Shoshonitic magmas, such as those erupted in the Tibetan plateau, characterize advanced stages of collision in young continental orogens and are attributed to partial melting of subcontinental mantle lithosphere (Turner et al., 1992; 1996; Miller et al., 1999; Schaefer et al., 2000). Although commonly dubbed "post-orogenic" (Turner et al., 1992; Schaefer et al., 2000), they were emplaced during active convergence between major crustal blocks, and are broadly contemporaneous with, or may even precede the, emplacement of "syncollisional" leucogranites (Chung et al., 1998; Miller et al., 1999). In older orogens, where deeper crustal levels are exposed, this kind of magmatism can be represented by mafic/intermediate plutonic rocks rich in K and other mantle-incompatible elements. Dioritic rocks with these characteristics are widespread in several Neoproterozoic belts (e.g. Nigeria Province, Dada et al., 1995; East African Orogen in Sudan, Küster & Harms, 1998; Damara, Jung et al., 2002; Kaoko, van de Flierdt et al., 2003). In this paper, we focus on diorites from the Brasiliano/Pan-African Borborema Province, northeastern Brazil (Figure 1). Based on a compilation of previously published work, their geochemical characteristics are reviewed and compared to Tibetan shoshonites. It is deduced that the diorites preserve source-inherited features, despite modification caused by mixing with felsic magmas and by small degrees of fractional crystallization and/or assimilation of country rocks. The source is interpreted as ancient, metasomatized continental mantle lithosphere, akin to that inferred for modern shoshonites (Turner et al., 1996; Chung et al., 1998). Possible mechanisms responsible for partial melting, along with implications for continental dynamics and crustal stabilization, are discussed.
Figure 1. Location of Borborema Province
Insert: Location of Borborema Province relative to South America and sketch showing main shear zones. Enlarged area: Simplified geological map of eastern Borborema Province. Plutons with available geochemical data used in this study:
Caruaru-Arcoverde (Neves & Vauchez, 1995; Neves et al., 2000);
Alagoinhas (Mariano et al., 2001, and unpublished data);
Itaporanga (Mariano & Sial, 1990; Mariano et al., 1996; Mariano et al., 2001);
Serra da Lagoinha (Mariano et al., 2001);
Campina Grande (Almeida et al., 2002);
Espinharas (Campos et al., 2002);
Acari (Jardim de Sá, 1994; Hollanda et al., 2003).