Evolução do magmatismo neoproterozóico registrado nos ortognaisses Cerro Bori, cinturão Dom Feliciano no Uruguai

Content Provider	Semantic Scholar
Author	Lenz, Cristine
Copyright Year	2010
Abstract	Geochemistry of major and trace elements, Sm-Nd and Pb-Pb isotopic composition and UPb zircon ages (SHRIMP) led to the understanding of tectonic and magmatic evolution of the Cerro Bori orthogneisses, Eastern Domain of the Dom Feliciano Belt in Uruguay, during the Brasiliano Orogenic Cycle. The Cerro Bori orthogneisses are composed of tonalitic and granodioritic gneisses which involve boudins of mafic gneisses, with composition of gabbroic diorite and diorite. These rocks intruded the Chafalote paragneisses and both units were affected by a high grade metamorphic event (high P and T) and intense deformation. The orthogneisses were separated into three groups, based on gechemical and isotopic data, Type I, II and III. Type I is composed of mafic gneisses with tholeiitic affinity and signature of transitional back-arc magmas. U-Pb zircon ages (SHRIMP) were obtained in five samples of this group, in which zircons with oscillatory zoning reveal crystallization ages between 794 ± 8 Ma and 771 ± 6 Ma Type II is composed mainly of tonalitic gneiss with cal-alkaline affinity with and geochemical signature of continental magmatic arc rocks. Ages were obtained in five samples and zircons with oscillatory zoning reveal crystallization ages between 802 ± 12 Ma and 786 ± 9 Ma Type III is composed of biotite rich mafic gneiss with geochemical characteristics of potassic to utrapotassic rocks. A population of zircon was found in one sample, resulting in an age of 767 ± 9 Ma. Type I, II and III have negative values of ƐND (between -2.12 and 6.67) and old TDM ages (between 1.2 and 2.0 Ga), indicating processes of crustal assimilation/ contamination associated to fractional crystallization in the generation of these rocks. An Andean type arc setting is suggested for these rocks, with an arc setting near the back-arc. Younger mafic rocks were found as well in associated to the ortogneisses Cerro Bori (between 659 and 637) Ma. They can occur as boudins, in the tonalitic and granodioritic gneiss or as dikes. The mafic rocks as boudins have ages of 659 ± 5 Ma, an typpical metamorphic zircons replect a crystallization near the peak, beeing this probably the best estimate of metamorphic peak age. Later partial melting event was determined on zircon rims of the Type I, II and III rocks, in 654 ± 3 Ma. The dikes represent post-collisional magmas. Geochemical signatures of an E-Morb were found in the mafic granulite dike, crystallized at 647 ± 7 Ma. The amphibolite has characteristics of a high-K calc-alkaline to shoshonitic magma, crystallized at 637 ± 4 Ma and characterizing the post-collisional setting. The emplacement of these dikes was possibly related to the shear zones found in the area.
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Language	English
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Content Type	Text
Resource Type	Article