ALBERT

Description: In Scotland and Ireland, a Laurentian passive margin sequence, the Dalradian Supergroup, was deformed during the c. 470-460 Ma Grampian orogeny, resulting in the formation of crustal-scale recumbent nappes. In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasediments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian nappe fabric acting as a decollement-like slip surface in both cases. As the orientation of these foliation slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers. 40Ar-39Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 {+/-} 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at c. 400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulationslip morphologies combined with geochronology can thus be used to constrain the reactivation history of a major crustal-scale shear zone.

Description: The Yangtze River is the largest fluvial system draining the Tibetan Plateau, yet its time of formation—pre-Miocene versus Pleistocene—has been debated for more than a century, with a particular focus on the incision of the Three Gorges. In this study, we used laser-ablation–multiple collector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) to measure Pb isotope compositions of detrital K-feldspars from Pliocene–Pleistocene sediments in the Jianghan Basin, located just downstream of the Three Gorges. Our new Pb results indicate that feldspars from the Songpan-Ganzi terrane were already being delivered to the Jianghan Basin by ca. 3.4 Ma. Therefore, we suggest that the Three Gorges was incised prior to the late Pliocene. Moreover, the Pb isotopic data also record the first delivery of detritus from the Hanjiang River, one of the Yangtze’s largest tributaries in the middle reaches, into the Jianghan Basin at ca. 1.8 Ma. Most K-feldspar grains from the mid-late Pleistocene samples have similar Pb isotopic compositions to those from the major tributaries in the upper reaches, i.e., Minjiang and Jialingjiang Rivers, indicating that the eastern Tibetan Plateau, as the major sediment supplier to the Jianghan Basin and mid-lower Yangtze River, has played a key role in the sediment source-to-sink transport of this river. This study sheds light on the factors that control the supply of detrital K-feldspar in large-scale drainage systems.

Description: Ophiolites allow spatial and temporal assessment of the causes and length-scales of upper mantle compositional heterogeneity because they permit field-based observations to be coupled with geochemical investigations of upper mantle lithologies. The ~497 Ma Leka Ophiolite Complex (Norway) comprises a section of early Palaeozoic (Iapetus) oceanic lithosphere with well-exposed mantle and lower crustal sections and generally low degrees of serpentinization. The Leka upper mantle section is heterogeneous at the centimetre to metre scale, manifested by abundant dunite lenses and sheets in harzburgitic host-rock, especially within ~500 m of the palaeo Moho. Abundant chromitite (≥60 vol. % Cr-spinel) and pyroxenite lenses and layers also occur in the uppermost 200–300 m of the mantle section. These diverse mantle lithologies probably developed in a suprasubduction-zone (SSZ) setting, as a result of fluid-assisted melt extraction, offering an opportunity to interrogate the nature of chemical heterogeneities developed in such rocks. At ~497 Ma, the Os isotopic compositions of Leka harzburgites averaged ~2% more radiogenic than the projected average for abyssal peridotites at that time, yet they exhibit nearly chondritic relative abundances of the highly siderophile elements (HSE). Several of the harzburgites are characterized by low initial 187 Os/ 188 Os (〈0·121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient (〈0·5 to 2 Ga) melt depletion episodes is a common characteristic of melt-depleted oceanic peridotites. There is no clear evidence that SSZ melt extraction had a discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle, as represented by the Leka harzburgites. By contrast, non-harzburgitic lithologies are generally characterized by more radiogenic initial 187 Os/ 188 Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of their trace element geochemistry and the Re-Os isotope errorchrons that they define, yielding ages of 485 ± 32 Ma and 589 ± 15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 ± 2 Ma). The meaning of the latter age is uncertain, but possibly corresponds to the early stages of Iapetus opening. The Leka ophiolite reveals the importance of oceanic lithosphere formation processes for mantle heterogeneity at metre to kilometre scales, but also emphasizes the robustness of Os isotopes in recording older melt-depletion events.

Description: Chromium-spinels have been widely used as petrogenetic indicators to infer parent melt compositions and the tectonic setting of their formation. This study integrates petrographic, quantitative textural and geochemical analyses of Cr-spinel seams within the Dawros Peridotite, NW Connemara, Ireland to determine the composition of their parental magmas. Calculation of Cr no. (Cr/(Cr + Al)) (0.50–0.77) values and TiO2 (0.18–0.36 wt%) contents of the Cr-spinel seams, coupled with an estimation of the Al2O3 and TiO2 contents (~11.86 wt% and ~0.39 wt%, respectively) of their parental melts, indicates that they probably formed from boninitic melts sourced from a highly depleted mantle. This implies that the Cr-spinel seams formed in a supra-subduction zone undergoing high degrees of partial melting. The Cr-spinel data support tectonic models for the formation of the Dawros Peridotite (and Connemara Metagabbro-Gneiss Complex) during island arc collision, immediately prior to Grampian orogenesis at ~470 Ma. The occurrence of the Dawros chromitite seams at the approximate transition between the lower harzburgite sequence and the upper lherzolite sequence bears marked similarities to the positions of such seams in larger anorogenic layered mafic-ultramafic intrusions, and implies that the Dawros Peridotite behaved as an open-system magma chamber.

Description: Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore SW UK, and the East Irish Sea Basin, some 350 km to the north, show that the same grain populations are present. This indicates that the drainage system (the ‘Budleighensis’ river) feeding these basins originated from the same source/s, most probably the remnant Variscan uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of 400 km, with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundreds of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the NW of the Irish and Scottish massifs, where the remnant Variscan uplands apparently exerted no influence on drainage or sand supply.

Description: : Twenty-two wells on the Norwegian continental margin have penetrated underlying basement. We present U–Pb zircon, whole-rock geochemical, and Sm–Nd and Rb–Sr isotopic data from nine wells in the North Sea and Norwegian Sea with relevance to the offshore continuation of the Norwegian Caledonides, and their correlation throughout the Caledonian–Appalachian orogenic belt. Palaeozoic magmatism in the North Sea can be divided into two groups. The older group consists of 460 Ma calc-alkaline granites with evolved isotopic compositions, correlative with similar rocks in the Uppermost Allochthon. The younger group consists of a 430 Ma dacite and a 421 Ma leucogabbro, with less evolved isotopic compositions. In the Norwegian Sea, isotopically evolved granitic magmatism at 437 Ma and more juvenile dioritic magmatism at 447 Ma are correlative with magmatism in the Bindal and Smøla–Hitra Batholiths in the Uppermost Allochthon. Metasedimentary basement rocks from the North Sea and Norwegian Sea, dominated by Late Palaeoproterozoic and Mesoproterozoic grains, resemble rocks found in the Caledonides of Scotland, Greenland and Svalbard. The new data, along with studies elsewhere along the belt, suggest that similar rocks may exist along much of the orogen. Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18471 .

Description: 〈p〉The development of laser ablation techniques using inductively coupled plasma mass spectrometry has enabled the routine and fast acquisition of 〈i〉in situ〈/i〉 U–Pb and Pb–Pb isotope ratio data from single detrital grains or parts of grains. Detrital zircon dating is a technique that is increasingly applied to sedimentary provenance studies. However, sand routing information using zircon analysis alone may be obscured by repeated sedimentary reworking cycles and mineral fertility variations. These biases are illustrated by two clear case studies from the Triassic–Jurassic of the Barents Shelf where the use of U–Pb geochronology on apatite and rutile and Pb–Pb isotopic data from K-feldspar is highly beneficial for provenance interpretations. In the first case study, U–Pb apatite ages from the (Induan – Norian) Havert, Kobbe and Snadd formations indicate an evolving provenance and identify possible episodes of storage within foreland basins prior to delivery onto the Barents Shelf. In the second case study, U–Pb rutile and Pb isotopic analyses of K-feldspar from the Norian–Pliensbachian Realgrunnen Subgroup provide a clear distinction between north Norwegian Caledonides and Fennoscandian Shield sources and suggest that a similar approach may be used to test competing models for sand dispersal for this Subgroup in regions farther north than this study.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 U–Pb and trace element data for detrital apatite and detrital rutile, Pb isotopic data for detrital K-feldspar, U–Pb data for detrital zircon and a detailed analytical method is available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4363838"〉https://doi.org/10.6084/m9.figshare.c.4363838〈/a〉〈/p〉

Description: In the Caledonides of northwestern Ireland there is little evidence for the crystalline basement to the Dalradian Supergroup, though ascertaining its nature is a critical prerequisite for palaeogeographical reconstructions. In Donegal a lamprophyre intrusion has sampled material with isotopic composition, age and fabrics that permit the investigation of the hidden basement of this region. U–Pb and Lu–Hf isotopes in concert with structural analysis are used to elucidate the origin of three morphologically distinct zircon populations from the lamprophyre. The first of these comprises c . 828 to c . 1660 Ma rounded inherited grains. The second population, of 476 ± 3 Ma large xenocrystic grains, displays oscillatory zoned cores embayed by cross-cutting overgrowths, implying a multi-stage magmatic genesis. The third population, of 437 ± 5 Ma high aspect ratio euhedral grains, is interpreted to date the magmatic crystallization of the lamprophyre. Hf isotopic data from the Silurian and Neo- to Mesoproterozoic zircons yield crustal residence ages of 1.0–2.3 Ga. However, the Ordovician grains have Hf i values of c . –20 and model ages of 2.7–2.8 Ga. These data are consistent with the presence of either Archaean crust or an underthrust Ordovician magmatic rock with an Archaean source component, at depth in NW Ireland. Felsic xenoliths, interpreted to be derived from the country rock Dalradian metasediments, exhibit a pre-entrainment foliation and are randomly oriented relative to the foliation within the lamprophyre matrix, indicating deformation prior to c . 437 Ma, probably related to the Grampian orogeny. The host lamprophyre contains a magmatic foliation parallel to a foliation in the country rock, implying synkinematic emplacement during regional NW–SE compression at 437 Ma. Supplementary material: Details of the analytical methods are available online at www.geolsoc.org.uk/SUP18614 .