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The Montesbelos mass-flow (southern Amazonian craton, Brazil): a Paleoproterozoic volcanic debris avalanche deposit?
No full textThe present contribution documents the extremely well-preserved Paleoproterozoic architecture of the Montesbelos breccia (named here for the first time), which is interpreted as a rare example of a subaerial paleoproterozoic (>1.85 Ga) granular-dominated mass-flow deposit, few of which are recorded in the literature. Montesbelos deposit is part of the andesitic Sobreiro Formation located in the Sao Felix do Xingu region, southern Amazonian craton, northern Brazil. The large volume, high variability of textural features, presence of broken clasts, angular low sphericity fragments, mono- to heterolithic character, and the size of the outcrops point to a volcanic debris avalanche flow. Fluviatile sandy material and debris flows are associated with the deposit as a result of post-depositional reworking processes
Características microtexturales como indicadores del transporte y emplazamiento de dos depósitos de avalancha de escombros del Volcán de Colima (México)
No full textDuring its history Colima volcano has experienced numerous partial edifice sector collapses with the emplacement of debris avalanche deposits (DAD) of contrasting volume, morphology and texture. A previous detailed stratigraphic study in the southeastern sector of the volcano described by Roverato et al. (2011, J. Volc. Geoth. Res., 207, 33-46) allowed the recognition of two debris avalanche deposits named San Marcos DAD (SM-DAD V= ~1.3 km3) and Tonila DAD (T-DAD V= ~1 km3). Radiocarbon dates of organic material, directly associated with these deposits, gave ages of >28 kyr cal. BP for the SM-DAD and ~15 kyr cal. BP for the T-DAD. The San Marcos collapse was promoted by ongoing volcanotectonic deformation. The failure event triggered a "dry" debris avalanche (water content <10%). In contrast, the Tonila failure occurred in "wet" paleoclimate conditions during a period characterized by high humidity; the fact that the volcanic system was partially water-saturated was an important factor in the volcanic instability and transportation processes. This work sheds light on the transport and emplacement mechanisms of debris avalanches based on a detailed granulometric and microtextural analysis of the studied deposits. SM-DAD and T-DAD had a massive dynamic behavior during their emplacement, characterized by a lack of selective depositional process. The SM-DAD originated from a "dry" inertial granular flow in which high-energy grain-grain interaction prevailed. Abundant fines and fluids within the mass before the collapse could have enhanced its mobility. In fact, the water content in the flowing mass promoted positive pore-water pressure that reduced the frictional energy dissipation enhancing flow mobility. Both DADs show partially broken crystals/particles, percussion marks and fractures of different intensity that are the result of punctual, rapid and high-energy impacts. In general, the observed microscopic features suggest that the particle-particle interaction occurred in a collisional regime characterized by grains subjected to short, high velocity impacts, while evidence of frictional interaction is less common than that of the collisional type
First evidence of hydromagmatism at Colima volcano (Mexico)
No full textHere we report for the first time evidences of hydromagmatism at Colima volcano (western sector of Trans-Mexican Volcanic Belt). The studied location exposes 9 m of fine ash deposits dated at around 15 cal kyr BP by C-14 measurement on organic matter collected at the very base of the stratigraphic succession. The ash deposits rest directly on the top of a debris avalanche deposit. The morphology and grain size distribution of ash samples from the basal part of the stratigraphic succession testify for an explosive magma-water interaction, which produced finely bedded layers with blocky fragments characterized by adhering particles, quenching cracks and chemical pitting. The availability of ground water was hypothesized to be in close relationship with the amelioration of climate conditions at the end of the Last Glacial period, with rising temperatures that induced melting of glaciers and snow previously accumulated on the volcano summit slopes. This recognition demonstrates how, even a "dry" volcano like Colima, can experience hydromagmatism under adequate climatic conditions. (C) 2012 Published by Elsevier B.V
The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data
No full textThe study of Paleoproterozoic rocks is crucial for understanding Earth's tectonic evolution during the time when most of the modern crust and ore deposits were formed. The rocks of the Brazilian Amazonian Craton record some of the most-complete and best-preserved Paleoproterozoic magmatic and volcanic episodes on Earth. Following previous investigations, we present new lithofaciological and stratigraphic records of the felsic rocks of the Tapajos Mineral Province (TMP) (similar to 2-1.88 Ga) and the Sao Felix do Xingu region (SFX) (similar to 1.88 Ga) which, combined with new petrological and geochronological data, help providing a more complete understanding of the tectonic, magmatic and volcanological evolution of the Amazonian Craton. This magmatism/volcanism is thought to be formed in a late-/post-orogenic to extentional regime confirmed by the new geochemical data presented here. The transition from late-convergent to extensional tectonic setting could register the beginning of the taphrogenesis that marked the Amazonian Craton throughout the Mesoproterozoic. The volcanological approach of this contribution can serve as a strategy for the modelling of the evolution of Precambrian volcano-sedimentary basins around the world. The large amount of rocks analyzed are divided into primary and secondary volcaniclastic products depending on if they resulted from a direct volcanic activity (pyroclastic) or processes that reworked pyroclastic fragments. Furthermore, the deposits are subdivided into massive and stratified, depending on their primary mechanisms of transport and emplacement. By confirming the results from previous studies, our study permits to depict a more precise paleo-environmental picture of the processes that occurred in the Amazonian Craton during the Late-Paleoproterozoic. In particular, the presence of large regional-scale fissural systems and caldera collapses produced large silicic explosive volcanic eruptions, also accompanied by the emission of large volume effusive products. Although studies on the Amazonian Craton are still scarce and controversial, the present study provides new evidence that this volcanism may have formed one of the largest Silicic Large Igneous Provinces (SLIP) on earth. Our data also confirm that at least two major Paleoproterozoic periods of formation of volcanic rocks exist in the Amazonian craton. This point is of great relevance for any future interpretation of the geological evolution of this craton. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved
Textural features as indicators of debris avalanche transport and emplacement, Taranaki volcano
No full textThe Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto-Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto-Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, "megaclasts," were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper plug-like mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by similar to 23-25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to similar to 25-27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes
Stratigraphic reconstruction of two debris avalanche deposits at Colima volcano (Mexico): insights into pre-failure conditions and climate influence
No full textPaleoproterozoic felsic volcanism of the Tapajós Mineral Province, Southern Amazon Craton, Brazil
No full textAmazonian rocks record one amongst the most complete and best-preserved Paleoproterozoic magmatic episodes on Earth. The present contribution documents the extremely well preserved paleoproterozoic architecture of a series of felsic rocks found in the Tapajos Mineral Province (TMP), located in the western part of Path State, southern Amazon Craton, north of Brazil. These rocks are the first to be investigated to comprehend, based on their textural evidences, their emplacement mechanisms. Textural characterization allowed to identify three main facies with, as following reported, 1) chaotic ("Breccia") group, 2) eutaxitic ("Eutax") group and 3) parataxitic ("Paratax") group vitrophyric textures. Given the superb preservation of our samples, the investigated rocks are grouped, according to their grade of welding, into a wide variety of lithofacies from very low-grade to high-grade and rheomorphic ignimbrites. In the "Paratax group" strong similarities with banding in lava flows are observed. Based on the presented data we discuss the effusive or explosive origin of the observed flow mechanisms. (C) 2015 Elsevier B.V. All rights reserved
The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data
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