ALBERT

Description: Fragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses. Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying values, a reduced sample set defines a 147 Sm/ 143 Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a 176 Lu/ 176 Hf regression age of 2867 ±160 Ma (MSWD = 5.5). Initial Nd 2970Ma values of the least-altered amphibolites range from 0.0 to +5.7 and initial Hf 2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources. Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

Description: Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene (~30–19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of ~35 km (NW–SE) by ~25 km (SW–NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO 2 -undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW–SE-striking zone, whereas the more SiO 2 -undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions ( 87 Sr/ 86 Sr 0·70335–0·70371; Nd +3·1 to +4·5; Hf +6·5 to +8·0; 206 Pb/ 204 Pb 19·46–19·69; 207 Pb/ 204 Pb 15·63–15·66; 208 Pb/ 204 Pb 39·34–39·62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel–garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (La N /Yb N = 20–25), whereas the alkaline basalts (La N /Yb N = 14–18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Löwenburg Volcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels and Wolkenburg protrusions, are more common in the central portion of the SVF. These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lower- to mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting that intraplate volcanism in Central Europe is directly linked to regional lithospheric thinning and asthenospheric upwelling. Overall, the SVF constitutes an exceptionally well-preserved magmatic assemblage to illustrate these tectono-magmatic relationships.

Description: The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a "dome-and-keel" vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ~4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ~50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Description: Potassium-enriched mafic lavas, and in particular basalts of the high-K series, are important end-members of subduction zone volcanism. Two petrogenetic models can explain the generation of mafic K-rich lavas, involving either melting of ancient, enriched lithospheric mantle sources (single-stage model) or melting triggered by recent refertilization by subduction-related components derived from subducted sediments or oceanic crust (multi-stage model). These two models are tested for post-collisional high-K rocks of Eocene–Oligocene age from the Rhodopes, SE Bulgaria, based on new major element, trace element and Sr–Nd–Hf–Pb isotope data. The single-stage model is evaluated by Sr–Nd isotope modelling assuming the presence of ancient enriched lithospheric domains whereas the multi-stage model is assessed by comparing the compositions of the Bulgarian lavas with those of lavas from Santorini. Santorini lavas are considered to sample the current trace element and isotope inventory of the long-lived Aegean subduction zone system. This northward facing system has been active most probably since late Jurassic or early Cretaceous times and was potentially involved in refertilizing the mantle sources of the Bulgarian lavas. In addition to the data for Bulgarian lavas, we present new major element, trace element and Sr–Nd–Hf–Pb isotope data for Santorini. These are evaluated together with previously published data to infer the mode of source enrichment in the Aegean realm. The Bulgarian lavas exhibit a broad range of compositions from medium-K to high-K and shoshonitic with radiogenic 87 Sr/ 86 Sr (0·706–0·709), and unradiogenic Nd (–5·7 to –1·9) and Hf (–3 to +3) isotope signatures. The trace element budget was apparently well buffered against shallow-level crustal assimilation as documented by the trace element and Sr–Nd isotope systematics. Modelling of the Sr–Nd isotope compositions of the Bulgarian lavas argues for recent (Mesozoic to Cenozoic) source enrichment. Therefore single-stage models involving melting of ancient (〉1 Ga) lithospheric mantle can be confidently ruled out, in agreement with tectonic models for the region. The enriched isotope signatures, together with a pronounced enrichment of incompatible elements, rather indicate mantle refertilization by a subduction component similar to continent-derived sediments subducted at the Hellenic Trench at present. The Bulgarian lavas record a predominant influx of subduction fluid components in comparison with the Santorini lavas. Collectively, the data presented for the Bulgarian lavas are clearly in favour of a multi-stage model.

Description: Marine chemical sediments from the Temagami banded iron formation (BIF) in Canada exhibit nonchondritic Zr/Hf and Y/Ho ratios and seawater-like rare earth element patterns, indicating that their Hf and Nd are not detrital, but derived from seawater. This is confirmed by Sm-Nd and Lu-Hf isochron ages of 2605 ± 140 Ma (initial Nd +0.03 ± 4.1) and 2760 ± 120 Ma (initial Hf +7.2 ± 5.3), respectively, that overlap within error the 2.7 Ga U-Pb age of associated igneous rocks. The Temagami BIF is therefore an excellent archive of the Nd-Hf isotopic composition of Neoarchean seawater. Whereas values cluster around +1, values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an Hf - Nd distribution is typical of modern seawater, plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks. The only mechanism known to produce natural waters with decoupled Nd and Hf isotope compositions is the incongruent mobilization of Hf from continental crustal material. Therefore, input of such highly radiogenic Hf into seawater requires substantial amounts of evolved Neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering.