Petrologia do magmatismo anorogênico Cariris Velhos no Sistema Orogênico Sergipano: batólito Serra Negra.

Abstract

The Serra Negra Batholith (BSN), covering an area of 155 km², is intrusive into the metavolcano-sedimentary terrains of the Marancó Domain and is located in the northwestern region of the State of Sergipe and northeastern Bahia. This batholith is oriented SE-NW, forming a range of hills extending for 40 km with an average width of 3.8 km. The BSN has an age of 933 ± 7 Ma and, to date, is the sole representative of Tonian magmatism identified within the terrains of the Sergipano Orogenic System, structured during the Neoproterozoic. The majority of the rocks in the BSN are augen gneissic granites, with porphyritic granites devoid of gneissic structure occurring locally within the deformed granites. Occasional pegmatitic and granitic dykes are present, along with sporadic amphibolitic xenoliths from the metasedimentary rocks. Representative samples of the BSN were selected for petrographic, whole-rock geochemical, and mineral-chemical studies. These rocks were examined using a petrographic microscope to describe their constituent mineralogy, textures, and modal compositions, enabling their classification on the QAPF diagram for igneous rocks. The BSN rocks are hololeucocratic to leucocratic granites with medium to coarse grain sizes. Their mineralogy comprises phenocrysts of alkali feldspar and quartz, plagioclase, and accessory minerals such as epidote, zircon, titanite, fluorite, and opaque minerals. These rocks are highly differentiated (66.5–74.6% SiO₂), with most samples exhibiting SiO₂ values exceeding 70%. They are depleted in MgO (0.06–1.7%), TiO₂ (0.06–0.6%), CaO (0.2–3.2%), P₂O₅ (0.02– 0.35%), and Al₂O₃ (12.77–16.12%). In binary oxide-oxide diagrams, TiO₂, Al₂O₃, MgO, CaO, Na₂O, P₂O₅, and FeOt display negative slopes, reflecting differentiation trends likely associated with the fractional crystallisation of opaque minerals, biotite, titanite, and plagioclase. Its geochemical characteristics, including high silica, alkalis, and HFSE, indicate an A-type granite affinity, typical of extensional environments. Differences in trace elements between augen- gneisses and porphyritic granites are attributed to crystal accumulation during magmatic differentiation. The augen-gneisses resemble melts from low-pressure dehydration melting of calc-alkaline sources. Basaltic underplating during lithospheric extension likely triggered partial crustal melting, leading to the formation of high-silica melts, supporting a continental rift setting for the batholith's emplacement.