The Caraguatai syenitic suite, a ca. 2.7 Ga-old alkaline magmatism (petrology, geochemistry and U-Pb zircon ages). Southern Gaviao block (Sao Francisco Craton), Brazil

摘要

The Gaviao 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 Abaira-Jussiape (BA), two main igneous suites, called Caraguatai and Jussiape, are exposed in the core of an anticline. The Caraguatai 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 U-Pb zircon dating using the Laser Ablation ICPMS method was carried out for five samples of the Caraguatai 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 ~(207)Pb/~(206)Pb age of 2696 ± 5 Ma (±2σ) 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 T_(DM) model ages (ca. 3.2-3.8 Ga) and the corresponding negative e(t) values (-4 to -6), suggest that the alkaline magmas are mainly derived from partial melting of Paleo-archean 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 Gaviao Block an important event of crustal recycling occurred ca. 2.7 Ga ago. Similar ages have been found in other parts of the Sao Francisco Craton.