ALBERT

Beschreibung: : The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c . 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c . 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c . 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus. Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467 .

Beschreibung: The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c. 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c. 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c. 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus.Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467.

Beschreibung: Recent studies on shallow-level arcuate intrusions have identified numerous examples of horizontal mineral fabrics. These are commonly interpreted as reflecting considerable lateral flow during magma emplacement, thus querying established ‘semi-vertical’ ring-dyke models. We question the recent lateral emplacement model proposed for the Palaeocene Slieve Gullion Ring-complex, NE Ireland, where the absence of steep fabrics in parts of the ring-complex has been used to support a shallow, semi-horizontal sheet intrusion mechanism. We argue that such simple flow models cannot be applied to explosive ring-fissure eruptions and that fabric data alone do not warrant rejection of the ring-dyke model. Moreover, the apparent ‘absence of steep intrusive contacts’ along the intrusion’s perimeter is readdressed and we present numerous examples of outcrops (27) with steep-sided geometries. The Camlough Breccias are reinterpreted as the product of gas-driven tuffisites injected along the active ring-fault (rather than of purely tectonic origin). Crucially, the porphyritic microgranite and porphyritic rhyolite ring-dyke rocks exhibit geochemical and petrographic signatures of contamination by the geographically restricted Palaeozoic Newry granodiorite and are best explained through crustal interaction vertically beneath the ring-complex. Subsequently, these silicic magmas rose into ignimbrite feeders along a caldera ring-fault system that was emplaced into near-surface vent-filling breccias.

Beschreibung: We present here a low-temperature thermochronological study that combines the apatite fission-track and (U + Th)/He dating methods with a pseudo-vertical sampling approach to generate continuous and well-constrained temperature–time histories from the onshore Irish Atlantic margin. The apatite fission-track and (U + Th)/He ages range from the Late Jurassic to Early Cretaceous and the mean track lengths are relatively short. Thermal histories derived from inverse modelling show that following post-orogenic exhumation the sample profiles cooled to c . 75 °C. A rapid cooling event to surface temperatures occurred during the Late Jurassic to Early Cretaceous and was diachronous from north to south. It was most probably caused by c . 2.5 km of rift-shoulder related exhumation and can be temporally linked to the main stage of Mesozoic rifting in the offshore basins. A slow phase of reheating during the Late Cretaceous and Early Cenozoic is attributed to the deposition of a thick sedimentary sequence that resulted in c . 1.5 km of burial. Our data imply a final pulse of exhumation in Neogene times, probably related to compression of the margin. However, it is possible that an Early Cenozoic cooling event, compatible with our data but not seen in our inverse models, accounts for part of the Cenozoic exhumation. Supplementary material: Details on the apatite fission-track and (U + Th)/He methods are available at www.geolsoc.org.uk/SUP18765 .

Beschreibung: Extract The Caledonian orogen can be traced for several thousand kilometres from the Arctic region southward on both sides of the North Atlantic Ocean (Fig. 1). The orogen formed in the Palaeozoic by convergence and collision of Laurentia, Baltica and Avalonia. Subsequently during the Variscan orogeny, Gondwana collided with the Laurasian continent, forming the supercontinent Pangaea (e.g. Cocks & Torsvik 2006). In terms of scale and processes the Caledonian orogen is often compared with the Himalayan–Alpine orogenic belt that extends from western Europe to eastern Asia (e.g. Milnes 1998; van Staal et al. 1998; Labrousse et al. 2010; Streule et al. 2010). The Himalayan–Alpine orogenic belt is currently still undergoing contraction, which will probably result in the future amalgamation of Eurasia with Africa and Australia (e.g. Hsü 1994). ... This 250-word extract was created in the absence of an abstract.

Beschreibung: Three new U–Pb zircon chemical abrasion isotope dilution thermal ionization mass spectrometry dates obtained from late Viséan Belgian bentonites are reported and are used to estimate the periodicity of early Warnantian shallowing-upwards carbonate parasequences that are interbedded with the dated bentonites. Early Warnantian parasequences exhibit mean cycle periodicity values that are consistent with the c . 100 ka Milankovitch cycle, which is the dominant Milankovitch frequency recognized from recent Pleistocene glacial records, and thus strengthen the arguments for (1) these sedimentary cycles being of glacio-eustatic origin and (2) the initiation of the main phase of late Palaeozoic glaciation before the start of or during earliest Warnantian times. The new dates also provide additional high-precision age constraints for the improved calibration of the Mississippian time scale. Using the new dates, the stratigraphical age of the Clyde Plateau Volcanic Formation, Midland Valley of Scotland, is revised from Holkerian to early Asbian. Supplementary material: U–Pb zircon CA-ID-TIMS data tables and the method of the mean cycle periodicity calculations are available at www.geolsoc.org.uk/SUP18747 .

Beschreibung: 〈span〉〈div〉Abstract〈/div〉The Paleo-Yangtze Basin is hypothesized to have connected the Sichuan, Xichang, and Chuxiong basins during the Late Mesozoic to Early Cenozoic. The Paleo-Yangtze Basin subsequently fragmented later in the Cenozoic, which makes it challenging to decipher its provenance shifts and the exhumation of the surrounding hinterland. Heavy mineral analyses combined with detrital apatite, rutile, and zircon U-Pb dating were employed to elucidate the stratigraphic signatures and exhumation patterns along the eastern margin of the Tibetan Plateau during the Late Cretaceous to Paleogene. Two key horizons have been identified across these basins based on their detrital mineralogy: a garnet-rich horizon within the southern Sichuan Basin, and a garnet-poor horizon in the Xichang and Chuxiong basins. Furthermore, there is a distinct increase in garnet-zircon (GZi) and apatite-tourmaline (ATi) indices in the Upper Cretaceous Gaokan Formation in the Sichuan Basin, indicating increased sediment input from metamorphic and granitic sources as a result of enhanced uplift and exhumation during the Late Cretaceous. A subsequent decrease in these indices in the Sichuan Basin during the Early Paleogene is attributed to the tectonic quiescence of the western Yangtze region during the Early Cenozoic. It should be noted that a contemporaneous decrease in ATi and RZi indices in both the Xichang and Chuxiong basins sections is observed from the Upper Cretaceous to the Paleogene. Detrital apatite, rutile and zircon U-Pb ages from the Sichuan, Xichang and Chuxiong basins share similar characteristics, and the zircon U-Pb data are dominated by age peaks of ca. 2600–2200 Ma, ca. 1900–1700 Ma, ca. 900–600 Ma, ca. 500–380 Ma, and ca. 260–200 Ma. These data indicate a dominant source from the northwestern and northern Yangtze areas and the region around the Kangdian rift, consistent with predominant southeast- and southwest-directed palaeocurrents in these basins. We thus argue that detritus derived from the northwestern and northern Yangtze areas and Kangdian rift region fed the basins located on the western Yangtze Block throughout Late Cretaceous to Early Paleogene times. The Lanping-Simao and Khorat basins on the Indo-China Block were also fed by these sedimentary source regions, indicating a major transcontinental drainage system across the western Yangtze Block.〈/span〉

Beschreibung: 〈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〉