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RESUMO:O Granito Chasqueiro aflora como campos de matacões e lajeados, que em planta constituem um corpo alongado segundo N50 o E com área de aproximadamente 400 km 2 , localizado no sul da porção oriental do Escudo Sul-Rio-Grandense. Compreende de monzo a sienogranito, leucocrático de cor cinza-clara e textura porfirítica com megacristais de K-feldspatos em matriz equigranular, hipidiomórfica grossa, composta por quartzo, K-feldspato, plagioclásio, biotita, hornblenda, minerais opacos e acessórios. Os megacristais atingem proporções modais entre 30,0 e 60,0%, e variam entre 4 e 8 cm, que com os minerais máficos da matriz evidenciam uma foliação de fluxo magmático subvertical bem desenvolvida que transaciona lateralmente para uma foliação tectônica nos bordos do granito próximo às zonas de cisalhamento onde ocorrem extensas faixas marcadas por intensa deformação dúctil e geração de protomilonitos. O granito apresenta enclaves máficos microgranulares de composição diorítica, apresentando diferentes formas e tamanhos. Geoquimicamente, é caracterizado por um magmatismo subalcalino do tipo cálcio-alcalino de alto potássio, metaluminoso a levemente peraluminoso, com assinatura característica de granitos gerados em ambiente pós-colisional. Processos de fusão crustal, mistura de magmas e cristalização fracionada são sugeridos para a sua evolução. Análises geocronológicas obtidas pelo método U-Pb (LA-ICP-MS) e geoquímica isotópica de Lu-Hf (LA-ICP-MS) em zircões indicaram, respectivamente, idade de cristalização 574 ± 3 Ma e valores negativos para εHf, sugerindo assim uma relação do Granito Chasqueiro com o evento deformacional D2 e fontes magmáticas dominantemente crustais com participação de componente mantélico subordinado. PALAVRAS-CHAVE:Petrologia; Granito Chasqueiro; U-Pb; Lu--Hf; Escudo Sul-Rio-Grandense. ABSTRACT: Chasqueiro Granite comprises an

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The LA-MC-ICP-MS method applied to U-Pb in situ dating is still rapidly evolving due to improvements in both lasers and ICP-MS. To test the validity and reproducibility of the method, 5 different zircon samples, including the standard Temora-2, ranging in age between 2.2 Ga and 246 Ma, were dated using both LA-MC-ICP-MS and SHRIMP. The selected zircons were dated by SHRIMP and, after gentle polishing, the laser spot was driven to the same site or on the same zircon phase with a 213 nm laser microprobe coupled to a multi-collector mixed system. The data were collected with a routine spot size of 25 µm and, in some cases, of 15 and 40 µm. A careful cross-calibration using a diluted U-Th-Pb solution to calculate the Faraday reading to counting rate conversion factors and the highly suitable GJ-1 standard zircon for external calibrations were of paramount importance for obtaining reliable results. All age results were concordant within the experimental errors. The assigned age errors using the LA-MC-ICP-MS technique were, in most cases, higher than those obtained by SHRIMP, but if we are not faced with a high resolution stratigraphy, the laser technique has certain advantages.

The Porongos Metamorphic Complex (PMC) is composed of supracrustal rocks, predominantly pelitic micaschist, quartzite and felsic metavolcanic rocks. It represents the remnants of a Neoproterozoic sedimentary basin developed in the western Gondwana, now located in the central section of Dom Feliciano Belt in southern Brazil. Isotope signature comparison and U-Pb ages shows that there are at least three main source-areas for the Porongos sediments: (i) one that can be related to Paleoproterozoic terrains; (ii) a younger component with Mesoproterozoic (1.5-1.2 Ga) ages; and (iii), a Neoproterozoic source with magmatic zircons (0.8 Ga), with relative age distribution mostly between 1.7 and 1.0 Ga, TDM ages between 2.58 and 1.71 Ga, and younger ages of ca. 1.21 Ga, which can be related to 1.4-1.0 African signature sources. Ten concordant analysis obtained in zircon grains from a metarenite rock yield 206 Pb/ 238 U median age of 802 +26/-33 Ma, which is related to metavolcanic samples from PMC and correlates with the Neoproterozoic Cerro Bori Continental Arc, indicating a syn-depositional volcanism in the sedimentary record. The depositional setting for these sequences can be understood as an evolution between a passive margin developed after 1.6 Ga and a terrain-collisional setting (ca. 570 Ma).

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