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  • 1
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    The chilled margin of the Ben Buie layered gabbro, Isle of Mull (1979)
    Geological Society of London
    Details
    Publication Date: 1979-06-01
    Print ISSN: 0036-9276
    Electronic ISSN: 2041-4951
    Topics: Geosciences
    Published by Geological Society of London
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  • 2
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    Fracturing and fluid flow in a sub-decollement sandstone; or, a leak in the basement (2015)
    Geological Society of London
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    Publication Date: 2015-06-24
    Description: Crack-seal texture within fracture cements in the Triassic El Alamar Formation, NE Mexico, shows that the fractures opened during precipitation of quartz cements; later, overlapping calcite cements further occluded pore space. Previous workers defined four systematic fracture sets, A (oldest) to D (youngest), with relative timing constrained by crosscutting relationships. Quartz fluid inclusion homogenization temperatures are higher within Set B (148 ± 20°C) than in Set C (105 ± 12°C). These data and previous burial history modelling are consistent with Set C forming during exhumation. Fluid inclusions in Set C quartz have higher salinity than those in Set B (22.9 v. 14.2 wt% NaCl equivalent, respectively), and Set C quartz cement is more enriched in 18 O (20.2 v. 18.7 VSMOW). Under most assumptions about the true temperature during fracture opening, the burial duration, the amount of cement precipitated and fluid-flow patterns, it appears that the fracture fluid became depleted in 18 O and enriched in 13 C. This isotopic evolution, combined with increasing salinity, suggests that throughout fracture opening there was a gravity-driven influx of fluid from upsection Jurassic evaporites, which form a regional décollement. Fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading, rather than increases in fluid pressure.
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 3
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    Characteristics and geological significance of particles on fractures from the Dongshengmiao polymetallic pyrite deposit, Inner Mongolia, China (2015)
    Geological Society of London
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    Publication Date: 2015-10-27
    Description: Fault gouge samples were collected from deep-seated fractures in the Dongshengmiao polymetallic pyrite deposit. Fault gouge particles were analysed by transmission electron microscopy. Many particles were identified with sizes of 200 nm or less, such as carbon-bearing, gold, Pb-bearing or Zn-bearing mineral particles. Carbon-bearing particles included graphite, carbonate, organic matter and carbonate mixed with organic matter. Gold particles were found in fault gouge samples in the form of natural gold, which had not been previously observed. Other particles, such as Pb- and Zn-bearing particles, directly reflect the ore-forming elemental characteristics of the Dongshengmiao deposit. Based on analysis of these particles, the following conclusions made: (1) carbon-bearing particles were deduced to be able to provide additional information related to the deposit formation; (2) fracture-induced stress acted on particles based on the observed lattice deformations in the gold, graphite, dolomite and Fe-bearing particles and the uneven edges of many particles; and (3) information about concealed ore bodies can be more easily obtained through the study of fault gouge particles, which will make ore body exploration more efficient.
    Print ISSN: 1467-7873
    Electronic ISSN: 1467-7873
    Topics: Chemistry and Pharmacology , Geosciences
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  • 4
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    An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprian massif (NW Spain) (2016)
    Geological Society of London
    Details
    Publication Date: 2016-03-02
    Description: Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U 〈 0.1) and magmatic (Th/U 〉 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprián massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U 〉 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U 〈 0.1. Hence, the San Ciprián massif presents an uncommon but unambiguous example of magmatic zircon mantles with Th/U values typical of metamorphic zircon. The most likely causes for the unusually low-Th/U zircon values in the San Ciprián massif are a combination of a U-rich magma composition (owing to a fractionation process) and the absence of other U-enriched accessory minerals. Our work in determining Th/U ratios substantiates the warning previously made by some researchers and precludes the use of Th/U values in zircon as an unequivocal indicator of metamorphic origin in the absence of other chemical, zircon morphology or field-based independent criteria. Supplementary materials: Code and data to generate Figures 1 and 5 are available at http://www.geolsoc.org.uk/SUP18885 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 5
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    Messinian productivity changes in the northeastern Atlantic and their relationship to the closure of the Atlantic-Mediterranean gateway: implications for Neogene palaeoclimate and palaeoceanography (2014)
    Geological Society of London
    Details
    Publication Date: 2014-04-25
    Description: The stable isotope composition of planktic and benthic foraminifera and the distribution of selected benthic foraminiferal species from a Messinian record of the lower Guadalquivir Basin, northeastern Atlantic Ocean, show that regional productivity changes were linked to glacioeustatic fluctuations. Glacial periods were characterized by poorly ventilated bottom waters as a result of weak Atlantic Meridional Overturning Circulation (AMOC), and by phases of high productivity related to intensified upwelling. In contrast, well-ventilated bottom waters owing to strong AMOC, the presence of degraded organic matter in the upper slope, and high input of degraded terrestrial organic matter derived from fluvial discharge to the outer shelf were recorded during interglacial periods. Before closure of the adjacent Guadalhorce Corridor at 6.18 Ma, which was the final active Betic Atlantic–Mediterranean gateway, the study area was alternately influenced by well-ventilated Mediterranean Outflow Water (MOW) and poorly ventilated Atlantic Upwelled Water (AUW). Following closure of the corridor, cessation of the MOW reduced the AMOC and promoted glacial conditions in the northern hemisphere, resulting in the establishment of local upwelling cells.
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 6
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    Thermotectonic history of SE China since the Late Mesozoic: insights from detailed thermochronological studies of Hong Kong (2014)
    Geological Society of London
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    Publication Date: 2014-07-01
    Description: The late Mesozoic Yanshanian volcanic arc affected an extensive region of SE China, but the conclusion of magmatism and later evolution are not fully understood. Widespread Yanshanian ignimbrites and their contemporaneous granites exposed in Hong Kong represent a microcosm of this magmatic arc. To constrain the post-magmatic thermal history of the region, we present zircon and apatite fission-track analyses from these rocks. Double dating using laser ablation inductively coupled plasma mass spectrometry U–Pb and fission-track techniques on detrital zircons from post-volcanic Cretaceous sediments is used to further constrain the tectonothermal evolution. The resulting dataset and thermal modelling suggest that the igneous rocks and Cretaceous sediments together experienced post-emplacement or post-depositional heating to 〉250 °C, subsequently cooling through 120–60 °C after c . 80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. We interpret the enhanced gradient to represent continuing Yanshanian magmatic activity until c . 100–80 Ma, much later than previously considered. Our data also indicate a long-term, slow cooling ( c . 1 °C myr –1 ) since the early Cenozoic, linked to c . 2–3 km of erosion-driven exhumation. The thermotectonic history of Hong Kong reflects the mid-Cretaceous transition of SE China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. Supplementary material: Descriptions of samples, operating conditions of the laser ablation inductively coupled plasma mass spectrometry system and the full dataset of U–Pb dating of detrital zircons are available at www.geolsoc.org.uk/SUP18750 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 7
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    An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprian massif (NW Spain) (2016)
    Geological Society of London
    Details
    Publication Date: 2016-04-15
    Description: Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U 〈 0.1) and magmatic (Th/U 〉 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprián massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U 〉 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U 〈 0.1. Hence, the San Ciprián massif presents an uncommon but unambiguous example of magmatic zircon mantles with Th/U values typical of metamorphic zircon. The most likely causes for the unusually low-Th/U zircon values in the San Ciprián massif are a combination of a U-rich magma composition (owing to a fractionation process) and the absence of other U-enriched accessory minerals. Our work in determining Th/U ratios substantiates the warning previously made by some researchers and precludes the use of Th/U values in zircon as an unequivocal indicator of metamorphic origin in the absence of other chemical, zircon morphology or field-based independent criteria. Supplementary materials: Code and data to generate Figures 1 and 5 are available at http://www.geolsoc.org.uk/SUP18885 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 8
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    The late Ordovician Soom Shale Lagerstätte: an extraordinary post-glacial fossil and sedimentary record (2017)
    Geological Society of London
    Details
    Publication Date: 2017-02-23
    Description: Fossils of the Late Ordovician Soom Shale Lagerstätte are characterized by exceptional preservation of their soft tissues in clay minerals. The low-diversity community lived in an unusual cold-water setting, dominated by anoxic bottom waters, in the immediate aftermath of the Hirnantian glaciation. Giant conodonts represented by complete tooth sets, and one with trunk musculature and liver preserved, unarmoured jawless fish, lobopods and enigmatic taxa are some of the more important fossils. Furthermore, this Lagerstätte also preserves biomineralized Ordovician taxa such as brachiopods, orthoconic nautiloids and trilobites. It is important in capturing the only known examples of many taxa, extending temporal ranges of others and providing a unique glimpse of a post-glacial refugium, at a time when other Lagerstätten are unknown.
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 9
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    A biogeochemical orientation survey around a low-grade uranium deposit in black shale in the Goesan district, Korea (2013)
    Geological Society of London
    Details
    Publication Date: 2013-08-10
    Description: Biogeochemical methods have been widely applied for uranium mineral exploration, but there have been few applications for low-grade uranium deposits. This paper describes a biogeochemical method of exploration for U in black shale areas. Sampling was performed in October 2010 and the following May along three survey lines at 15 sampling points 30 m apart. Daimyo oak ( Quercus dentata ) leaves and twigs, Japanese red pine ( Pinus densiflora ) twigs, and soils were collected at each sampling site, and the same suite of samples from five sites in a control area. Soil samples were analysed by INAA and four-acid digestion ICP-AES, and plant samples by ICP-MS after ashing to calculate values levelled to a dry-weight basis. The range of U in soils is from 2.7 to 694 ppm (mean 91.8 ppm; median 34.5 ppm); in plant samples, the ranges in Quercus dentata leaves and twigs, and Pinus densiflora twigs are from 0.007 to 2.687 ppm (mean 0.160 ppm; median 0.063 ppm), from 0.002 to 0.53 ppm (mean 0.037 ppm; median 0.012 ppm), and from 0.002 to 0.2 ppm (mean 0.014 ppm; median 0.009 ppm), respectively. In general, profiles of U concentrations in soil and plant samples coincide relatively well with the mineralized zone, although an inferior relationship at some sites may be due to down-slope displacement of soil. Quercus dentata twigs are considered to be useful as a biogeochemical medium to explore for low-grade U deposits in the study area.
    Print ISSN: 1467-7873
    Electronic ISSN: 1467-7873
    Topics: Chemistry and Pharmacology , Geosciences
    Published by Geological Society of London
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  • 10
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    The late Ordovician Soom Shale Lagerstätte: an extraordinary post-glacial fossil and sedimentary record (2017)
    Geological Society of London
    Details
    Publication Date: 2017-01-07
    Description: Fossils of the Late Ordovician Soom Shale Lagerstätte are characterized by exceptional preservation of their soft tissues in clay minerals. The low-diversity community lived in an unusual cold-water setting, dominated by anoxic bottom waters, in the immediate aftermath of the Hirnantian glaciation. Giant conodonts represented by complete tooth sets, and one with trunk musculature and liver preserved, unarmoured jawless fish, lobopods and enigmatic taxa are some of the more important fossils. Furthermore, this Lagerstätte also preserves biomineralized Ordovician taxa such as brachiopods, orthoconic nautiloids and trilobites. It is important in capturing the only known examples of many taxa, extending temporal ranges of others and providing a unique glimpse of a post-glacial refugium, at a time when other Lagerstätten are unknown.
    Print ISSN: 0016-7649
    Topics: Geosciences
    Published by Geological Society of London
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