Geochemical and isotopic data are presented for 32 Ma-old high-K andesites and dacites from the Alpine Chain. The samples consist of plagioclase, amphibole, titanomagnetite and rare biotite and quartz. Geochemical and isotope data indicate that slab-derived fluids, sediment melts and presumably AFC processes involving continental crust played a key role in the petrogenesis of the high-K rocks. A contribution of fluids is suggested based on the overall enrichment of large-ion lithophile elements and related high Ba/La, Ba/Zr, Ba/Th, Ba/Nb and Pb/Nd, sometimes distinctively higher than average continental crust. Positively correlated Ba/Nb–Th/Nb relationships, low Ce/Pb, low Nb/U and a negative correlation of Pb isotopes with Ce/Pb and Nb/U and positive ∆ 7/4 and ∆ 8/4 values similar to GLOSS imply the additional involvement of a sediment-derived melt. Negatively correlated Nb/Ta–Zr/Hf ratios at overall low Nb/Ta (13–7.5) are best explained by parental magma differentiation involving amphibole and biotite in a continental arc system. The samples have moderately unradiogenic Nd (εNd: – 2.0 to – 6.7) and radiogenic 87Sr/86Sr isotope compositions (0.7085–0.7113), moderately radiogenic Pb isotope compositions (206Pb/204Pb: 18.50–18.72; 207Pb/204Pb: 15.59–15.65; 208Pb/204Pb: 38.30–38.67), and elevated δ18O values (+ 6.5 to + 9.1 ‰). Epsilon Hf isotope values range from + 2.5 to – 4.0. Negative εHf(t) and εNd(t) values and 206Pb/204Pb ratios are correlated with elevated K2O abundances that indicate enrichment in K2O is related to AFC processes. The offset of εHf at a given εNd points to involvement of aged garnet-bearing crustal lithologies. The latter feature is qualitatively consistent with modification of unexposed primary basaltic andesites by AFC processes involving deep crustal material. In conclusion, in an Alpine context, inferred unexposed primitive high-K basaltic to andesitic melts are generated in the mantle wedge through fluid infiltration from the descending slab where fluids may have caused also partial melting of sedimentary rocks that mixed with evolving andesite–dacite compositions towards shallow-level intrusive and extrusive rocks. High-K and related trace element and isotope features thus result from a combination of already elevated values with participation of fluids and melts and probably AFC processes.

The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033–0.7042; 143Nd/144Nd: 0.51279–0.51287; 206Pb/204Pb: 19.1–19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebriá and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite±plagioclase±alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd~0.51280 and 87Sr/86Sr~0.7035. The other process involved an assimilation–fractional crystallization (AFC) process where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037 to 0.7089 at constant 143Nd/144Nd~0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (<100ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62–15.63) in the differentiated rocks than in the primitive basanites (15.58–15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63–15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13–19.35) and the primitive basanites (19.12–19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province.

Basanites from the Tertiary Siebengebirge area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg# (> 0 center dot 60), moderate to high Cr (> 300 ppm) and Ni (> 200 ppm) contents and strong light rare earth element enrichment, but systematic depletion in Rb and K relative to trace elements of similar compatibility in anhydrous mantle. Rare earth element melting models can explain the petrogenesis of these basanites in terms of partial melting of a spinel peridotite source containing residual amphibole. It is inferred that amphibole, indicated by the relative K and Rb depletion and the melting model, was precipitated in the spinel peridotite lithospheric mantle beneath the Siebengebirge, by metasomatic fluids or melts from a rising mantle diapir or plume. Alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe-Ti oxides, amphibole and plagioclase. Most of the basanites and alkali basalts approach the Sr-Nd-Pb isotope compositions inferred for the European Asthenospheric Reservoir component. Trace element constraints (i.e. low Nb/U and Ce/Pb ratios) and the Sr-Nd-Pb isotope composition of the differentiated rocks indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. High Pb-207/Pb-204 ratios in the differentiated lavas point to assimilation of ancient lower crustal components having high U/Pb and Th/Pb ratios. Relatively shallow melting of inferred amphibole-bearing spinel peridotite sources may suggest an origin from the metasomatized part of the thermal boundary layer. Application of new thermobarometric equations for the basaltic magmas indicates relatively normal mantle potential temperatures (1300-1400 degrees C); thus the inferred mantle 'baby plume' or 'hot finger' is not thermally anomalous.

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ18O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (87Sr/86Sr: 0.7031–0.7035, ε Nd: ca. +1, δ18O: 7‰, 206Pb/204Pb: ca.18.00, 207Pb/204Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (87Sr/86Sr: 0.7034, ε Nd: ca. +1, δ18O: 6.5‰, 206Pb/204Pb: ca.18.00, 207Pb/204Pb: 15.59) whereas more differentiated samples have higher 87Sr/86Sr ratios (0.709–0.714), slightly higher δ18O values (7.0–7.1‰), less radiogenic ε Nd values (−1.1 to −1.4) and more radiogenic 206Pb/204Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.

New oxygen isotope data for metaluminous granites from the basement-dominated part of the Damara orogen (Namibia) range from 9.1 to 11.9‰. These data, together with previously published Sr, Nd and Pb isotope data indicate that these granites and associated peraluminous granites originated from felsic meta-igneous basement sources. New and unusually low oxygen isotope data for metaluminous granodiorites extend now the range of δ18O values from ca. 12 to 6‰ for this rock type. These low oxygen isotope values approach the values observed in mafic quartz diorites for which a model of derivation from depleted mafic lower crust has been established. In view of the higher Pb isotope ratios but lower oxygen isotope values of the granodiorites relative to the mafic quartz diorites, it is concluded that the granodiorites represent partial melts of an undepleted but strongly altered mafic lower crust. Most of the peraluminous and metaluminous granites and the metaluminous granodiorites have identical U–Pb monazite, allanite and zircon ages of ca. 510–500 Ma implying partial melting of distinct basement rocks of Archaean to Proterozoic age at the peak of regional high-grade metamorphism.

Deerite, a typical mineral of Fe-rich metacherts metamorphosed under blueschist conditions, is not rare, but known occurrences have up to now been restricted mainly to the Tethyan collisional zone and the Western Cordillera of North America. We describe a first occurrence in the high-pressure nappes of the Escambray Massif, Cuba, in the assemblage deerite + Mg-Al-poor riebeckite + magnetite + quartz garnet +/- phengite +/- aegirine. This assemblage typically forms during exhumation and accompanies late, stress-free annealing of the quartz matrix. Mg-Al-poor ricbeckite overgrows older, large, oriented crystals of glaucophane, ferroglaucophane and Mg-Al-rich riebeckite ('crossite') during deerite formation. Early-formed hematite was largely replaced by magnetite. Deerite is very close to ideal 2 composition, attaining > 99% Fe122+Fe63+Si12O40(OH)(5), allowing direct application of the experimentally determined P-T-f(O2) stability field (Lattard and Le Breton, 1994). In combination with oxygen-isotope thermometry on magnetite-quartz, the crystallization conditions of the deerite-bearing assemblage can be constrained to similar to 470 degrees C, > 15 kbar, and an oxygen fugacity restricted closely to the quartz-fayalite-magnetite buffer (f(O2) approximate to 10(-23) bar). Thus, the late-stage P-T path of the metacherts mirrors a steep P-T gradient of 10 degrees C/km or less, requiring subduction of this part of the Antillean Island Arc to be still active during exhumation of the Escambray nappes.

Geochemical and Nd-Sr-O whole rock isotope data and Pb isotope data from leached feldspars for a suite of synorogenic (ca. 540 Ma) syenites and granites from the northern part of the Proterozoic Damara orogen ( Namibia) constrain their sources and petrogenesis. Major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr, and P on a primitive mantle - normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites back to a hypothetical parental melt with 8 wt% - 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (Sr-87/Sr-86 = 0.7055, epsilon(Nd) = -4, delta(18)O = 5 parts per thousand, Pb-206/Pb-204 = 17.60, Pb-207/Pb-204 = 15.58, Pb-208/Pb-204 = 37.50). More evolved quartz syenites show virtually unchanged Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 and Pb-208/Pb-204 ratios and epsilon(Nd) values. The most distinctive feature of the quartz syenites is their higher delta(18)O values up to 10.5 parts per thousand. These features seem to indicate that, unlike most other Damaran igneous rocks, the syenites described here were entirely formed within the lithospheric mantle and were not modified by assimilation processes of basement and/or Damaran metasedimentary rocks. In order to account for the substantial variation in O isotopes, the most likely source for these rocks is a heterogeneous association of mantle rocks and subducted upper crustal material, most likely altered basaltic rocks from subducted ocean floor. Additionally, low temperature alteration may have affected the quartz syenites. In contrast, although coeval granites with A-type affinities are interpreted as fractionation products of the syenites, their distinctly more negative epsilon(Nd epsilon) values ranging from - 9 to - 10 indicate that deep crustal assimilation processes of local Proterozoic basement rocks play an important role in the evolution of these granites.

We introduce a new approach for tracing the origin submerged entry nozzle precipitates (clogging) in continuous casting processes using stable oxygen isotopes. The isotopic signatures of possible oxygen sources are compared with the isotope ratios in the precipitates. We use infra-red laser fluorination in combination with continuous flow gas mass spectroscopy to determine the oxygen ratios in the highly refractory, alumina-rich precipitates and possible oxygen sources. Our isotope data indicate that atmospheric oxygen is not the major source of oxide precipitates in the submerged entry nozzles. Also process oxygen from the converter can be excluded to be source of oxygen in clogging. A possible origin of oxygen in alumina-rich precipitates from olivine from refractory material is discussed.

Drill core samples from the depth interval between 19.4 and 32.0 m of the laminated central lake facies of the Eocene Eckfeld Maar were investigated for biomarker and stable isotope composition. Bulk organic geochemical parameters (C/N, HI) and the molecular composition of the soluble organic matter indicate a dominance of particulate organic matter from land plants and microbially derived lipids in the lower part of the sedimentary succession. An angiosperm-dominated vegetation is indicated from the terpenoid biomarker composition. Abundant 4-methylsteroids in the 25.6-30.8 m section of the oil shale sequence reflect a contribution of algal-derived biomass. Samples with high concentrations of methylsteroids are characterized by low amounts of triterpenoids related to the arborane skeleton, and vice versa. This pattern is interpreted as reflecting differences in autochthonous organic matter production vs. microbial activity.In the lowermost section (32.0-30.6 m), a trend towards heavier delta O-18 and delta C-13 values indicates the evolution of permanently meromictic conditions in the lake and an increase in methanogenesis. High delta C-13 values of siderites (> 10 parts per thousand) throughout most of the sequence are consistent with permanently anoxic conditions at the sediment-water interface. The lighter delta O-18 values of siderites from turbidites, relative to siderites from biogenic laminites, are postulated to have been caused by temporary phases of increased precipitation, followed by landslides and improved circulation within the lake. Depletion of the organic matter in C-13 (average delta C-13 = -29.4 parts per thousand), in comparison with the fossil wood (ranging from -23.1 parts per thousand to -26.6 parts per thousand), is explained by the dominance of waxy, lipid-rich land plant material (e.g., leaf waxes, resins, bark) over wood supplied to the lake. Carbon cycling during anoxic decomposition of organic matter is assumed to further affect the delta C-13 values through the activity of anaerobic (e.g., methanogenic) bacteria, resulting in a depletion of the biomass in C-13. The overall trend in the isotopic composition of organic carbon towards heavier values in the depth interval between 26.0 and 3.2 m is accompanied by decreasing C/N ratios, indicating an increase in aquatic organic matter production.

New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence ( Dongargarh Group ) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO 2 , Na 2 O + K 2 O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an ‘A-type granite’ melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2–3 times more than the upper continental crust (UCC). LREE is high (La/Yb ∼ 20) and HREE 20–30 times chondritic. δ 18 O whole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰). The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al 2 O 3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ 18 O whole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting.

The distribution of oxygen isotopes between minerals, a parameter that is widely used to determine characteristics of metamorphic fluids, is susceptible to overprinted secondary processes. These isotopic ratios were proven to often be more stable in whole-rock samples than in monomineralic separates, a fact which presents the possibility of evaluating the fluid regime during early evolutionary stages. The factors of the temperature dependence of oxygen isotopic fractionation between a rock and fluid can be calculated from the modal composition of the rock. Judging from certain isotopic ratios, the granulite-facies fluid of the Rayner Complex, East Antarctica, consisted of at least two components (with delta(18) = 8.0-8.5 and 6.1-6.7parts per thousand). The isotopic exchange between the rocks of different composition (homogenization of the fluid) proceeded at temperatures higher than 700-730degreesC over a distance of no less than tens of meters. At temperatures below 700-730degreesC, the granulite-facies fluid was mostly removed from the rocks, although local isotopic exchange between minerals in some rocks continued to temperatures of 500-600degreesC. Rocks with pronounced amphibolite-facies mineral transformations at temperatures of 500-600degreesC were in equilibrium with the fluid of nearly constant oxygen isotopic composition (8.0-8.5parts per thousand), a fact testifying that the fluid could interact with the rocks at distances of hundreds of meters to a few kilometers. Conceivably, this fluid was formerly a constituent of the granulite-facies fluid that was accommodated in tectonically weakened zones. The oxygen isotopic composition of the fluid contained in the diaphthorized rocks (420-450degreesC, delta(18)O = 6.5parts per thousand) is significantly different from the isotopic composition of the fluid of the earlier retrograde metamorphic stage, perhaps because a new fluid could inflow into these rocks from some deep-seated source.

Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle ( 87 Sr/ 86 Sr: 0.705, ε Nd: −2, δ 18 O: 6‰, 206 Pb/ 204 Pb: 19.40, 207 Pb/ 204 Pb: 15.82). More evolved quartz syenites show increasing 87 Sr/ 86 Sr ratios, increasing δ 18 O values but less radiogenic ε Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic ε Nd, high 87 Sr/ 86 Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition ( 87 Sr/ 86 Sr: 0.712, ε Nd: −13), with strongly unradiogenic Pb isotope ratios ( 206 Pb/ 204 Pb: 17.40, 207 Pb/ 204 Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing 87 Sr/ 86 Sr, increasing δ 18 O values, more radiogenic ε Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous homblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U-Pb zircon age of 540 +/- 3 Ma.Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (epsilon(Nd) ((540) (Ma)) = -6.3 to - 19.8; delta(18)O = 9.0-11.6parts per thousand) but rather low and uniform initial Sr isotope compositions (Sr-87/Sr-86(initial) = 0.70794-0.70982). Two leucogranites and one aplite have higher initial Sr-87/Sr-86 ratios (0.70828-0.71559), but similar initial epsilon(Nd) (-11-9 to -15.8) and oxygen isotope values (10.5-12.9parts per thousand). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (< 0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material. (C) 2003 Elsevier Science B.V. All rights reserved.

Major and trace element and Nd, Sr, O and Pb isotope data from granites of the high-grade central part of the Damara orogen (Khan and Outlet gorge areas, Namibia) indicate a dominantly lower crustal origin. Based on their appearance in field, the granites can be divided into red granites, white granites and grey granodiorites. Red granites and some of the white granites from the Khan area are isotopically evolved (initial εNd: −12.5 to −18.9) and were likely derived from metaigneous sources with late Archaean to early Proterozoic crustal residence ages. Other white granites are less evolved (initial εNd: −6.5 to −8.8) and were likely derived from metasedimentary sources that are similar to the country rock metapelites. Grey granodiorites from the Khan and Outlet gorge area are also isotopically evolved (initial εNd: −9.9 to −13.1) but are derived from metaigneous sources with younger, late Proterozoic crustal residence ages. Major and trace element data do not support closed-system fractional crystallization processes for all samples; however, some chemical features (i.e., decreasing Rare Earth Element (REE) abundances with increasing SiO2) underline the importance of crystal fractionation processes for each distinct magmatic pulse. Isotope data do not support mixing of different crust-derived melts or assimilation of crustal rocks by a mafic magma. Instead, highly evolved Pb isotope compositions, strongly negative εNd values and radiogenic Sr isotope compositions argue for an undepleted basement as a potential source. The most likely model involves mainly partial melting of different basement rocks of Archaean to Proterozoic age at different levels within the crust. Only some granites could be derived by melting of metasedimentary rocks of the Pan-African cover sequence. The consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening; however, some of them may have been emplaced during extensional tectonics c. 40–50 Ma later than the main period of crustal thickening. The heating events that promoted melting of fertile deep-crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny, or by delamination of mantle lithosphere during the final extensional stages of the Pan-African orogeny.

Basic unshaped refractories are used as tundish lining in the continuous casting steelmaking process. These refractories are composed of forsteritic olivine and periclase. Slag covers the steel bath in the tundish in order to protect the melt from reoxidation. Using TEM, we demonstrate that the fayalite component of the olivine is reduced to metallic iron giving the olivine a dusty appearence in transmitted light. The reduced olivine appears stable under high temperature reducing conditions. We suggest that diffusion of Mg2+ from the outer part of the olivine into empty Fe2+ lattice positions in the interior prohibits formation of SiO2 as by-product of the fayalite reduction. Some of the silica may also have left the olivine in form of gaseous SiO. Periclase reacts with Al2O3 from the tundish slag to form spinel. We show that Mn from the steel melt is oxidized and enriched in tundish slag as well as in the refractory. The steel melt is the only source of Mn in the slag. Thermodynamic calculations show that equilibrium fractionation of Fe and Mn between steel melt and slag results in effective separation of Fe (in steel) and Mn (in oxides/silicates).

The most prevalent Phanerozoic glaciation occurred during the Carboniferous-Permian on the Southern Hemisphere Gondwana supercontinent. Sediments from the Pennsylvanian Dwyka Group deposited in the Karoo Basin of South Africa provide a complete record of glaciation and deglaciation phases. The direct correlation of glaciation events in southern Gondwana basins with the well-studied climate evolution of equatorial regions was previously hampered by lack of precise radiometric dating. As dating has now become available for the Karoo Basin, the Gondwana glaciation can be viewed in a global paleoclimatic framework with high temporal resolution. Element geochemical proxies (CIA [chemical index of alteration], Zr/Ti, Rb/K, V/Cr) record three confined shifts in climate and paleoenvironment of the Karoo Basin. These shifts were induced by changes in sea level, weathering rate, provenance, and redox conditions. Because of the low availability and diagenetic overprint of carbonates, ocean and atmosphere pCO(2) variations had to be reconstructed from delta(13)C(org) values of marine organic matter. The delta(13)C(org) signatures are affected by variable proportions of marine versus terrestrially derived organic matter and its state of preservation. Organic geochemical investigations (TOC [total organic carbon], C/N, lipid biomarkers) indicate the organic matter in the central Karoo Basin was primarily of algal origin. In agreement with element proxies, the varying delta(13)C(org) values mirror shifts in pCO(2), rather than variations of organic-matter type. A covariation trend between carbon isotope signatures of equatorial carbonates and delta(13)C(org) values from the Karoo Basin argues against local forcing factors and instead implies a global climate-control mechanism. The 5-7 m.y. duration of a complete glacial cycle is not in tune with any known orbital frequency. Processes such as changes in equator-pole temperature gradients or newly developing atmosphere-ocean circulation pathways can be regarded as controlling factors.

Oxygen isotope fractionation between synthesised zircon and water has been experimentally quantified at 700, 800, 900, and 1000 °C. The results are interpolated by: Δzm-H20 = −3.70 + 2.74 ± 0.19x2, where x = 103/T (K). Combined with the fractionation between quartz and water (Bottinga & Javoy, 1973) this yields: Δqtz-zm= 1.36x2. Theoretical evaluations of the reduced partition function ratios for zircon and two (a- and β-) modifications of quartz are expressed in terms of the following polynomials: 1000 lnfzm= 8.3306x2+ 1.9402 x − 0.6896 (400 < T < 1100°C) 1000 lnfa-qtz= 7.8963 x2+7.4091 x-3.6015 (200°C < T < a-quartz stability field) 1000 lnfβ-qtz= 9.3362 x2+ 2.4514 x − 0.7844 (β-quartz stability field up to 1100 °C). These expressions are in excellent agreement both with the experimentally derived factors of oxygen isotope fractionation for β-quartz and zircon, and the incremental calibrations for a-quartz and zircon (Hoffbauer et al., 1994). The effect of a-β-quartz transition on oxygen isotope fractionation implies, that those calculations, anchored to the theoretically evaluated reduced partition function ratios of quartz ( e.g., Zheng, 1993), can predict fractionations only within the P-T stability field of the respective modification of quartz ( i.e. a-quartz).

. Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K 2 O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO 2 ) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P 2 O 5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87 Sr/ 86 Sr: 0.70943–0.71285), Nd (initial ε Nd: –9.1 to –15.2 ) and O (δ 18 O: 6.8–8.1‰) isotope compositions. Neodymium model ages (T DM ) that range from 1.7 to 2.2 Ga and large variation in 207 Pb/ 204 Pb relative to 206 Pb/ 204 Pb indicates involvement of ancient crustal material. Lead ( 206 Pb/ 204 Pb: 17.08–17.23, 207 Pb/ 204 Pb: 15.53–15.62, 208 Pb/ 204 Pb: 37.71–38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.

The geodynamic evolution of the Kaoko belt, northwest Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. In the coastal area of this belt, Pan-African dioritic to granitic melts intruded at similar to650 Ma and between similar to580 and similar to550 Ma into sedimentary sequences of the Kuiseb Formation. Field observations suggest that the later intrusive episode coincided with the peak of high-grade metamorphism in this region, which is characterised by granulite facies conditions and anatexis. Franz et al. (1999) state that both intrusive episodes can be connected with high-grade metamorphic events.The delta(18)O values of the Pan-African granitoids range from +8.5 to +10.3parts per thousand and tend to be slightly higher than those of the Palaeoproterozoic and late Archaean basement gneisses (+7.6 to +9.9parts per thousand and +7.4 to +9.0parts per thousand, respectively). The Pan-African granitoids have low to moderate initial Sr-87/Sr-86 ratios between similar to0.7036 and similar to0.7121. Sm-Nd isotopic data of the Pan-African granitoids have moderate initial epsilon(Nd) values ranging from -1.2 to -6.8. They are distinct from Palaeoproterozoic as well as late Archaean orthogneisses from the adjacent Congo Craton, which reveal much lower epsilon(Nd)values between -18 and -28 during the time of emplacement of the Pan-African granitoids. Thus, the derivation of these granitoid melts exclusively from much older crust can be precluded. Although the Pan-African granitoids and the Pan-African high-grade metasedimentary rocks have similar initial epsilon(Nd) values of -1.2 to -6.8 and also similar mean crustal residence ages between 1.2 and 1.6 Ga, the derivation of the granitoids from these metasedimentary rocks alone seems also improbable since the granitoids reveal significantly lower initial Sr-87/Sr-86 ratios than the metasediments. It is here suggested that the Nd, Sr and O isotopic features of the Pan-African granitoids reflect mixing between upper crustal material (metasediments) and remelted basement gneisses from lower crustal levels.