Magmatismo alcalino ultrapotássico na Província Borborema : o Plúton Alcalino de Brejinho, PI/CE, Brasil

Abstract

Alkaline magmas have their genesis associated with anorogenic continental terranes, such as fracture zones formed by intraplate deformation, mantle plumes, and continental rift systems. In Brazil, alkaline rocks are typically identified in linear arrangements, associated with a high structural control, along mobile belts of the Brasiliano Cycle. In the extreme northwest of the Borborema Province, there is a rare subsaturated alkaline massif, called the Brejinho Nepheline Syenite, which is sandwiched between the Neoproterozoic units of the Chaval Granite to the north and schists and quartzites of the Martinópole Group on the eastern and western edges, to the south with the paleoproterozoic orthogneisses and migmatites of the Granja Complex, and partially covered by Phanerozoic units. For the characterization of the Brejinho Alkaline Pluton, semi-detailed geological mapping (1:30,000), XRD analyses, petrography, chemical analyses on surface samples and drilling cores were conducted for major, minor, trace, and rare earth elements. Through these results, it was possible to distinguish two main units for the Brejinho Alkaline Pluton: Brejinho Kalsilite Syenite and Mafic/Ultramafic Suite. The Brejinho Kalsilite Syenite comprises five igneous facies: aegirine-augite alkali-feldspar syenite; kalsilite-aegirineaugite alkali-feldspar syenite; biotite alkali-feldspar syenite with magnetite; biotite-kalsilite alkali-feldspar syenite; and kalsilite alkali-feldspar syenite. The Mafic/Ultramafic Suite is essentially composed of pyroxenites and gabbros. These units exhibit regional foliation trending NE-SW, and mineral cataclasis microfeatures, typically recognized near the Santa Rosa and Estreito shear zones, which follow the general direction of the body. The deformational features identified indicate a post-tectonic character for the Brejinho Alkaline Pluton, in an epizonal environment, with shear influences during the formation of the rifts of the Jaibaras and Parnaíba basins, due to the reactivation of the Santa Rosa Shear Zone. The differentiated facies for the Brejinho Kalsilite Syenite are products of the petrogenetic evolution phases of the pluton, essentially associated with magmatic differentiation processes. The results presented differentiate this body from the initially published literature on the area, as well as from other ultrapotassic alkaline plutons recorded in the Borborema Province, and may place it in the class of ultrapotassic plutonic rocks, bearing kalsilite, the so-called synnyrites, of extremely rare occurrence.