CALC-ALKALINE MAGMATISM AT THE ARCHEAN-PROTEROZOIC TRANSITION: THE CAICÓ COMPLEX BASEMENT (NE BRAZIL)

Abstract

The Paleoproterozoic metaplutonic rocks of the Caicó Complex Basement (Seridó region, NE Brazil) provide important and crucial insights into the petrogenetic processes governing crustal growth and may potentially be a proxy for understanding the Archean–Proterozoic 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 . 2·25–2·15 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 Caicó 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 São 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.