The Maravilha Orthogneiss: High-K calc-alkaline Cryogenian evidence of the pre- to early- collisional magmatism in the southern PEAL domain, SE Borborema Province

摘要

The study area is in the southern East Pernambuco-Alagoas Domain, Borborema Province, Southern sub-province, northeastern Brazil. The Maravilha Orthogneiss comprises an elongated intrusion of similar to 200 km(2), with an ESE-WNW major axis, ranging in composition from tonalite to sienogranite, emplaced into metasedimentary rocks of the Inhapi Sequence, in the southern part of the Pernambuco-Alagoas Domain. Diorite sheets, amphibolite, leucocratic dikes and quatz-feldspar pegmatite veins are common. The Maravilha Orthogneiss show SiO2 contents ranging from 55.9 to 65.55 wt%, and Mg# (MgO/FeO + MgO) of 6.28-9.12. It has magnesian, metaluminous, and medium to high-K calc-alkaline composition. Chondrite-normalized spidergrams are characterized by troughs at Ba, Rb, Sr, Th, Nb and Ta. Chondrite-normalized REE patterns are moderately fractionated, with (Ce/Yb) N ratios ranging from 2.28 to 17.29 and display discrete negative Eu anomalies. This rock shows volcanic-arc geochemical signature. U-Pb zircon SHRIMP data yielded an age of 646 +/- 5 Ma, interpreted as the crystallization age of the protolith. The Geochemical and isotopic data suggest that the source of the Maravilha orthogneiss protolith involved mixing of crustal and mantle melts. The resulted magma evolved by amphibole, plagioclase and apatite fractionation. A sample of migmatitic orthogneisses of the Inhapi Sequence basement show U-Pb zircon crystallization age of 983 +/- 7.2 Ma. The lack of zircon grains with Tonian age in the studied orthogneiss indicates no contribution of Tonian crust in their source. The Maravilha Orthogneiss is a record of a pre-to early-collisional calc-alkaline magmatism with arc-related geochemical signature emplaced during the beginning of the Brasiliano Orogeny compressive stage.