| dc.description | Tonian granitoids, today augen-gneisses and migmatites, showing crystallization ages ranging from 870
Ma to 1000 Ma occur in the Borborema Province, NE Brazil. The majority of them have ages within the
925e970 Ma interval. Few intrusions with ages of ~1.0 Ga and <900 Ma occur in the Transversal and
South subprovinces. The Tonian granitoids constitute the most expressive magmatic rocks of the Cariris
Velhos event. The studied granitoids (herein CVG -Cariris Velhos granitoids) intrude slightly older
bimodal (but mostly felsic) volcanic successions and metasedimentary sequences in the Transversal and
South subprovinces. Tonian granitoids are unknown in the North subprovince. The CVG comprise mainly
coarse-grained augen-gneisses of granite to granodiorite composition. Fe-rich biotite (annite) is the main
mafic mineral phase, constituting up to 15% of the modal composition. Garnet, muscovite and tourmaline
occur as accessory phases in many plutons.
The CVG augenegneisses have high SiO2 (>71%) and alkali contents, they vary from slightly peraluminous
to slightly metaluminous, and from slightly magnesian to typical ferrroan rocks. In the
migmatized orthogneisses the SiO2 contents are usually <70%. Trace element variations in the CVG are
extensive, reflecting the migmatization recorded in some plutons and/or distinct sources. They are Ca-,
Sr- and Nb-poor, showing variable Ba (100e1260 ppm), Rb (164e400 ppm) and Zr (144e408 ppm)
contents, and high abundances of Y (>40 ppm). The chondrite normalized REE patterns are characterized
by strong to moderate negative Eu anomalies (Eu/Eu* ¼ 0.23e0.70). In general, the spidergram patterns
show deep troughs at Ti, P, Ba and Sr and less pronounced NbeTa troughs. These patterns are similar to
those reported for anorogenic granites evolved from mixtures of magmas from both crustal and mantle
sources. The CVG exhibit TDM model ages ranging from 1.9 to 1.1Ga, with slightly negative to slightly
positive ƐNd(t) values, suggesting the involvement of distinct proportions of mantle and crustal components
in the source of their protoliths.
There is no consensus in the literature about the tectonic setting of the CVG ie they have been related
to either continental margin magmatic arc, with possible backearc association, or extention-related
setting, with generation of A-type granites. However, all the available geochemical data suggest that
the CVG represent extension related magmatism. The geochemical signature associated to bimodal
volcanism, including pyroclastic rocks, with similar ages, and absence, up to now, of evidence for
metamorphism of Tonian age, support the hypothesis of extension - related magmatism. | |
