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Petrography
About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.
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01 Jan 1973
TL;DR: In this article, the authors presented the petrographic and mineralogic results together with all the pertinent data which may help to solve the problem of the peri-Mediterranean evaporites and showed how these observations have led to the choice of a shallow-water, shallow-basin desiccation model for the Messinian Mediterranean evaporites.
Abstract: Jacques Bourcart was the first to draw attention, in the middle of the nineteen fifties, to the knolls of the Ligurian Abyssal Plain which are the superficial topographic expression of the piercement structures of the Mediterranean evaporites (Bourcart et al., 1958, Bourcart 1959). Later, Alinat and Cousteau (1962), followed by Hersey (1965) interpreted them as salt diapirs, followed by Glangeaud et al. (1966). Since that time, various solutions were proposed regarding the petrographic nature and the age of the Mediterranean deep-sea diapirs (Mauffret 1969, Watson and Johnson 1968, etc.). The problem of the peri-Mediterranean evaporites is not a simple one. Important salt deposits of Triassic age are known throughout the western Alps. In addition, elevated shreds of evaporitic series of Messinian age are observed around the different Mediterranean basins. In this context, the relationship of the deep sea diapirs to the geological outcrops of evaporites of different ages was not obvious. The scientific shipboard party of Leg 13 was well aware of this situation and consider confirmation of the evaporitic nature of the deep-sea diapirs and the discovery of their Messinian age a major step to an understanding of the Cenozoic geological history of the western Alps. Our drilling proved the existence of a huge geological formation of Messinian Mediterranean evaporites well defined by its lithology and time boundaries (Figure 1). All of the shreds of Messinian evaporites scattered around the Mediterranean, and the deep-sea diapirs, belong to this formation, which is younger in age than, and distinct from, the Triassic evaporites of the same region. Since discovery of the deep-sea diapirs and the corroboration of their salt nature (Ewing, Worzel, et al, 1969), the problem of the de positional mechanism of the salts, now underlying several thousand meters of water, has never ceased to attract attention. Traditionally, authors considered that the evaporites precipitated under a very thin layer of water. Recently however, Schmalz (1969) proposed several models of precipitation in deep water that could account for deep deposits such as those which have been discovered in the Gulf of Mexico. In this context, the interpretation of Leg 13 findings becomes all the more important because none of the existing theories account completely for our results. Thus we felt the need for a new explanation. In this chapter, we present the petrographic and mineralogic results together with all the pertinent data which may help to solve this problem. These different results are organized into a coherent group of observations which, however, may be interpreted in two different ways. In the discussion, we show how these observations have led us to the choice of a shallow-water, shallow-basin desiccation model for the Messinian Mediterranean evaporites.
41 citations
TL;DR: In this paper, the spatial distribution of the dominant matrix minerals present in the middle-Proterozoic Athabasca Group sandstone (kaolin, illite, sudoite, dravite, hematite) was studied at a regional scale in the Shea Creek region (Saskatchewan, Canada), in which two epigenetic unconformity-type uranium deposits have been discovered.
Abstract: The spatial distribution of the dominant matrix minerals present in the middle-Proterozoic Athabasca Group sandstone (kaolin, illite, sudoite, dravite, hematite) was studied at a regional scale in the Shea Creek region (Saskatchewan, Canada), in which two epigenetic unconformity-type uranium deposits have been discovered. 3D models of matrix mineral distribution were derived from normative mineral calculations and 3D interpolation using whole-rock geochemical analyses of sandstone samples collected from both mineralized and barren areas. The calculations were constrained by information obtained from petrographic and crystal-chemical clay mineralogical studies on representative samples. The 3D mineral distribution models were compared to the lithostratigraphy and structural features of the Athabasca Group sandstone to ascertain the source and mobility of the main elements involved in the sandstone host-rock alteration processes related to the U mineralization. The distribution of Al is conformable with the lithostratigraphy throughout the studied area, regardless of proximity to basement-rooted structures and U ore bodies. The distribution of illite displays similar features, but the intensity of the illitization of kaolin decreases with increasing distance from the structures and U ore bodies. Hematite bleaching and neoformation of sudoite and dravite were restricted to the vicinity of the fault zones above the U ore bodies. The spatial configurations of the mineral anomalies show that syn-ore fluids flowed from the basement towards the sandstone cover via the fault zones, as described in current metallogenic models. Although Al remained immobile (mass transfer), the anomalous K, B and Mg present in the host-rock alteration haloes were probably imported from the basement rocks (mass transport). Unlike B and Mg, K migrated laterally at least several kilometers from the basement-rooted faults. The mineral distribution models were used to quantify the volume of altered sandstone (10−2−10−1 km3) and the amounts of K, Mg and B which were imported to the alteration haloes above the Shea Creek U ore bodies: 186,000 t of K, 66,000 t of Mg, and 11,000 t of B above the Anne ore body, and 24,000 t of K, 185,000 t of Mg, and a similar 11,000 t of B above the Colette ore body.
41 citations
TL;DR: In this article, the authors used the THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrained the P-T conditions of the M1, M2 and M3 assemblages to P = 1.28-1.28 and T = 0.73, respectively.
41 citations
TL;DR: Gneissic clasts are found in Carboniferous volcanic vents in the Scottish Midland Valley as discussed by the authors, where they exhibit mineral assemblages and textures typical of granulite-facies metamorphism.
Abstract: Gneissic clasts occur in Carboniferous volcanic vents in the Scottish Midland Valley. Samples from two widely separated areas are closely similar in petrography and are interpreted as fragments of the same gneiss complex. They exhibit mineral assemblages and textures typical of granulite-facies metamorphism. Mineral chemistry is used to suggest that pressure and temperature of metamorphism exceeded 11 kbar and 850°C in part of the complex. The gneisses are geochemically similar to Lewisian granulites in their depletion in K, Rb, and Y. Seismic evidence indicates derivation from a basement now lying 7 to 8 km down which is unconformably overlain by Ordovician and later rocks. This basement complex may have been an area of positive relief in late Precambrian and early Palaeozoic times, and its existence provides an important constraint on plate tectonic models of the Lower Palaeozoic.
41 citations
TL;DR: In this article, eight sandstone samples were collected for zircon U-Pb and Lu-Hf isotopic analyses combined with petrographic analysis, which supported the interpretation that the southeastern CAOB entered stages of extension and rifting during the Late Paleozoic.
41 citations