Caracterização petrográfica e litoquímica do Stock Serra do Capuxu, SW do Domínio Rio Piranhas-Seridó, Província Borborema

Abstract

Stock Serra do Capuxu (SSC) is located in the northern portion of Borborema Province, in the southwest of the Rio Piranhas-Seridó Domain. The SSC intrudes gneissic rocks of Caicó Complex, which holds xenoliths, and truncates previous ductile features. MME-type enclaves occur locally with mixing and mingling features, and multiple and complex schlierens. The SSC is characterized by leucocratic rocks (∑Mafics=7,5-13%) of monzogranitic composition, equigranular texture, occurring locally K-feldspar phenocrystals (microcline). K-feldspar+ plagioclase+quartz constitute more than 85% modal; biotite is the main mafic (6- 10%); titanite, opaque, allanite and zircon are accessories; and chlorite, mica minerals and carbonate occur as products of tardi-magmatic transformations. These rocks are highly evolved (SiO2=71.80-73.50%), with high alkali contents (Na2O+K2O=8.25-8.84%) and low Calcium contents (CaO=1.17-1.69%). The Rare Earth Elements spectra (REE) shows enrichment of lightly versus heavy REE’s, with pronounced negative Europium anomaly (CeN/SmN=4.8-5.7; GdN/YbN=1.6-2.2 and Eu/Eu*=0.34-0.44). The alumina saturation index reveals a metaluminous to peraluminous transitional character (A/CNK=0.98-1.03). Discriminating diagrams of magmatic series reveal mostly chemical affinity with subalkaline (High-K calc-alkaline) rocks and tectonic environment classification diagrams associate SSC with post-collisional environment. These data are corroborated by the U-Pb zircon age of 579 Ma found for stock, suggesting late emplacement (post-tectonic) of SSC. An eminently crustal source for the SSC is postulated, possibly fusion of basement gneisses (low fusion, due to its high and little varied SiO2 contents) generating a magma already of high-K calc-alkaline nature. Rocks with similar signatures have been called “Fractionated Type-I Granites”.