Paleoproterozoic granitic magmatism in the northern São Francisco Craton, NE Brazil : new perspectives from geochemistry, zircon U–Pb geochronology and Hf isotopes.

Abstract

The Contendas-Mirante metavolcano-sedimentary sequence (CMS) has been studied by diverse authors but they did not emphasize the petrogenesis of the granitoid rocks that occur in the area. A set of eight fine-to-medium grained granitic plutons intrude the 3.2-3.4 Ga TTG rocks from the Gavião Block (GB) as well the 2.09-2.17 Ga meta-sedimentary rocks of the Contendas-Mirante metavolcano-sedimentary sequence. Two mineral assemblages were identified: i) quartz, K-feldspar, plagioclase and biotite as main phases and muscovite, zircon, apatite, chlorite, sericite, ilmenite and magnetite as accessories and ii) quartz, K-feldspar, plagioclase, biotite and muscovite as main phases and zircon, apatite, chlorite, sericite and ilmenite as accessories. Thus, it was possible to divide them between biotite-monzogranites and two-mica granites. Our results reveal that these granitoids are sodic and potassic peraluminous, leucocratic, calc-alkalic to alkali-calcic and are enriched in LREE in relation to the HREE. The biotite-monzogranites have slightly negative Eu anomalies whereas the two mica granites portray pronounced negative Eu anomalies. Both biotite-monzogranites and two-mica granites have Zr/Hf ratios between 21.90 and 35.59 which allowed their classification as moderately-to-highly fractionated granites. The biotite-monzogranites show geochemical similarities with Archean TTGs while the two-mica granites are akin to the post-Archean granitoids. The Rb and Y+Nb values indicate that the biotite-monzogranites are syn-collisional and the two-mica granites are post-collisional. The Rb/Sr and Rb/Ba ratios allowed the distinction of between clay-poor sources, related to the biotite-monzogranites and clay-rich sources related to the two-mica granites. U-Pb geochronological analysis indicated Journal Pre-proof 2 that the biotite-monzogranites have U-Pb concordia age of 2003 ± 10 Ma whereas the two mica granites portray U-Pb concordia age of 2056.7 ± 14 Ma, suggesting that both are Paleoproterozoic. Lu-Hf isotopic analysis in zircon showed that both granites have negative ƐHf(t) values (-33.5 - -8.1 for the biotite-monzogranites and -26.3 - -7.4 for the two-mica granites) which evidences their crustal origin. The biotite-monzogranites have TDM2(Hf) varying between 2.99 and 3.98 Ga while the two-mica granites have TDM2(Hf) between 3.02 and 3.64 Ga, suggestive of their derivation from Archean source rocks.