No description available.

The unique Selukwe Subchamber of the Great Dyke is bounded by the Shurugwi greenstone belt (SGB) on the west side for approximately 25 km and granitoids on the east side, as compared to other subchambers of the Great Dyke that are bounded on both sides by granitoids. It is also the narrowest section of the entire Great Dyke. The extensive xenolith suite is found on the western flank and the central zone of the subchamber. This study focuses on the PAR 11 borehole and the surface xenoliths in the Selukwe Subchamber (SSC). The PAR 11 core was drilled into an anomalous sequence of ultramafic rocks situated in the Mafic Succession of the SSC. There are basically two rock types in the PAR 11 borehole: peridotites and pyroxenites. Comparison of the major and trace element geochemistry of the PAR 11 body with the MR 92 data of Coghill (1994) for the SSC reveals that they are similar but less evolved. The mineral assemblages and proportions of phases in the PAR 11 borehole samples are indicative of essentially the same composition as that which formed the layered sequence of the Great Dyke. Therefore, on the basis of the rock types and chemical compositions, the PAR 11 body and the Great Dyke cumulates appear to be petrologically and chemically similar and had the same petrogenesis. There are three rock types in the xenolith suite that have been observed in the mafic succession of the Unki area: peridotites, pyroxenites and gabbros. Major and trace elements show a wide range of compositions that have CaO/Al2O3 ~ 1, which are dissimilar to both PAR 11 and MR 92 borehole data. REE patterns show depletion of LREE, with flat HREEs indicating a different magma to that which gave rise to the Great Dyke. Such flat patterns are typical of a primitive mantle source similar to that of komatiite magma. Stowe, (1974) describes dunite and chromite in the SGB and does not describe pyroxenites and gabbros. Therefore, it is not clear in the first instance that the xenoliths were derived from the SGB. It also does not necessarily mean that these rock types did not occur in the SGB and, if they did, maybe they were derived from an intrusion within the SGB that is at depth and never been seen iv before. The xenoliths do not have mineral compositions that are similar to the Great Dyke and therefore precludes them as having been derived from the Great Dyke Marginal Facies, a possible source of such rocks. Therefore, it is concluded from this study that they were inherited from another source which also does not appear to be the SGB because there is no report of such rock types (other than peridotite) in the SGB. They are also not mantle derived. The metasedimentary rocks that occur as xenoliths are banded iron formation and quartzites and are all clearly derived from the different formations of the SGB. The quartzites are from the Mont d’Or Formation and Wanderer Formation. The BIFs are from the Upper Greenstone and Wanderer Formation. The Shurugwi Greenstones were stripped off from the western flank whereas the Archean granitoids to the eastern flank of the Great Dyke remained. The conclusion from this study is that the Shurugwi greenstones and Archean granitoids of the Selukwe area were intruded by the large volume of new magma that was the parental magma to the Great Dyke. The hot parental magma carried up with it xenoliths from outside the Great Dyke and large blocks from within the Great Dyke to the uppermost rocks of the level of the P1 pyroxenite layer and mafic unit.

The petrogenetic relationships of the different varieties of kimberlite in the De Beers Mine and Letseng-Ia-terai composite diatremes have been investigated using petrographic and chemical methods. Kimberlites in the Letseng-Ia-terai diatreme were found to be strongly contaminated by crustal material, mainly basalt. A method to correct for the effects of the contamination has been developed and applied to these kimberlites. Using the corrected data, the four kimberlite types in each group appear to be related to each other by crystal/liquid fractionation models. However the two groups cannot be related to each other. The De Beer Mine has two varieties of kimberlite, a monticellite apatite and calcite rich variety which intruded first, and a phlogopite rich type forming a discrete cylindrical body within the earlier kimberlite. These two kimberlites do not appear to be related by any of the fractionation models discussed. An examination of the data from this work and published sources, suggests that kimberlites are derived from below the low velocity zone by small degrees of partial melting involving garnet lherzolite with subordinate phlogopite and carbonate. Diamonds are probably incorporated as xenocrysts in the magma. Upward movement and emplacement of kimberlite appears to have been very rapid. The diatremes were probably eroded and shaped by gas, derived from the kimberlite magma, escaping to surface along weak zones in the earth's crust. Xenoliths of crustal material incorporated in the kimberlite on intrusion have also been studied and various features due to alteration by the magma are described, including the formation of natrolite and cebollite. The latter is a rare mineral that has not been described from kimberlite before. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in

No description available.

Mantle xenoliths allow us to investigate the geochemical and dynamic evolution of the mantle beneath the western margin of Antarctica and reconstruct a timeline of geologic events that are obscured on the surface. For this study, mantle xenoliths, brought to the surface by recent volcanism, were collected along a transect from the rift shoulder and into the rift basin in the western margin of the West Antarctic Rift System (WARS), thus providing a recent snapshot of the lithospheric mantle after major episodes of rifting. The second chapter of my thesis focuses on determining the age and persistence of the mantle within the rift. The rhenium-osmium (Re-Os) isotope system has proven to be an invaluable tracer of the tectonic history of the lithospheric mantle and can constrain the age of melt extraction and subsequent stabilization of the lithospheric mantle. This allowed us to track the age of the lithospheric mantle across this rifted margin. Os isotopes, combined with major element compositions, reveal widespread Paleoproterozoic (1.7-2.4 Ga) stabilization of the lithosphere and subsequent preservation, suggesting the lithosphere has dynamically thinned in response to rifting. Major element data allowed us to place temperature (T) constraints on the mantle and characterize the thermal history in the WARS. This study also revealed the oldest lithosphere ages recorded in Antarctica (3.3 Ga) and is the first to report ages that coincide with adjacent crustal ages, thus confirming the coupled relationship between the lithospheric mantle and continental crust. An integral factor controlling the composition of magmas generated at Earth’s surface is the composition of the SCLM. Magmas generated at depth must pass through it, and subsequently may take on geochemical signatures of the lithosphere, or may leave behind geochemical imprints of the migrating magma in the SCLM. Trace elements provide a means to investigate both the depletion and re-enrichment history of the SCLM. The third chapter of my thesis investigates the metasomatic overprinting of the Paleoproterozoic SCLM. Metasomatism, which is the chemical alteration of a rock by a migrating melt and/or fluid, leaves behind diagnostic signatures of the metasomatizing agent (e.g. subduction related fluids or carbonated melts). This can occur cryptically, where a melt percolates through the rock, changing the composition of the rock, but not the lithology. Modal metasomatism produces new mineral phases that are not typically expected in the rock. In xenoliths, trace elements enable us to decode geochemical signatures, and determine the sources of metasomatism. The WARS lithosphere has experienced varying degrees of re-enrichment, broadly characterized by low high field strength element (HFSE) abundances and rare earth element (REE) enrichments that correspond with carbonatite metasomatism. In addition, the presence of secondary hydrous phases (e.g. amphibole and phlogopite) imparted distinct geochemical signatures, revealing that the SCLM beneath the WARS was modified by reactive porous flow with an evolving metasomatic fluid/melt. Widespread Cenozoic rift-related volcanism (<20 Ma) is observed throughout the western margin of the East Antarctic Craton. It has been proposed that the Cenozoic basaltic volcanism in the region of our study site originated from a SCLM source that had been metasomatized during subduction along the paleo-Pacific margin of Gondwana, and subsequent extension in the WARS during the Late Cretaceous (~90 Ma). The fourth chapter of my thesis utilizes strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopes to date depletion and refertilization events in the lithosphere, as well as understand the role of the SCLM in the formation of WARS volcanism. Together with lithologic features (e.g. presence of hydrous phase additions), Sr and Nd isotopic ratios in WARS xenoliths provide a geochemical link to the Cenozoic rift-related magmatism, and supports the SCLM’s role in the formation of diffuse alkaline magmatism throughout the region. Lu-Hf isotope model ages add a constraint on the timing of melt depletion, and establish a relationship between depleted and refertilized domains. Sr isotopes constrain a genetic link between the metasomatized Archean lithosphere sampled on the rift shoulder and the highly radiogenic character of the Ferrar flood basalts, and indicate long-term storage of subduction modified mantle domains in the SCLM. The Sm-Nd isotope system is variably overprinted by metasomatism throughout the WARS. The most highly metasomatized location produces a well-correlated isochron that indicates that the SCLM acquired its trace element metasomatic signature about 130 Ma ago, during the late stages of subduction along the paleo-Pacific margin of Gondwana.

Magma recharge events and subsequent mixing processes are understood to precede volcanic eruptions. Textural evidence of intrusion of hot, mafic magma into a cooler, rheologically locked silicic magma is commonplace. Solidified "blobs" of injected magma, called enclaves, are evidence of magma mixing, but the petrological and mechanical conditions during their formation are debated. Mount Hood, Oregon consistently erupts andesite bearing compositionally similar enclaves. These enclaves are evidence of mingling and mixing of two magmas. However, due to the compositional similarity between enclave and host lava (e.g. ~1-5 wt.% difference in SiO2), it is unclear whether the preserved enclaves represent; 1) partially hybridized mafic melt remaining after mixing with significant crystal exchange with the host magma or 2) the preserved remnants of the intruding magma during recharge, with no homogenization or crystal exchange with the host magma. The aim of this study is to understand how and why enclaves form in compositionally similar host magmas, such as those at Mount Hood. Building off previous research, we utilize a combination of field observations, chemical analyses, and numerical modeling to constrain the rheology of the magmas prior to and during mixing. The degree of magma mixing is dependent on the viscosity contrast between the host and intruding magmas. Since these magmas are similar compositionally, variations in other magmatic properties such as crystallinity, and therefore temperature, and density may drive the viscosity differences between the host and intruding magmas needed for enclave formation. The enclaves at Mount Hood are vesicular (13-28%), coarse-grained; made up of mainly groundmass crystals (200-450 µm) with sparse microlites (< 200 µm), glass (450 µm) proportions, and rarely contain quenched margins. Additionally, crystals within the host magma show preferential alignment along the margins between host and enclave, suggesting a fluid behavior of the host magma during mixing. Based on textural and compositional evidence, we hypothesize that the intruding magma was buoyant, viscous, and crystalline, due to decompression-induced crystallization and exsolution of volatiles, during recharge and ascent to the shallow magma reservoir. Injection and underplating of the viscous crystalline intruding magma into a hot convecting host magma induces enclave formation. Crystallization temperatures differ by only 6-15 °C between host and enclave lavas, derived by the two pyroxene geothermometry method by Putrika (2008). These crystallization temperatures are consistent with crystallization in compositionally similar magmas. However, with such similar crystallization and liquidus temperatures, maintaining a viscosity contrast between the mixing magmas for enclave survival after formation suggests other properties, apart from temperature, must explain the viscosity contrast needed for enclave survival after enclave dispersal and thermal equilibration occurs. The presence of bubbles, from exsolution during crystallization, within the enclave magma increases the viscosity while simultaneously decreasing the density. Therefore, the presence of bubbles increases the viscosity of the intruding magma and maintains the viscosity contrast during the mixing process after thermal equilibration occurs. Additionally, if degassing occurs, rapid crystallization maintains the high viscosity of the enclaves. The enclaves observed at Mount Hood represent the solidified remnants of the last recharge event prior to eruption. The presence of compositionally similar enclaves and host lavas suggest a transient precursor event just prior to eruption at Mount Hood and can be applied to other recharge-driven arc volcanic systems.

No description available.

Mapping of the north-eastern portion of the Bird1s River Complex revealed that two large xenoliths composed of pyroclastic rocks and sandstone of the Cave Sandstone Stage have reacted with the dolerite. All the reaction phenomena normally associated with Karroo Dolerite are encountered. These are: (a) Metasomatism during the stage of iron enrichment of the dolerite with the production of a pyroxene-plagioclase metasomatic granophyre (b) Metasomatism during the stage of alkali enrichment of the dolerite with the production of a potassium feldspar adinole C c) Assimilation 'vi th the production of contaminated doleri tes Cd) Fusion 'vi th the production of glassy rocks including buchi tes (e) The production of rheomorphic veins

Kimberlite, an ultrabasic diamond-bearing hypabyssal rock-type which has its origin in the Earth's upper mantle, characteristically contains rare, well-rounded xenoliths of peridotite and eclogite. These xenoliths, which undoubtedly originate from some considerable depth below the Earth's surface, possibly represent samples of upper mantle material. They have received much attention from earth scientists and numerous theories as to their origin have been proposed. Forty-two selected peridotite xenoliths from the Bultfontein, Wesselton, Dutoitspan and Roberts Victor kimberlite pipes of the Kimberley area, South Africa, and 24 eclogite xenoliths from the Roberts Victor pipe have been examined in detail using a variety of petrological and mineralogical techniques. The petrologic research comprises conventional petrographic studies, the determination of accurate modal compositions and the presentation of 22 new whole-rock chemical analyses, nine of which are of garnet peridotite, four of spinel peridotite and nine of eclogite, one being a diamondiferous specimen. Detailed mineralogical studies of the constituent minerals of the xenoliths comprises descriptive mineralogy, in most cases an estimation of the compositions of these minerals from the measurement of physical properties, X-ray powder diffraction data and the presentation of 21 new chemical analyses of pure mineral separates. This includes five analyses of clivine, five of orthopyroxene, eight of garnet, one of chrome diopside and two of omphacite. The results of the investigation have shown that the peridotites consist essentially of forsterite and enstatite with minor or trace amounts of one or more of pyrope-rich garnet, chrome diopside, chrome spinel, phlogopite and rarely graphite, and often exhibit features consistent with plastic movement and tectonic deformation. The peridotites are believed to be derived from an ultrabasic upper mantle, which is both chemioally and physically zoned. The eclogite xenoliths, which are composed mainly of pyrope-almandine garnet and omphacitic clinopyroxene and occasionally contain kyanite, corundum and diamond, are not samples of a primary eclogitic upper mantle nor the products of an eclogite fractionation related to kimberlite genesis. Chemically they are not typical of extrusive basalts and probably either represent pockets of partially fractionated basic magma trapped at mantle-level in an eclogite-stable environment or samples of high-grade crustal metamorphic eclogite accidentally incorporated into the Roberts Victor kimberlite.

he estimation of post-emplacement kimberlite erosion in South Africa through the study of upper-crustal xenoliths is relatively unexplored; however the presence of these xenoliths has been recognized for well over 100 years. Post-emplacement erosion levels of a small number of South African kimberlite pipes have been inferred through the study of the degree of country-rock diagenesis, the depth of sill formation, the depth of the initiation of the diatreme and fission track studies. Through these studies, several estimates were proposed for the Group I Kimberley kimberlites. Although the 1400 m estimate of erosion remains widely accepted today, this estimate relies on the presence of Karoo-like basalt xenoliths in the Group I Kimberley kimberlites, as their presence proves that basalt existed in the Kimberley area when the kimberlites were emplaced. Basaltic xenoliths were described during the early stages of mining in Kimberley, though only one of these descriptions suggests that the ‘basaltic’ boulders correlate with the Karoo basalts. Because of the discrepancy between these early documentations of upper-crustal xenoliths and because the occurrence of Karoo-like basalt xenoliths in the Group I Kimberley kimberlites is under question, a re-investigation of the erosion levels and the upper crustal xenolith suites in South African, Cretaceous-aged kimberlites, including Melton Wold, Voorspoed, Roberts Victor, West End, Record Stone Quarry, Finsch, Markt, Frank Smith, Pampoenpoort, Uintjiesberg, Koffiefontein / Ebenheuyser, Monastery, Kimberley (Big Hole), Kamfersdam , Jagersfontein, Kaal Vallei, De Beers, Bultfontein, Lushof, Britstown Cluster, Hebron and Lovedale, was conducted. This study presents the analytical results for upper-crustal sandstone and basalt xenoliths collected from dumps, excavation pits and borehole core at the above-mentioned kimberlites, and demonstrates that they correlate with stratigraphic units of the Karoo Supergroup on the basis of mineral and geochemical compositions. These upper-crustal xenoliths are incorporated into kimberlites and down-rafted to levels below their stratigraphic position during kimberlite emplacement, consequently recording the broad stratigraphy into which each kimberlite is emplaced. Therefore, the Cretaceous lateral extent of the Karoo Supergroup is inferred and post-emplacement erosion estimated by reconstructing the stratigraphy based on upper-crustal xenolith suites for each kimberlite and calculating the total thickness of the now-eroded units. The distribution of sandstone xenoliths indicates that during the Cretaceous the lateral extent of the Dwyka, Ecca and Beaufort Groups encompassed all of the examined kimberlites, while the ‘Stormberg’ Group was constrained to an area outlined by the Voorspoed and Monastery kimberlites. Similarly, basalt xenoliths occur in all of the Group II and transitional (143 – 100 Ma) kimberlites but only in the Group I (90 – 74 Ma) kimberlites that lie within close proximity to the western outcrop margin of the outcrop area of the Drakensberg Group basalts (Lesotho Remnant), namely Monastery, Jagersfontein and Kaal Vallei. This trend implies an eastward-retreat of the inland erosion front of the Karoo basalts between 140 and 90 Ma and subsequent erosion of the underlying sedimentary units. It also suggests that a thicker succession of Karoo strata was present at the time of Group II and transitional kimberlite emplacement and that there has been more post-emplacement erosion in these kimberlites than the younger Group I kimberlites, except for Monastery, Jagersfontein and Kaal Vallei. Estimates are unique to each kimberlite as they are dependent on both stratigraphic location, elevation and present country rock, and range from approximately 1000 – 2500 m for the older kimberlites and less than 700 m to 1400 m for the younger kimberlites. Furthermore, the upper-crustal xenoliths found at the Group I Kimberley kimberlites and the coinciding trend of basalt erosion demonstrate that Karoo basalts were eroded from the Kimberley area by the time the Group I Kimberley kimberlites erupted (~85 Ma). Therefore, basalts are omitted from the Group I Kimberley kimberlites post-emplacement erosion estimate, and the upper Beaufort Group is considered the upper limit of the stratigraphy that was present at the time of the eruption of the Group I Kimberley pipes. Therefore, the erosion estimates decrease from a previous estimate of 1400 m down to 400 to 1100 m, where 850 m is considered a dependable intermediate estimate.