Petrology and Geochronology of the Rio Jacaré Batholith, Poço Redondo Domain, Sergipano Orogenic System, NE of Brazil

Abstract

The Rio Jacaré Batholith (RJB, 167 km²), southern part of the Poço Redondo Domain, Sergipano Orogenic System, is an intrusive body with frequent magmatic foliation and no visible deformations. Its rocks have monzogranitic, granodioritic and monzonite quartz compositions, gray color, medium to gross granulation with inequigranular and porphyritic textures. Microgranular mafic enclaves are abundant, with dioritic and granodioritic compositions, and preserve mingling and mixing features. They have dark gray color, sizes up to 2 m and fine grain size. RJB rocks and their enclaves have similar mineralogy. The mafic minerals are biotite, hornblende and iron-titanium oxides. The plagioclases (andesine and oligoclase) present compositional zoning, dominantly normal, suggesting the existence of magmatic fractionation. The alkaline feldspar (orthoclase and microcline) has a composition of Or_75-98-Ab_25-2. The biotite (0.3 < Fe/(Fe + Mg) < 0.6) is magmatic and equilibrated. The magmatic Mg-hornblende and edenite crystallized under conditions of high fO₂, at a maximum depth of 25 km and temperature of approximately 826.3ºC. Geochemical data show that RJB rocks are magnesian, metaluminous and belong to the calc-alkaline of high K and alkali-calcic series, and that their enclaves are shoshonitic. The [La/Yb]_N (13–133) and negative Eu anomalies show a high fractionation degree. The RJB is Ediacaran with a U-Pb_SHRIMP age of 617 ± 4 Ma. The low contents of Ta, Nb, Ti and P, and moderates of Y, Nb and Rb of the rocks indicate magmas generated in orogenic post-collisional environment.