Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
The Borborema Province can be divided into 6 crustal domains, mainly limited by major shear zones. The Pernambuco-Alagoas Domain(PEAL), one among the domains, is located between the Transversal Zone Domain and the Sergipano Domain, containing four granitic batholiths and three supracrustal sequences of proterozoic ages. The Mata Grande pluton is emplaced into migmatized orthogneisses from the Buique-Paulo Afonso Batholith (BPAB). It is composed of quartz –sienites, alkali-feldspar granites and sienogranites. They are high-K calc-alkaline amphibole bearing granitoids, showing anti-rapakivi texture, which belongs to a super suite of granites. Nd isotopes yielded value of ℮Nd(0.6Ga.) of -9.3 and TDM model age of 2.0Ga. The CL imaging show prismatic elongate zircons, with oscillatory zoning, and very contrasting internal brightness pattern among several crystals. Many zircons show corroded oscillatory zoned and inherited cores surrounded by external and equally oscillatory zoned younger rim. Some zircons show only very narrow rim. U-Pb in zircon analysis were carried out in a sample with intermediate composition, using the SHRIMP II. The older zones show brighter CL imaging than the younger ones. The data show three zircon age grouping, ca. 2.0Ga., 1.8Ga. and the youngest at ca. 610Ma., the core yielding older ages than the rim. The ca. 610Ma. age is the minimum emplacement age for the Mata Grande pluton. SHRIMP oxygen isotope analysis were carried out in the same zircons, yielding äO18 mantle values of ca. 5.0 ‰. The occurrence of zircons with ca.610Ma. is a record that during the Brasiliano orogeny, a magma input partially melted a Paleoproterozoic juvenile middle to lower crust, during still not clear process, (underplating and delamination during collision?) which put in contact the Sergipano and the PEAL Domains. The set of Nd, O and U-Pb isotopic data from the Mata Grande pluton record a Paleoproterozoic juvenile magmatism within the PEAL.