ABSTRACT In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn- to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.

RESUMO: Este trabalho apresenta o estudo de petrografia, química mineral e geocronologia de escarnitos gerados no contato de mármores do Complexo Paraíba do Sul com diques metamáficos e félsicos, no sul do Espírito Santo. Os mármores foram metamorfizados sob condições de pressão e temperatura da fácies granulito durante a fase sin-colisional do orógeno neoproterozoico Araçuaí. Os corpos metamáficos são compostos de anfibolito e hornblenda granofels, enquanto os diques félsicos consistem de álcali-feldspato granito, monzogranito ou sienogranito. Do mármore para o dique, escarnitos associados com os diques metamáficos são compostos das zonas carbonato + olivina e diopsídio + hornblenda. Escarnitos associados com os diques graníticos são compostos de três zonas mineralógicas distintas: carbonato + tremolita, diopsídio e escapolita + diopsídio. Variações na composição química mineral ao longo das zonas metassomáticas sugerem introdução de Mg e Ca dos mármores, Fe dos diques metamáficos e Na dos granitos. A presença de espinélio nos diques metamáficos e em seus escarnitos indica que ambos foram metamorfizados sob condições de fácies granulito durante o estágio sin-colisional (580-560 Ma). A geocronologia U-Pb via LA-ICP-MS em zircões de um dique de álcali-feldspatogranito resultou em uma idade de cristalização de ca.540 Ma, o que sugere que seus escarnitos são, portanto, mais novos que os escarnitos associados com os diques metamáficos sin-colisionais.

ABSTRACT: This paper concerns the study of petrography, mineral chemistry and geochronology of skarns generated at the contact of marbles of the Paraíba do Sul Complex with felsic and metamafic dykes in the southern Espírito Santo State. The marbles were metamorphosed under P-T granulite facies conditions during the syn-collisional stage of the Neoproterozoic Araçuaí orogen. Metamafic bodies are composed of amphibolite and hornblende granofels, while felsic dykes consist of alkali-feldspar granite, monzogranite or syenogranite. From marble towards the dyke, skarns related to the metamafic bodies are composed of carbonate + olivine and diopside + hornblende zones. Skarn associated to the granitic dykes are composed of three different zones: carbonate + tremolite, diopside, scapolite + diopside. Variations in mineral chemical compositions along the metasomatic zones suggest introduction of Mg and Ca from the marbles, Fe from the metamafic dykes and Na from the granitoids. The presence of spinel in the metamafic dykes and their skarns indicates that both were metamorphosed under granulite facies conditions during the 580-560 Ma syn-collisional stage. U-Pb zircon geochronology (LA-ICP-MS) of an alkali-feldspar granite dyke resulted in a crystallization age of ca.540 Ma, which suggests that its skarns are therefore younger than skarns associated with the syn-collisional metamafic dykes.

Abstract The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulites as well as lower grade gneisses and mylonites. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometric calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulites based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 ºC and pressures between of 5.0 and 9.9 kbar and a retrometamorphic path characterized by near-isobaric cooling. Part of the granulites was affected by anatexis. The melting of felsic granulites resulted in the generation of pegmatites and two aluminous lithotypes. These are: i) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and ii) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronologically related to the lower grade Mantiqueira Complex.

Na região de Lagoa Dourada, inserida no Cinturão Mineiro, ocorrem granitóides paleoproterozóicos, que intrudem seqüências supracrustais arqueanas do greenstone belt Rio das Velhas. Os granitóides são cortados por rochas máficas anfibolitizadas, que portam xenólitos de granulitos. Trata-se de granulitos máficos caracterizados pela textura granoblástica e pela paragênese ortopiroxênio-clinopiroxênio-anfibólio-plagioclásio. Como fases secundárias, registram-se actinolita, cummingtonita, carbonato, epídoto, granada e quartzo. Determinações geotermométricas baseadas nos pares minerais ortopiroxênio-clinopiroxênio e anfibólio-plagioclásio produziram valores de temperatura entre 700 e 853ºC. As fases secundárias são produtos de um processo metamórfi co posterior.

Paleoproterozoic granitoids intruding the Archean greenstone belt of Rio das Velhas are found in the region of Lagoa Dourada, which belongs to the Mineiro Belt. The granitoids are cut by amphibolitized mafi c rocks which contain xenoliths of granulite facies rocks. The rocks are mafi c granulites, characterized by their granoblastic texture and by parageneses orthopyroxene-clinopyroxene-amphibole-plagioclase. As secondary minerals, actinolite, carbonate, epidote, cummingtonite, garnet and quartz can be found. Geothermometric determinations based on the mineral pairs orthopyroxene-clinopyroxene and amphibole-plagioclase yielded temperature values between 700ºC and 853ºC. The secondary mineral phases are interpreted as a result of a younger metamorphic event.

O Complexo Acaiaca abrange uma região composta, predominantemente, por rochas de fácies granulito e essa região está localizada próximo à cidade de Acaiaca, Minas Gerais. O Complexo se estende por, no mínimo, 36 km na direção norte-sul e atinge cerca de 6 km de largura na porção central. Granulitos félsicos (biotita granulitos e charnockitos), granulitos máficos (piroxênio ± hornblenda granulitos), granulitos ultramáficos (olivina-piroxênio granofels) e granulitos aluminosos (granada-sillimanita ± cordierita granulitos e granada-ordierita-cianita granulitos) são litotipos encontrados nesse Complexo. Os granulitos félsicos são derivados de rochas de composição riolítica. Os máficos são derivados de rochas quimicamente semelhantes a basaltos de ambientes de arco de ilhas. Os granulitos aluminosos apresentam protólito sedimentar pelítico a grauvaquiano. O olivina-piroxênio granofels possui composição química semelhante a harzburgitos. As paragêneses minerais indicam metamorfismo de fácies granulito de pressão intermediária para formação das mesmas. Os gnaisses de fácies anfibolito encontrados na área do Complexo Acaiaca comumente apresentam microestruturas miloníticas e evidências de geração por retrometamorfismo a partir dos granulitos em zonas de cisalhamento.

The Acaiaca Complex, located near the town of Acaiaca, Minas Gerais, is composed predominantly of granulite facies rocks. The Complex extends for at least 36 km in the N-S direction and is about 6 km wide in the central area. Felsic (biotite granulites and charnockites), mafic (pyroxene ± hornblende granulites), ultramafic (olivine-pyroxene granofels) and aluminous granulites are the main rock types. Felsic granulites are derived from rocks of rhyolitic composition. Mafic granulites are derived from rocks geochemically similar to island arc basalts. Aluminous granulites are derived from pelitic rocks and greywackes. The olivine-pyroxene granofels present a chemical composition similar to harzburgites. The mineral assemblages of granulites indicate an intermediate-pressure granulite facies metamorphism for their formation. Metamorphic rocks of lower grade found in the area of the Acaiaca Complex commonly present mylonitic microstructures and evidences of generation by retrometamorphism of the granulites in shear zones.