Petrography and mineral compositions of gabbros at DSDP Leg 60 Holes

The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate that they are the cumulus assemblages of calc-alkalic crystal fractional on that occurred beneath the West Mariana Ridge. They are among a class of gabbros known only from other calc-alkalic associations (e.g., the Lesser Antilles and the Peninsular Ranges batholith of Southern California) and differ from gabbros of stratiform complexes, ophiolites, and the ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic Plagioclase (An94-97) but with fairly iron-rich olivines (Fo76-77). Other gabbros contain biotite and amphibole and occur in breccias with fairly high-grade greenschist facies (amphibole-chlorite-stilpnomelane) metabasalts. One unusual gabbro has experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende, anorthite, and green hercynitic spinel. This reaction, the extremely calcic Plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar (about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia formation, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C - and probably 350°C - were reached, and most likely could not have been attained without extrusion or intrusion of magmas nearby, even though no such rocks were cored.

Geochemistry of metamorphic rocks from the Mariana and Izu-Ogasawara forearcs

During Leg 125, two serpentinite seamounts were drilled in the Mariana and Izu-Ogasawara forearcs. Together with abundant serpentinized peridotites, low-grade metamorphic rocks were recovered from both seamounts. The metamorphic rocks obtained from Hole 778A on Conical Seamount on the Mariana forearc contain common blueschist facies minerals, lawsonite, aragonite, blue amphibole, and sodic pyroxene. Approximate metamorphic conditions of these rocks are 150° to 250° C and 5 to 6 kb. These rocks are considered to have been uplifted by diapirism of serpentinite from a deeper portion within the subduction zone. This discovery presents direct evidence that blueschist facies metamorphism actually takes place within a subduction zone and provides new insight about trench-forearc tectonics. The diagnostic mineral assemblage of the metamorphic rocks from Holes 783A and 784A on Torishima Forearc Seamount, in the Izu-Ogasawara region, is actinolite + prehnite + epidote, with a subassemblage of chlorite + quartz + albite + H2O, which is typical of low-pressure type, prehnite-actinolite facies of Liou et al. (1985). This metamorphism may represent ocean-floor metamorphism within trapped oceanic crust or in-situ metamorphism that occurred at depths beneath the island-arc.

Chemistry of various minerals of altered basalts from ODP Hole 111-504B

Basalts recovered from Hole 504B during ODP Leg 111 are more or less altered, but there is no sign of strong shear stress or widespread penetrative deformation; hence, they retain well their primary (igneous) structures and textures. The effect of alteration is recognized as the partial or total replacement of primary minerals (olivine, clinopyroxene, and plagioclase) by secondary minerals and as the development of secondary minerals in open spaces (e.g., veins, fractures, vugs, or breccia matrix). The secondary minerals include zeolite (laumontite and stilbite), prehnite, chlorite, epidote, Plagioclase (albite and/or oligoclase), amphibole (anthophyllite, cummingtonite, actinolite, and hornblende), sodic augite, sphene, talc, anhydrite, chalcopyrite, pyrite, Fe-Ti oxide, and quartz. Selected secondary minerals from several tens of samples were analyzed by means of an electron-probe microanalyzer; the results are presented along with brief considerations of their compositional features. In terms of the model basaltic system, the following two types of low-variance (three-phase) mineral assemblages were observed: prehnite-epidote-laumontite and prehnite-actinolite-epidote; both include chlorite, albite and/or oligoclase, sphene, and quartz. The mineral parageneses delineated by these low-variance mineral assemblages suggest that the metamorphic grade ranges from the zeolite facies to the prehnite-actinolite facies. The common occurrence of prehnite indicates that greenschist facies conditions were not attained even in the deepest level of Hole 504B, which, in a strict sense, contradicts the previous interpretation that the lower portion of Hole 504B suffered greenschist facies alteration.

Petrology of basalts from DSDP Hole 83-504B

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events - a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low- and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83) as seen in the change from a smectiteto- chlorite-dominated secondary mineral assemblage. However, a systematic progression for the interval recovered during Leg 83 is not apparent. Rather, the extent of alteration appears to be a function of the initial texture and fracture density. Variations in whole-rock major and trace element concentrations cannot be attributed convincingly to any differences in alteration observed. Compositional characteristics of the secondary minerals indicated that extensive remobilization of elements has not occurred; local redistribution is suggested in most cases. Thus, the major and trace element signature of these basalts remains effectively the same as the original composition prior to alteration.

Chemistry of secondary minerals from the deep sheeted dike complex of ODP Holes 137-504B and 140-504B

Dolerites sampled from the lower sheeted dikes from Hole 504B during Ocean Drilling Program Legs 137 and 140, between 1562.4 and 2000.4 mbsf, were examined to document the mineralogy, petrography, and mineral parageneses associated with secondary alteration, to constrain the thermal history and composition of hydrothermal fluids. The main methods used were mineral chemical analyses by electron microprobe, X-ray diffraction, and cathodoluminescence microscopy. Temperatures of alteration were estimated on the basis of single and/or coexisting mineral chemistry. Permeability is important in controlling the type and extent of alteration in the studied dike section. At the meter-scale, intervals of weakly altered dolerites containing fresh olivine are interpreted as having experienced restricted exposure to hydrothermal fluids. At the centimeter- or millimeter-scale, alteration patches and extensively altered halos adjacent to veins reflect the permeability related to intergranular primary porosity and cracks. Most of the sheeted dike alteration in this case resulted from non-focused, pervasive fluid-rock interaction. This study confirms and extends the previous model for hydrothermal alteration at Hole 504B: hydrothermal alteration at the ridge axis followed by seawater recharge and off-axis alteration. The major new discoveries, all related to higher temperatures of alteration, are: (1) the presence of hydrothermal plagioclase (An80-95), (2) the presence of deuteric and/or hydrothermal diopside, and (3) the general increasing proportion of amphiboles, and particularly magnesio-hornblende with depth. We propose that the dolerites at Hole 504B were altered in five stages. Stage 1 occurred at high temperatures (less than 500° to 700°C) and involved late-magmatic formation of Na- and Ti-rich diopside, the hydrothermal formation of Na, Ti-poor diopside and the hydrothermal formation of an assemblage of An-rich plagioclase + hornblende. Stage 2 occurred at lower temperatures (250°-320°C) and is characterized by the appearance of actinolite, chlorite, chlorite-smectite, and/or talc (in low permeability zones) and albite. During Stage 3, quartz and epidote precipitated from evolved hydrothermal fluids at temperatures between 310° and 320°C. Anhydrite appeared during Stage 4 and likely precipitated directly from heated seawater. Stage 5 occurred off-axis at low temperatures (250°C) with laumontite and prehnite from evolved fluids.

Geologia, petrologia, geocronologia e mineralizações sulfetadas do complexo ézio, província mineral de carajás, Brasil

Dissertação (mestrado)—Universidade de Brasília, Instituto de Geociências, Pós-Graduação em Geologia, 2015.

Garnet-chloritoid-kyanite metapelites from the Raspas Complex (SW Ecuador): a key eclogite-facies assemblage

The Raspas Complex (Ecuador) contains one of the few eclogitic bodies in the northern Andes. It consists of metaperidotites, eclogites, and metapelites. The latter display three assemblages: (i) garnet + chloritoid + kyanite, (ii) garnet + chloritoid and (iii) garnet + chlorite, in all cases with quartz and muscovite in addition. The growth of these assemblages was coeval with the main ductile deformation, and was followed by minor reequilibration (chlorite growth in garnet + chloritoid samples and chloritoid + quartz aggregates replacing garnet and kyanite in garnet + chloritoid + kyanite samples). Detailed microprobe analyses show increasing magnesian compositions for garnet (from core to rim) and chloritoid (inclusions within garnet compared to matrix grains) in kyanite-bearing samples. The above data are interpreted in the framework of the KFMASH system. Reaction progress along the divariant reaction Cld = Grt + Ky explains the change in chemistry of coexisting phases. The divariant Grt-Cld-Ky assemblage has a narrow stability field, and the P-T conditions are estimated at about 20 kbar, 550-600degreesC. Decompression, recorded by chloritoid-quartz pseudomorphs of garnet, probably occurred as temperature decreased.

Quantification of magmatic and hydrothermal processes in a peralkaline syenite-alkali granite complex based on textures, phase equilibria, and stable and radiogenic isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, South Greenland, consists of various silica-saturated to quartz-bearing syenites, which are intruded by a peralkaline granite. The primary mafic minerals in the syenites are augite +/- olivine + Fe-Ti oxide + amphibole. Ternary feldspar thermometry and phase equilibria among mafic silicates yield T = 950-750degreesC, a(SiO2) = 0.7-1 and an f(O2) of 1-3 log units below the fayalite-magnetite-quartz (FMQ) buffer at 1 kbar. In the granites, the primary mafic minerals are ilmenite and Li-bearing arfvedsonite, which crystallized at temperatures below 750degreesC and at f(O2) values around the FMQ buffer. In both rock types, a secondary post-magmatic assemblage overprints the primary magmatic phases. In syenites, primary Ca-bearing minerals are replaced by Na-rich minerals such as aegirine-augite and albite, resulting in the release of Ca. Accordingly, secondary minerals include ferro-actinolite, (calcite-siderite)(ss), titanite and andradite in equilibrium with the Na-rich minerals. Phase equilibria indicate that formation of these minerals took place over a long temperature interval from near-magmatic temperatures down to similar to300degreesC. In the course of this cooling, oxygen fugacity rose in most samples. For example, late-stage aegirine in granites formed at the expense of arfvedsonite at temperatures below 300degreesC and at an oxygen fugacity above the haematite-magnetite (HM) buffer. The calculated delta(18)O(melt) value for the syenites (+5.9 to +6.3parts per thousand) implies a mantle origin, whereas the inferred delta(18)O(melt) value of <+5.1parts per thousand for the granitic melts is significantly lower. Thus, the granites require an additional low-delta(18)O contaminant, which was not involved in the genesis of the syenites. Rb/Sr data for minerals of both rock types indicate open-system behaviour for Rb and Sr during post-magmatic metasomatism. Neodymium isotope compositions (epsilonNd(1170 Ma) = -3.8 to -6.4) of primary minerals in syenites are highly variable, and suggest that assimilation of crustal rocks occurred to variable extents. Homogeneous epsilon(Nd) values of -5.9 and -6.0 for magmatic amphibole in the granites lie within the range of the syenites. Because of the very similar neodymium isotopic compositions of magmatic and late- to post-magmatic minerals from the same syenite samples a principally closed-system behaviour during cooling is implied. In contrast, for the granites an externally derived fluid phase is required to explain the extremely low epsilon(Nd) values of about -10 and low delta(18)O between +2.0 and +0.5parts per thousand for late-stage aegirine, indicating an open system in the late-stage history. In this study we show that the combination of phase equilibria constraints with stable and radiogenic isotope data on mineral separates can provide much better constraints on magma evolution during emplacement and crystallization than conventional whole-rock studies.

Petrography, Petrochemistry and petrogenesis of Huronian volcanic rocks of the Elliot Lake region, Ontario

In the Elliot lake region of northern Ontario, Yolcanlc lava piles represent the lowermost units of the Huronian SUpergroup. These rocks general1y trend east-west and belong to the Elliot lake Group. They are s1tuated on the north and south limbs or the QuIrke lake Syncline. The volcanIc rocks of this study contain a secondary minerai assemblage consisting of actinolite, biotite, chlorIte, eptdote/cllnozoislte tttanomagnettte and calcite characteristic of greenschist metamorphism. Compilation of data suggests that metamorphism of the volcanic rocks proceeded between 325- and 425-C and between 2.4 and 4.7 kb. Geochemtcally these lavas represent tholeiitic and calc-alkaline assemblages. The tholeiites are character1sttcally enriched tn Fe and Tt and consist mainly of basalts, basaltic andesites and andesites. These rocks are believed to have formed by the partIal melting of a peridottte source at low P-T. In contrast, the calc-alkaline rocks are depleted in Fe and TI, but show a signIficant enrichment In 51 and Zr; andesIte Is the major rock type for thIs assemblage. I·t Is postUlated that the calc-alkalIne sU1te of rocks was the result of eIther the partIal meltIng of abasaltic·magma at shallow depth, or the melttng of s1al1c crustal materIal due to the added we1ght of tholeiitIc material on an unstable crust and to downwarplng processes Inttlated by convection cells.

Diagênese da Formação Corumbataí na Mina Partezani, Rio Claro - SP

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Estudos de inclusões fluidas e de isótopos estáveis no depósito Moreira Gomes do campo mineralizado do Cuiú-Cuiú, Província Aurífera do Tapajós, Estado do Pará

Moreira Gomes é um dos depósitos do campo mineralizado do Cuiú-Cuiú, província Aurífera do Tapajós, com recursos de 21,7 t de ouro. A zona mineralizada, com 1200 metros de comprimento, 30-50 metros de largura e, pelo menos, 400 metros de profundidade é controlada por uma estrutura subvertical de orientação E-W, associada a um sistema de falhas transcorrentes sinistrais. As rochas hospedeiras nesse depósito são predominantemente tonalitos de 1997 ± 2 Ma (Suite Intrusiva Creporizão). O estilo da alteração hidrotermal relacionado à mineralização é predominantemente fissural e localmente pervasivo. Os tipos de alteração hidrotermal são sericitização, carbonatação, cloritização, sulfetação, silicificação e epidotização, além da formação de veios de quartzo de espessuras variadas. Pirita é principal sulfeto e contém inclusões de galena, esfalerita, calcopirita e, em menor quantidade, de hessita e bismutinita. O ouro ocorre mais comumente como inclusão em cristais de pirita e, secundariamente, na forma livre em veios de quartzo. Ag, Pb e Bi foram detectados por análise semi-quantitativa como componentes das partículas de ouro. Estudo de inclusões fluidas identificou fluidos compostos por CO₂ (Tipo 1), H₂O-C O₂-sal (Tipo 2) e H₂O-sal (Tipo 3). O volátil CO₂ é predominante na fase carbônica. O fluido do Tipo 2 apresenta densidade baixa a moderada, salinidade entre 1,6 e 11,8 % em peso equivalente de NaCl e foi aprisionado principalmente entre 280° e 350°C. No fluido do Tipo 3 o sistema químico pode conter aCl₂ e, talvez, MgCl₂, e a salinidade varia de zero a 10,1% em peso equivalente de NaCl. Apenas localmente a salinidade atingiu 25% em peso equivalente de NaCl. Esse fluido foi aprisionado principalmente entre 120° e 220°C e foi interpretado como resultado de mistura de fluido aquoso mais quente e levemente mais salino, com fluido mais frio e diluído. Globalmente, o estudo das inclusões fluidas indica estado heterogêneo durante o aprisionamento e ocorrência de separação de fases, mistura, flutuação de pressão e reequilíbrio das inclusões durante aprisionamento. A composição isotópica do fluido em equilíbrio com minerais hidrotermais (quartzo, clorita e calcita e pirita) e de inclusões fluidas apresenta valores de δ¹⁸O e δD entre +0,5 e +9,8 ‰, e -49 a -8 ‰, respectivamente. Os valores de ³⁴S de pirita (-0,29 ‰ a 3,95 ‰) são provavelmente indicativos da presença de enxofre magmático. Pares minerais forneceram temperaturas de equilíbrio isotópico em geral concordante com as temperaturas de homogeneização de inclusões fluidas e compatíveis com as relações texturais. Os resultados isotópicos, combinados com os dados mineralógicos e de inclusões fluidas são interpretados como produto da evolução de um sistema magmático hidrotermal em três estágios. (1) Exsolução de fluido magmático aquoso e portador de CO₂ entre 400°C e 320-350°C, seguido de separação de fases e precipitação principal da assembleia clorita-sericita-pirita-quartzo-ouro sob pressões menores que 2,1 kb e a 6-7 km de profundidade. (2) Resfriamento e continuação da exsolução do CO₂ do fluido magmático geraram fluido aquoso, mais pobre a desprovido de CO₂ e levemente mais salino, com aprisionamento dominantemente a 250°-280°C. A assembleia hidrotermal principal ainda precipitou, mas epidoto foi a principal fase nesse estágio. (3) Mistura do fluido aquoso do estágio 2, mais quente e mais salino, com um fluido aquoso mais frio e menos salino, de origem meteórica. Carbonatação está associada com esse estágio. A assembleia hidrotermal e os valores isotópicos indicam que fluido foi neutro a levemente alcalino e relativamente reduzido, que H₂S (ou HS-) pode ter sido a espécie de enxofre predominante, e que Au(HS) ^-2 deve ter sido o complexo transportador de ouro. A deposição do ouro em Moreira Gomes ocorreu em resposta a diversos mecanismos, envolvendo a separação de fases, mistura e reações fluido-rocha. O depósito Moreira Gomes é interpretado como o produto de um sistema magmático-hidrotermal, mas não possui feições clássicas de depósitos relacionados a intrusões graníticas, tanto oxidadas como reduzidas. A idade de deposição do minério (1,86 Ga) sugere que o sistema magmático-hidrotermal pode estar relacionado com a fase final do extenso magmatismo cálcio-alcalino da Suíte Intrusiva Parauari, embora o magmatismo transicional a alcalino da Suíte Intrusiva Maloquinha não possa ser descartado.