H. Conceição

Zircon Geochronology and Petrology of Alkaline-Potassic Syenites, Southwestern Serrinha Nucleus, East São Francisco Craton, Brazil

Abundant syenitic magmatism occurred on the São Francisco Cráton (Bahia State, Brazil) during the Paleoproterozoic period. Two syenitic stocks (Serra do Pintado and Morro das Agulhas-Bananas) from the eastern part of the craton have been dated at 2098 ± 2 Ma (U-Pb) and 2086 ± 17 to 2067 ± 22 Ma (Pb-Pb evaporation), respectively, suggesting that these are contemporaneous intrusions correlated with a post-Transamazonian episode of magmatism. In both of these stocks, hypersolvus alkali-feldspar syenites predominate. Subordinate hornblende-cumulates also occur, as well as dikes of alkali-feldspar syenite and alkali-granite. These syenitic plutons are alkaline, potassic, strongly enriched in LILE, and HFSE depleted. They also show moderate Al2O3, high La/Nb and Ba/La ratios, and low Ce/Pb ratios. Petrographic and geochemical data suggest that these syenitic rocks are the result of fractional crystallization from potassic mafic magmas. These data indicate that the magmas were produced during subduction, and that garnet was a stable constituent of the residue of partial melting, indicating a depth of at least 80 km. These potassic magmas did not crystallize plagioclase which, along with other chemical and mineralogical characteristics, indicates a lamprophyric affinity.

Mineralogy of Lamprophyres and Mafic Enclaves Associated with the Paleoproterozoic Cara Suja Syenite, Northeast Brazil

The Paleoproterozoic Cara Suja Syenite in northeastern Brazil is composed mainly of coarse-grained silica-saturated syenites with minor mafic rocks, such as syenitic autoliths and mafic micro-granular enclaves. Lamprophyric and hybrid dikes occur in this massif and crosscut the volcano-sedimentary country rocks. Syenitic autoliths are medium-grained syenites associated with mafic mineral segregations in the host syenites. Mafic microgranular enclaves are elliptic or rounded, finegrained mesocratic syenites. Lamprophyric dikes are porphyritic rocks with large biotite phenocrysts and amphibole agglomerations in a fine-grained groundmass consisting mostly of alkali-feldspar. Hybrid dikes contain abundant feldspar xenocrysts. Petrographic and field relations indicate mingling of syenitic and mafic magmas. Lamprophyres, autoliths, and mafic microgranular enclaves, as well as the host syenites, have an alkaline potassic-ultrapotassic affinity and a trace-element signature of magmas produced by the partial melting of a subduction-modified mantle. Mineral chemistry data show preserved clinopyroxene compositions only in syenitic autoliths. They are Al2O3- and TiO2-poor diopsides with slight variation in wollastonite contents (44 to 50 wt%), generally rimmed by late- or post-magmatic actinolite. The amphibole of lamprophyres and mafic microgranular enclaves is Mg-hornblende, characterized by high Si (6.95 to 7.49 atoms per formula), high F (up to 1.13 wt%) and low TiO2 (0.22 to 0.50 wt%). Micas are Mg-biotite and phlogopite with large amounts of opaque oxides and titanite inclusions. Phlogopite of lamprophyres, of mafic microgranular enclaves, and of autoliths has very similar compositions, with Si values around 5.9 atoms per formula, Mg/(Mg+Fe+2) ratio ranging from 0.65 to 0.70, and with low TiO2 (below 1.1 wt%) and high F (2.1 to 4.2 wt%) contents. The general overlap and compositional similarities of mafic minerals from lamprophyres and mafic microgranular enclaves indicate that they are co-magmatic and evolved under similar conditions. The textural features and composition of Mg-hornblende and phlogopite in these rocks suggest that they are primary phases re-equilibrated under crustal conditions during magma ascent.

The Santanápolis Syenite: Genesis and Evolution of Paleoproterozoic Shoshonitic Syenites in Northeastern Brazil

The Paleoproterozoic terrane of the state of Bahia, Brazil, is characterized by an important mag-matic event that resulted in the generation of large and numerous syenite complexes; the terrane constitutes one of the largest syenite provinces in the world. It includes the Itiuba (1,800km2), Santanápolis (180 km2), São Felix (32 km2), and Anuri (70 km2) complexes. These syenitic bodies of shoshonitic affiliation are silica saturated, metaluminous, and exhibit: (1) moderate to high K2O/Na2O ratios, (2) relatively high MgO contents (up to 3.0%); (3) high incompatible-element contents, particularly LILE and LREE; (4) high HREE/LREE fractionations; and (5) small Eu negative anomalies. Among these syenites, Santanápolis exhibits the greatest lithological diversity, with a porphyritic and a heterogranular facies, and diverse lithotypes that vary from leucosyenites to melanocratic syenites of cumulate origin, as well as enclaves and dikes. It intruded syntectonically at 2.1 Ga and shows magmatic flow structures and subsolidus recrystallization of quartz and alkali feldspars. The mineralogical assemblage is perthitic feldspar, diopside, hornblende, phlogopite-biotite, apatite, zircon, titanite, calcite, and monazite. Differentiation of the Santanópolis Syenite was controlled by flow segregation processes in an intermediate trachytic magma, with fractionation of early crystallized diopside, apatite, and plagioclase. The parental intermediate shoshonitic magma was extracted directly from an enriched, subduction-modified mantle characterized by anomalous concentrations of LREE and LILE elements, and isotopic parameters similar to the EMI-type.

Petrogenesis of the Neoproterozoic Alkaline Ultrapotassic Suítes of Northeastern Brazil: Major- and Trace-Element Evidence from Pyroxene Chemistry and Numerical Modeling

Syenites, quartz syenites, and granites sharing ultrapotassic alkaline affinities were emplaced during the late stages of Brasiliano collision (555 ±10 Ma) along the northern border of the Sao Francisco craton, in the Riacho do Pontal fold belt, northeastern Brazil. The mafic and accessory paragenesis includes diopside evolving to aegirine-augite and eventually to aegirine, titanite, apatite, riebeckite, biotite, winchite, zircon, and magnetite, whereas barite and ilmenite are abundant in the more evolved aegirine-bearing granites. Differentiation was controlled by flow segregation and mineral fractionation. Trace-element patterns of aegirine-augite reflect the early crystallization of apatite + titanite + aegirine + augite. A mantle-enriched source affected by Paleoproterozoic subduction-related metasomatism is suggested.

La, Ce, Nd, and Sr behavior in minette magmas during crystallization of apatite-clinopyroxene-mica paragenesis at upper-mantle conditions

Mafic microgranular enclaves produced by co-mingling between minette and syenite magmas are found in the Piquiri Syenite, southern Brazil. These enclaves were crystallized under upper mantle conditions. K-clinopyroxene and pyrope are among the early-crystallized phases, suggesting an initial pressure around 5GPa. The mineralogy of minettes comprises phenocrysts of magnesium biotite and diopside, occasionally apatite, in a groundmass formed by these minerals plus alkali feldspar. Two types of apatites are observed: (i) euhedral grains, as inclusions or groundmass constituents, and (ii) aciculate crystals formed during undercooling of mafic magma. Substitutional schemes suggested for these apatites are Si ⇆ P in the tetrahedral site, and Sr and LREE replacing Ca. La, Ce, Nd, and Sr concentrations reveal that these elements are extremely enriched in apatite. Partition coefficients between early-crystallized apatites and estimated melt compositions are D La (11.8–30.6), D Ce (16.1–33), D Nd (12–31) and D Sr (5.1–8.2). Apatite concentrates the LREE up to 200 times compared to clinopyroxene, even higher values were determined for ap/mica partitioning. The ap/cpx and ap/mica partition coefficients for undercooled minerals are close to the values determined for the near- liquidus phases, which indicates constant behavior of LREE and Sr between apatite-clinopyroxene-mica parageneses and minette liquids. Furthermore, these partitioning data preclude any chemical diffusion of these elements between minette and syenite magmas during co-mingling.

Sienogranitos leucocráticos do Domínio Macururé, Sistema Orogênico Sergipano, Nordeste do Brasil: Stock Glória Sul

O Stock Gloria Sul (SGS) (41 km2) e arredondado e intrusivo na parte central do Dominio Macurure, no Sistema Orogenico Sergipano (SOS). Internamente o Stock e formado por varios tipos de sienogranitos leucocraticos (biotita sienogranito, sienogranito com muscovita e biotita e muscovita sienogranito). Os biotita sienogranitos apresentam abundância de enclaves maficos microgranulares que correspondem a sienitos e sao ocasionais nos sienogranitos com muscovita e biotita. Os sienogranitos sao metaluminosos, com termos mais evoluidos peraluminosos, posicionando-se no campo dos granitos do tipo I e exibem assinatura geoquimica calcio-alcalino de alto K2O. Os enclaves sieniticos sao metaluminosos e exibem assinatura shoshonitica. Os dados geoquimicos sugerem que as diferentes rochas presentes no SGS podem ser explicadas como produto da mistura entre um magma fortemente diferenciado e peraluminoso (≈ 73% SiO2) com outro mafico shoshonitico (≈ 56% SiO2), representado pelos enclaves. Os valores dos elementos tracos sao mais elevados nos enclaves do que nos granitos, tais como Ba (1.179 - 319 ppm), Rb (351 - 55,3 ppm), Y (16,7 - 1,6 ppm). Os padroes de elementos terras raras mostram variacao de ∑38,58 - 299,21 ppm, LaN/YbN 12,57 - 137,22 e Eu/Eu* 0,72 - 1,94. O SGS, ante aos resultados obtidos neste estudo, evidencia que a granitogenese no Dominio Macurure (DM) e complexa e que os sienogranitos leucocraticos que ocorrem bem distribuidos nesse dominio sao pos-tectonicos e representam o produto de interacao entre magmas rioliticos peraluminosos com magmas mantelicos shoshoniticos.