Caetano Juliani

40Ar/39Ar ages from blueschists of the Jambaló region, Central Cordillera of Colombia: implications on the styles of accretion in the Northern Andes

This paper presents the first argon dating of blueschists from the Jambalo area (Cauca Department) in the Central Cordillera of the Colombian Andes. Step-heating 40Ar/39Ar spectra were obtained for mica from several lenses of blueschists including greenschist facies rocks. The blueschists are mainly constituted of preserved lenticular cores in strongly mylonitic rocks, which resulted from retrometamorphic processes that affected the high pressure rocks during their exhumation. The majority of 40Ar/39Ar data points to metamorphic ages close to 63±3Ma, but some ages are older than 71Ma. These Maastritchtian–Danian ages correspond to the timing of exhumation of the blueschists near metamorphic peak conditions, because the dated paragonite and phengite crystallized during development of the mylonitic foliation. The continuous exhumation of this blueschist belt between 71–63Ma reflects the flow on an accretionary system/subduction channel environment that was interrupted by the collision of an intra-oceanic arc with the continental margin. Regional geological correlations suggest that this arc–continent collision also took place in Ecuador. This collisional event, although synchronous with other arc–continent collisions in the Northern Andes, was apparently not related to the formation of the great Caribbean arc, but to an arc built in the southeastern margin of the Caribbean plate

The nature of the Palaeozoic oceanic basin at the southwestern margin of Gondwana and implications for the origin of the Chilenia terrane (Pichilemu region, central Chile)

The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane.

Revisão da litoestratigrafia da faixa São Roque/Serra do Itaberaba - SP

A great number of publications on the geology of the Sao Roque/Serra do Itaberaba Belt (Sao Paulo State, Brazil) introduce and use diverse lithostratigraphic denominations, creating difficulties to a clear understanding of the geological evolution and lithostratigraphy of the metamorphic belt. This paper seeks an organization of the various nomenclatures and litostratigraphic suggestions for the Belt. A review of theessential publications about the subject is made, including those where the first litostratigraphic denominations were cited. After acomparative and critical analysis of the bibliography and, taking as a basis the geologic map 1:25.000 of the Itaberaba and Pedra Branca inountains, a sepatation of the supracrustal mcks which constitute the Belt in two major litostratigraphic units is proposed: Serra do Itaberaba Group and Sao Roque Group. The Serra do Itaberaba Group is subdivided into the Morro da Pedra Preta, Nhangucu and Pirucaia formations. The Morro da Pedra Preta Fotmation is a volcano-sedimentary sequence, and is basal to the Nhangucu, mainly composed by andalusite-bearing and manganesiferous metapelites, with some smalllenses of carbonatic and calc-silicatic rocks. The Pirucaia Formation is predominantly quariizitic and probably represents lhe lithoracies deposited in the margins of the two other formations, partieularly oft he Nhangucu Formation. In the Sao Roque Group the following formations can be recognized: Piragibu, composed by metapelites with few lenses of metaconglomerates and metavolcanics; Pirapora do Bom Jesus, essentialy volcanic/volcaniclastic, with bioherms and calc-phyllites; Estrada dos Romeiros, with rhythmic metapelites and metarenites and, finally, the metarenitic arcosean . Boturuna Formation. The Piragibu Formation is basal in the area of the Itaberaba, Pedra Branca and Morro Doce Mountains. In the Pirapora Synclinorium, the homonymic formation is basal to the Estrada dos Romeiros Formation, which corresponds to the more distal facies of the Piragibu Formation, covered by the Boturuna Formation.

Geochemistry of tourmalines associated with iron formation and quartz veins of the Morro da Pedra Preta Formation, Serra do Itaberaba Group (São Paulo, Brazil)

Tourmalines of intermediate schorl-dravite composition occur in iron formation (including metachert and tourmalinites), metasediments, calc-silicate and metabasic/intermediate rocks of the Morro da Pedra Preta Formation, a volcanic-sedimentary sequence of the Serra do Itaberaba Group (northeast of Sao Paulo City, southeastern Brazil). The Morro da Pedra Preta Formation is crosscut by quartz veins that contain both intermediate schorl-dravite and an alkali-deficient, Cr-(V-)bearing tourmaline, in which the occupancy of the X-site is ϑ0.51Ca0.33Na0.15, characterizing it as intermediate to foitite and magnesiofoitite end-members. Mg# values for this tourmaline are higher than those for intermediate schorl-dravite. Raman spectroscopy also confirms the presence of two groups of tourmalines. Stable isotope data indicate sediment waters as fluid sources, rather than fluids from magmatic/post-magmatic sources. Delta18O compositions for tourmalines, host metachert, and quartz veins are similar, showing that fluid equilibration occurred during crystallization of both quartz and tourmaline. Syngenetic, intermediate schorl-dravite tourmalines were formed under submarine, sedimentary-exhalative conditions; amphibolite-grade metamorphism did not strongly affect their compositions. Younger tourmalines of compositions intermediate to foitite and magnesiofoitite reflect the composition of the host rocks of quartz veins, due to fluid percolation along faults and fractures that caused leaching of Cr (and V) and the crystallization of these alkali-deficient, Cr-(V-)bearing tourmalines.

Geochemical constraints on blueschist- and amphibolite-facies rocks of the Central Cordillera of Colombia: the Andean Barragán region

Subduction zones are one of the most characteristic features of planet Earth. Convergent plate junctions exert enormous influence on the formation and recycling of continental crust, and they are also responsible for major mineral resources and earthquakes, which are of crucial importance for society. A subduction-related geologic unit containing high-pressure rocks occurs in the Barragán area (Valle del Cauca Department) on the western flank of the Central Cordillera of the Colombian Andes. Blueschists and amphibolites, serpentinized meta-ultramafic rocks, graphite-chlorite-muscovite-quartz schists, protocataclasites, and graphite-chlorite-andalusite-andesine-garnet-muscovite ±  titanite schists are exposed in this region. In spite of the petrotectonic importance of blueschists, the high-pressure metamorphism of the Central Cordillera of Colombia has been rarely studied. New geochemical data indicate that protoliths of the blueschist- and amphibolite-facies rocks possessed normal mid-ocean ridge basalt bulk compositions. 40Ar/39Ar geochronology for a metapelite rock associated with the blueschists shows a plateau age of ∼120 million years. We suggest that high-P/T conditions were present from ∼150 to 125 Ma, depending on the model of generation and exhumation considered.

A 100-m.y.-long window onto mass-flow processes in the Patagonian Mesozoic subduction zone (Diego de Almagro Island, Chile)

Diego de Almagro Island was formed by the subduction and accretion of several seafloor-derived tectonic slices with very heterogeneous ages and pressure-temperature-time (P-T-t) paths. The highest element of the pile (the Lazaro unit) evidences subduction in the high-P granulite field (∼1.3 GPa, 750 °C) at ca. 163 Ma. Below it, a thin tectonic sliver (the Garnet Amphibolite unit) preserves eclogite-facies remnants (∼570 °C and ∼1.7 GPa) formed at ca. 131 Ma (in situ U-Pb zircon rim ages). Peak assemblages were nearly fully amphibolitized during decompression down to ∼1.2 GPa and ∼600 °C at 125–120 Ma (Rb-Sr multimineral dating). The underlying Blueschist unit has ∼50 m.y. younger metamorphic ages and exhibits slightly cooler peak burial conditions (∼520 °C, 1.7 GPa; ca. 80 Ma, in situ white mica Ar-Ar ages and multimineral Rb-Sr dating) and is devoid of amphibolitization. The mylonites from the sinistral strike-slip Seno Arcabuz shear zone bounding Diego de Almagro Island to the east also exhibit amphibolite-facies (∼620 °C and ∼0.9 GPa) deformation at ca. 117 Ma (multimineral Rb-Sr ages). In situ white mica Ar-Ar dating and multimineral Rb-Sr dating of low-T mylonites (∼450 °C) along the base of the Lazaro unit reveal partial resetting of high-T assemblages during tectonic displacement between 115 and 72 Ma and exhumation of the slice stack. Detrital zircon U-Th-Pb ages indicate that the material accreted on Diego de Almagro Island has been mostly recycled from a Permian–Triassic accretionary wedge (Madre de Dios accretionary complex) exposed along the subduction buttress. Geological and geochronological constraints suggest that the rocks of the Seno Arcabuz shear zone and the Lazaro unit were tectonically eroded from the buttress, while the underlying Garnet Amphibolite and Blueschist units instead derive from the subducted oceanic basin, with increasingly younger maximum depositional ages. The very long residence time of the rocks (∼90 m.y. for the Lazaro unit) along the hanging wall of the subduction interface recorded long-term cooling along the Patagonian subduction zone during the Mesozoic. Diego de Almagro Island therefore represents a unique window onto long-term tectonic processes such as subduction interface down-stepping, tectonic erosion, and episodic underplating near the base of an accretionary wedge (40–50 km).

Química mineral do vulcano-plutonismo paleoproterozoico da região de São Félix do Xingu (PA), Cráton Amazônico

As formacoes Sobreiro e Santa Rosa sao resultado de intensas atividades vulcânicas paleoproterozoicas na regiao de Sao Felix do Xingu (PA), SE do Craton Amazonico. A Formacao Sobreiro e composta por rochas de facies de fluxo de lava andesitica, com dacito e riodacito subordinados, alem de rochas que compoem a facies vulcanoclastica, caracterizadas por tufo, lapilli-tufo e brecha polimitica macica. Essas rochas exibem fenocristais de clinopiroxenio, anfibolio e plagioclasio em uma matriz microlitica ou traquitica. O clinopiroxenio e classificado predominantemente como augita, com diopsidio subordinado, e apresenta caracterisiticas geoquimicas de minerais gerados em rochas de arco magmatico. O anfibolio, representado pela magnesiohastingsita, foi formado sob condicoes oxidantes e apresenta texturas de desequilibrio, como bordas de oxidacao vinculadas a degaseificacao por alivio de pressao. As rochas da Formacao Santa Rosa foram extravasadas em grandes fissuras crustais de direcao NE-SW, tem caracteristicas de evolucao polifasica e compoem uma facies de fluxo de lava riolitica e riodacitica e uma facies vulcanoclastica de ignimbritos, lapilli-tufos, tufos de cristais felsicos e brechas polimiticas macicas. Diques metricos e stocks de porfiros graniticos e granitoides equigranulares completam essa suite. Fenocristais de feldspato potassico, plagioclasio e quartzo dispersos em matriz de quartzo e feldspato potassico intercrescidos ocorrem nessas rochas. Por meio de analises quimicas pontuais dos fenocristais em microssonda eletronica, foram estimadas as condicoes de pressao e temperatura de sua formacao, sendo que o clinopiroxenio das rochas intermediarias da Formacao Sobreiro indica profundidade de formacao variavel entre 58 e 17,5 km (17,5 - 4,5 kbar), a temperaturas entre 1.294 e 1.082 oC, enquanto o anfibolio cristalizou-se entre 28 e 15 km (7,8 - 4,1 kbar), o que sugere uma evolucao polibarica. Assim, propoe-se um modelo de geracao de magma basaltico hidratado com base na fusao parcial de cunha mantelica e no acumulo na crosta inferior em uma zona quente, a partir da qual os magmas andesiticos e daciticos sao formados pela assimilacao de crosta continental e cristalizacao fracionada.

Petrografia de zonas de alteração hidrotermal mesoproterozóicas do tipo Kuroko no Grupo Serra do Itaberaba (SP) e seu uso na exploração mineral

In the volcano-sedimentary Serra do Itaberaba Group, located within the central part of the Ribeira Fold Belt, Mesoproterozic paleo-hydrothermal systems developed surrounding small bodies of andesites and rhyodacites. These systems were responsible for the hydrothermal alteration of basic and intermediate igneous and volcaniclastic rocks. Despite the overprinting of two medium-grade and one low-grade metamorphic-deformational events, well-defined zones generated by different types and intensities of hydrothermal alteration are still recognizable. An initial large chloritic alteration zone (ZC1) was overprinted by restricted chloritic (ZC2) alteration zones, similar to those associated with Kuroko-type base metal deposits. The metamorphic products from ZC1 have magnesium amphibole(s) in different proportions (cummingtonite-anthophyllite rocks) and those from ZC2, Mg-chlorite (meta-chloritites). These rocks are composed of exotic mineralogic associations and can be used as exploration guides for Kuroko-type base metal deposits present in metamorphosed volcano-sedimentary sequences.

Características das mineralizações auríferas no grupo Serra do Itaberaba, Guarulhos-SP

The metavolcanic-sedimentary sequence of the Serra do ltaberaba Group are host to several gold occurrences, the best known being Tapera Grande. There are two main types of gold mineralization: a syngenetic, stratabound, in which gold is scattered in basic and intermediate metavolcaniclastic rocks and meta-exhalites (metacherts with sulfides and turmalinites), and a hydrothermal, epigenetic one, within quartz veins with sulfides, superimposed to the former or associated with shear zones. The former is characterized by the assemblage of pyrrhotite, pyrite and subordinate chalcopyrite as well as by the very fine granulation of the gold. ln the latter, gold occurs free ar associated with copper sulfides (chalcopyrite, covellite and chalcosine) and with quartz. Chemical analyses showed higher contents of copper, lead, zinc and silver in the epigenetic type, while in the syngenetic one are found palladium and high contents of tungsten. Arsenium is absent, in disagreement with other similar deposits.

Produtos metamórficos de sistemas high-sulfidation oceânicos mesoproterozoicos, grupo Serra do Itaberaba, SP

In the central portion of the Ribeira fold belt, southeastern Brazil, the Mesoproterozoic volcanosedimentary Serra do Itaberaba Group was affected by two medium-grade regional metamorphic events and by a third low-grade retrometamorphic event. This succession was deposited in an ocean basin having N-MORB type basalts that evolved to a back-arc environment. Within the group four occurrences of alumina-rich rocks are now known (Guavirituba, Pedra Branca, Itaberaba and Pico Pelado), which crop out as small lenses intercalated between metabasites and metamorphosed volcanoclastic rocks, tuffs and pelites. Their genesis is related to oceanic magmatic-hydrothermal activity in the backarc environment that was associated with the emplacement of small rhyolitic bodies and high-sulfidation gold mineralization. X-ray diffraction analysis (XRD) of 15 very fine-grained alumina-rich samples permitted corundum, topaz, margarita, rutile and an undifferentiated mica (possibly sericite) to be identified. Mineral associations allow recognition of two advanced argillic alteration events and a third carbonation or retrometamorphic event that affected rocks of the Serra do Itaberaba Group: the first event generated protolith 1 rich in aluminum oxides ± alunite that after metamorphism produced dark blue lithotypes composed of corundum ± sericite; the second event was a silicification event that produced protolith 2 composed of topaz + zunyite + alunite ± rutile or of andalusite + alunite, with metamorphic products corresponding to brown and whitish lithotypes (Pico Pelado occurrence); during carbonatization or retrometamorphism, margarite crystalized after andalusite, cyanite or sillimanite. The sum of these three events produced, after the metamorphism or retrometamorphism, brown, whitish, and heterogeneous lithotypes composed of margarite or of margarite + sericite ± corundum (Guavirituba and Pedra Branca occurrences). The identification of similar lithotypes during field work represents a potentially very valuable tool in mineral exploration works as they comprise rock guides for finding gold deposits in medium-grade metamorphosed volcanosedimentary sequences.