Sediment routing and recycling through multiple basins from Palaeozoic to Mesozoic times: a provenance study of the Devonian Old Red Sandstone of southern Ireland and neighbouring offshore Mesozoic basins

This study presents the first dataset of detrital zircon U-Pb ages from the Devonian Old Red Sandstone (ORS) in the Dingle and Munster Basins as well as the offshore Mesozoic North Celtic Sea Basin (NCSB), South Celtic Sea Basin (SCSB), Goban Spur Basin (GSB) and Fastnet Basin (FB). This large dataset is complimented by new detrital white mica Ar-Ar ages in both onshore and offshore basins as well as apatite U-Pb ages in the Dingle and Munster Basins. Previous work (Ennis et al., 2015; Soper and Woodcock, 2003) has indicated the possibility of recycling of Lower ORS (LORS) into the Upper ORS (UORS) due to mid-Devonian Acadian deformation and basin inversion. By exploring the possible sediment sources for each basin, the study aims to investigate the role of recycling in sediment provision to answer two main questions: 1)Does the UORS in southern Ireland represent large-scale recycling of LORS from the Dingle Basin? 2)Does the southern Irish ORS act as a major source of detritus for Triassic to Cretaceous sediments of the NCSB, SCSB, GSB and FB? Sedimentary rocks in the LORS have similar detrital zircon age distributions which are dominated by ca. 1.2 Ga zircons as well as late Neoproterozoic grains. These age distributions suggest a dominant contribution of detritus of Laurentian affinity as well as contributions from westerly and southerly derived Ganderian (peri-Gondwanan) detritus. Caledonian uplift of the area north of the Iapetus Suture would have allowed for a large contribution of Laurentian material. The majority of UORS samples contain very few late Neoproterozoic grains and are instead dominated by early Palaeozoic and ca. 1.1 Ga zircons. These detrital zircon age distributions represent recycling of northerly-derived Ordovician to Silurian sedimentary rocks of the Southern Uplands – Longford Down terrane, which are of Laurentian affinity, and not recycling of LORS as previously suggested. The western Toe Head and Old Head Sandstone Formations, which represent the transition from terrestrial ORS to Carboniferous marine environments, are dominated by zircons of Acadian (410-390 Ma) age as well as late Neoproterozoic zircons, suggesting a western offshore Acadian granite source. Clastic rocks of the eastern Old Head Sandstone Formation have a Laurentian provenance like that of the underlying UORS. Detrital apatite and white micas from the ORS indicate Late Caledonian (430-420 Ma) to Acadian sources which were generally not recorded by detrital zircon ages. The majority of samples in the offshore basins of southern Ireland have complex detrital zircon age distributions which indicate a mixture of sources. Most samples contain an abundance of late Neoproterozoic zircons of peri-Gondwanan affinity. Potential candidates for these zircons include Ganderia, Megumia, Avalonia and Cadomia but distinguishing between these is problematic – especially given the likely mixture of sources. The abundance of late Mesoproterozoic detrital zircons in some samples indicates the presence of a Laurentian source with the most likely delivery method for such zircons being recycling of UORS sedimentary rocks, simply due to their close proximity (forming the footwall to the NCSB) to the Mesozoic basins. Two detrital white mica samples have dominant Late Caledonian (~430 Ma) ages, one sample has mostly Acadian (~405 Ma) and one sample has a dominant late Neoproterozoic age. Thus, sediments in the southern Irish offshore basins were predominantly sourced from peri-Gondwanan terranes, Caledonian and Acadian granites and recycling of UORS sedimentary rocks

The role of hydrothermal activity in the formation of karst-hosted manganese deposits of the Postmasburg Mn Field, Northern Cape Province, South Africa

The Postmasburg Manganese Field (PMF), Northern Cape Province, South Africa, once represented one of the largest sources of manganese ore worldwide. Two belts of manganese ore deposits have been distinguished in the PMF, namely the Western Belt of ferruginous manganese ores and the Eastern Belt of siliceous manganese ores. Prevailing models of ore formation in these two belts invoke karstification of manganese-rich dolomites and residual accumulation of manganese wad which later underwent diagenetic and low-grade metamorphic processes. For the most part, the role of hydrothermal processes and metasomatic alteration towards ore formation has not been adequately discussed. Here we report an abundance of common and some rare Al-, Na-, K- and Ba-bearing minerals, particularly aegirine, albite, microcline, banalsite, sérandite-pectolite, paragonite and natrolite in Mn ores of the PMF, indicative of hydrothermal influence. Enrichments in Na, K and/or Ba in the ores are generally on a percentage level for most samples analysed through bulk-rock techniques. The presence of As-rich tokyoite also suggests the presence of As and V in the hydrothermal fluid. The fluid was likely oxidized and alkaline in nature, akin to a mature basinal brine. Various replacement textures, particularly of Na- and K- rich minerals by Ba-bearing phases, suggest sequential deposition of gangue as well as ore-minerals from the hydrothermal fluid, with Ba phases being deposited at a later stage. The stratigraphic variability of the studied ores and their deviation from the strict classification of ferruginous and siliceous ores in the literature, suggests that a re-evaluation of genetic models is warranted. New Ar-Ar ages for K-feldspars suggest a late Neoproterozoic timing for hydrothermal activity. This corroborates previous geochronological evidence for regional hydrothermal activity that affected Mn ores at the PMF but also, possibly, the high-grade Mn ores of the Kalahari Manganese Field to the north. A revised, all-encompassing model for the development of the manganese deposits of the PMF is then proposed, whereby the source of metals is attributed to underlying carbonate rocks beyond the Reivilo Formation of the Campbellrand Subgroup. The main process by which metals are primarily accumulated is attributed to karstification of the dolomitic substrate. The overlying Asbestos Hills Subgroup banded iron formation (BIF) is suggested as a potential source of alkali metals, which also provides a mechanism for leaching of these BIFs to form high-grade residual iron ore deposits

The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland

The Lower Old Red Sandstone in southern Ireland is hosted in the Early Devonian Dingle Basin, which lies immediately south of the Iapetus Suture on the Dingle Peninsula, County Kerry. The basin developed as a post-Caledonian pullapart structure prior to Acadian deformation, which in turn was followed by end-Carboniferous Variscan deformation. Detrital zircon U–Th–Pb geochronology is complemented by mica Ar–Ar and apatite U–Pb geochronology to gain a comprehensive understanding of the provenance of the Lower Devonian Lower Old Red Sandstone of the Dingle Basin and assess contributions of major tectonic components (e.g. Laurentia, Ganderia). Sedimentary rocks in the Lower Old Red Sandstone have similar detrital zircon age distributions, which are dominated by c. 1.2 Ga zircons as well as late Neoproterozoic grains. This indicates a dominant contribution of detritus of Laurentian affinity as well as contributions from westerly and southerly derived Ganderian detritus. Caledonian uplift of the area north of the Iapetus Suture would have facilitated a large contribution of (peri-)Laurentian material. The Upper Old Red Sandstone on the Dingle Peninsula has a distinctly different detrital zircon character including few late Neoproterozoic zircons and abundant zircons of c. 1.05 Ga age, indicating sediment derivation only from Laurentia and no recycling from the Lower Old Red Sandstone

Erratum for 'The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland,' Journal of the Geological Society, London, 175, 411-424

Samples in this paper have been assigned formations based on the Geological Survey of Ireland shapefile released prior to the commencement of the study. However, the authors were not aware that, since obtaining the samples, an updated shapefile had been released. This update affects three of the four apatite samples assigned to the Lower Devonian Ballymore Formation. The location of samples Mb-1, Mb-4 and Mb-5 now places them well within the undifferentiated, Upper Devonian Slieve Mish Group. As outlined in our paper, the apatite ages were originally produced concurrently with apatite fission track analysis and were later used in our study to provide additional provenance information in support of the detrital zircon geochronological data. In the second paragraph of the discussion section we say the following: "Williams et al. (1999) obtained an age of 411 Ma for the Cooscrawn Tuff Bed in the Ballymore Formation, which is older than 22 of the 70 detrital apatites analysed in this formation". The reassignment of the three samples to the Upper Devonian Slieve Mish Group nullifies the above statement. However, our interpretation that the depositional age of the Ballymore Formation is younger than the 411 Ma age given by Williams et al. (1999) is predominantly based upon the evidence given by the six youngest detrital zircons from the formation which underlies the Ballymore Formation (i.e. the Slea Head Formation). These zircons give a concordia age of 405 ± 4 Ma. This suggests that the Ballymore Formation was more than likely deposited after 409 Ma. We do not believe that the reassignment of the three samples has any major impact on our provenance interpretations. The ∼420 Ma age of the majority of the apatites in samples Mb-1, Mb-4 and Mb-5 actually fits with the range of Palaeozoic detrital zircons in sample AK-17 which was taken from the Slieve Mish Group, thereby supporting minor input of rocks affected by end-Scandian metamorphism

Full detrital U–Pb zircon dataset. The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland

Full detrital U–Pb zircon datase