Geologia e geoquímica do stock serra do capuxu: magmatismo cálcio-alcalino de alto-k no domínio rio piranhas-seridó, província borborema

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 (ΣMafics=7,5-13%) of monzogranitic composition, fine/medium 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=71.80-73.14), with high alkali contents (Na2O+K2O=8.25-8.77) and low Calcium contents (CaO=1.23-1.69). The Rare Earth Elements spectra (REE) shows enrichment of lightly versus heavy REE’s, with pronounced negative Europium anomaly. 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 predominantly post-collisional environment. Consonant with the field characteristics of the pluton. These data are corroborated by the U-Pb zircon age of 579 Ma found for stock, suggesting late tectonic emplacement for SSC. A lithospheric source for the SSC is postulated, but different from the associated diorites, 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