U–Pb zircon geochronology of plagiogranites from the Lough Nafooey (= Midland Valley) arc in western Ireland: constraints on the onset of the Grampian orogeny

<p>The timing of peak Grampian metamorphism is well constrained from the detrital record of the adjacent fore-arc basin fill and geochronology of synorogenic intrusive rocks, but the onset of collision is less certain. Proximal Silurian conglomerates contain plagiogranite boulders unequivocally derived from the Lough Nafooey arc, two of which yield U–Pb secondary ionization mass spectrometry zircon ages of 489.9 ± 3.1 Ma and 487.8 ± 2.3 Ma. Nd isotopic evidence (ϵ_Nd(490) <em>c</em>. 0) demonstrates that the plagiogranites assimilated significant amounts of old continental crust. This provides an absolute age constraint on a previously poorly constrained and inferred event, demonstrating that the arc had encountered subducting Laurentian margin sediments by 490 Ma. </p

The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland

<p>Field relations, structure and geochronology demonstrate that controversial rocks on Akilia, SW Greenland, cannot host evidence of Earth's oldest life. Mafic–ultramafic gneiss that contains a layer of quartz–amphibole–pyroxene gneiss, a few metres thick, with purported biogenic graphite is not cross-cut by the protoliths of >3.82–3.65 Ga tonalitic gneiss. The contact between these gneisses is tectonic so their relative ages are unknown. Multiple episodes of intrusion and deformation in the tonalitic gneiss preceded the earliest deformation fabrics and structures seen in the mafic–ultramafic gneiss. Although previously interpreted as a volcano-sedimentary stratigraphy, the mafic–ultramafic gneiss displays no evidence of such an origin, and could be partly or wholly derived from igneous intrusions. Existing geochronology indicates that the mafic–ultramafic and qtz–am–px gneisses have ages <3.67 Ga. Metamorphic zircon in the qtz–am–px gneiss grew at <em>c</em>. 2.68 Ga during late Archaean high-grade events that complicate any protolith interpretation. U–Pb dating of apatite from a variety of rocks on Akilia shows that this mineral crystallized, or possibly recrystallized, at <em>c</em>. 1.75 Ga and it thus provides no indication of an early Archaean age for any associated graphite, regardless of whether or not the latter is biogenic. </p

Neoproterozoic evolution of the eastern Arabian basement based on a refined geochronology of the Marbat region, Sultanate of Oman

<p>New high spatial resolution secondary ion mass spectrometry (SIMS) U–Pb zircon data from the Sadh gneiss complex and the intruding Marbat granodiorite of the Marbat region, southern Sultanate of Oman, yield Cryogenian magmatic protolith ages for gneisses ranging from <em>c.</em> 850 to 830 Ma. Zircon ages record a <em>c.</em> 815–820 Ma period of deformation and migmatization, followed by intrusion of a hornblende gabbro/diorite and the undeformed Marbat granodiorite at <em>c.</em> 795 Ma. Following break-up and rifting of Rodinia at <em>c.</em> 870 Ma, crustal growth in the Marbat region occurred via arc accretion at <em>c.</em> 850–790 Ma, possibly in the easternmost part of the Mozambique Ocean based on earlier cessation of accretion here compared to the Arabian–Nubian Shield. Similarity of the new zircon geochronology to peaks of detrital zircon ages in the unconformably overlying Ediacaran Marbat sandstone suggests relatively local derivation from uplifted basement for the latter. </p

Table S1: XRF whole-rock major (wt%) and trace (µg/g) element data and CIPW norm calculations. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S1: XRF whole-rock major (wt%) and trace (µg/g) element data and CIPW norm calculations from the granites at Glenmacnass Waterfall

Table S4: LA–MC–ICPMS zircon Lu–Hf isotopic data. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S4: LA–MC–ICPMS zircon Lu–Hf isotopic data for the granites from Glenmacnass Waterfall

LA–MC–ICPMS zircon Lu–Hf isotopic data. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S4: LA–MC–ICPMS zircon Lu–Hf isotopic data for the granites from Glenmacnass Waterfall

XRF whole-rock major (wt%) and trace (µg/g) element data and CIPW norm calculations. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S1: XRF whole-rock major (wt%) and trace (µg/g) element data and CIPW norm calculations from the granites at Glenmacnass Waterfall

Table S5: LA–MC–ICPMS K-feldspar Pb isotopic data. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S5: LA–MC–ICPMS K-feldspar Pb isotopic data for the granites from Glenmacnass Waterfall

Pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern Baltic Sea

<p>Holocene postglacial lacustrine clays in the northern Baltic Sea were studied for ichnofossils, petrography, and microscale elemental and ³⁴S/³²S composition to understand the impact of benthos-produced organic matrices on the early diagenesis of sediments. X-radiographs of these clays display intense burrow mottling with discrete <em>Palaeophycus</em>, ‘Mycellia' and rare <em>Arenicolites</em>. Small burrow-like and irregularly shaped concretions were separated from these clays. The burrow-like concretions are composed of framboidal pyrite and poorly crystalline FeS₂ cement. They formed within worm-produced burrows by the decomposition of mucous coatings on the burrow walls by sulphate reduction. δ³⁴S_CDT composition of the framboids indicates saline inflows from the North Sea as the sulphate source. The irregular concretions are microcrystalline pyritic masses that formed in pore spaces with less reactive organic compounds. Their δ³⁴S_CDT values reflect precipitation from the ambient porewater sulphate by sulphate reduction. The burrow-like and irregular concretions are microcrystalline barite in the lower parts of the clays. ‘Mycellia' are mineral-replaced filaments produced by large sulphur-oxidizing bacteria or mycelial fungi. They concentrate in monosulphide bands in the lower substrate levels, but are associated with organic patches upward. This study demonstrates that substantial amounts of pyrite formed within organic matrices close to the sediment–water interface. This differs from previous studies that proposed a deeper-burial origin and allochthonous sulphur source. </p

Table S3: Ion-microprobe zircon oxygen isotopic data. Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

Table S3: Ion-microprobe zircon oxygen isotopic data for the granites from Glenmacnass Waterfall