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Structure and stratigraphy of the metamorphic rocks in part of northwest County Mayo, MICHAEL JOHN CROW, MICHAEL DAVID MAX & JOHN STUART SUTTON

CONTENTS i. Introduction ...... 579 2. Stratigraphy ...... 579 3. Structure .... 581 4. References ...... 583

SUMMARY A new geological map covering over IOOO Rocks of the Complex were tectonically square km of N.w. Co. Mayo is presented. emplaced within the metasediments as major Metasediments, probably Moine and Dalradian anticlinal cores, and as transgressive thrust in age, have been deformed and metamorphosed wedges during the first Caledonian deformation. during the Caledonian orogeny together with A significant structural and metamorphic the pre-Moine Erris Complex (Laxfordian? break occurs across the Corraun Fault which is rocks). The gneissose Erris Complex is com- probably the south-westward continuation of monly preserved except where significant Cale- the Leannan Fault of . donian reconstitution to schists has occurred.

I. Introduction

T nE AREA constitutes over 60 % of the metamorphic rocks in northwest County Mayo, Ireland (Fig. t). Blanket and drift largely obscure the rocks inland, especially in the Ballycroy area where the boundaries of the Kinrovar Schist are not well known, and along the bog over the Corraun Fault northwest of Corraun. Full outcrop maps on a six inch to one mile scale are available at the Geological Survey of Ireland. We generally agree with Kilroe (i9o7) and Trendall & ElweU (I963) but not with that part of Phillips et al. (t969) which includes this area. In the semi-petites, kyanite-staurolite-andesine/oligoclase-garnet-biotite assem- blages in the north gradually give way to albite-biotite-garnet assemblages in the south with a metamorphic break across the Corraun Fault.

2. Stratigraphy

The oldest metasediments in the area occur on the west side of the Blacksod Fault (Sutton I97Ia ). The Doonaneanir Formation is a thin dolomitic semi- pelitic to pelitic schist underlying the Doonamoo Formation which is a heavy mineral banded psammite of almost gneissose appearance. The contacts of the heavy mineral banded, semi-pelitic, I OO metre thick Nakil Formation with the

Jl geol. So¢. vol. x27, I97I, PP. 579--584, 3 figs. Printed in .

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underlying Erris Complex to the north and the overlying Formation on islands to the southwest of Fig. x (Max et al. 197 o) are not seen, and the strati- graphic position of the Nakil Formation is not certain. As it is the oldest observed metasediment south of the Erris Complex, it may be in part equivalent to the Doonaneanir Formation. These metasediments have all been correlated with the Morar Division of the Scottish Moinian (Johnstone et al., I969) and are about 85o metres thick. The Erris Group has been divided into five formations although other formations may be present in areas of pooi" exposure. In the northern part the oldest unit is the grey, banded, cross-bedded Belderg Quartzite. This is succeeded by the white to tan, banded and gneissose Broadhaven Psammite and the more finely banded and quartzose Ben More Quartzite. The youngest unit is the cross-laminated and more massive Srahlaghy Quartzite. The Annagh Formation is composed of tan, banded, gneissose psammites which in part strongly resemble the Broadhaven Psammite to the north. The Erris Group is correlated with the upper Moine, Loch Ell Division (Johnston et al. op. cit.; Max 197oa) and all except possibly the Annagh Formation are probably high in the Moine succession. At the maximum the Erris Group is about 5,ooo metres thick but it is so poorly exposed that fold and slide repetition could exist, which would reduce the true thickness. The Dalradian is separated into two successions. North of the Corraun Fault there are three map units, the oldest of which is the Inver Group of semi-pelitic schists with locally two limestone horizons in the lower part and banded quartzites in the upper part. From the middle upwards occasional zones of granitoid clasts occur although no prominent boulder horizons were seen except at those localities shown on the map. Thickness is difficult to estimate as both tectonic repetition and thinning must occur (Max i97o ). Above this is the Corslieve Group with a boulder bed in the lower part (c.f. Elwell I955). The IOOO metres thick Corslieve Group is largely quartzitic with massive orthoquartzites rarely containing bedded units of granitoid clast conglomerate above the boulder bed in the lower part. It is characterized by alternating quartz rich pelitic schists in the upper part. Lateral facies changes and rarity of exposure have prevented the delineation of mappable formations. The upper part of the sequence is the Corraun Group of alternating semi-pelitic and fine-grained white orthoquartzites of which only the lower 5oo metres is exposed. South of the Corraun Fault are four units. The lowest is the Knockcorraun Formation composed of pelitic schists and a limestone and above this is the 35 ° metre thick Cullydo Formation of banded psammites and orthoquartzites, which is well exposed on Corraun. The Anaffrin Formation, which is I ooo metres thick on Corraun and thickens to 2 5oo metres to the east, is mostly banded psammites with a general decrease in grain size upwards in the less schistose units. Occasional pelitic schists occur. The contact with the overlying 2oo metre thick White- heather Formation is nowhere exposed. This youngest unit is composed of gritty quartzites, banded limestones, semi-pelitic schists and actinolitic greenbeds. The northern Dalradian succession is typical of the Lower and lower Middle Dalradian (Rast I963). South of the Corraun Fault the gritty quartzites, and limestones, and greenbeds are more similar to an upper Middle or Upper Dalradian succession. It is possible that the Knockcorraun Formation is equivalent

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Fxo. 1. Geological map of a part of northwest County Mayo, Ireland

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to part of the Inver Group and if so the Corslieve and Corraun Groups could correlate with the Cullydo and the Anaffrin Formations, but there is no evidence to confirm this. 3. Structure

The area is composed of 3 structural sub-areas. West of the Blacksod Fault is the lowest structural level, east of the Blacksod Fault and north of the Corraun Fault is an intermediate structural level, and south of the Corraun Fault is an inter- mediate to higher level (Fig. I). West of the Blacksod Fault the dominant structural element is a D x dome modified by D2 antiformal doming containing the pre-Moine Erris Complex (Fig. 2a). The Annagh Gneisses (Sutton & Max I969; Max I97oa; Sutton i97I ) are marginaly reconstituted in zones designated the Carricklahan, the Tiraun, and the Scotchport Schists. These are uniform, grey semi-pelitic schists con- taining small tectonic inclusions, and relic gneissose features (Sutton I97 I b). The The Annagh Gneiss-Scotchport Schist Complex and all other pre-Moine rocks in northwest County Mayo, from the Erris Complex (Max 197 ~) which is regarded as Laxfordian in age. The Nakil, Doonaneanir, and Doonamoo Formations (Sutton i97Ia) are all upward facing on $2 which is the dominant fabric element, which generally strikes east and dips to the north at 25 ° to 30 °. The Termon Granite is post- tectonic (Sutton I97O) and has been dated at 368 4- ~o m.y. (Leutwein i97o ).

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FIO. 2a. Diagrammatic north-south tectonic profile across the on the west side of the Blacksod Fault (A-B on Fig. x) b. Diagrammatic north-south tectonic profile across the mainland on the east side of the Blacksod Fault (C-D on Fig. i). The fold south of the Corraun Fault is F I.

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East of the Blacksod Fault and north of the Corraun Fault the dominant structural elements are of different ages in different parts. The main F I anticline north of and the D I emplaced Kinrovar Schist wedge, have been little deformed during D2. To the north, however, the dominant structure is the F2 Bangor- synclinorium and the large F2 anticline in the Erris Group farther north (Fig. 2b). In general the D I effects are dominant south of the synclinorium and although $2 and minor F2 folds occur there are no major D2 folds. $2 forms a 'fan" which is southerly dipping in the north part of the area and passes through the vertical in the hinge of the Bangor-Belmullet synclinorium becoming northerly and northeasterly dipping in the south. In the north $2 strikes about east, but in the east and southeast the strike swings to south- easterly and southerly. The dip of $2 is most shallow in the southwest. South of the Corraun Fault the FI folds are upward facing and the FI fold hinges generally strike to the north or northeast but swing to the east on the southwest part of Corraun. $2 gradually changes from steeply eastward dipping in the east to moderately eastward dipping in the western part of Corraun. There are no major F2 folds but $2 is often the dominant planar fabric. The near-vertical Blacksod Fault has both wrench and normal components and is clearly defined. The west block has been upthrown and moved about 6 km to the north. The Fault zone is marked by widespread shattering and limited movement on often more than one fault plane. However, the most signifi- cant dislocation is the Corraun Fault along which displacement is difficult to estimate as no distinct repetition of structures occurs across it. The older Dalradian sediments could pass up into, or be in part equivalent to, part of the younger Dalradian. This is not likely however, as the sudden change in structures across the Corraun Fault, indicates that a significant dislocation exists.

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FI o. 3. Suggested extension of the Leannan Fault through County Mayo. The Corraun Fault A and a probable extension beneath the Carboniferous toward the seaward continuation in Donegal Bay. The Leannan Fault may continue into Bay slightly south of C where it would be covered by Carboniferous sediments. The lightly stippled ornament designates post-metamorphic rocks. The metamorphic rocks are unshaded. Several faults occur in Malin Head but none is here correlated with the Leannan Fault. The thickness of the line showing the position of the fault is exaggerated.

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To the north there is a complex D i/D2 hinge zone. The wedge of the Kinrovar Schists is probably truncated by the Corraun Fault, as is the larger scale hinge area, but lack of exposure in the small critical area prevented unequivocal interpretation. To the south all the ground is on the same limb of a large scale F I structure which has upward facing, asymmetrical F I folds with their shorter limbs to the northwest. The orientation of $2 is not significantly different across the fault but prominent F2 folds in the range of hills around Corslieve do not appear to be as well developed across the fault to the south. In any case this could be a juxtaposed relation. It is highly probable that the Corraun Fault (Fig. 3) is related to, or is a west- ward continuation of, the Leannan Fault in Donegal (Pitcher et al. 1964) although no direction or amount of displacement can be estimated in North Mayo at this time.

ACKNOWLEDGEMENTS.Part of this work was done at Trinity College, and we thank those at that College who helped us. Crow mapped south of the Tarsaghaunmore River, Sutton the Mullet peninsula and Max the remainder of the area. Crow and Sutton acknowledge financial support from the Irish Government and Trinity College, Dublin. This paper is published with the per- mission of the Director of the Geological Survey of Ireland.

4. References

ELWELL, R. W. D. I955. The lithology and structure of a boulder bed in the Dalradian of Co. Mayo, Eire. Q. Jl geol. Soc. Lond. hi, 7I-84. JOHNSTONE, G. S., SMITH, D. I., & HARMS, A. L. I969. The Molnian Assemblage of . Mere. Am. Assoc. Petrol. Geol. x2, I59-I8o. KmRO~, J. R. ~9o7 . The Silurian and metamorphic rocks of Mayo and North . Proc. R. Ir. Acad. 26, B, x29-x6o. LEUTWXn~, F. 197 o. Preliminary remarks on some geochronological analyses of Irish granites and gneisses. Ir. Nat. J., I6, 3o6--3o8. MAX, M. D. I97oa. Stratigraphy of some metasediments in part of northwest County Mayo, Ireland. Sci. Proc. R. Dubl. Soc. 3A, 303-317. , i97ob. Mainland gneisses southwest of Bangor in Erris, northwest County Mayo, Ireland. Sci. Proc. R. Dubl. Soc. ~ 275-29 x. , i97I. A comparison of two gneiss complexes in Ireland. Bull. Geol. Sum. Ireland 2 (in press). , PHILLIPS, W. E. A., & BROCK, I970. Geology of the DuviUaun Islands. Sci. Proc. R. Dubl. Soc. ~k, 257-268. PHILLIPS, W. E. A., KemCEDY, M. J., & DUNLOP, G. M. x969 . Geologic comparison of western Ireland and northeastern Newfoundland: xo: North Atlantic--Geology and Continental Drift. Mem. Am. Assoc. Petrol. Geol. x2, x94-'2 x x. PITCHER, W. S., ET.WEL~., R. W. D., TOZER, C. F., & C.~BR-~Y, F. W. I964. The Leannan Fault. Q. Jl geol. Soc. Lond. x2o, 241-273. I~ST, N. I963. Structure and metamorphism of the Dalradlan rocks of Scotland, in: The British Galedonides, ed. Johnson, M. R. W. and Stewart, F., 0liver and Boyd, Edin. I23-I42. SUTTON, J. S., x97o. The Termon Granite and associated minor intruslves, County Mayo. Sci. Proc. R. Dubl. Soc. 3A, 293-3o2. , x97ia. The stratigraphy and structure of the Moinian Dalradian metasediments of the Mullet peninsula, Co. Mayo, Ireland. Sci. Proc. R. Dubl. Soe. ,1A, I-I 3. ~-, i97Ib. The pre-Caledonlan rocks of the Mullet peninsula, County Mayo, Ireland. Sci. Proc. R. Dubl. Soc. in press.

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StrrroN, J. S. & MAX, M. D. 1969. Gneisses in the north-western part of County Mayo, Ireland. Geol. Mag., xo6, 284-29o. TR~NDAnr., A. F. & ELWFJn, R. W. D. x963. The metamorphic rocks of northwest Co. Mayo, Ireland: Proc. R. Ir. Acad. 62, B, 217-247.

Manuscript received March x9, x97I.

M. J. Crow, Geology Dept., Trinity College, Dublin, Ireland.

M. D. Max, Irish Geological Survey, x4 Hume Street, Dublin, Ireland.

J. S. Sutton, Geology Dept., Memorial University of Newfoundland, St. John's, Newfoundland, Canada.

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