New U-Pb (SHRIMP) and first Hf isotope constraints on the Tonian (1000-920 Ma) Cariris Velhos event, Borborema Province, NE Brazil

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 Afeição 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 Afeição Suite. Analyzed samples yielded εHf(t) in a narrow range between-1.51 and +2.41, with associated T DMHf of 1.6-1.4 Ga, similar to previously obtained Nd isotope data with εNd(t) =-1.0 to +3.1 and T DMNd 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.

Accretionary models for the Neoproterozoic evolution of the Borborema Province: advances and open questions

“…Other evidence for Neoproterozoic plate tectonic processes in the BP are more contentious and highly discussed , Caxito et al 2020b. For example, the Cariris Velhos belt of the Transversal Zone has been proposed as an accretionary orogen, developed in a continental arc setting (Santos et al 2010(Santos et al , 2019, including metavolcanics, metasedimentary, and metaplutonic members.…”

“…The lack of a well-defined metamorphic event related to continental accretion at this stage is also debated (Neves 2015). Regarding the first point, Caxito et al (2020b) propose a useful distinction between Tonian rocks related to the Cariris Velhos event at 1.0 Ga-920 Ma and those related with crustal rifting at ca. 900-860 Ma, and discuss the occurrence of extensional settings related to accretionary orogens such as back-arc basins (one representative would be the Riacho Gravatá subdomain of the Alto Pajeú domain; Kozuch 2003, Santos et al 2010, Caxito et al 2020b.…”

“…Regarding the first point, Caxito et al (2020b) propose a useful distinction between Tonian rocks related to the Cariris Velhos event at 1.0 Ga-920 Ma and those related with crustal rifting at ca. 900-860 Ma, and discuss the occurrence of extensional settings related to accretionary orogens such as back-arc basins (one representative would be the Riacho Gravatá subdomain of the Alto Pajeú domain; Kozuch 2003, Santos et al 2010, Caxito et al 2020b. Regarding the second point, Caxito et al (2020b) discuss the probability of soft-collision events and reworking during the extensive Brasiliano Orogeny.…”

“…900-860 Ma, and discuss the occurrence of extensional settings related to accretionary orogens such as back-arc basins (one representative would be the Riacho Gravatá subdomain of the Alto Pajeú domain; Kozuch 2003, Santos et al 2010, Caxito et al 2020b. Regarding the second point, Caxito et al (2020b) discuss the probability of soft-collision events and reworking during the extensive Brasiliano Orogeny. In addition, restricted metamorphic zircon crystals and recrystallized rims still poorly dated at ca.…”

The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

“…1 Ga) of tectonic stability. Igneous activity in the Neoproterozoic produced a ~50 km wide NE-trending belt of dominantly coarsegrained granites to granodiorites and subordinate volcanosedimentary sequences between 1000 and 920 Ma ( [29][30][31] and the references therein) and more restricted episodes of intraplate magmatism at 900-850 Ma [32,33] and 700-640 Ma [34,35] in the central and southern subprovinces. Between 670 and 640 Ma, a large batholith, interpreted as a magmatic arc, was emplaced in the western part of the northern subprovince [36,37].…”