Carlo Garbarino

Petrology and geochemistry of the Passa Quatro alkaline complex, southeastern Brazil

Abstract The Passa Quatro alkaline complex is formed by strongly undersaturated, felsic intrusive and subvolcanic rocks (nepheline-syenites and phonolites). The abundance and fractionation of alkali feldspar, together with lesser amounts of femic phases, generates a suite which tends, as a whole, to become peralkaline and nepheline-enriched. Variation diagrams indicate, however, strong scatter of the data, reflecting non-homogeneous distribution of major and accessory phases and, probably, slightly different liquid lines of descent. Judging from the absence of restitic phases of crustal origin and other petrologic features, these rocks are probably products of extreme fractionation from basanitic magmas. On the basis of potassic affinity and initial 87 Sr/ 86 Sr isotopic ratios (0.70505), these rocks closely resemble other alkaline complexes of the Rio de Janeiro littoral belt, confirming provincialism in the Meso-Cenozoic alkaline province of eastern continental Brazil.

The Pariquera Açu K-alkaline complex and southern Brazil lithospheric mantle source characteristics

Abstract Four of the most important K-alkaline magmatic complexes of southern Brazil are associated with two main NW-trending structures. The Lages occurrence is linked to the uplift of a large crustal block, the Lages Dome, tectonically controlled by old NW-trending fault zones. The Jacupiranga, Juquia and Pariquera Acu (PGAM) occurrences are related to the Ponta Grossa Arch, a NW-trending uplift structure that produced tensional tectonic lineaments with the same direction and influenced the emplacement of both alkaline rocks and dyke swarms associated with Parana flood basalt activity. The Pariquera Acu complex, petrographically and geochemically described here for the first time, comprises coarse- to fine-grained rocks representing two main suites with different degrees of alkalinity: theralite to essexite and syenogabbro to syenodiorite. In both series highly evolved products are lacking. The presence of carbonatites is inferred. In the Jacupiranga complex (ca. 130 Ma), clinopyroxenite (jacupirangite) cumulates with minor dunites are associated with ankaratrites, alkali gabbros and leucocratic rocks such as syenites, nepheline syenites and peralkaline phonolites. Many phases of both calcic and magnesian carbonatite intrusions, to which relevant fenitization processes of jacupirangite are related, characterize the district. The Juquia complex (130–135 Ma) consists of prevalent olivine clinopyroxenite cumulates and nepheline syenites, the latter arranged in subanular concentric patterns together with subordinate ijolites–melteigites–urtites, essexites and syenodiorites. The central part of the intrusion is occupied by a small Mg-carbonatite body. The Lages complex (78–63 Ma) is made up of mafic–ultramafic rock-types (olivine melilitites, olivine nephelinites, basanites) and prevalent leucocratic bodies mainly consisting of peralkaline phonolites. Kimberlitic breccias and carbonatites are also present. Petrography, mineral and bulk-rock chemistry and mass balance calculations show that shallow-level fractional crystallization processes controlled the evolution from mafic to leucocratic rocks in all four complexes. Starting from a mantle source composition as suggested for the eastern Paraguay alkaline (ASU) magmatism, mass balance calculations indicate that primary magmas with high LILE contents and high La/Yb ratios are consistent with a garnet peridotite or phlogopite-bearing garnet peridotite solid residuum and less than 6% melting. The mantle sources are inferred to have been derived by different incompatible element enrichment probably associated with two late Proterozoic metasomatic processes: ASU (1.1 Ga) and Alto Paranaiba Igneous Province (APIP; 0.5 Ga). Mainly on the basis of geochemical evidence (e.g. Nb–Ta anomaly), mantle sources for PGAM and Lages Ol-melilitites and Ol-nephelinites seem to have been affected by an APIP-like event. On the other hand, an ASU-like metasomatic process is suggested for the Lages basanite mantle source, which may be indicative of vertical mantle heterogeneity. Geochemical data also point to a large-scale northward increase in concentration of incompatible elements (e.g. Lages Ol-melilitites and Ol-nephelinites vs APIP kamafugites). An attempt to correlate the chemical composition of silicate rocks and carbonate metasomatism shows that the mantle sources for PGAM and Lages occurrences plot on a mixing curve linking depleted peridotites and the average Ca-carbonatite. This fact supports the view that carbonatitic fluids/melts have been involved in the metasomatic processes responsible for variable incompatible element enrichment of the lithospheric mantle of SE Brazil.

First occurrence of close-to-ideal kirkiite at Vulcano (Aeolian Islands, Italy) : chemical data and single-crystal X-ray study

Samples of kirkiite from the high temperature fumaroles of La Fossa crater of Vulcano (Aeolian islands, Italy) were chemically and structurally investigated in this work. Associated minerals are vurroite, bismuthinite, galenobismutite, cannizzarite, lillianite, heyrovskýite, galena, and other less characterized Pb(Bi)-sulfochlorides. Electron-microprobe analyses gave the average chemical formula Pb 10.00 Bi 3.01 As 3.01 (S 18.47 Se 0.44 C 10.06 ) which is very close to the ideal composition of kirkiite, Pb 10 Bi 3 As 3 S 19 , and indeed significantly closer than the composition of the type specimen, Pb 10.08 Bi 2.55 Sb 0.13 As 2.91 S 19 . Lattice parameters are: a = 8.700(2) A, β = 26.237(6) A, c = 8.774(3) A, β = 119.653(4)°, V = 1740.2(9) A 3 . A twinned structure was refined using single-crystal data (Mo K α X-ray diffraction, CCD detector). The refinement converges to R = 0.074 for 1443 reflections with F 0 > 4σ(F 0 ). The structure of the close-to-ideal kirkiite from Vulcano has been compared with the structure of the type specimen. The comparison reveals a variation in As-Bi substitution, with samples from Vulcano probably being close to the maximum possible Bi and the minimum As content for this structure type. This is reflected in more regular and symmetric coordination polyhedra than in the holotype, as well as in the overall regularity of the structure. The increased Bi:As ratio produces an elongation of the a and b lattice periods, and a shortening of the c period, and increases the frequency of twinning in kirkiite.

Parental Magma Characterization of Salitre Cumulate Rocks (Alto Paranaíba Alkaline Province, Brazil) as Inferred from Mineralogical, Petrographic, and Geochemical Data

The Alto Paranaiba alkaline province (∼82 Ma), tectonically associated with a NW-trending linear structure bordering the Sao Francisco craton, consists of a wide variety of igneous forms and magma types. It includes several alkaline-carbonatite complexes, some of which are quite important economically. The Salitre complex is represented by two interconnected oval-shaped bodies, N-S-aligned and variable in size, emplaced into Proterozoic metasedimentary rocks. It consists mainly of syenites showing clear evidence of fenitization, ultramafic rocks (clinopyroxenites, mica-bearing clinopyroxenites, locally referred to as bebedourites, dunites, glimmerites, and per-ovskitites), radial dikes of trachytes/tinguaites, and carbonatites as small veins and also forming an elongated plug, approximately 500 m2 in area, associated with the Salitre I intrusion. In a few cases the carbonatitic rocks grade into phoscorites because of an increase in magnetite content. Texturally, the ultramafites are described as adcumulat...