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Este trabalho baseou-se no estudo de xenólitos de rochas metamórficas na região de Piratini e Pinheiro Machado, sudeste do Escudo Sul-rio-grandense. Partindo de estudos de campo, microscopia ótica, geoquímica de elementos maiores e traços e geocronologia U-Pb em zircão, essa associação de rochas gnáissicas, denominada de Gnaisses Piratini, apresenta uma composição intermediária a ácida, predominantemente granodiorítica a tonalítica, subalcalina cálcio-alcalina médio-K, peraluminosa a levemente metaluminosa. Estes dados, juntamente com sua idade magmática de 784 ± 4 Ma, leva esta associação a ser interpretada como originada em um ambiente de arco magmático continental, e metamorfisada em fácies anfibolito em torno de 664 a 688 Ma. Nesta dissertação é apresentado um estado da arte dos estudos publicados sobre os Gnaisses Piratini, além de considerações sobre a importância desta unidade como um embasamento de sua região. Após, é apresentado o artigo denominado “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submetido para publicação no periódico Brazilian Journal of Geology, onde são descritos os dados e interpretações obtidos sobre os Gnaisses Piratini. / This work has based on the study of xenoliths of metamorphic rocks in the region of Piratini and Pinheiro Machado, southeastern Sul-rio-grandense Shield. Through field studies, optical microscopy, geochemistry of major and trace elements and U-Pb zircon geochronology, this association of gneissic rocks, named Piratini Gneisses, features an intermediate to felsic composition, predominantly granodioritic to tonalitic, medium-K calc-alkaline subalkaline, peraluminous to slightly metaluminous. These data, together with the magmatic age of 784 ± 4 Ma lead this rock association to be originated on an active magmatic arc environment, and metamorphosed at around 664 to 688 Ma in amphibolite facies. This dissertation presents a state-of-the-art of studies published about Piratini Gneisses, in addition to considerations about the importance of this unit as a basement of your region. After, it is featured the article entitled “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submitted for publication in the “Brazilian Journal of Geology”, where are described the data and interpretations obtained on the Piratini Gneisses.

Este trabalho baseou-se no estudo de xenólitos de rochas metamórficas na região de Piratini e Pinheiro Machado, sudeste do Escudo Sul-rio-grandense. Partindo de estudos de campo, microscopia ótica, geoquímica de elementos maiores e traços e geocronologia U-Pb em zircão, essa associação de rochas gnáissicas, denominada de Gnaisses Piratini, apresenta uma composição intermediária a ácida, predominantemente granodiorítica a tonalítica, subalcalina cálcio-alcalina médio-K, peraluminosa a levemente metaluminosa. Estes dados, juntamente com sua idade magmática de 784 ± 4 Ma, leva esta associação a ser interpretada como originada em um ambiente de arco magmático continental, e metamorfisada em fácies anfibolito em torno de 664 a 688 Ma. Nesta dissertação é apresentado um estado da arte dos estudos publicados sobre os Gnaisses Piratini, além de considerações sobre a importância desta unidade como um embasamento de sua região. Após, é apresentado o artigo denominado “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submetido para publicação no periódico Brazilian Journal of Geology, onde são descritos os dados e interpretações obtidos sobre os Gnaisses Piratini. / This work has based on the study of xenoliths of metamorphic rocks in the region of Piratini and Pinheiro Machado, southeastern Sul-rio-grandense Shield. Through field studies, optical microscopy, geochemistry of major and trace elements and U-Pb zircon geochronology, this association of gneissic rocks, named Piratini Gneisses, features an intermediate to felsic composition, predominantly granodioritic to tonalitic, medium-K calc-alkaline subalkaline, peraluminous to slightly metaluminous. These data, together with the magmatic age of 784 ± 4 Ma lead this rock association to be originated on an active magmatic arc environment, and metamorphosed at around 664 to 688 Ma in amphibolite facies. This dissertation presents a state-of-the-art of studies published about Piratini Gneisses, in addition to considerations about the importance of this unit as a basement of your region. After, it is featured the article entitled “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submitted for publication in the “Brazilian Journal of Geology”, where are described the data and interpretations obtained on the Piratini Gneisses.

Este trabalho baseou-se no estudo de xenólitos de rochas metamórficas na região de Piratini e Pinheiro Machado, sudeste do Escudo Sul-rio-grandense. Partindo de estudos de campo, microscopia ótica, geoquímica de elementos maiores e traços e geocronologia U-Pb em zircão, essa associação de rochas gnáissicas, denominada de Gnaisses Piratini, apresenta uma composição intermediária a ácida, predominantemente granodiorítica a tonalítica, subalcalina cálcio-alcalina médio-K, peraluminosa a levemente metaluminosa. Estes dados, juntamente com sua idade magmática de 784 ± 4 Ma, leva esta associação a ser interpretada como originada em um ambiente de arco magmático continental, e metamorfisada em fácies anfibolito em torno de 664 a 688 Ma. Nesta dissertação é apresentado um estado da arte dos estudos publicados sobre os Gnaisses Piratini, além de considerações sobre a importância desta unidade como um embasamento de sua região. Após, é apresentado o artigo denominado “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submetido para publicação no periódico Brazilian Journal of Geology, onde são descritos os dados e interpretações obtidos sobre os Gnaisses Piratini. / This work has based on the study of xenoliths of metamorphic rocks in the region of Piratini and Pinheiro Machado, southeastern Sul-rio-grandense Shield. Through field studies, optical microscopy, geochemistry of major and trace elements and U-Pb zircon geochronology, this association of gneissic rocks, named Piratini Gneisses, features an intermediate to felsic composition, predominantly granodioritic to tonalitic, medium-K calc-alkaline subalkaline, peraluminous to slightly metaluminous. These data, together with the magmatic age of 784 ± 4 Ma lead this rock association to be originated on an active magmatic arc environment, and metamorphosed at around 664 to 688 Ma in amphibolite facies. This dissertation presents a state-of-the-art of studies published about Piratini Gneisses, in addition to considerations about the importance of this unit as a basement of your region. After, it is featured the article entitled “Geology of the Piratini Gneisses: medium-K calc-alkaline magmatism of 784 ma (U-Pb SHRIMP) on the southeastern of Dom Feliciano Belt, RS”, submitted for publication in the “Brazilian Journal of Geology”, where are described the data and interpretations obtained on the Piratini Gneisses.

Deformation and partial melting have long been recognized to occur together, but differentiating which actually occurred first has remained enigmatic. Prevailing theories suggest that partial melting typically occurs first, and deformation is localized into melt-rich areas because they are rheologically weak. However, evidence from three different areas, suggests the role of strain has been underestimated in localizing partial melting. The Wet Mountains of central Colorado provide evidence for synchronous partial melting and deformation, with each process enhancing the other. Throughout the Wet Mountains, deformation is concentrated in areas where melt producing reactions occurred, and melt appears to be localized along deformation-related features. Melt microstructures present within the Wet Mountains correlate well with crustal-scale plutons and magmatic bodies and provide a proxy for crustal-scale melt flow. Granitic gneisses from the Llano Uplift, central Texas, provide evidence for partial melting occurring within small-scale shear zones and surrounding country rocks, synchronously. In the field, shear zones appear to contain former melt, whereas the country rock does not provide macroscopic evidence for partial melting. However, detailed microstructural investigation of shear zones and country rocks indicates the same density of melt microstructures, in both rock types. Melt microstructures are important for understanding the full melting history of a rock and without detailed structural and petrographic analysis, erroneous conclusions may be reached. Granulite-facies migmatites of the Albany-Fraser Orogen, southwestern Australia, have undergone partial melting, synchronous with three phases of bidirectional extension. Four major groups of leucosomes, including: foliation-parallel, cross-cutting, boudin neck and jumbled channelway leucosomes and late pegmatites were analyzed via whole-rock geochemistry, and there is evidence for fluid-saturated and -undersaturated biotite- and amphibole-dehydration melting. Migmatites from these three locations contain pseudomorphs of melt along subgrain and grain boundaries, areas of high dislocation density, in quartz and plagioclase. For these rocks that involve multicomponent systems, the primary cause for preferential melting in high strain locations is enhanced diffusion rates along the subgrain boundary because of pipe diffusion or water associated with dislocations. / text

La tsavorite, grossulaire vert à V-Cr-Mn, est contenue dans des gneiss et roches calco-silicatées graphiteux, souvent associés à des marbres dolomitiques, et appartenant à la ceinture métamorphique néoprotérozoïque mozambicaine. La tsavorite se trouve soit dans des nodules ou des veines de quartz (gisements primaires), soit dans des placers (gisements secondaires). L'étude minéralogique des tsavorites propose un nouveau protocole de certification de leur origine géographique, à partir du rapport V/Cr, de la teneur en Mn et du [delta]18 O. L'étude des gisements de Lemshuku et Namalulu en Tanzanie montre que le métamorphisme des protolithes sédimentaires riches en matière organique et évaporites s'est effectué à P = 7,0 ± 0,4 kbar et T = 677 ± 14°C, à 634 ± 22 Ma (datation U-Th-Pb sur monazite). Le bâti métamorphique s'est refroidi vers 500 Ma (datation 40Ar-39Ar sur muscovite). Deux stades de métasomatose sont reliés à la formation de la tsavorite : (i) une métasomatose de diffusion formant les nodules à P = 5,0-7,4 kbar et T = 580-691°C; (ii) une métasomatose calcique d'infiltration contemporaine de la formation des veines de quartz à P = 3,6-4,9 kbar et T = 505-587°C. Ces dernières sont datées in situ par la méthode Sm-Nd à 606 ± 36 Ma. Les évaporites continentales, déposées dans une sabkha de côte marine avec des sédiments silico-calcaires, sont transformées en tsavorite dans le cas des nodules, alors que les sels fondus sont associés à la formation des veines de quartz. Les minéralisations sont contrôlées par la lithostratigraphie et la tectonique / Tsavorite, a (V, Cr, Mn)-bearing green grossular, is hosted by graphitic gneisses or calc-silicates, often asssociated with dolomitic marbles, and belonging to the Metamorphic Neoproterozoic Mozambique Belt. Tsavorite is found either as nodules or in quartz veins (primary deposits), or in placers (secondary deposits). The mineralogical study of tsavorites suggests a new protocol to certificate their geographical origin, based on the V/Cr ratio, Mn content and delta18O. The study of the Lemshuku and Namalulu deposits in Tanzania has shown that the metamorphism of organic matter-rich and evaporites-rich sedimentary protoliths occurred at P = 7.0 ± 0.4 kbar and T = 677 ± 14°C, at 634 ± 22 Ma (U-Th-Pb dating on monazite). The metamorphic series cooled down at around 500 Ma (40Ar-39Ar dating on muscovite). Two metasomatic stages are linked to the formation of tsavorite : (i) diffusion metasomatism forming nodules at P = 5.0-7.4 kbar and T = 580-691°C; (ii) calcitic infiltration metasomatism forming quartz veins at P = 3.6-4.9 kbar and T = 505-587°C. These last have been dated in situ with Sm-Nd dating at 606 ± 36 Ma. Continental evaporites, deposited in a coastal marine sabkha with (Si, Ca)-bearing sediments, transformed into tsavorite in the case of the nodules, while the molten salts are associated with the formation of the quartz veins. The mineralisations are controlled by lithostratigraphy and structure