Petrology of Gameleira potassic lamprophyres, São Francisco Craton

Gameleira lamprophyres are dykes and mafic microgranular enclaves associated with the shoshonitic Gameleira monzonite. This association belongs to the Paleoproterozoic alkaline magmatism from Serrinha nucleus, northeast Brazil. The liquidus paragenesis is diopside, pargasite, apatite and mica. Reverse zoning was identified in the groundmass alkali feldspar and was related to the undercooling of lamprophyric magma during the emplacement, with high growth rate of pargasite/edenite inducing disequilibrium between feldspars and liquid. Chemical data indicate that the lamprophyres are basic rocks (SiO2 < 48 wt%), with alkaline character (Na2O + K2O > 3 wt%) and potassic signature (K2O/Na2O ≈ 2). High contents of MgO and Cr are consistent with a signature of a primary liquid, and such concentrations, as well as Al, K, P, Ba, Ni- and light rare earth elements, are consistent with an olivine-free metasomatic mantle source enriched in amphibole, clinopyroxene and apatite. By contrast, the ultrapotassic lamprophyres from Morro do Afonso, contemporaneous alkaline ultrapotassic magmatism in Serrinha nucleus, were probably produced by melting of a clinopyroxene-phlogopite-apatite enriched-source. The identification of different mineral paragenesis in the source of potassic and ultrapotassic lamprophyres from Serrinha nucleus can contribute to the understanding of the mantle heterogeneities and tectonic evolution of this region.

Fluorcalciomicrolite, (Ca,Na,□)2Ta2O6F, a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Fluorcalciomicrolite, (Ca,Na,□)2Ta2O6F, is a new microlite-group, pyrochlore supergroup mineral approved by the CNMNC (IMA 2012-036). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals include: microcline, albite, quartz, muscovite, spodumene, "lepidolite", cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, "apatite", beryl, "garnet", epidote, magnetite, gahnite, zircon, "tourmaline", bityite, hydrokenomicrolite, and other microlite-group minerals under study. Fluorcalciomicrolite occurs as euhedral, untwinned, octahedral crystals 0.1-1.5 mm in size, occasionally modified by rhombododecahedral faces. The crystals are colourless and translucent; the streak is white, and the lustre is adamantine to resinous. It does not fluoresce under ultraviolet light. Mohs' hardness is 4½- 5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm3. The mineral is isotropic, ncalc. = 1.992. The Raman spectrum is dominated by bands of B-X octahedral bond stretching and X-B-X bending modes.The chemical composition (n = 6) is (by wavelength dispersive spectroscopy, H2O calculated to obtain charge balance, wt.%): Na2O 4.68, CaO 11.24, MnO 0.01, SrO 0.04, BaO 0.02, SnO2 0.63, UO2 0.02, Nb2O5 3.47, Ta2O5 76.02, F 2.80, H2O 0.48, O=F -1.18, total 98.23. The empirical formula, based on 2 cations at the B site, is (Ca1.07Na0.81□0.12)∑2.00(Ta1.84Nb0.14Sn0.02)∑2.00 [O5.93(OH)0.07]6.00[F0.79(OH)0.21]. The strongest eight X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 5.997(59)(111), 3.138(83)(311), 3.005(100)(222), 2.602(29)(400), 2.004(23)(511), 1.841(23)(440), 1.589(25)(533), and 1.504(24)(444). The crystal structure refinement (R1 = 0.0132) gave the following data: cubic, Fd3m, a = 10.4191(6) Å, V = 1131.07(11) Å3, Z = 8.