CONDITIONS OF FORMATION OF CHARNOCKITIC MAGMATIC ROCKS FROM THE VÁRZEA ALEGRE MASSIF, ESPÍRITO SANTO, SOUTHEASTERN BRAZIL

Abstract

The Várzea Alegre intrusive massif, in the central portion of the Espírito Santo State, is related to the late to post-tectonic Brasíliano magmatic arc in this region. It is an inversely zoned pluton with an almost circular shape, enclosed by ortho and paragneisses metamorphosed to the amphibolite and granulite facies. The pluton has a gabbroic center surrounded by diorites/quartz diorites, monzodiorites and megaporphyritic granites. All these magmatites are involved by a large and irregular ring of charnockitic rocks (opdalites, jotunites, opx-quartz diorites and quartz mangerites). The charnockites are composed of plagioclase, perthitic alkali feldspar/mesoperthite, orthopyroxene, biotite, hornblende, ilmenite, magnetite, pyrite, apatite, zircon, rare allanite and hematite. Chemical composition of these rocks points towards a high-K calc-alkalic magmatism enriched in LIL and HFS elements. Orthopyroxene (Wo 1.6-2.5 , En 30,4-41.2 to Fs 57-67.4 ) is partly replaced by biotite, amphibole and opaque minerals. Amphiboles (Mg-hastingsitic hornblende to magnesian hastingsite) show limited variations in the Al-contents. Biotite has compositions close to the annite end member, with high Ti-contents. The X Mg of orthopyroxene, amphibole and biotite are larger than those calculated for host whole rocks. Megacrysts feldspar and from the matrix present similar compositional variation. Plagioclase (Ab 65 An 32 to Ab 57 An 40 ) show small variations in the Or molecule. Compositional variations in the alkali feldspar ((Or 89 Ab 11 to Or 69 Ab 31 ) are associated to an unequal exsolution of the Na component. Zr and P 2 O 5 saturation levels of the Várzea Alegre charnockites indicate magmatic crystallization temperature around 950°C. Subsolidus temperatures, estimated through the ilmenite-magnetite and plagioclase-alkali feldspar pairs, are between 550°C and 630°C. Estimated crystallization pressure ranges from 6.5 to 7 kbar. Calculated fO 2 is consistent with highly reducing conditions, which is confirmed by the low Mg/(Mg+Fe) ratios of the rocks.