Detrital zircon provenance and Ordovician terrane amalgamation,

19 Detrital-zircon analysis of sandstones interbedded with ca 464 Ma ignimbrites in 20 the lower Mweelrea Formation of the South Mayo Trough, western Ireland, 21 suggests Ordovician source-rock provenance that corresponds to two distinct 22 volcanic-arc phases on the Laurentian margin. East-derived sandstones contain a 23 suite of zircons with a mean age of ca 487 Ma that suggests derivation from the 24 Cambrian to early Ordovician Baie Verte Oceanic Tract arc/ophiolite complex, 25 locally represented by the Lough Nafooey arc rocks and the Clew Bay Complex. 26 Zircons from south-derived sandstones within the Bunnacunneen conglomerate 27 fan have average ages of ca 467 – 474 Ma, and correspond to the Notre Dame arc 28 and locally the Connemara metagabbro and orthogneiss suite. Granite clasts in 29 the Bunnacunneen conglomerate are similar to the Connemara orthogneiss suite, 30 both in terms of their geochemistry and age (ca 471 Ma). The southerly derived 31 sedimentary strata also include Archaean and Proterozoic zircon age spectra 32 consistent with a Dalradian source. A southern provenance from the Notre Dame 33 arc and Dalradian suggests that the Connemara terrane lay to the south of the 34 South Mayo Trough during middle Llanvirn times, from at least 464 Ma. 35 36 Supplementary material: U-Pb LA-MC-ICPMS data for detrital zircons and details of 37 analytical methods for U-Pb LA-MC-ICPMS and U-Pb ID-TIMS analyses are available 38 at http://www.geolsoc.org.uk/SUP00000. 39 40 The Caledonian – Appalachian orogen in eastern North America, Ireland and Britain is a 41 complex mosaic of terranes and has been inferred to represent the collision of western42 Pacific-type tectonic elements (van Staal et al. 1998). Within such large-scale orogens, 43 local geological histories will vary, due, for example, to the presence of microcontinents 44 or to diachronous collisions. The first-order sequence of events, however, should 45 conform to the large-scale plate dynamics of the margin. 46 47 Successions preserved in eastern North America indicate that Cambrian to early 48 Ordovician oceanic volcanic arcs and supra-subduction ophiolites were obducted and 49 accreted onto Laurentia (Baie Verte Oceanic Tract), followed by subduction-polarity flip 50 and development of the 488 Ma to 456 Ma Notre Dame magmatic arc within the 51 Laurentian accretionary margin (van Staal et al. 1998 and references therein). A similar 52 history of successive Ordovician magmatic arcs is evident in western Ireland (Ryan and 53 Dewey 1991, Dewey 2005). Early Ordovician, oceanward-directed subduction formed 54 the Lough Nafooey oceanic volcanic arc (Figs 1, 3; Clift and Ryan 1994, Chew et al. 55 2007); this arc and the associated South Mayo Trough fore-arc basin collided with the 56 Laurentian margin from approximately 475 Ma, causing the Grampian (Taconic) orogeny 57 (Soper et al. 1999, Dewey 2005). A change in subduction polarity then produced ca 475– 58 462 Ma magmatic arc intrusions within the Neoproterozoic metasedimentary sequence 59 (Dalradian) of the deforming Laurentian margin (Friedrich et al. 1999a, Leake 1989). 60 61 Connemara is a piece of Dalradian Laurentian crust that now lies to the south of the 62 obducted-accreted Lough Nafooey arc terrane and South Mayo Trough (Fig. 1). The 63 relative position of Connemara during sedimentation in the South Mayo Trough is crucial 64 to understanding the development and paleogeography of the margin. The emplacement 65 of Connemara adjacent to the Lough Nafooey arc terrane is an important episode in the 66 Grampian Orogeny, but the timing and mechanism of that emplacement are unresolved. 67 Although there is evidence that it occurred before deposition of the late Ordovician or 68 early Silurian Derryveeny conglomerate (Graham et al. 1991; Fig. 3), the earliest 69 contribution of Connemara to sedimentary successions along the Laurentian margin has 70 not previously been constrained. 71 72 The South Mayo Trough sedimentary basin (Figs 1, 2, 3) occupied a key position in this 73 history. Its 8 km thick fill (Pudsey 1984a) preserves Arenig to early Llanvirn (ca 478– 74 464 Ma) turbidites that provide evidence of unroofing of ophiolitic and Dalradian rocks 75 to the north (Wrafter and Graham 1989, Dewey and Mange 1999). These turbiditic 76 successions are overlain by Llanvirn to Caradoc (ca 464–460 Ma) fluvial to shallow 77 marine sandstones of the Mweelrea Formation, which record volcanic arc and 78 metamorphic source areas (Dewey and Mange 1999). Paleocurrents predominantly 79 indicate east-derived flow (Pudsey 1984b), but the formation also contains a southerly 80 derived alluvial fan facies that has been overlooked as a source of important data in the 81 evolving history of terrane amalgamation of this section of the Laurentian margin. 82 83 In this paper we present the results of a study of Ordovician rocks in the South Mayo 84 Trough that combines detrital-zircon analysis with conglomerate-clast petrology and 85 geochemistry, building on an established geological base (e.g., Pudsey, 1984a, b; Graham 86 et al., 1991). Strata of the Mweelrea Formation contain detrital zircons and 87 conglomerates that provide evidence of the earliest amalgmation of the Connemara block 88 with the South Mayo Trough. Our results strongly suggest that the Connemara block was 89 in a position to the south of the South Mayo Trough by Mweelrea (middle Llanvirn, ca 90 464 Ma) time and that the uplifting and eroding Connemara block provided primary 91 volcanic material (ignimbrites), deeper magmatic arc rocks (granite clasts) and Dalradian 92 metasedimentary detritus. Our new data provide a clearer picture of the similarities 93 between disparate parts of the margin and further illucidate the palaeogeographic 94 relations within the Caladonian Appalachian orogen now exposed in western Ireland 95 and northeastern North America. 96 97