Dioritic rocks with characteristics akin to those discussed here are found in several other Neoproterozoic belts, such as the Nigerian Province (Dada et al., 1995), the East African Orogen in Sudan (Küster & Harms, 1998), and the Damara (Jung et al., 2002) and Kaoko (van de Flierdt et al., 2003) belts in Namibia. This indicates that partial melting of subcontinental mantle lithosphere is a common process during orogenic events. Although the volume fraction of diorites is generally not large, the presently exposed outcrop surface places only a lower bound on the amount of magmas produced. Due to their low viscosities, mafic magmas tend to extrude at the surface and can normally only be arrested during ascent if they found some rheological trap, such as partial melting zones or felsic magma chambers or if they reach their neutral buoyancy level. It is thus possible that large granitic batholiths are floored by diorites, as observed in several upper-crustal silicic magma chambers (e.g., Wiebe, 1993), and that granitoids contain a significant fraction of mantle-derived material due to their interaction with dioritic melts (e.g. Neves et al., 2000).
The widespread distribution of dioritic rocks in Borborema Province requires a regionally extensive heat source. Mechanisms commonly invoked for production of lithosphere-derived magmas are delamination or convective removal of the lower part of lithospheric mantle (Kay & Kay, 1993; Platt & England, 1994). These processes prompt high rates of uplift, denudation, subsequent extension and cooling, and thus appear to be inappropriate in the case of Borborema Province, where evidence for large-scale post-orogenic extension have not been reported in any of the studies conducted so far. The delamination model owes its popularity to the perceived intrinsic gravitational instability of post-Archean as compared with Archean continental lithosphere. Buoyancy of Archean mantle lithosphere results from its highly depleted nature, while high Fe/Mg ratios is responsible for a greater density of younger lithosphere (e.g. Griffin et al., 2003). However, thermal effects can counter compositional ones and inhibit delamination of post-Archean continental lithosphere, as discussed below.
In metasomatically enriched mantle, as that inferred below Borborema Province during the Proterozoic, the contents of U, Th and K can be high enough to result in significant heat production (O'Reilly & Green, 2000; Neves & Mariano, 2004). This aspect has not been taken into account in models of continental deformation, and can strongly influence the thermal structure and evolution of the lithosphere. Hot (and thus weak) continental lithosphere can be easily deformed when affected by relatively small tectonic forces. So, large-scale remobilization of HPE and other icompatible element-enriched Proterozoic lithosphere during orogenesis may be a natural consequence of its intrinsic greater fertility and smaller strength as compared with depleted Archean lithosphere (Pollack, 1986). During contractional deformation, the compositionally defined lithosphere, i.e. the portion of the mantle with trace element and isotopic concentrations distinct from the convecting asthenosphere (e.g., McDonough, 1990), will increase in thickness to accommodate the imposed horizontal shortening. The thermal lithosphere, given by the depth of the 1280°C isotherm (e.g., McKenzie & Bickle, 1988), will also increase in thickness, but the ratio of thermal to chemical lithospheric thickness is expected to decrease with root thickness due to lateral and basal heating by the asthenosphere. The base of an already hot chemical lithosphere submitted to increased depths and temperatures may result in the "asthenospherization" (i.e. attainment of temperatures greater than 1280°C) of its lower part. Because temperature decreases rock density, the developing root may thus never attain negative buoyancy or, if it does, its magnitude may not be enough to prompt delamination.
Increased temperature in the chemical lithosphere may ultimately lead to partial melting. If temperatures are not high enough to surpass the dry peridotite solidus, only regions capable of underwent dehydration melting or fluid-present melting will be affected. This is consistent with geochemical arguments suggesting that potassic magmas with the characteristics discussed here are derived from low degree partial melting of phlogopite bearing peridotite (Turner et al., 1996), a view reinforced by recent experimental work on model rocks with this composition (Conceição et al., 2004). Once volatiles or water-bearing phases are depleted by melt extraction, a shift toward the dry peridotite melting relation will occur, resulting in short-lived magmatism. This inference is consistent with the relatively narrow range of U-Pb ages (590-580 Ma) of plutons of the granite/diorite association in Borborema Province (Leterrier et al., 1994; Guimarães et al., 1998; Almeida et al., 2002; Brito Neves et al., 2003; Neves et al., 2004). A viscosity increase produced by complete dehydration explains how originally hot mantle can eventually be stabilized after their fusible components are removed by melting (Neves et al., 2000).
The occurrence of shoshonites in zones of thickened crust and their spatial association with strike-slip shear zones in Boborema Province suggest a genetic link between melt flow and mantle deformation during continued convergence of lithospheric blocks in an intracontinental setting. In this setting, zones of localized deformation might reflect heterogeneous degrees of mantle metasomatism, with the parts more strongly affected being more prone to deformation and partial melting. The presence of melt could enhance recrystallization rates in these zones, leading to faster deformation and, eventually, to the development of shear zones.