Abstract The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U–Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf 470 values of −7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similar material. Rims from the Tyrone Central Inlier have extremely negative ϵHf 470 values of approximately −39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland.
This study integrates U‐Pb zircon geochronology (from LAM‐ICP‐MS, SHRIMP, and TIMS) with Nd isotopic data from orthogneisses and metasedimentary rocks of the pre‐Mesozoic basement of the eastern Peruvian Andes to provide new information on the tectonic evolution and Neoproterozoic‐Paleozoic paleogeography of this segment of the proto‐Andean margin. A high‐grade orthogneiss unit yields U‐Pb zircon protolith crystallization ages of ∼613 Ma. It was metamorphosed and intruded by an Early Ordovician granitoid. Subsequently, two different volcano‐sedimentary sequences were laid down and metamorphosed, probably as a consequence of terrane accretion. The older sequence was deposited and metamorphosed between 450 and 420 Ma, and the younger one was deposited after 320 Ma and metamorphosed at 310 Ma. U‐Pb detrital zircon age patterns from the two sequences are within the age intervals 315–480, 480–860, 960–1400, and >1400 Ma. These data strongly suggest geological and spatial links between the different units, implying the existence of active magmatism contemporaneous with the reworking of previously formed orogenic complexes. Mesoproterozoic and older ages suggest that the detrital sources are on the western margin of Gondwana, near the Amazonian Craton and/or other Grenvillian‐type domains, such as those found within the Andean belt. Neoproterozoic to Ordovician zircons suggest that this crustal segment was formed on an active margin along the western side of the Amazonian Craton. Whole‐rock Nd isotope data from metasedimentary rocks of the two younger units yield εNd (450 Ma, 310 Ma) values between −6.3 and −13.2 and Sm‐Nd TDM model ages between 1.6 and 2.1 Ga. The comparison of the Nd isotope record with the U‐Pb detrital zircon data suggests that recycling of older crust was an important factor in the growth of the central Peruvian segment of the proto‐Andean margin during the Proterozoic and the Early Paleozoic. Different tectonic and paleogeographic models are discussed in light of the new data presented here.
LA ‐ ICP ‐ MS U–Pb detrital zircon studies typically analyse 50–200 grains per sample, with the consequent risk that minor but geologically important age components (e.g., the youngest detrital zircon population) are not detected, and higher abundance age components are misrepresented, rendering quantitative comparisons between samples impossible. This study undertook rapid U–Pb LA ‐ ICP ‐ MS analyses (8 s per 18–47 μm diameter spot including baseline and ablation) of zircon, apatite, rutile and titanite using an aerosol rapid introduction system ( ARIS ). As the ARIS resolves individual single pulses at fast sampling rates, spot analyses require a high repetition rate (> 50 Hz) so the signal does not return to baseline and mass sweep times (> 80 ms) that span several laser pulses (i.e., major undersampling of the signal). All rapid U–Pb spot analyses employed 250–300 pulses, repetition rates of 53–65 Hz (total ablation times of 4.1–5.7 s) and low fluence (1.75–2.5 J cm −2 ), resulting in pit depths of ca . 15 μm. Zircon, apatite, rutile and titanite reference material data yield an accuracy and precision (2 s ) of < 1% for pre‐Cenozoic reference materials and < 2% for younger reference materials. We present a detrital zircon data set from a Neoproterozoic tillite where > 1000 grains were analysed in < 3 h with a precision and accuracy comparable to conventional LA ‐ ICP ‐ MS analytical protocols, demonstrating the rapid acquisition of huge detrital data sets.
Abstract 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 décollement-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. 40 Ar- 39 Ar 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.
Marine phosphatization events cause episodic carbonate fluorapatite (CFA) precipitation on seamounts, and are commonly linked to growth hiatuses in ferromanganese (Fe-Mn) crusts. However, the complete record of these events and their paleoenvironmental significance remains poorly understood, in large part due to poor age constraints. Here, we apply U-Pb dating to CFA in Fe-Mn crusts from Western Pacific seamounts. These data exhibit good alignment with Sr isotope ages, revealing six potential phosphatization events. This established CFA chronology tightens the timespan of phosphatization events and refines the age framework of Fe-Mn crusts. We subsequently utilize a multiproxy approach to demonstrate that the phosphatization events occurred coeval with the expansion of oceanic oxygen minimum zones. The Western Pacific Fe-Mn crusts thus document major perturbations in global oceanic phosphorus cycling, which appear to have been driven by climate-induced increases in primary productivity linked to changes in global ocean circulation.
Abstract The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.
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