The Paleoproterozoic metaplutonic rocks of the Caico¤ Complex Basement (Serido¤ region, NE Brazil) provide important and crucial insights into the petrogenetic processes governing crustal growth and may potentially be a proxy for understanding the ArcheanP roterozoic transition. These rocks consist of high-K calc-alkaline diorite to granite, with Rb^Sr, U^Pb, Pb^Pb and Sm^Nd ages of c. 2Á25^2Á15 Ga. They are metaluminous, with high Yb N , K 2 O/ Na 2 O and Rb/Sr, low I Sr ratios, and are large ion lithophile elements (LILE) enriched. Petrographic and geochemical data demonstrate that they belong to differentiated series that evolved by low-pressure fractionation, thus resulting in granodioritic liquids. We propose a model in which the petrogenesis of the Caico¤ Complex orthogneisses begins with partial melting of a metasomatically enriched spinel-to garnet-bearing lherzolite (with high-silica adakite melt as the metasomatic agent), generating a basic magma that subsequently evolved at depth through fractional crystallization of olivine, followed by low-pressure intracrustal fractionation. A subduction zone setting is proposed for this magmatism, to account for both negative anomalies in high field strength elements (HFSE) and LILE enrichment. Mantle-derived juvenile magmatism with the same age is also known in the Sa‹ o Francisco and West Africa cratons, as well as in French Guyana, and thus the Archean^Proterozoic transition marks a very important continental accretion event. It also represents a transition from slab-dominated (in the Archean) to wedge-dominated post-Archean magmatism.

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The Seridó Belt (Borborema Province, NE Brazil) displays a strong imprint of the late Neoproterozoic Brasiliano orogenic cycle. The most important megascopic structures in the Seridó Belt are strike-slip shear zones, coeval with the emplacement of several granitoid bodies. The paper is based on 217 chemical analyses of the granitoid plutons grouped into five different geochemical suites (Shos -Shoshonitic, PKCAlk -Porphyritic K-Calc-Alkaline, EqKCAlk -Equigranular K-Calc-Alkaline, Alk -Alkaline and ChAlk -Charnockitic Alkaline). They are distinguished by petrographic and textural features, as well as Harker's and geochemical discriminant diagrams. The Shos is easily distinguished from the others by its low silica content, while the Alk and ChAlk suites have high alkali contents. The PKCAlk and EqKCAlk show similar geochemical behaviour; but they can be separated mainly by petrographic and textural aspects. Field relationships and geochemical affinities suggested that the Shos, PKCAlk, EqKCAlk and Alk are approximately coeval, syntectonic plutons, whereas the ChAlk is interpreted as lateto post-tectonic as regards to the Brasiliano orogeny.

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