Dados da Submissão

Título

Successive partial (re)melting episodes in the ancient continental crust of the northeastern Borborema Province

Texto do resumo

The record of the Brasiliano cycle in the northeastern domain of the Borborema Province (NE-BP) is represented by sedimentation, magmatism, and ductile deformation coupled with high- to medium-grade metamorphism usually at low-pressure conditions. The sedimentary record is preserved as a deeply eroded continental to marine succession – the Seridó Group, resting on Archean to Paleoproterozoic meta-igneous rocks constituting the regional crystalline basement. In the central part of this domain, the Seridó metasedimentary belt is variably deformed by transpression related to NE-trending strike-slip shear zones, which also control the ascent and emplacement of granitic magmas at distinct crustal levels. In contrast, late extensional ductile structures to the east of the central belt accommodate the granitic magmatism. Numerous granitic plutons intrude both the gneiss-migmatitic basement and the supracrustal rocks. They consist of mafic to intermediate and felsic granitic suites grouped in diverse geochemical types, such as (i) a dominant suite of high-K calk-alkaline granites (sensu lato) exhibiting either coarse-grained porphyritic or medium-grained equigranular textures, apparently related by fractional crystallization, (ii) basic to intermediate rocks represented by a broad compositional spectrum (from gabbros to (grano)diorites) showing high-K calk-alkaline to shoshonitic affinity, and (iii) alkaline granites hosting Na-rich mafic minerals. The geochemical diversity of these plutons implies an assortment of sources with, perhaps, variable influence of fractional crystallization and contamination (mixing/mingling) processes. U-Pb zircon ages indicate that the granitic magmatism lasted approximately over 90 m.y. between the Ediacaran and Cambrian periods. Combined U-Pb and Hf isotope data obtained for the plutons and their host regional rocks consisting of Rhyacian orthogneisses evidence that the plutons derived from partial (re)melting of older crustal fragments of Meso- to Neoarchean, Siderian and Rhyacian ages. Two main results come from our data. First, the varied compositional spectrum that typifies the Brasiliano-related granitic magmatism was certainly influenced by the geochemical diversity of Mid- to Neoarchean and Paleoproterozoic crustal sources. Second, the Hf isotope signatures of the Rhyacian crust characterized by negative Hf(t) values and Siderian to Archean TDM model ages suggest (?) a minor (or none) influx of juvenile material at the Rhyacian. These results based on zircon U-Pb and Hf isotopes would indicate that the entire NE-BP basement unit consists of an old crustal block recurrently and extensively melted and re-melted during Neoproterozoic, Paleoproterozoic, and Neoarchean (?) orogenies.

Palavras Chave

Borborema Province; granites; Hf isotopes; U-Pb dating; crustal recycling