Crystallization conditions of two adjacent epidote + diopside-bearing granitic stocks, northeastern Brazil

Abstract

The Tamboril and Olho d’Água granitic stocks are part of the abundant calc-alkalic magmatic epidote-bearing granitic rocks in the Cachoeirinha–Salgueiro Terrane (CST) in the Transversal Zone Domain, northeastern Brazil. The equigranular Olho d’Água stock is composed of medium-grained clinopyroxene-amphibole–biotite tonalite; the porphyritic Tamboril stock is medium- to coarse-grained amphibole biotite ± clinopyroxene granodiorite. Abundances of clinopyroxene and epidote vary inversely in both stocks. Amphibole–rich clots are regarded as fragments from the source region captured by granodioritic/tonalitic magma during its ascent. Epidote composition in the Olho d’Água stock (Ps₁₈–₂₆) and in Tamboril stock (Ps_17-20) is consistent with crystallization under oxygen fugacity between QFM and HM buffers. In the Olho d’Água stock, calculated values of pressure range from 5.1 to 6.6 kbar and in the Tamboril stock from 6.2 to 7.0 kbar. Solidification temperatures estimated from plagioclase-hornblende pairs in the Olho D´Água stock range from 637 to 679 °C and for Tamboril from 587 to 641 °C. Zr-saturation temperature estimates are 788 to 819 °C (Olho d’Água) and 807 to 829 °C (Tamboril). Altogether our data suggest that the studied stocks crystallized from two distinct magmatic pulses formed from fractional melting of a single amphibolitic source. These two magma pulses underwent subsequent crystallization, in a convective magmatic chamber, at rather high pressure.

Key words
calc–alkalic granites; magmatic epidote; clinopyroxene; thermobarometry; oxygen fugacity