Julian S. Marsh

The timing and duration of the Karoo igneous event, southern Gondwana

A volcanic event of immense scale occurred within a relatively short period in early Jurassic time over large regions of the contiguous Gondwana supercontinent. In southern Africa, associated remnants of thick volcanic successions of lava flows and extensive dike and sill complexes of similar composition have been grouped together as the Karoo Igneous Province. Correlative volcanic and plutonic rocks occur in Antarctica and Australia as the Ferrar Province. Thirty-two new 40Ar-39Ar incremental heating experiments on feldspars and whole rocks from Namibia, South Africa and East Antarctica produce highly resolved ages with a vast majority at 183±1 Ma and a total range of 184 to 179 Ma. These are indistinguishable from recent, high-resolution 40Ar-39Ar and U-Pb age determinations reported from the Antarctic portion of the province. Initial Karoo volcanism (Lesotho-type compositions) occurred across the entire South African craton. The ubiquitous distribution of a plexus of generally nonoriented feeder dikes and sills intruding Precambrian crystalline rocks and Phanerozoic sediments indicates that these magmas penetrated the craton over a broad region. Lithosphere thinning of the continent followed the main pulse of igneous activity, with volcanism focused in the Lebombo-Nuanetsi region, near the eventual split between Africa and Antarctica. Seafloor spreading and dispersion of east and west Gondwana followed some 10–20 m.y. afterward. The volume of the combined Karoo-Ferrar province (∼2.5×106 km3) makes it one of the largest continental flood basalt events. The timing of this event correlates with a moderate mass extinction (Toarcian-Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian-Triassic or Cretaceous-Tertiary boundaries associated with the Siberian and Deccan flood basalts events, respectively. The difference may be due to the high southerly latitude and somewhat lower eruption rates of the Karoo event.

The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region

Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.

REE FRACTIONATION AND CE ANOMALIES IN WEATHERED KAROO DOLERITE

Abstract Analyses of samples from a weathering profile on Karoo dolerite allow elements to be divided into three groups depending on their behaviour. Si, K, Na, Mg, Ca, Sr, Ba and V are mobilized and removed from weathered products. Fe, Al. Ti, Zr, Hf, Zn, Cu, Sc, Co and Ni are immobile. REE, Y, and to a lesser extent Cr, are mobile and redistributed within the profile without a net loss of these elements from the profile. Large positive Ce anomalies are developed in oxidized weathered products by preferential leaching of the other REEs. Negative Ce anomalies and REE enrichment is a feature of less altered dolerite.

Basalt geochemistry and tectonic discrimination within continental flood basalt provinces

Abstract Continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings. In one well-studied, classic, flood basalt province, the Mesozoic Karoo province of southern Africa, magmatism occurred in the following tectonic settings: (a) continental rifting leading to ocean-floor spreading in the South Atlantic Ocean (Etendeka suite of Namibia); (b) stretched continental lithosphere and rifting not leading directly to ocean-floor formation (Lebombo suite of southeastern Africa); and (c) an a-tectonic, within-plate, continental setting characterized by an absence of faulting or warping (Lesotho highlands and Karoo dolerites of South Africa). By means of spidergrams of the elements Rb, Ba, Th, Nb, K, La, Ce, Sr, Nd, P, Hf, Zr, Sm, Ti, Tb, Y, V, Ni and Cr, uncontaminated tholeiites from (c) above [i.e. the Lesotho-type continental flood basalts (LTCFB)] are compared with mid-ocean ridge basalts (MORB), ocean-island tholeiites (OIT), and tholeiites and calc-alkali basalts from subduction environments. The comparison reveals the LTCFBs are geochemically distinct. The differences are reflected in relative enrichments or depletions of the more incompatible elements (Rb-Ce) to less incompatible elements (Ce-Y), i.e. the overall slope of the spidergrams, and in anomalous enrichments or depletions of one or more of the elements Th, K, Nb, Sr, Ti, Hf, and Zr. The distinctive geochemical character of the Lesotho LTCFBs is interpreted in terms of a lithospheric mantle source for the basalts. This is supported by isotopic data. There are no major geochemical differences between Lesotho CFBs and basalts of the rift-related Etendeka and Lebombo suites, although the latter are somewhat enriched in Rb, Ba and K. However, unlike the Lesotho basalts, the Lebombo and Etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late-stage dolerites having MORB/OIT (i.e. asthenospheric) geochemical characteristics. The flood basalt/silicic magmatism/late-stage dyke swarm association is characteristic of several rift or thinned lithosphere environments (e.g., Ethiopia, Skye, eastern Greenland) but in many of these the flood basalts have ocean-island basalt (OIT) geochemical characteristics. The Lesotho-type CFB geochemistry is exhibited by the Grande Ronde Basalt of the Columbia River Group (a possible subduction-related flood basalt province) and the basic rocks associated with Mesozoic rifting in the North and South Atlantic. Basalt geochemistry alone is unhelpful in determining the tectonic setting of CFBs although the rift-related environments may be identified by the petrology and geochemistry of the whole igneous suite. A two-source model is proposed for the mantle-derived basic rocks in rift-related CFB provinces. Early enriched basalts are derived from the lithosphere and, following pronounced lithospheric attenuation or rifting, later MORB-like melts are emplaced from the rising asthenosphere. The presence of both Lesotho- and OIT-type geochemical patterns in rift-related CFBs suggests that the lithosphere exhibits different styles of enrichment.

Magma flow inferred from AMS fabrics in a layered mafic sill, Insizwa, South Africa

The Insizwa sill, is a 25-km-diameter, >1000-m-thick layered mafic intrusion, part of the Karoo Igneous Province in South Africa. The peridotitic and gabbronoritic rocks are undeformed and mineral fabrics demonstrably result from magma flow. A horizontal, centimeter-scale model layering is visible in numerous outcrops. Plagioclase crystals are both tabular and elongated. Their preferred orientation, parallel to the layering, forms a foliation and a NW–SE lineation, respectively interpreted as the magma flow plane and flow direction. Throughout the 78 stations of this study (699 specimens), magnetic susceptibilities (Km) range from 750 to 10,000 � 10 � 6 SI. The magnetic anisotropy (Pj) ranges from 1.03 to 1.08. Magnetic ellipsoids are both prolate and oblate (average Tjc0). Anisotropy of magnetic susceptibility (AMS) fabrics are dominated by multidomain to pseudo-single domain magnetite. High-field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50% for low susceptibility rock types. The anisotropy results from magnetite grain shape solely as shown by no significant increase in Pj with increasing Km. The magnetic lineation (305j ,0 5j) is consistent throughout the sill at various scales and coincides with the mineral lineation in average. In contrast, the magnetic foliation (125j NE 10j) is generally perpendicular to the mineral foliation and to the layering. Several explanations for this odd configuration are discussed. The variations of magnetic parameters across the layering and field observations point to a multiple injection. The magnetic lineation is consistent with the presence of a single feeder dike situated to the SE of the sill. D 2002 Elsevier Science B.V. All rights reserved.

Geochemical constraints on coupled assimilation and fractional crystallization involving upper crustal compositions and continental tholeiitic magma

Abstract The commonly analyzed trace elements in tholeiites can be subdivided into three groups depending on their sense of enrichment or depletion in upper continental crust in relation to fractional crystallization. Lithophile incompatible elements are enriched in crustal rocks and by fractional crystallization, whereas compatible transition elements such as Ni and Cr are depleted. A small third group comprising Ti, V, Fe, and sometimes P, enrich with crystallization but are depleted in the crust. The behaviour of elements from these three groups is influenced in different ways by the variables in assimilation-fractional crystallization processes (AFC). An evaluation of these influences suggests that lithophile incompatible element variations may not be as useful for constraining AFC in tholeiites as the elements V, Ti, etc. AFC modelling should use elements from all of these groups. In addition, the R and F parameters obtained by trace element and isotopic modelling should be checked for consistency with major element data by using published numerical phase equilibria models. The latter can also provide estimates of the fractionating mode which is necessary for calculating bulk distribution coefficients for AFC models. As an example, the Etendeka suite of Namibia is used to demonstrate the power of V-isotope ratio plots over those of Rb-isotope ratio for constraining AFC parameters, and the importance of checking R and F against major element variations.

Volcanic rocks of the Witwatersrand triad, South Africa. I: Description, classification and geochemical stratigraphy

Abstract The Witwatersrand triad contains thick volcanic sequences confined largely to the Dominion Group at the base and the Ventersdorp Supergroup at the top. These volcanic sequences are of late-Archaean to early-Proterozoic age and are amongst the oldest supracrustal volcanic sequences erupted onto the Archaean Kaapvaal craton. The volcanic rocks have suffered low-grade greenschist facies metamorphism but primary textures and, in some samples, primary mineralogies are well preserved. Major and trace element analyses of a large number of samples of volcanic rocks from boreholes in the Klerksdorp area, Western Transvaal, indicate that the chemical compositions of the lavas are largely unmodified by secondary processes and can be used for classification, stratigraphic studies and petrogenetic interpretation. The volcanics of the Dominion Group are a bimodal tholeiitic basalt-rhyolite association with considerable compositional variation from basalt to tholeiitic andesite within the basaltic suite. The Ventersdorp Supergroup is a weakly developed bimodal tholeiitic basalt-dacite suite. Basic rocks in the Klipriviersberg Group at the base and in the overlying Platberg Group are largely basaltic, whereas those of the Allanridge Formation at the top are largely tholeiitic andesites. Within the Klipriviersberg Group the more primitive samples occur at the top of the sequence. Phenocryst-rich porphyries of the Makwassie Formation in the Platberg Group are largely of dacitic composition and have textural features suggestive of an ash flow origin. A distinct geochemical stratigraphy is present within the volcanic sequences and application of discriminant function analysis and an empirical discrimination grid based on ratios of immobile elements P, Ti, and Zr allows for unknown samples to be placed within their correct stratigraphic unit.

Aenigmatite Stability in Silica-Undersaturated Rocks

AbstractAenigmatite is common in many trachytes, phonolites and agpaitic nepheline syenites. Petrographic evidence suggests that the aenigmatite in these rocks arises by the reaction of Ti-magnetite with a peralkaline silica-undersaturated liquid, and it is postulated that a no-oxide field, where aenigmatite is stable, exists in alkaline undersaturated magmas. This field is similar to that found in silicic liquids but lies below the FMQ buffer curve in

The Marinkas Quellen Carbonatite Complex, southern Namibia; carbonatite magmatism with an uncontaminated depleted mantle signature in a continental setting

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