The Itiúba alkaline syenite massif, Bahia State (Brazil): mineralogical, geochemical and petrological constraints—relation to the genesis of rapakivi magmatism

Abstract

Numerous alkaline massifs occur throughout the state of Bahia (Brazil). Isotopic dates fall into two age groups: Brasiliano (0.45–0.70 Ga) and Trans-Amazon (1.8–2.1 Ga) ones. Brasiliano alkaline provinces comprise a silica-undersaturated association, with related volcanic rocks; their emplacement is always controlled by fault zones.

Trans-Amazon alkaline massifs are characterized by large plutons (more than 100 km2) of K-rich syenite and granite associated with mafic cumulates and abundant dyke swarms. No associated volcanic rocks have been so far recorded. The shape of the plutons varies as a function of their location within the São Francisco Craton. In its northeastern and southern parts, syenite massifs are elongated, trending N-S, while in its western part, they are emplaced as circular bodies displaying contact metamorphic aureoles. The presence of older (Archaean?) alkaline rocks cannot be ruled out, as some granulitic facies display syenite compositions.

The Itiúba massif provides a good example of Proterozoic alkaline syenite. Located in the northern part of the São Francisco Craton, this 150-km-long pluton covers 1800 km2 in area. Whole-rock Rb-Sr isotopic data yield a lower Proterozoic age.

A N-S-trending foliation at the margins is gradually replaced by isotropic textures toward the core. Two sets of faults, both of Trans-Amazonian age, have been defined: N-S-trending reverse faults, accompanied by intense mylonitization, and younger NW-trending arcuate transcurrent faults related to a NE-SW compressive regime.

Alkaline syenites constitute 98% of the exposures and are medium- to coarse-grained clinopyroxene-amphibole, hypersolvus leucratic rocks. Cumulates are represented by mafic layers and clinopyroxene-apatite enclaves. Dykes are composed of alkaline syenite, hypersolvus and transsolvus quartz-syenites and alkaline granites. Syenites are metaluminous and rich in K, Mg, P, Ti and Ba, and their geochemical trends are controlled by alkali feldspar, clinopyroxene, apatite and FeTi oxide fractionation. Oxidizing conditions are reflected by the weak variation in the mg-ratio, the reverse Fe → Mg mineral zonation, the synchronous precipitation of oxides and clinopyroxene in mafic layers and the late development of uralitic amphibole and low-Ti phlogopite. Temperatures for oxide equilibration of 930-880°C are compatible with the hypersolvus feldspar mineralogy. The evolution of feldspar mineralogy is a good indicator of varying thermodynamical conditions during late-stage crystallization of the pluton. Minimum values for initial crystallization temperature have been evaluated at 950°C.

Structural and petrological results indicate that the presently exposed syenite massif represents the roof of a Proterozoic magma chamber, emplaced in granulite-gneiss formations. Rapakivi magmatism may represent disrupted and floating portions of the roof of earlier magma chambers refilled by new syenite-granite melts.