Lima, Rayane Gois de; https://orcid.org/0000-0002-4547-0263; http://lattes.cnpq.br/1827815479918697
Resumo:
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.