Journal of the Geological Society, London, Vol. 146, 1989, pp. 213-215, 2 figs. Printed in Northern Ireland

Short Paper: Ophiolitic detritus in the Ordovician sediments of South Mayo, Ireland

JOHN P . WRAFTER' & JOHN R. GRAHAM2 Department of Geology, University College, Dublin 4, Ireland 2Department of Geology, Trinity College, Dublin 2, Ireland

Sandstones from Ordovician sediments of the South Mayo Trough, western Irish Caledonides, show marked stratigraphic variation in their major and trace element geochemistry. High Cr, Ni, and Mg concentrations of ultramafic derivation succeed high Ti and Fe concentrations of mafic provenance. This sequence is interpreted as the result of the progressive unroofing of an ophiolite. A possible sedimentary linkage with ophiolite lithologies in the Clew Bay Domain to the north would indicate minimal post-Arenig transcurrent displacement. The South Mayo flysch with its ophiolitic detritus is typical of Caledonian flysch sequences which pre-date the mid-Ordovician Taconic deformation, but differs in character from those of late Ordovician age.

Geological setting. Lower Palaeozoic rocks crop out in South Mayo in the western part of the Irish Caledonides (Fig. 1). This area is unique in the Caledonides in that it has late Proterozoic to possible Cambrian Dalradian rocks lying to both north and south. The relative position of these two Dalradian domains and South Mayo during the Ordovician has been the subject of much recent debate (Bluck & Leake 1986; Hutton C% Dewey 1986; Graham 1987). Lying between South Mayo and the Dalradian rocks to the north is a narrow (c . 5 km) belt of rocks (Clew Bay Domain) comprising a southern unit of schists and dismembered ophiolite (Deer Park Complex; Ryan et al. 1983) which is in tectonic contact with a northern unit of low grade metasediments of partly Cambrian age (Rushton & Phillips 1973). These Clew Bay metasediments have been assigned to the Dalradian (Phillips et al. 1975) but have also been interpreted as part of the Highland Border Terrane (Hutton 1987; Graham 1987). The present contact with the northern Dalradian is a major fault (Achill Beg Fault). South Mayo displays a 6 km thick volcano-sedimentary succession of Tremadoc to Llanvirn age which developed close to the southeastern margin of the Laurentian craton and was deformed some time in the mid- to Late Ordovician. The contact between the Clew Bay and South Mayo Ordovician rocks is everywhere concealed beneath Upper Llandovery to Wenlock overstep sequences.

Structure in the South Mayo Ordovician is dominated by a major E-W fold, the Mweelrea Syncline (Fig. 1; Graham et al. 1985). In contrast to the succession on the south limb, the north limb succession is sparsely fossiliferous (see Graham 1987, fig. 11; Graham et al. in press for summary). The lowest reliably dated horizon on the north limb lies within the Sheeffry Formation (Pudsey 1984), over 2 km stratigraphically above the lowest exposed strata, and indicates a level close to the Arenig-Llanvirn boundary. Thus the age of the lower part of the succession (Letterbrock and Derrymore formations) is poorly con- strained but seems likely to be Arenig given the rapid sedimentation rates demonstrable for the rest of the

sequence. Fauna1 control above this level is much better and marked facies changes in rocks of Llanvirn age across the Mweelrea Syncline can be demonstrated (Archer 1977; Graham 1987) (Figs l and 2). Southerly and easterly derived conglomerates occur on the southern limb and generally finer grained sandstones and mudrocks with only minor conglomerates on the northern limb. The southerly conglomerates represent both alluvial and submarine fan deposits whereas most of the rocks of the north limb succession represent a deep water flysch sequence. Both sequences are capped by a fan delta and fluvial complex (Mweelrea Formation) of Upper Llanvirn age (Pudsey 1984). The lateral continuity and equivalence of these sequences can be demonstrated at the eastern culmination of the Mweelrea Syncline where alluvial fan conglomerates of the Maumtrasna Formation pass laterally into coarse grained submarine fan deposits of the Derrylea Formation (Graham 1987).

Early work on the South Mayo Ordovician established evidence for both lateral and axial palaeocurrents within the flysch sediments (Dewey 1962). Pudsey (1984) presented limited palaeocurrent data supporting a northerly and easterly source for the north limb succession. Derivation directions for the south limb succession are better constrained from the S to SE (Archer 1977; Graham 1987).

We present evidence here for ophiolite derivation of Ordovician flysch as recorded by anomalously high concentrations of Cr and Ni as well as the presence of detrital chromite and Mg-rich chlorite. Regional facies patterns and palaeocurrent data indicate a derivation of this detritus from the north where ophiolitic rocks of the Deer Park Complex presently crop out. The possibility of a sedimentological linkage between this northern Clew Bay zone and the southern South Mayo block has implications for regional tectonic models.

Petrological data and interpretation. Whole rock geo- chemistry is useful in characterizing the provenance of sandstones, especially in cases where the original detrital mineralogy is only partly preserved during diagenesis (Bhatia 1983). Sandstone geochemistry reflects both the composition of source rocks and also weathering, deposi- tional and diagenetic processes (Sawyer 1986). However the concentration of certain elements such as Mg, Fe, Cr, Ni and Ti are controlled primarily by the nature of the source rocks, because such elements are either contained within physically resistant minerals or are readily adsorbed by clays. In particular, high concentrations of Cr and Ni are good indicators of an ophiolitic source (Hiscott 1984).

Most of the sandstones of South Mayo have high (up to 60%) matrix contents, much of which is of diagenetic origin. Thus provenance information, no longer discernible in thin section, may be revealed by whole rock geochemistry. To

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214 J O H N P . W R A F T E R & J O H N R . G R A H A M

minimize the chemical effects of grain size sorting during deposition, sediments from a restricted grain size range (medium to coarse sand) were selected. We have analysed 48 such sandstones by EDXRF for a range of major and trace elements from various localities and stratigraphic levels in South Mayo (Fig. 2).

Results show extremely high concentrations of Cr, Ni, and Mg in the northerly derived late Arenig-early Llanvirn sandstones (upper part of the Sheeffry Fm. and Derrylea Fm.) and indicate provenance from an ultramafic source, while high Ti and Fe, but low Mg, Cr and Ni concentrations in the older Letterbrock and Derrymore formations suggest an important contribution from a mafic source. In contrast the southerly and easterly derived early Llanvirn lateral equivalents (Rosroe-Maumtrasna Fm. on the southern limb and eastern outcrops of the Derrylea Fm.) show very low

Fig. 1. Simplified geological map of South Mayo based on Graham et al. (1985). Inset shows the position of South Mayo with respect to some major structural elements of the British Cale- donides after Hutton (1987). C, Con- nemara; CB, Clew Bay; GT, Grampian Terrane; HBF, Highland Boundary Fault; IS, Iapetus Suture; L, Lewisian Craton; LD, Longford Down; MV, Midland Valley of Scotland; NHT, Northern Highlands Terrane; SM, South Mayo; SU, Southern Uplands; SUF, Southern Uplands Fault.

concentrations of Cr, Ni and Mg and clearly indicate a different provenance.

Identification of abundant detrital chromite and Mg-rich chlorite, both confirmed by microprobe analyses, in the Sheeffry and Derrylea formations and mafic igneous clasts in the Letterbrock and Derrymore formations allow us to characterize the cause of the geochemical variation and adds weight to our interpretation. A mean (6 grains) composition of (Fe,4,Mn,.,3M~.4,)(Cr~.~,Fe,,Al~.~~)~O~ (assuming R,03: R 0 = 1) for the chromite is consistent with a derivation from the ultramafic parts of an ophiolite suite (Press 1986). The chemistry of the chlorite (MgO, 19-22%; Cr up to 1850 ppm; Ni up to 2120 ppm) also indicates a source in ultramafic rocks.

We interpret the stratigraphic variation in the chemistry and petrography of the Ordovician sediments on the

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F@. 2. Stratigraphic variation in chem- istry and petrography of Ordovician sandstones in South Mayo. MW, Mweelrea Fm; GL, Glenummera Fm; R/M, Rosroe and Maumtrasna Fms; DL, Derrylea Fm; SH, Sheeffry Fm; DM, Derrymore Fm; LB, Letterbrock Fm. Fe is total iron plotted as Fe,O,; Mg plotted as MgO. Arrows indicate generalized palaeocurrent directions. Symbols and bars indicate mean values and range; U, north limb; V , south limb; 0, eastern Derrylea Fm.

SHORT PAPER 215

northern limb of the Mweelrea Syncline as representing the progressive unroofing of an ophiolite sequence from its upper mafic part down to its lower ultramafic components. As far as we know this is the first clear documentation of the geochemical variation in sediments produced by such an unroofing. The source ophiolite for the South Mayo sediments must, therefore, have been obducted by the early Arenig. The fact that all of these formations contain significant proportions of continental debris is consistent with a source ophiolite that had been obducted onto continental crust. Strongly contrasting compositions of coeval successions on the south limb, which are dominated by granitic, acid volcanic and metamorphic debris (Archer 1977; Graham 1987), together with the palaeocurrent data (Pudsey 1984), lead us to suggest that this ophiolitic source lay to the north. At this time South Mayo was probably a back-arc basin (Graham 1987) and varied sources of clastic detritus might be expected. . Discussion. In western Ireland the simplest explanation of the observed geometry and geochemistry is that the ophiolitic detritus is derived from the presently adjacent source. Nothing in the data requires any relative strike-slip motions. However the inadequacy of simple orthogonal cross sections of the British and Irish Caledonides to explain many of their features has recently been highlighted by Hutton (1987) and Hutton & Dewey (1986). Unfortunately this has also led to an inherent expectation of major strike-slip displacements even in the absence of associated proof. The presence of sinistral displacements across many major faults in the Irish and Scottish Caledonides is not now in dispute but their magnitude is. Thus the detritus could well be derived from the Clew Bay ophiolite or its physical continuation and could represent an early Ordovician sedimentary linkage. Given an apparent linkage we feel the onus is on the proponents of major strike-slip displacements in this area to offer proof of their existence.

In a wider context, the presence of ophiolitic detritus in the Lower Ordovician of South Mayo has parallels to both SW and NE within the Caledonides (Bluck 1985; Dewey et al. 1983; Hiscott 1984; Sturt et al. 1984) as does the post-Llanvirn deformation of South Mayo in both North America (Dewey et al. 1983) and Scotland (Curry et al. 1984).

Flysch sedimentation which post-dates this widespread Taconic deformation and uplift also commonly contains ophiolitic detritus (Kelling 1962; Hiscott 1984; Sturt et al. 1984) but in contrast invariably contains abundant arc debris lacking from the earlier ophiolitic flysch, such as that described above. The fact that these two distinct periods of ophiolitic flysch formation are traceable along such great

strike lengths in the orogen suggests that they are responses to events which affected most of the Laurentian margin.

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--- , , In press. The Geology of South Mayo, Scottish Academic

Received 22 September 1988; revised typescript accepted 27 October 1988.