A complex history of extension, subduction and collision in west Gondwana: Clues from the Riacho do Pontal orogen, Borborema Province (NE Brazil)

Abstract

The geodynamic understanding of the Riacho do Pontal Orogen, in the Borborema Province, NE-Brazil, has progressed substantially in the last decade, mainly from geochemical and isotopic studies. However, the role of the Neoproterozoic magmatism has not totally been unraveled and has left some crucial questions unanswered. In order to contribute to this debate, we present petrographic, geochemical and isotopic (U–Pb, Sm–Nd and Sr–Sr) data obtained for a set of granitoids and metavolcanic rocks from the Internal and Central domains of the Riacho do Pontal Orogen. The Neoproterozoic granitoids comprise a major alkaline pluton of the Serra da Aldeia suite; two syenitic plutons – Nova Olinda and Caboclo –, and high-K, sub-alkaline granitoids from the Betânia granite. The alkaline pluton from the Serra da Aldeia suite yielded a zircon U–Pb age of 578 ± 5 Ma, while the Nova Olinda and Caboclo syenites yielded ages of 613 ± 2 Ma and 620 ± 3 Ma, respectively. The Betânia granite, displaying a volcanic-arc geochemical signature, yielded the age of 627 ± 4 Ma. Tonian ages were also obtained between 956 ± 8 Ma and 965 ± 3 Ma for three unclassified orthogneisses from the Internal Domain of the Orogen. A sample of metarhyolite from the Paleoproterozoic basement yielded an age of 1631 ± 21 Ma. The isotopic data for the Nova Olinda and Caboclo syenites revealed εNd(620 Ma) values between −7.72 and −8.45 and TDM values from 1.76 to 1.97.87Sr/86Sr ratios of 0.71594 ± 2 and 0.74432 ± 1 suggest mixing of crustal and mantle materials during their evolution. In contrast, the Betânia granite presented a ratio of 0.70669 ± 1, reinforcing the correlation with mantle sources. The integrated analysis of the data indicate that the Betânia granite is part of an Ediacaran magmatic-arc system, with the Nova Olinda and Caboclo syenites characterized as products of the late-orogenic magmatism, while the alkaline rocks of the Serra da Aldeia suite represent the final of post-orogenic stages of one of the multiple collisions of the West Gondwana.