímica / Magmatica evolution of the Limeira Sill: petrography and chemistry

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The Limeira Sill exhibits a wide and continuous compositional variation, between basalt at the chilled margins and coarse-grained quartz monzodiorite in the innermost part currently exposed in the quarried where it is exploited. Below the top basalt border there is a layer rich in amygdales filled by hydrothermal minerals, followed downwards by the appearance of quartz monzonitic occelli. Throughout the sill occur rhyolitic (less often quartz monzonitic) veins oriented preferentially normal to the chilled margins. The rocks are composed mostly of plagioclase, clinopyroxene (augite ± pigeonite) and/or amphibole, Ti-magnetite, ilmenite, plus quartz and alkali feldspar (in the more differentiated rocks). Accessory minerals include apatite, filossilicates, zircon, baddeleyite, sphalerite, pyrite and allanite; hydrothermal minerals are zeolites, calcite and apophylite. Augite compositions vary from Fs~20 in the more primitive rocks to Fs40 in the more differentiated (quartz monzodiorite to rhyolite). Plagioclase varies from labradorite to oligoclase, with predominance of andesine An50-30 in the more abundant rocks. The whole rock chemistry reveals a continuous differentiation trend with compositions between the border basalt (~48 wt% SiO2) and the quartz monzodiorite (~61 wt% SiO2); a gap between quartz monzodiorite and rhyolite is identified in the 61-69 wt% SiO2 interval, but quartz monzonites with 63-64 wt% SiO2 appear as small-volume veins and occelli. The Ca, Mg, Ti and Fe contents show a trend of continuous decrease with differentiation, while K shows a continuous increase, and Na and Al are nearly constant, reaching maximum value in the quartz monzonites. Ba, Rb and Zr show incompatible behavior, while Co, Cr and Sr are typically compatible. The REE patterns are fractionated (LaN/YbN~12), and show enrichment up to the quartz monzodiorite; in more differentiated rocks they begin to decrease, especially the medium REE, reflecting extraction of clinopyroxene. The differentiation of the Limeira Sill appears to be a reflection of crystal fractionation, as suggested by consistent results in mass balance calculations, both for the initial stage (basalt to quartz monzodiorite) and for the generation of residual liquids (quartz monzonite and rhyolite), the latter probably involving some sort of filter pressing. In a model of magma chamber where crystallization occurs at the margins and residual liquids are generated in the solidification fronts, the occelli appear to be products of upward migration of these liquids. Later in the evolution of the chamber, the residual liquids extracted from these fronts would have percolated fractures in portions already solidified, forming the rhyolitic veins.

ASSUNTO(S)

sill de limeira petrografia rock chemistry química de rocha limeira sill petrography

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