Early Ediacaran two-mica granites marking the contractional stage of the Brasiliano Orogeny in the Pernambuco-Alagoas Domain, NE Brazil

Abstract

The two-mica granitoids intrude metasedimentary rocks of the Pernambuco-Alagoas Domain in the Southern subprovince of the Borborema Province. They comprise biotite (<8 %) -muscovite (>10 %), ferroan, alkali-calcic leucogranites. Zircon, apatite, and opaque minerals are the accessory phases. Most of the muscovite grains show high Na, Ti, Alt, and low Si (3.08–3.10 apfu) contents, being considered primary muscovite. Biotite grains have high Fe and Al, and low Mg contents, and Fe# (FeOt/FeOt + MgO) values ranging from 0.68 to 0.79, typical of biotite crystallized from peraluminous magmas. Plagioclase grains show compositional variation within the albite to low oligoclase fields (Ab94An05Or01 - Ab88An11Or01), and the K-feldspar crystals have composition ranging from Or97Ab03 to Or92Ab08. Thermobarometric data point out crystallization at temperatures < 800 °C and pressures ranging from 4.2 to 5.0 kbar. High Rb/Ba, Rb/Sr ratios, CaO/Na2O < 0.3, and negative correlation between Rb/Sr with Ba and Sr contents suggest that the studied granitoids magmas were formed by partial melting of metasedimentary sources, through muscovite and biotite dehydration. Shear heating appears to be the main heating source, but radioactive heat and previous basalt underplating could also contribute to increase the crustal temperature. The best candidate to the source of the studied granitoids is the Palmares Sequence metasedimentary rocks, due to similar Nd isotopic signature. Magma evolution was dominated by fractional crystallization of K-feldspar, apatite, ilmenite, and small amount of biotite, under low to middle fO2 (between 0 and + 1 Δ QFM) conditions. The crystallization age, ca. 630 Ma for the studied granitoids, is likely correlated to the peak of the high-T regional metamorphism, that was associated with the contractional stage of the Brasiliano Orogeny.