No description available.

The South Qȏroq Centre is one of four high-level, major intrusive centres comprising the Igaliko Nepheline Syenite complex. Three elliptical stocks of foyaite were emplaced in fairly rapid succession by ring fracture and block subsidence, followed by a partial ring dyke of augite syenite. Petrographic and mineralogical data shows that the intrusions become successfully less differentiated with time. Inward-dipping microsyenite sheets appear to be associated with the ring dyke and four earlier, satellitic stocks occur around the periphery of SS2. Fieldspar, nepheline, iron-titanium oxide, clinopyroxene and amphibole have been investigated in each of the principal rock units by electron-microprobe. Most minerals show a considerable range in composition often with chemical zoning, forming continuous series from the augite syenites through the foyaites. Variations within the three individual foyaites are usually slight, but the alkai feldspar compsitions. Alkali feldspars provide estimates of P(_H(_2)0) and mafic minerals give an indication of f(0(_2)) conditions. Major and trace element analyses of the rocks were made by X-Ray florescence. Variation diagram trends are interpreted mainly in terms of fractionation of feldspar and the mafic phases. Trace element distributions are highly characteristic of fractional crystallisation series, but may not be compatible with progressive partial melting. The analyses are compared with phase equilibria in the experimental systems Q – Ne – Ks and Na(_2)0 – Fe(_2)0(_3) – Al(_2)0(_3) – Si0(_2). It is suggested that the centre evolved from an underlining, differentiated magma chamber, formed by crystal accumulation, possibly with associated liquid fractionation. Successively lower portions of the chamber were tapped, producing batches of fractionated magma. Post-emplacement differentiation was restricted mainly to slight outward diffusion of alkalis and volatiles under a thermal diffusion gradient. Physico-chemical conditions during recrystallisation near to the Igdlerfigssalik Centre are interpreted from textural, geochemical and mineralogical changes.

Three bitumen-hosted ore deposits from Peru, Argentina and Chile are described. These deposits (now abandoned) contain diverse mineral assemblages and were mined for vanadium (Minasragra, Peru), uranium (Cerro Huemul, Argentina) and copper (Copiapó, Chile). Highly integrated geological and geochemical studies have enabled the construction of paragenetic sequences and genetic models for these deposits. The Minasragra deposit was a bitumen-hosted vanadium orebody located within the Central Peruvian Andes. It was characterised by a highly unusual and world-unique mineral assemblage which included the vanadium sulphide mineral, patronite. Paragenesis is divided into four distinct phases and deposit genesis is linked to the extreme alteration of an <I>in situ</I> vanadium and sulphur-rich bitumen vein catalyzed by the intrusion of a high-level dyke suite. Cerro Huemul is a bitumen-hosted sandstone-type U-Cu deposit located within continental facies in the Neuquén Basin. Petrographic studies of bitumens and ore species coupled with fluid inclusion analysis enable reconstruction of fluid flow through this sandstone unit. Mineralization is divided into three paragenetic stages and is integrated into a regional and deposit-specific genetic model for ore deposition. The Copiapó study focuses on an extensive andesite lava horizon of Lower Cretaceous age which contains a diverse suite of copper sulphide minerals in intimate association with solid bitumen. Petrographic studies of ore and bitumen species enable the construction of paragenetic sequence which is divided into three mineralizing stages. All three bitumen-ore deposits formed due to the presence of solid bitumen and each case study investigates the role that bitumen played in the mineralizing process. Collectively these studies provide detailed insight into mineralization processes at bitumen-hosted ore deposits and highlight the versatility of bitumens to economically-concentrate a wide range of metals and minerals. A synopsis of the formation of all bitumen-type deposits (solid bitumen veins and bitumen-hosted ore bodies) is presented and ideas regarding the potential of mineral exploration for organo-metallic deposits within the Andean Cordillera are outlined.

No description available.

The Eastern Anatolia Region exhibits one of the world's best exposed and most complete transects across a volcanic province related to continent-continent collision. Within this region, the Erzurum-Kars Plateau is of particular importance since it contains the full record of post-collision volcanism from 11 to 1.5 Ma. The volcanics of the Erzurum-Kars Plateau cover a broad compositional range from basalts to rhyolites displaying a calc-alkaline character. They show a distinctive subduction signature represented by selective enrichment of the large ion lithophile and light rare earth elements. Among trace elements, Y behaves in a quite different way forming two distinct trends against silica named as the low- and high-Y series. Lavas of the high-Y series are characterised by a distinct bimodal volcanism (from basalt to rhyolite), in contrast to the low- Y series which comprises an unimodal andesitic volcanism. Trace element systematics together with modelling of theoretical Rayleigh fractionation vectors suggest that the low-Y series underwent a hydrous crystallisation dominated by amphibole as the mafic phase, whereas the high-Y series was dominated by anhydrous (POAM) crystallisation. Al-in- amphibole geobarometer calculations on the plateau volcanics reveal that the low-Y series evolved in magma chambers located between 20 and 28 km. In contrast, magma chambers of the high-Y series were much shallower, around 14-22 km. The high-Y series dominates early and late stages of the volcanic activity, whereas the low-Y series dominates the middle (between 7.5 and 5 Ma) stage, probably coinciding with the most intensive stage of crustal thickening. Sr, Pb, Nd and δ(^18)O isotopic systematics also show significant differences between the high- and low-Y series. Lavas of the high-Y series are always more radiogenic with respect to the lavas of low-Y series. Results of assimilation combined with crystallisation (AFC) modelling suggests that the low-Y series assimilated a lower crustal material which is compositionally similar to the granulitic xenoliths from the Pannonian Basin in Hungary and from Central Europe. In contrast, the high-Y series assimilated two different upper crustal materials. In both the low- and high-Y series, the maximum assimilation rate was around 40% of the fractional crystallisation rate. Trace element and isotopic differences between the low- and high-Y series are not significantly dependent upon variations in the source. These differences appear to have been extensively controlled by the AFC processes.

No description available.

The Holkerian (Lower Carboniferous, Mississippian) Hunts Bay Oolite (HBO) was studied in the south of Gower, along a section 15km long, up to 300m thick, at three locations: Langland Bay, Three Cliffs Bay and Port Eynon Bay. The HBO has been divided petrographically into four main associations (A, B, C and D), each with up to three lithofacies. Association A comprises crinoidal plates and bimodal grains of ooid, grapestone and aggregate grains it represents open-marine offshore deposits. Association B consists of the main oolite factory in an active shoal setting. Association C, deposited in back-shoal settings, and represents mainly grapestones and aggregate. Associations B and C are the main sandbody constituents. D is the lagoonal part of the HBO and includes bioclasts and peloids which are associated with sponges. The High Tor Limestone to HBO transition is gradational, without sharp boundaries. These associations accumulated in shoaling-upward sequences: offshore via active shoal to back shoal or lagoonal settings. Each sequence shows a thickening ESE to WNW trend. These cycles are not glacio-eustatic in origin but were likely tectonically-controlled and local, so that the sea-level curve of the HBO does not conform to the Holkerian global signal. Ramsay's (1987) and Scott's (1988) overall HBO-sandbody models are inapplicable to Ball's (1967) and Handford's (1988) marine sand-belt model as they lack differentiation between ramp and shield, and large bipolar cross-stratification. Considering all the HBO features exhibited, the deposystem lacks analogy to any modern or ancient examples, and represents a range of small sandbodies, and not major oolite shoals, as the source. A paragenetic sequence has been derived and fifteen diagenetic events identified. Most HBO diagenetic features differ from those of the well documented Chadian Gully Oolite of Gower, especially in the early diagenesis phase Gully Oolite contains ranges of emergence phases, and associated features such as vadose cementation and compaction, oomouldic fabrics, development of palaeosol and palaeokarsts.

The identification of post-Jurassic arc and plateau rocks in Jamaica has helped constrain the tectonic evolution of the Caribbean plate by identifying when, where and how the different volcanic rocks formed. This research therefore not only evaluates the existing models of Caribbean plate evolution, it also presents for the first time, a detailed gcochcmical and geochronolgical analysis of the igneous rocks on Jamaica. This study has focussed on the igneous rocks of the Blue Mountains, Central, Above Rocks and Benbow Cretaceous Inliers and the Tertiary Wagwater belt. Major and trace element data, Sr, Nd, Pb and Hf isotope analysis and argon radiometric ages have confirmed the presence of a Cretaceous oceanic plateau section within the Blue Mountains inlier and a number of primitive and evolved Cretaceous island arc sequences in the remaining inliers. Rare high-Nb basalts (HNB) and adakites have also been discovered in the Tertiary Wagwater belt. Many elements have been mobilised because of intense tropical weathering, and so the tectonic setting and petrogenesis of most of the analysed samples were interpreted using immobile trace elements which has led to the development of the Co- Th, Th/Zr-La/Yb. Cc/Lu-Sm/Yb, La'Hf-Sm/Y and Th/Hf-Sm/Yb discrimination diagrams. These diagrams have been used to classify the Jamaican volcanic arc rocks and thus identity their extent of fractionation and incompatible trace element enrichment. Immobile trace element and isotope geochemistry has identified at least 5 mantle wedge components and 8 slab components in the Jamaican island arc rocks. Additionally, the Bath-Dunrobin plateau lavas, the adakites and the HNBs represent at least three other chemically distinct source regions. The Bath-Dunrobin plateau lavas are derived from a - 90 Ma heterogeneous mantle plume source region which is distinct from the source regions for other Caribbean oceanic plateau lavas as it contains a larger HIMU component giving it more radiogenic Pb isotope ratios. The adakites have been derived from the combination of complex post-eruptive alteration, partial melting, fractional crystallisation and hybridisation processes. Rather than being related to a melt from a subducting slab, the Jamaican adakites appear to be derived from a melt of lower crustal garnet amphibolite with inter-bedded sedimentary material. The HNBs are derived from a HIMU-type source, which contained garnet and amphibole and so is distinct from the source region of the oceanic plateau. Using the new geochemical and geochronological data, together with the stratigraphic information and temporal location of the Jamaican igneous rocks, it is possible to place Jamaica in the Pacific model of Caribbean plate evolution. From 120- 75 Ma Jamaica formed the northernmost part of the Great Arc of the Antilles and erupted bimodal tholeiitic and calcalkaline magmas, which eventually evolved into calcalkaline and shoshonitic lavas after the collision of the Caribbean oceanic plateau -90 Ma. From -55 Ma Jamaica collided with the Yucatan peninsula and was subsequently tectonically transported to the east by transtensional opening of the Cayman Trough. This extension enabled decompression melting of the underlying asthenosphere to form the HNBs and adakites.

No description available.

No description available.