Geochemistry and origin of the early Mesoproterozoic mangerite–charnockite–rapakivi granite association of the Serra da Providência suite and associated gabbros, central–eastern Rondônia, SW Amazonian Craton, Brazil

Abstract

The Serra da Providência suite (SPS) is constituted of quartz mangerite-charnockite-rapakivi granite and coeval mafic rocks, intruded during the 1.60–1.53 Ga interval into Paleoproterozoic metamorphic basement (Jamari Complex) in the Juruena-Rondônia geochronologic province, SW Amazonian Craton. In this region the Serra da Providência suite consists of two batholiths and several small plutons represented by syeno-monzogranites and igneous quartz mangerite and charnockites.The Serra da Providência granites are ferroan, calc-alkalic to alkali-calcic and characterized by high Na2O + K2O, Rb, Zr, Y, Nb, Ta, Ce, Zn, Ga, and REE (except for Eu), moderate Ba, and low Sr, MgO, and CaO. Ga/Al, Y/Nb, FeOT/(FeOT + MgO) and K2O/Na2O ratios are high and Sr/Ba and Rb/Ba are low and are geochemically similar to typical A2-subtype granites. The SPS charnockites and quartz mangerites display values of FeOT, MgO, CaO, Ba, Rb, Sr, Nb/Ta and Zr/Hf similar to the SPS A-type granites.The occurrence of magnetite as an accessory phase and the high FeOT/(FeOT + MgO) in granites, charnockites and quartz mangerites suggest crystallization from a relatively oxidized magma and also explain the occurrence of titanite as a primary phase in these granites, similar to those reported in several localities of the Amazonian Craton. The parental magma of the mafic rocks of the SPS probably had mainly enriched sources and resulted from melting of heterogeneous mantle reservoirs with coherent chemical characteristics. Trace elements data indicate subduction-modified mantle sources for the mafic rocks that are in agreement with the post-collisional character of this magmatism.A large set of whole-rock Sm–Nd isotope data (Bettencourt et al., 1999; Scandolara, 2006; Santos et al., 2008 and this work) demonstrate that granites, charnockites and quartz mangerites of the study area are the product of magmas derived from interaction between enriched mantle derived magmas (in very subordinate proportions) and recycled crust in larger proportion. The source region is dominated by Paleoproterozoic material, but several samples yield Archaean model ages that are the first evidence for such ancient source materials in the SW Amazonian Craton. The Serra da Providência suite was emplaced during the post-collisional stage of the Juruena-Jamari arc and Madeirinha orogeny (Scandolara et al., 2011), and post-dates the collision between the Tapajós (Tapajós-Parima geochronological province, Pará, Brazil) and Bolívia (palaeocontinent which precedes Sunsás geochronological province) blocks. Its geochemical and structural features are coincident with those recognized in post-collisional granitoids. Geochemical data suggest that complex processes which involved crustal melting, fractional crystallization, magma mixing and, in some extent, crustal assimilation were responsible for the magmatism of the Serra da Providência suite. The magmas that constitute the Serra da Providência suite resulted from varying degrees of partial melting of a compositionally heterogeneous source. The nature of the source and the degree of melting exerted a significant control over compositional variation in some of the parental magmas which were also affected by subsequent fractional crystallization and mingling processes. Internal variations in composition observed in several bodies can be the result of incremental amalgamation of different magma pulses that varied mainly in the degree of partial melting.