Browsing by Subject "geological sciences"

Now showing 1 - 4 of 4
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    A multi-isotopic geochemical investigation of the Lower Zone, Bushveld Complex, South Africa: implications for a crustal component for parental magmas
    (2020) Edwards, Hunter R; Howarth, Geoffrey H
    The current study focuses on the Lower Zone of the Bushveld Complex, South Africa using multiple geochemical and isotopic systems to determine the origin of crustal signatures, i.e. crustal assimilation or recycled crust in the mantle source, present throughout the Rustenburg Layered Suite (RLS) such as elevated  18O values. These geochemical and isotopic systems include major elements, trace elements, highly siderophile elements, oxygen isotopes, 87Sr/86Sr, 143Nd/144Nd, and Os-Os isotopes. Samples come from the Nooitgedacht Borehole 2 (NG2) at Union Section of the western limb of the Bushveld Complex, which sampled the Lower Zone. The 87Sr/86Sri (0.7043 – 0.7086) and Ndi values (-7.40 - -4.97) calculated in this study are in agreement with published data for the Bushveld. The majority of NG2 samples contain  18O greater than mantle peridotite (5.50‰) and MORB (5.70‰), in which NG2  18O ranges from 5.60 up to 8.00‰ for olivine, orthopyroxene, and clinopyroxene separates. These high  18O values suggests the Bushveld magmas sourced a crustal reservoir, either through crust assimilation or recycled crustal materials in the mantle source. This is the first study utilizing the Re-Os isotope system for the Lower Zone. The Osi values for the NG2 suite range from -4.37 to +35.9, which overlap with published data for the Critical Zone and the Platreef, the only previous Re-Os studies on the Bushveld. However, there are no previously reported negative Osi values for the Bushveld. The range in Osi values for the NG2 samples suggest mixing of at least two geochemical reservoirs. In addition to Lower Zone NG2 samples, sample NG2-773.65 is a chilled margin sample at the base of the NG2 borehole that contains high  18O ( 18O = 9.42 – 9.78‰) and radiogenic Osi (Osi = +62.5), in which the crust and/or recycled crust in the mantle source caused these high values. Sample NG2-734.64 contains the second lowest  18O ( 18O = 5.67‰) and most unradiogenic Osi (Osi = -4.37) values for the NG2 suite, evidence for a harzburgitic SCLM (H-SCLM) mantle source component. A lack of correlations for Osi values with  18O and 87Sr/86Sri values are indices for crustal assimilation processes. This lack in correlation may suggest a crustal component in the mantle is more likely than assimilation of the crust during ascent of the magmas toward the surface. The geochemical data presented in this study suggest the Lower Zone parent magma had a H-SCLM mantle source component in addition to the assimilation of the crust and/or the eclogitic SCLM (E-SCLM).
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    Megacryst suite from the Salpeterkop carbonatite complex, Sutherland, Northern Cape, South Africa: an in-depth geochemical study
    (2020) Peel, Chad; Janney, Philip Edward
    Presented here are major and trace element, stable (oxygen and hydrogen) and radiogenic (Sr-Nd-Pb) isotope analyses for a Cr-poor megacryst suite from the Salpeterkop complex, South Africa. The clinopyroxene, amphibole, phlogopite and ilmenite megacrysts all appear to be cogenetic, and based on known mineral relationships and intergrowths from xenoliths in the complex, the apparent order of mineral crystallisation is as follows: phlogopite → ilmenite → amphibole → clinopyroxene. Megacrysts of amphibole and phlogopite exhibit δD and δ18O values that are aligned with these grains having crystallised from melt originating from the upper mantle. Additionally, the amphibole and phlogopite megacrysts appear have experienced dehydration styled degassing, possibly related to their exhumation. Calculated P-T conditions have the megacrysts crystallising in the lower crust, under conditions ranging from 1 to 1.5 GPa (35 to 45 km depth) and 1000 to 1250 ℃. Calculated REE melts in equilibrium with the megacryst as well as radiogenic isotope results suggest that the Salpeterkop ultramafic lamprophyres are genetically related the the SPKC megacryst suite, however, the calculated parent melt to the megacryst appears to have mixed with a HIMU component. These findings primarily affect higher Mg-number megacrysts, suggesting that this assimilation or mixing occurred during initial stages of crystallisation. Lower Mg-number megacrysts lack the variations noted in their more primitive counterparts and present more tightly defined trends. A model of formation for the megacryst suite of the Salpeterkop complex sees grains having crystallised from an SPKC ultramafic lamprophyre-like melt originating from sublithospheric/asthenospheric conditions. During ascension the melt episodically assimilates material with a HIMU signature. The high Mg-number megacryst population crystallises from this melt at lower crustal depths. Soon after assimilation halts the megacryst parent melt homogenises (or re-homogenises), with grains to crystallise from this melt forming the low-Mg megacryst population.
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    Reinterpreting vintage geophysical data from the Algoa and Gamtoos Basins, South Africa: an integrated sequence stratigraphic framework since the middle Mesozoic
    (2021) Makhubele, Marvel Hope; Bordy, Maria Emese
    Sequence stratigraphy is a branch of stratigraphy that is concerned with how genetically related geological successions are deposited in time and space. This requires the integration of diverse types of datasets (drill core, outcrop, wireline, reflection seismic surveys, etc.) to build robust depositional models, the cornerstones of stratigraphic frameworks. Although the application of sequence stratigraphy has been a successful tool to predict the lithology of geobodies in the petroleum industry, terminology is inconsistently used by the different schools of thought to define stratigraphic surfaces. This has resulted in multiple sequence stratigraphic models that interpret the same data differently. The limited exploration, to-date, and poor dataset quality have impeded the understanding of the geological evolution of the offshore Algoa and Gamtoos Basins in the southern Cape region of South Africa. To reconstruct the main geological events in the area since the late Early Jurassic, we integrated vintage borehole and seismic data as well as key outcrop observations, generated contemporary gross depositional environment models for the basin fill, and tested the applicability of different sequence stratigraphic models. The studied stratigraphic interval formed since the inception of Gondwana break-up, in syn- and post-rift systems that were increasingly dominated by marine processes, especially in the distal hanging walls. Marine incursions are detected in the Upper and Middle Jurassic in the Algoa and Gamtoos Basins, respectively. However, the severely eroded Algoa Basin syn-rift succession, exacerbated by poor data quality, makes it challenging to understand the timing of the marine incursion in this compartmentalized half-graben. Sedimentation within these half-grabens primarily occurred above the hanging walls, whilst the footwalls (i.e., basement highs) formed the dominant sediment source areas. The geological characteristics of the studied synrift succession prevents the application of the depositional sequence stratigraphic or the tectonic system tracts models. Because subaerial unconformities (SUs) in the distal syn-rift sequence are not detectable, a diachronous, northward advancement of the shoreline until the late Valanginian can be postulated. The observations in the syn-rift sequence, which is bound by a basal SU, followed by third and fourth-order transgressive and regressive cycles and a second-order maximum flooding surface at the top, can be explained with a modified genetic sequence stratigraphic model. In the transitional to drift phase interval, from Hauterivian to Holocene, the successions are bound by third-order SUs and their correlative conformities. In the successions without evidence for subaerial exposure in the drift successions, flooding surfaces are used as sequence-bounding stratigraphic contacts, validating the applicability of the genetic and transgressive-regressive sequence stratigraphic models for this upper part of the studied stratigraphic interval. This study reaffirms the notion that while the sequence stratigraphic concept is model independent, sequence models are sensitive to depositional scale and data resolution. Moreover, it also reiterates that sequence boundaries should not be limited to subaerial unconformities, but rather to correlative surfaces that bound genetically related sedimentary successions.
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    Why are there no low-δ18O magmas In convergent margins? A case of the Central Andes, Northern Chile
    (2022) Sigauke, Connie; Harris, Chris
    It has long been thought that low-δ 18O magmas (<5.7‰) are only found in extensional tectonic settings. Low-δ 18O magmas are rare, worldwide, especially in subduction zone settings. The main objective of this study was to conduct a search for low-δ 18O magmas in the Central Volcanic Zone (CVZ) of the Andes, to verify if their rarity is due to under sampling. If no low-δ18O magmas were found, the question of why low-δ 18O magmas have appeared to be absent from the region would be addressed. This study has determined the variation in oxygen isotope composition of rocks across the CVZ, ranging in age from about 12 Ma to Recent. The δ18O values were measured in selected bulk rock samples and separated quartz phenocrysts in order to identify potential low-δ 18O rocks (from whole rock analyses) and magmas (from the quartz phenocrysts). The average δ18O values for quartz phenocrysts and whole rocks are 8.6‰ and 10.5‰ respectively, and no low-δ 18O magmas were found. Hydrogen isotope values range from -32 to -119‰, with the highest value in the most altered rock. The results from this study show no evidence for low-δ 18O magmas; the lowest value (5.0‰) was found in one sample (for both quartz and whole rock) and this sample appears to have been affected by interaction with meteoric water. The overall high δ18O values in the CVZ rocks are best explained as the result of alteration by fluids having high δ18O values. These were probably meteoric fluid whose δ18O were enriched due to fluid-rock exchange. The high δ18O values of the magmas must reflect the absence of low-δ 18O rocks that could be melted, and a relatively crustal input to magmas. This study agrees with the conclusions of Folkes et al. (2013), which explains the absence of low-δ 18O magmas as a result of tectonic history and climatic conditions of the central Andes; where low precipitation and high evaporation rates, high aridity, limited supply of meteoric waters, and high elevation all played significant roles in the lack of low-δ 18O magmas in the region.