William Randall Van Schmus

In the central part or central subprovince of the Borborema Province, in the so-called Transversal Zone, an assemblage of Paleoproterozoic (predominantly Rhyacian) rocks occurs together with sparsely distributed Neoarchean nuclei. These rocks constitute the main basement of the Tonian (Cariris Velhos Cycle) and Ediacaran (Brasiliano Cycle) mobile belts. They are older tectonostratigraphic terranes and par excellence ramifying elements (as “inliers”) of the Neoproterozoic orogenic constructions. This tectonostratigraphic terrane known as Alto Moxotó composes a litho-structural context typical of accretionary orogens, with the presence of granitic, granodioritic, and trondhjemitic orthogneisses (≈ arc-type associations) that stretch out for some hundreds of kilometers (Floresta Suite) and are partially covered by medium-grade pelitic-psammitic sequences (Sertânia Supergroup). This basement outline (Floresta Suite + Sertânia Supergroup) depicts an irregular sigmoidal shape, from the interior of Pernambuco (meridian 39ºW) to the Paraíba coastal area (meridian 34º45'W), along about 450 km, varying from 40 to 80 km in width, resulting in a total area of ca. 28,000 km2. This Rhyacian petrotectonic scenario is conditioned and delimited by extensive (shear + thrust) faults that configure a tectonostratigraphic terrane. Moreover, there are pieces of continuity evidence of this tectonostratigraphic terrane not only in the basement of the adjacent Neoproterozoic mobile belts to the North and South of the TAM in Brazil, but also in Northern Cameroon, in Africa. Within this broader predominantly Rhyacian belt, some small Neoarchean gneissic and migmatitic nuclei were identified. The Rhyacian lithologies reveal evidence (TDM ages) of sources from a Neoarchean paleosubstrate. During the Statherian, the whole Rhyacian context was affected by a series of small-scale magmatic manifestations of varied petrologic composition: syenitic, granitic, granodioritic, and gabbro-anorthositic. In this paper, a synthesis of the bibliographic data is presented, including a series of unpublished (geologic and geochronologic) data that together validate our conviction that we are dealing with an extensive (reworked) fragment (TAM) derived from the basement of a Paleoproterozoic supercontinent (Columbia).

O Sistema de Dobramentos Piancó-Alto Brígida (SPAB) situa-se na porção central a centro-ocidental da Zona Transversal, da Província Borborema, nos Estados da Paraíba e do Pernambuco e em parte do Estado do Ceará. Trata-se do produto de um desenvolvimento orogenético na passagem Criogeniano-Ediacarano (630–580 Ma, acresção seguida de colisão), levando à inversão vigorosa de uma pretérita bacia de sedimentação marinha costeira. O limite ao norte é marcado pelo Lineamento de Patos e o ao sul é balizado pelo Lineamento Pernambuco. O contexto paleogeográfico (e litoestratigráfico) original é de reconstituição difícil devido aos eventos de deformação (pelo menos três fases) e metamorfismo, lateralmente variável de muito baixo, baixo a médio graus, seguidos de uma importante fase de tectônica extrusional tardia. O SPAB se encontra hoje geológico-geograficamente ramificado em diferentes faixas, separadas por alguns basement inliers, paleoproterozoicos/“terrenos”, expressivos em área e importantes partícipes dos eventos de deformação. Os principais litotipos presentes no SPAB se enquadram nas típicas assembleias quartzito-pelito-calcário (QPC) com a característica primordial da grande ritmicidade. Os gradientes metamórficos são de muito baixo, baixo e médio graus (nesse último caso, para o sul, faixa do interior de Pernambuco, “Supergrupo Salgueiro”). Nas zonas proximais dos contatos com o embasamento, consoante a linhas de falhas importantes, são registrados brechas e conglomerados polimícticos, arenitos imaturos, metagrauvacas e metacórseos (Formação Olhos d’Água). Embora haja várias contribuições, de diversas fontes, ao longo dos últimos 50 anos, em diferentes tempos e propósitos, uma discriminação da litoestratigráfica satisfatória ainda não foi possível. De sorte que todos os termos litoestratigráficos devem ser encarados como informais e provisórios. Múltiplos trabalhos de reconhecimento em geocronologia foram realizados aleatoriamente nesse sistema nas

The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions). It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a) 1.97 Ga; (b) 0.59 - 0.53 Ga; (c) 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time.

Contribuição ao conhecimento da evolução geológica do Sistema Riacho do Pontal – PE, BA, PI

The Pernambuco–Alagoas (PE–AL) Domain contains major granitic batholiths typified by a wide range of T_DM model ages (Archaean to Neoproterozoic), reflecting the important role of quartzofeldspathic plutons attending the Brasiliano (Pan-African) Orogeny. U/Pb zircon data for eight syn- to post-collision to syn-transcurrent granitic intrusions of the PE–AL Domain allow the studied plutons to be divided into two groups: (1) granitoids with crystallization ages older than 600 Ma (Água Branca, Serra do Catú, Serra da Caiçara, and Mata Grande plutons) and (2) granitoids with ages of ca. 590 Ma (Correntes, Águas Belas, Viçosa, and Cachoeirinha plutons). The intrusions of group 1, except for the Mata Grande Pluton, all show Nd T_DM model ages ranging from 1.5 to 1.2 Ga, whereas the granitoids from group 2 and the Mata Grande Pluton have Nd T_DM model ages ranging from 2.2 to 1.7 Ga. The studied granitoids are in part high-K, calc-alkaline, shoshonitic, and in part transitional high-K calc-alkaline to alkaline in terms of their bulk chemistry. Volcanic arc signatures associated with the Palaeoproterozoic T_DM model ages are interpreted as inherited from the source rocks. The oldest ages and higher Nd T_DM model ages recorded in the granitoids intruded in the southwestern part of the PE–AL Domain suggest that these intrusions are associated with slab-tearing during convergence between the PE–AL and Sergipano domains. The investigated plutons are coeval with high-K granitoids intruded within the Transversal Zone Domain of Borborema Province and calc-alkaline granitoids of the Sergipano Domain. This suggests that these geologic realms belonged to the same crustal block during the Brasiliano Orogeny. However, such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Sergipano and Transversal Zone domains, suggesting differences in the crustal evolution of these three areas.