The Caraguataí syenitic suite, a ca. 2.7 Ga-old alkaline magmatism (petrology, geochemistry and UePb zircon ages). Southern Gavião block (São Francisco Craton), Brazil
Résumé
The Gavião Block comprises amphibolite- and granulite-facies gneisses and migmatites of tonalitic, granodioritic and granitic compositions and supracrustal sequences including volcanosedimentary layers metamorphosed up to the amphibolite facies. In the region of Abaíra-Jussiape (BA), two main igneous suites, called Caraguataí and Jussiape, are exposed in the core of an anticline. The Caraguataí suite encompasses alkali-feldspar granites, syenites and quartz syenites that contain biotite, magnetite/ hematite, apatite, titanite, hastingsite/pargasite and zircon as accessory minerals that were adjusted to the amphibolite facies. White mica and epidote minerals are related to retrograde greenschist facies. These rocks were deformed in dextral to reverse-dextral shear zones, giving origin to protomylonites and augen-mesomylonites to ultramylonites. The ultramylonites have a prominent banding parallel to the main foliation of the rocks. Lithogeochemical studies revealed subalkaline to alkaline, metaluminous to peraluminous, Fe-rich protolith for instead of to these rocks associated with A2-type magmatism and partial melting of igneous continental crust. In situ UePb zircon dating using the Laser Ablation ICPMS method was carried out for five samples of the Caraguataí alkaline suite. The ages obtained for an isotropic syenite (SCP 1470: 2680 24 Ma), a foliated syenite (SCP 2035: 2703 11 Ma), a syenitic augen gneiss (SCP 2017: 2706 34 Ma) and two ultramylonitic syenitic banded gneisses (SCP 1446: 2711 34 Ma and SCP 1809: 2698 10 Ma) fall in the same range. The average of the 62 concordant analyses obtained from the five samples allows to determine a mean 207Pb/206Pb age of 2696 5Ma ( 2s) interpreted as that of the alkaline plutonism. The geochronologic data obtained up to now have not helped to constrain an age for the metamorphism that affected the study area. The A2 type of magmatism, instead of and the TDM model ages (ca. 3.2e3.8 Ga) and the corresponding negative ε(t) values ( 4 to 6), suggest that the alkaline magmas are mainly derived from partial melting of Paleoarchean gneisses in an intraplate setting similar to those from the surrounding Aracatu region. The isotopic data obtained in this work together with published data suggest that in the southern sector of the Gavião Block an important event of crustal recycling occurred ca. 2.7 Ga ago. Similar ages have been found in other parts of the São Francisco Craton.