Petrology of Sítios Novos Batholith, Sergipano Orogenic System, Borborema Province, NE Brazil

Abstract

The Sítios Novos Batholith (110 km²) is an NE-SW elongated intrusion, with an age of 631 ± 4 Ma (which agrees with the regional orientation) and is located in the southern part of the Poço Redondo Domain, in the Sergipano Orogenic System. This batholith is constituted by leucocratic monzogranites with equigranular and unequigranular allotriomorphic texture, which occasionally present pegmatitic dikes. Mafic enclaves are rare. The batholith’s southern region is affected by the regional shear zone and the monzogranites developing milonitic and gneissic structures. Oligoclase and albite are the plagioclase present in these granites and the microcline is occasionally perthitic. Biotite is the dominant mafic mineral and its composition indicates affinity with orogenic suites (calcic-alkaline and peraluminous). Zircon, titanite, magnetite, ilmenite, apatite, thorite, allanite and monazite occur as accessory minerals. Bastnaesite and barite often occupy microfractures in these rocks. The monzogranites’ geochemistry shows that they are fractionated rocks (SiO₂ > 68%), peraluminous to metaluminous and are allocated in geochemical diagrams in the field of I-type granites. The REE spectra present moderate fractionation (15 < La_N/Yb_N < 70), with negative anomaly in Eu, indicating plagioclase fractionation. In multielementary diagrams, the samples of this batholith have pronounced valleys in Nb, P, Ba and Ti, and more or less accentuated peaks in Th, Pb and Zr. This geochemical signature is characteristic of orogenic magmas. These monzogranites have as probable source the partial melting of tonalitic crust.