Isotopic and trace element geochemistry of early Neoproterozoic gneissic and metavolcanic rocks in the Cariris Velhos orogen of the Borborema Province, Brazil, and their bearing on tectonic setting.

摘要

Metamorphic rocks in the Paraiba and Pernambuco states of northeast Brazil predate the Brasiliano-Pan African orogeny. Older gneisses of the assemblage are associated with Paleoproterozoic crustal formation. Protoliths of younger orthogneisses and coeval supracrustal rocks were formed during the early Neoproterozoic. The early Neoproterozoic magmatic event is termed the Cariris Velhos orogeny, which produced calc-alkaline to high-K calc-alkaline, metaluminous to mildly peraluminous magmas.;U-Pb geochronology indicates that Cariris Velhos metavolcanic rocks were initially erupted at 975 +/- 10 Ma. Granitic protoliths of gneisses were intruded at 960 +/- 5 Ma; later magmatism between 940 and 925 Ma (+/-15 Ma) produced both granites and rhyolites. There was no systematic inheritance of older xenocrystic components in Cariris Velhos igneous rocks. The average epsilonNd 1 Ga is -1.9 +/- 2.9 units and average T(DM) is 1.6 +/- 0.2 Ga for Cariris Velhos samples. Rb-Sr isotopes of Cariris Velhos rocks indicate that most have retained their original Rb-Sr isotopic relationships. Combined Nd-Sr modeling suggests Caririan rocks are products of depleted-mantle melts variably contaminated with Paleoproterozoic continental crust. Trace element abundances indicate that ocean-continent convergence dominated as the process that generated Cariris Velhos granites along the main orogen. Along the western convergent margin, extension played a part in arc magmagenesis. The Cariris Velhos orogeny is an example of Andean-type convergence, although its tectonic development may be more analogous to Cascadian than to Andean subduction.;The proposed scenario for the development of the Cariris Velhos orogeny involves rifting of Paleoproterozoic crust less than 20 m.y. prior to convergence. Convergence and subduction resulted in magmatism along the Paleoproterozoic continental edge, and ceased when the mid-ocean ridge reached the continental margin. Grenvillian collision sensu stricto by the end of the Mesoproterozoic resulted in the formation of the Rodinian supercontinent. Because the Cariris Velhos orogeny occurred in the early Neoproterozoic, it cannot be considered as part of a terminal Rodinian orogeny. Instead, the Cariris Velhos orogen formed somewhere along the periphery of Rodinia, and it may have been distally associated with extension approximately 1 Ga that almost divided the Sao Francisco-Congo craton into two different parts.