Abstract
Abstract Orthogneisses associated with metavolcanosedimentary successions related to the 1000 - 920 Ma Cariris Velhos event occur mainly in a ca. 700 km-long sigmoidal-shaped belt that crosscuts the Transversal Zone of the Borborema Province and extends to the fold belts located in its southern or external zone (part of the Riacho do Pontal and Sergipano orogens). Despite its importance, the tectonic setting and the role of those rocks in the geological evolution of the Borborema Province are contentious and not yet well understood. New zircon U-Pb SHRIMP data on an augen-gneiss sill of the Afeicao Suite intruding mica-schists mapped as part of the Santa Filomena Complex yielded a Concordia Age of 974 ± 11 Ma, indicating that at least part of the metasedimentary rocks in the internal zone of the Riacho do Pontal Orogen are Tonian or older and possibly related to the Cariris Velhos event. Hf-in-zircon isotope data are presented for the first time for Cariris Velhos-related ortho-derived rocks of the Afeicao Suite. Analyzed samples yielded eHf(t) in a narrow range between -1.51 and +2.41, with associated TDMHf of 1.6-1.4 Ga, similar to previously obtained Nd isotope data with eNd(t) = -1.0 to +3.1 and TDMNd of 1.5-1.2 Ga. A possible scenario to explain both the geochemical features and the moderately juvenile to slightly evolved, near-chondritic Hf and Nd isotope signatures is a continental arc setting, where fractionated melts produced in the supra-subduction zone mantle wedge carrying a Tonian juvenile signature became contaminated with discrete amounts of Archean-Paleoproterozoic continental crust during ascent, producing Mesoproterozoic model ages which represent the mixture of those two end-members.
Highlights
The Borborema Province of northeastern Brazil (Fig. 1) and its counterparts in northwestern Africa are part of the broad orogenic network involved in the amalgamation of West Gondwana during the Ediacaran-Cambrian Brasiliano/Pan-African Orogeny (Almeida et al 1981, Brito Neves et al 2000)
We present new U-Pb Sensitive High-Resolution Ion Microprobe (SHRIMP) data on zircon crystals from an augen-gneiss sill of the Afeição Suite that intrudes mica-schists mapped as part of the Santa Filomena Complex within the internal zone of the Riacho do Pontal Orogen, representing the Cariris Velhos event in the southern Borborema Province
The λ decay constant for 176Lu used is 1.867x10-11/yr (Söderlund et al 2004). This sample is from a meter-thick augen-gneiss sill of the Afeição Suite interleaved within garnet-mica schist mapped as part of the Santa Filomena Complex at Carnaíba creek, at the Acauã-Monte Orebe road (Figs. 2 and 3)
Summary
The Borborema Province of northeastern Brazil (Fig. 1) and its counterparts in northwestern Africa (the Tuareg Shield, the Benino-Nigerian Shield, and related provinces in Cameroon) are part of the broad orogenic network involved in the amalgamation of West Gondwana during the Ediacaran-Cambrian (ca. 630 – 500 Ma) Brasiliano/Pan-African Orogeny (Almeida et al 1981, Brito Neves et al 2000). 1000 – 920 Ma plutonic and metavolcanosedimentary rocks within the central and southern Borborema Province, encompassing the Cariris Velhos event (Brito Neves et al 1995, Kozuch 2003, Santos et al 2010). Rocks in this age range have not yet been unequivocally recognized in NW Africa, and interpretations of its geodynamic setting and role in the Borborema Province are contentious, resulting in the proposition of both continental arc (Kozuch 2003, Medeiros 2004, Santos et al 2010, Caxito et al 2014b) and intracontinental rift (Neves 2003, Guimarães et al 2012) models. The aim of the study is to contribute with new geochronological and isotopic data which will help to constrain the tectonic setting, petrological evolution and meaning of the Cariris Velhos event in the evolution of the Borborema Province
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