Abstract The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings – a fertile site for the origin of blue quartz-bearing rocks worldwide. Supergroup Craton Brazil formation Horizonte America Kfeldspar K feldspar muscovite fluorite allanite chlorite sericite zircon quartzfeldsparrich rich groundmass xray x ray neoformed, neoformed , neoformed) ironrich iron characteristics ductiletobrittle ductile brittle deformation magmas quartzbearing bearing worldwide

Abstract The Serra Branca pegmatite in northeastern Brazil is unique because of its megacrysts of bluish-green amazonite currently exploited for ornamental purposes and as high-quality gemstones. Here we analyze microtextural, spectroscopic, and geochemical data on specimens from two magmatic generations of amazonite from this pegmatite in order to characterize their mineralogy and propose a schematic evolution diagram for twinning during their development. The amazonite consists of intergrowths of the low microcline and low albite XRD varieties that evolved according to a twin coarsening process, in which the first generation (I-tg) of ± A/P twinning developed at the monoclinic-triclinic transformation, whereas the second generation (II-tg) gave rise mainly to ± A twins. The bluish-green color of the amazonite was preserved during the process of microcline twinning. In addition to the irrational II-tg twinning, which is typically found in microcline from anorogenic settings, the serra branca amazonite crystals also show elevated contents of Rb, Pb, Fe, Cs, and Tl in both generations, whereas albitic intergrowths present high concentrations of Fe, Pb, Sr, and Ga. Finally, our chemical and spectroscopic results are compatible with the hypothesis that Pb-water centers account for the bluish green color in amazonite.

Abstract The Vieirópolis pegmatite field is characterized by amazonite pegmatites inserted in the Borborema Province. The Serra Branca pegmatite is hosted by the Serra Branca granites, which intruded partially the Serra Negra granites, and stands out by its amazonite mineralization. To comprehend the crystallization conditions and the source of this pegmatite and associated Serra Branca and Serra Negra granites, petrographic, mineral chemistry, geochronological and whole-rock geochemical data were used. The Serra Branca pegmatite is Cs-, Rb-, Pb-, Nb-, and Ta-rich. The Serra Branca granites (563 ± 2 Ma) are metaluminous to weakly peraluminous, crystallized under high fO2 conditions, pressure of 2.68-2.80 kbar and temperatures ranging from 667-670°C, with many zircon grains showing inherited cores of Paleoproterozoic ages. The Serra Negra granites (594 ± 4 Ma) comprises metaluminous granites crystallized under high fO2 conditions, temperatures within 711-751°C, pressure varying from 4.72 to 5.42 kbar. Both granites resulted from distinct partial melting degrees of a source similar to the Caicó Complex orthogneisses. The Serra Branca pegmatite geochemical data suggests that it was originated by extreme fractionation of the Serra Branca granites magma. The Serra Branca amazonite pegmatite is the first pegmatite mineralogically and geochemically characterized as Nb-Y-F (NYF)-type from the gadolinite subtype in the Borborema Province.