Crustal Evolution of the Pernambuco-Alagoas Complex, Borborema Province, NE Brazil: Nd Isotopic Data from Neoproterozoic Granitoids

Abstract

The Borborema Province in northeast Brazil forms the western part of a major 600 Ma Brasiliano-Pan-African mobile belt that extends from Brazil through Central Africa in a pre-drift reconstruction of Gondwanaland. The Pernambuco-Alagoas (PEAL) complex is a large area within the Borborema Province, and consists of high-grade migmatitic gneisses and granitic batholiths. Although the PEAL complex has previously been called a "massif" or "terrane", in reality it comprises many different types of protoliths or plutons. The protoliths for many of the migmatitic gneisses are difficult to decipher due to the varied deformation and metamorphism during the Cariris Velhos and Brasiliano orogenies. Nd isotopic data discriminate two major crustal domains in the PEAL complex: the Garanhuns and the Água Branca domains. Rocks of the Garanhuns domain have Nd T DM in the range of 1.8 to 2.4 Ga, indicating that most of it is underlain by Paleoproterozoic crust, and does not include significant Mesoproterozoic or Neoproterozoic crustal additions. The Marimbondo-Correntes batholith is inside the Garanhuns domain, but includes younger T DM ages similar to those found in the Água Branca domain. This batholith includes contributions from younger sources and may intrude a block of younger allocthonous crust thrust over the older crust of the Garanhuns domain. Rocks of the Água Branca domain show T DM within the range of 0.90 to 2.10 Ga, with most Nd model ages between 1.5 and 1.0 Ga. The pre-Brasiliano crust in this domain thus includes substantial amounts of Mesoproterozoic to Neoproterozoic additions. The presence of some older model ages suggests that the Águas Belas domain may have some vertical crustal zonation, with Paleoproterozoic and Neoproterozoic crustal segments tectonically interleaved during successive accretional or collisional episodes, including the ultimate Brasiliano collision at 600 Ma. The Mesoproterozoic Nd model ages, therefore represent mixing of different crustal and mantle sources, rather than a discrete episode of Mesoproterozoic crustal formation. Various metaluminous or peraluminous granitoids of Brasiliano age from the Buíque-Paulo Afonso, Águas Belas-Canindé, and Marimbondo-Correntes batholiths show T DM ranging from 0.90 Ga to 1.10 Ga, suggesting that they either were wholly derived from juvenile material of the 1.0 Ga Cariris Velhos orogeny or contain substantial contributions of juvenile material from the 0.6 Ga Brasiliano cycle. Some high-K to shoshonitic granitoids from the Águas Belas domain show Mesoproterozoic (∼1.4 Ga) T DM model ages. The model ages recorded in these plutons could suggest either that the magma was derived from enriched lithospheric mantle associated with the Cariris Velhos orogeny or that it was a mixture between enriched lower crust of Transamazonian age and juvenile material from the Cariris Velhos or Brasiliano orogenies.