O complexo Juiz de Fora na região entre Mar de Espanha e Matias Barbosa, Minas Gerais: petrografia, litogeoquímica, geologia isotópica de Sm-Nd e geocronologia U-Pb

Abstract

The Juiz de Fora Complex in the region between Mar de Espanha and Matias Barbosa, Minas Gerais, is characterized by orthogranulites with a compositional range that includes indermediate and basic rocks. Geological features in field work related to the orthogranulites includes the shuffling of these rocks with cover associations related to the Andrelândia depositional cycle that took place on Neproterozoic. Granulite facies paragenesis was identified in basic rocks and includes clinopyroxe, orthopyroxene, quartz, plagioclase, hornblende and opaques, while in intermediate rocks includes orthopyroxene, quartz, plagioclase, k-feldspar and opaques. In areas closed to the boundaries between the basement rocks (orthogranulites) and the neoproterozoic sucessions there is a pattern of retrograde facies with hornblenda or/and biotite consuming ortho and clinopyroxene.Based on petrographic studies it was possible to characterize granodioritic, tonalitic and gabroic protolithes for the metamorphosed rocks in granulite facies. Based on elemental geochemistry studies it was possible distinguish the Juiz de Fora Complex rocks into four groups: I) Tholeiitic basic rocks II) Calc-alkaline intermediate rocks; III) high-K calc-alkaline intermediate rocks and IV) Tholeiitic intermediate rocks. It was possible to subdivide the granulites into several groups based on [La/Yb]n ratio. The calc-alkaline group that includes intermediate rocks was subdivided into six groups, the tholeiitic group that includes basic rocks was subdivided into 2 groups, the high-K calc-alkaline series that includes intermediate rocks was subdivides into 5 groups and the tholeiitic group that includes only the intermediate rock consists in a group itself. Tholeitic basic rocks were formed on ocean floor, due the midge ocean ridge basalts signature. Calc-alkaline and high-K alkaline rocks were formed in island arc setting. Tholeiitic intermediate rock enriched in TiO2 were formed in within plate setting. LA-ICPMS U-Pb data from 4 samples related to the orthogranulites of Juiz de Fora Complex are reported here. A charno-enderbite sample from the calc-alkaline series yielded concordant ages at 1984 ± 31 Ma and 604 ± 21 Ma. Discordant grains of zircons define a discordia line with upper intercept at 1981 ± 38 and lower intercept at 597 ± 30 Ma, interpreted as crystalization of the protolith and brasiliano metamorphic overprint ages, respectively. An enderbite sample from the calc-alkaline series yielded concordant ages at 2080 ± 77 Ma and 595 ± 15 Ma. Discordant grains define a discordia line with upper intercept at 2085 ± 48 Ma and 591 ± 20 Ma, interpreted as crystalization and metamorphic overprint, respectively. A charno-enderbite sample from the high-K calc-alkaline series yielded concordant ages at 1920 ± 19 Ma and 569± 11 Ma, interpreted as crystalization of the protolith and metamorphic overprint, respectively. Isotopic data obtained from Nd analyses shows extreme negative values to ɛNd, suggesting that the source of the orthogranulite rocks are more enriched than the chondritic uniform reservoir.