Timing of the magmatism of the paleo-Pacific border of Gondwana: U-Pb geochronology of Late Paleozoic to Early Mesozoic igneous rocks of the north Chilean Andes between 20° and 31°S

Indexación: Web of Science; ScieloABSTRACT. U-Pb zircon geochronological data provide record of about 130 Ma of igneous activity in the Andes of northern Chile, which extended episodically from the latest Early Carboniferous to Early Jurassic (328-194 Ma). The overall U-Pb data show that volcanism and plutonism were essentially synchronous and major episodes of igneous activity developed during the Late Carboniferous to Mid-Permian (310 to 260 Ma) and from Late Permian to Late Triassic (255-205 Ma), with less prominent episodes in the mid-Carboniferous (330 to 320 Ma), and Early Jurassic (200-190 Ma). Thus, from the Carboniferous to the Early Triassic dominantly silicic magmatism developed along the Chilean segment of the southwestern border of Gondwana supercontinent. Further magmatism developed during the Mid-Late Triassic (250-194 Ma) was bimodal and synchronous with rift-related, continental and/or marine sedimentary strata related to the early stages of break-up of Gondwana. Most of the silicic volcanic rocks of the Precordillera and Domeyko Cordillera of northern Chile (21°30' to 25°30'S) are older than the silicic rocks assigned to the Choiyoi succession in Argentina, being instead equivalent in age to Carboniferous to Early Permian marine sedimentary sequences present in the eastern Argentinean foreland. On the other hand, silicic volcanic successions exposed in the easternmost part of northern Chile are equivalent in age to the Choiyoi succession of the San Rafael Block of Argentina. An eastward expansion or migration of the volcanism during the Mid-Permian to Early Triassic is inferred, interpretation that is consistent with expansion of the volcanism at that time in Argentina. The timing of the Late Paleozoic to Early Jurassic magmatism is coincident with that of the Andes of Perú and of western Argentina according to the available U-Pb data, revealing a rather consistent evolution in time of the magmatism along the southwestern, paleo-Pacific border of Gondwana. Keywords: Geochronology, U-Pb, Andes, Gondwana, Choiyoi, Paleozoic, Carboniferous, Triassic. RESUMEN. Los datos de U-Pb en circón registran aproximadamente 130 Ma de actividad ígnea en los Andes del norte de Chile, la que se extendió episódicamente desde el Carbonífero temprano hasta el Jurásico temprano (328-194 Ma). Los datos globales de U-Pb indican que el volcanismo y plutonismo fueron esencialmente sincrónicos con episodios mayores desde el Carbonífero tardío al Pérmico Medio (310-260 Ma) y durante el Pérmico Tardío a Triásico Tardío (255-205 Ma) y episodios menos prominentes durante el Carbonífero medio (330-320 Ma) y el Jurásico Temprano (200-190 Ma). Desde el Carbonífero hasta el Triásico Temprano se desarrolló magmatismo predominantemente félsico a lo largo del borde suroccidental del supercontinente de Gondwana, mientras que durante el Triásico medio a tardío (250-194 Ma) se desarrolló magmatismo bimodal sincrónico con estratos sedimentarios continentales y/o marinos relacionados con extensión (rift), durante las etapas tempranas de la desintegración de Gondwana. La mayor parte de las rocas volcánicas silíceas de la precordillera y la cordillera de Domeyko en el norte de Chile (21°30' a 25°30'S) son más antiguas que las rocas silíceas asignadas a la sucesión Choiyoi en Argentina, y son, en cambio, equivalentes en edad con las secuencias sedimentarias marinas del Carbonífero al Pérmico temprano presentes en el antepaís argentino al oriente. Por otra parte, las sucesiones volcánicas félsicas expuestas en la parte más oriental del norte de Chile, de la cordillera de Domeyko y de la cordillera Frontal al sur de 25°S son equivalentes en edad a la sucesión Choiyoi de Argentina. Se infiere una migración o expansión del volcanismo hacia el este durante el Pérmico Medio a Triásico Temprano, interpretación que es consistente con la expansión del volcanismo en Argentina durante ese período. La temporalidad del magmatismo del Paleozoico tardío a Mesozoico temprano es coincidente con la de los Andes de Perú y del oeste de Argentina, de acuerdo a los datos U-Pb disponibles, lo que revela una evolución temporal del magmatismo concordante a lo largo del borde suroeste del Gondwana. Palabras clave: Geocronología, U-Pb, Andes, Gondwana, Choiyoi, Paleozoico, Carbonífero, Triásico

Ophiolites in the Eastern Cordillera of the central Peruvian Andes

A discoutinuous NNW-SSE trending belt of scattered ultraiuafic (UM) and subordínate mafic (M) rocks ís exposed alona some 250 km in the Eastern Cordillera of the peruvian Andes (Junin and Huanuco Departnients. -°"-12° S). New data questiou tlieír pieviousty assuuned [1.2] intrusive origin. Work, in progress shows tLat the essential geologic and tecronk featiires are comnion to most of them, as will t e shown on the southeniniost occurrences: Tapo and Acobaniba (Tarraa proviuce). The Tapo massif is the most conspkuoiis and the oaty one with chiomite mining history. It is a lens-shaped body, 5 km long [NV-SE direction) and 1 -2 km wide. lying on detritaí sedirnents of the Lower Carboniferous Ambo Group [3], and comprising extreniely tectomsed and serpentinised peridotiles wilh subordínate podiform chroirdtite bodies. nieta-gabbros or amphibolites The Acobamba oecurrences couiprise serpentinites aud subordínate meta-gabbros [A]. in contact with phyllites of the Precarnbnan (?) Huacar Group (Maraáón Complex)

Neoproterozoic ultramafic and mafic magmatism in the Eastern Cordillera of the central peruvian Andes: the Tapo Massif

A highly dismembered assemblage of ultramafic and mafic rocks is exposed in the Eastern Cordillera of the Central Peruvian Andes, extending along a discontinuous NW-SE belt over some 250 km between 12° and 9° S of latitude. One of the most important occurrences is the Tapo Mafic-Ultramafic Complex, which occurs at 3750 to 4200 m above sea level, 2 km to the west of Tapo locality, in the Tarma province, about 200 Km west of Lima. The Tapo complex is a lens-shaped body, 5 km long and 1-2 km wide, that consists mainly of strongly serpentinized peridotites and some gabbros. Several small open pits won chromite from podiform chromitite lenses ( ≥60 chromite) and from disseminated chromite in serpentinite. The main structural trend of the Tapo Complex is NW – SE and the massif is tectonically emplaced upon Lower Carboniferous sedimentary rocks. The rocks of the Tapo massif are overprinted by metamorphism reaching amphibolite facies (see Willner et al, 2010, for more information on the metamorphic conditions). The main purpose of this work is to constrain the age determination of the Tapo Complex, using Sm-Nd technique direct dating of chromites and, also, amphibole, plagioclase and whole-rock samples from the host gabbro. In addition K-Ar age determination on amphibole is presented to date the metamorphic overprint

Preliminary basin scale assessment of CO2 geological storage potential in Santos Basin, Southeastern Brazil: Merluza Field study case / Avaliação preliminar do potencial para armazenamento geológico de CO2 da Bacia de Santos, sudeste do Brasil: estudo de caso do Campo de Merluza

The global community has authorized, through the Paris Agreement, the choice of an ambitious goal of reducing greenhouse gas emissions during the 21st century. The high world dependence on fossil fuels indicates that a quick energy transition—based on the use of renewable fuels or even considering the reduction of total energy consumption—will not be possible without structural changes in the neoliberal way of production and consumption. Thus, carbon capture and storage technologies emerge as a relevant tool for emergency action to meet the Paris Agreement goals. The search for adequate reservoirs is a relevant part of the CO2 storage endeavor and the Santos Basin has a privileged location, being close to oil producing and consuming centers. This work culminated in the study of the Merluza Field zone. Geochemical data and compiled lithological descriptions suggest a possible use of Jureia Formation and Itajaí-Açu Formation. A storage capacity estimation study is also presented. This favorability is not only geological: the presence of infrastructure installed in the Merluza zone as a fixed platform and an exclusive gas pipeline suggests greater economy and less environmental damage from the adaptation of depleted fields for use as a geological reservoir of CO2

New data on the ophiolitic VMS deposits of Moeche (Cabo Ortegal Complex, NW Spain)

As a result of the variscan collision, several allochtonous complexes were emplaced on the Iberian margin in Devonian times, among them the Cabo Ortegal Complex comprising the Moeche ophiolitic sequence. Copper has been won from several mines (Piquitos I & II, Barqueira, Maruxa) from disseminated ores and thin massive sulphide layers in the Moeche Unit, a strongly deformed meta-volcanic sequence comprising mainly quartz-chlorite schists and mylonites, which defines the top of the ophiolite. The ores were metamorphosed and strongly deformed under brittle conditions (for pyrite), but their textures are often apparently post-deformational, due to very common solution-transfer processes; they are composed mostly of pyrite and chalcopyrite, with minor sphalerite, pyrrhotite, etc., and with traces of native gold and PGE. The geology, mineralogy, and geochemistry of the orebodies relate closely to VMS of the Cu-Zn (Cyprus) type. Fluid inclusion studies allowed an estimation of metamorphic conditions at pressures of 2/2’5 kb and T 325/350ºC. New determinations using the chlorite geothermometer yield temperatures around 320 ºC, corresponding to pressures near 2 kb according to the isochores deduced from the fluid inclusion study, although in the Barqueira mine higher temperatures, up to 350 ºC, are found, corresponding to presssures up to 2’5 kb. Pb isotopic compositions of pyrite point to a double source of Pb, i.e. a main mantle and a subordinate crustal source. The values for 87SR/86Sr in pyrite support this interpretation, but some results suggest later mobilization in an open system, corresponding to solution-transfer. Age determinations of pyrite deduced from the Pb isotope uranogenic graph, ≈ 480 Ma, do not fit with the metamorphic ages published for the Moeche Unit, and might point to the age of Pb extraction from the mantle

Inclusiones fluídas e isótopos de Pb y Sr en Chipmo y Poracota, región minera de Orcopampa, Perú. Implicaciones para la exploración

Chipmo y Poracota forman parte de los depósitos epitermales miocénicos de oro y plata de los Andes centrales en el sur del Perú, entre los 3900 y 4900 metros sobre nivel del mar. Figura 1. Políticamente pertenecen a las provincias de Castilla y Condesuyos en la región de Arequipa y actualmente representan la mayor producción subterranea de oro del Perú. La producción total, a finales del 2009 fue de 2.1 Moz Au – 2.0 Moz Chipmo y 0.1 Moz Poracota–, cuentan con reservas del orden de 0.7 Moz Au y recursos estimados superan los 1.0 Moz Au. Chipmo es el deposito mas importante de la región de Orcopampa, en los últimos 15 años importantes publicaciones se han venido desarrollando: Swanson, 1998; Noble, 1999; Caddey and Sabastizagal, 1999; Mayta, 1999, Salazar et al., 2009 y Salazar 2008 y en Poracota cabe destacar a Bradford, 1999; Miranda Vidal, 2006 y Sarmiento, 2008

Garnet-biotite diffusion mechanisms in complex high-grade orogenic belts : understanding and constraining petrological cooling rates in granulites from Ribeira Fold Belt (SE Brazil)

Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite. Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts, allowing chemical diffusion outside the garnet megablast – biotite inclusion system. However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during retrogression. Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (~100 °C/Ma) at lower temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5 °C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt

C-O-H isotopic evidences for fluid sources of granulites in Ribeira Belt, SE Brazil

Publicado em: Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A7