科研之友
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摘要
The Riacho do Pontal Orogen (RPO) is part of the extensive Brasiliano/Pan-African orogenic system in northeastern South America (Borborema Province). This region is crucial for understanding the Neoproterozoic evolution of West Gondwana because it occupies a strategic position between three of its most important constituents, namely the Sao Francisco/Congo, Amazon and West Africa cratons. Based on lithostratigraphic, lithochemical, geochronological and isotopic data, a five-stage model for the geodynamic evolution of the RPO and adjacent areas in the Borborema Province is proposed: (1) rift phase - Development of a triple junction rift system at ca. 900-820 Ma ago, probably related to the emplacement of a mantle plume and leading to intense mafic-ultramafic magmatism of the Brejo Seco layered intrusion (ca. 900 Ma). This was followed by the development of a continental rift system testified by the Paulistana Complex metavolcanosedimentary sequence (ca. 882 Ma); (2) drift phase - With continued lithospheric thinning, the rift system further evolved to a broad passive margin in the northern Sao Francisco craton edge, represented by the Barra Bonita Formation platformal sediments. The drift phase culminated in the development of true oceanic crust, represented by the Monte Orebe ophiolite, around 820-630 Ma ago; (3) convergence phase - At circa 630-620 Ma, onset of subduction in the Borborema Province caused the inversion of the basins, obduction of oceanic crust slices, and sedimentation of the Mandacaru Formation syn-orogenic greywackes; (4) collisional phase - Continental collision between the Sao Francisco craton (lower plate) and the Pernambuco-Alagoas block (upper plate) occurred around 620-590 Ma, with the stacking of the Casa Nova nappes upon the lower plate, crustal thickening, deformation, metamorphism, melt generation and intrusion of the syn-collisional Rajada Suite two-mica granites; (5) lateral escape phase - The collisional stage was followed by a lateral escape tectonics stage around 590-530 Ma, that generated the western branch of the E-W trending Pernambuco Shear Zone, which truncates the northern part of the orogen and displaces its counterparts in the Transversal Zone in a dextral-sense shear. This phase was accompanied by extensive alkaline magmatism in the form of the Serra da Aldeia/Caboclo suites. Thus, the RPO represents a complete plate tectonic cycle that spans the whole Neoproterozoic and preserve excellent time markers for both the break-up and dispersion of the Rodinia Supercontinent fragments and for the re-amalgamation of those fragments in West Gondwana. The interpretation of a complete Wilson Cycle in this area challenges current views of the Borborema Province as a coherent block since Paleoproterozoic times (part of the Atlantica Supercontinent); suggesting, instead, a dynamic setting where multiple plates interacted to form the heart of West Gondwana.
- 出版日期2016-9
