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JMU Ireland Summer Field Course 2018

An Introduction to the geology of Connemara’s metamorphic and igneous rocks. By Prof. Martin Feely, NUI, Galway, Ireland. Part 1.

A summary of the regional geology of Connemara

The rocks of Connemara contain petrified images of buckled crust and volcanic activity

that are many hundreds of millions of years old. The oldest rocks (~600-460 Ma) form

the dominant central sector, which is an east-west trending corridor of deformed and

metamorphosed igneous and sedimentary rocks collectively known as the Connemara

Metamorphic Complex (Leake and Tanner 1994 and references therein)-see Figure 1.

The Oughterard Granite (~463 Ma) occurs at the eastern end of the complex. The

complex is bounded to the north by igneous and sedimentary rocks of Ordovician and

Silurian age (~500-410 Ma) and, in the south by the Galway Granite (~425-380 Ma)

including the Roundstone, Inish and Omey Granites. Two other lithological units of

note are the metamorphosed volcanic rocks of the Delaney Dome Formation and the

sedimentary and volcanic rocks exposed on the islands of South Connemara

(Lettermullan and Gorumna)- both of these units are of similar age (~470 Ma).

The Connemara Metamorphic Complex is part of an ancient mountain belt that

stretched in NE direction from the Appalachians, through Canada, Ireland, Scotland

and Scandinavia. This mountain belt owes its present fragmented distribution pattern to

the birth (~200 million years ago) and ongoing growth (~2 cm/yr.) of the Atlantic

Ocean.

The metamorphic and igneous rocks form the solid mass of the landscape, but the

actual shapes of the mountains and valleys result from sculpting by ice. The past two

million years of geological history, in this part of the globe, have largely been

dominated by glaciations and interglacial periods (when the ice retreated). Interglacial

periods can last for 100,000 years and it seems that we are now living in an interglacial

period and maybe the ice will advance south again to cover most of Ireland. The ice

cover in Connemara may have been up to 1km thick at times. The birthplace of minor

glaciers can be seen in the scooped-out hollows (corries or cirques) on the sides of the

quartzite mountains (the Twelve Bens, Maumturks and Corcogemores) of the

Connemara Metamorphic Complex.

Of all the rock types to be found in Co. Galway, the Connemara marble is the most

celebrated. The marble stretches for about 30 km. in an E-W direction along the

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southern flank of the Twelve Bens. Its distinctive colour variations (shades of green,

sepia and white) developed as a result of the metamorphism of impure limestones ~

470 million years ago. It is ideal as an ornamental and decorative stone, and very

popular in the manufacture of jewellery. It has been widely used for panelling, pillars,

fireplaces and other interior work. The interior of Galway Cathedral offers a

spectacular example of the decorative use of Connemara marble (Feely, 2002).

The bedrock geology of south Connemara is dominated by granite. Many varieties are

encountered in the region. The granites range in colour from pink to dark grey

reflecting varying proportions of the essential minerals quartz, feldspar and mica. All of

these varieties are collectively termed the Galway Granite (Feely et al., 2006). The

Galway Granite is exposed over an area of approx. 600 sq. km. along the north shore of

Galway Bay. It extends offshore to the west, south and east reflecting a granite mass

that has a total surface area of ~ 1500 sq. km. Furthermore, it has an estimated

thickness of ~ 10 km. The Galway granites were emplaced into the upper 10 km of the

Earth's crust between ~425 and 380Ma (Feely et al., 2003, Feely et al., 2007 & Feely et

al., 2010). The younger Carboniferous limestones (~350-320 million years old) form a

boundary to the Connemara basement rocks to the east and south (Lees & Feely

2016)—see Figure 1 below.

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Figure 1. Map of Connemara and environs showing the distribution of the main geological units. The Galway Granite is undifferentiated while the earlier plutons are the Omey (O), Inish (I) and Roundstone (R) granites. The area to be visited on the fieldtrip is outlined.

The Connemara Metamorphic Complex (CMC).

The Connemara metamorphic complex forms part of a regional belt of rocks that

extend from the Appalachians, through Ireland and Scotland, to Norway. In Ireland,

the Connemara part of this belt occurs to the south of all other similarly aged rocks and

has been referred to as the Connemara allochthon i.e. block or nappe which has been

relocated from its site of formation. Recently, Friedrich and Hodges (2016) and

Dewey and Ryan(2016) however, argue that the complex was formed in situ and

therefore is not allothonous – see also Dewey and Ryan(2016) who also support this

revision. The early Ordovician Grampian Orogeny, responsible for the CMC, was the

result of collision between Laurentia’s margin and an oceanic island arc. The CMC

differs in position and character from all other parts of the exposed Dalradian rocks of

the Grampian Orogen in lying south of the Highland Boundary Fault (see Fig 6 below).

Furthermore, Dewey and Ryan (2016) state that Connemara is not a terrane, displaced

with respect to the remainder of the Grampian Orogen but was overridden, northwards,

by the arc (Lough Nafooey Volcanic Arc) and its fore-arc basin (South Mayo Trough)

its frontal ophiolite complex (Deer Park Complex) and accretionary complex

(Killadangan) the latter two exposed further north in the Clew Bay area.

The CMC is composed of two interfingered E-W trending metamorphic belts (- a

northern and southern belt) intruded by post tectonic granites e.g. in the east by the

Oughterard Granite (~463Ma) and in the south by the Galway Granite Complex (~425-

380Ma). It is overlain along its northern margin by unconformable Silurian strata while

to the east it is bounded by Carboniferous Limestones. Sediments and volcanics of late

Proterozoic age (~600Ma) that were metamorphosed during the Grampian orogeny

(~475-463Ma) form the northern belt see Fig. 2. Quartzites, schists, marbles and

amphibolites are the lithologies that characterise the northern belt and are collectively

termed the Connemara schists.

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Figure 2. Generalised geologic map of Connemara. Metamorphic isograds are shown

along with the MGGS, the Silurian unconformity, the trace of the D4 Connemara

antiform and the Mannin thrust responsible for the emplacement of the Connemara

Metamorphic Complex over the relatively low grade meta-rhyolites of the Delaney

Dome Formation (DDF). NS crossection A-A1 is shown in Fig 4 below. After Friedrich

& Hodges (2016)

Distribution of the major rock units in Connemara.

Dalradian lithostratigraphy reflects a continuum from the rocks of the Connemara

Metamorphic Complex, through Mayo and Donegal to the Central Highlands of

Scotland. They were deposited on the southeastern continental margin of Laurentia.

The older Dalradian rocks of Connemara (Appin Group) were deposited in a shelf

environment and the younger Argyll and Southern Highland Groups formed in a

series of fault bounded basins (Fig 3). Figure 3 displays the age relations of the

Dalradian linking sedimentation from Neoproterozoic to early Palaeozoic (~700-

520Ma) with deformation and metamorphism mainly associated with the Grampian

Orogeny (475-463Ma). The Dalradian (called after a Celtic tribe Dal Riada) is a

tectono-stratigraphic term with nomenclatures rooted in the Scottish Highlands.

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Fig 3. Geological sequence and events in the Grampian Highlands. From: Table 1 in Stephenson, D, and Gould, D. 1995. British regional geology: Grampian Highlands. Fourth edition. Keyworth, Nottingham: British Geological Survey. The southern belt is dominantly composed of meta-igneous rocks produced during synorogenic arc magmatism that spanned a period of ~12Ma (from ~475 to ~463Ma i.e. The Grampian Orogeny). This cycle of igneous activity, produced at the roots of a magmatic arc, started with the intrusion into the Dalradian metasediments of Lower Ordovician (~475-470Ma) gabbros followed by intrusions of intermediate to acid magmas up to at least 465Ma-all were metamorphosed during this period. The rocks comprising the southern belt are collectively termed the Metagabbro-Gneiss Suite (MGGS). The Metagabbros are essentially massive amphibolites that retain in places primary igneous minerals (olivine and pyroxene) and gabbroic textures. They are the most widely distributed occurring in both the northern and southern parts of Connemara. The gneisses comprise a suite of (a) orthogneisses derived from the calc-alkaline intermediate-acid magmatism that intruded the Dalradian metasediments and the metagabbros in the south and (b) paragneisses whose protoliths are the Dalradian metasediments that display evidence of partial melting (migmatites). The Connemara terrane is considered to have been exhumed from >18km to ~ 10km between ~470 and 463 Ma.

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Deformation The Connemara metamorphic complex displays evidence of at least three deformation events (D2 to D4) and the most obvious and regional scale structure, whose trace runs along the centre of the northern belt, is the Connemara Antiform (D4) that plunges gently (~20o ) to the east. No D1 folds are recognised and evidence for D1 comes from inclusion trails in garnet augened by a D2 schistosity S2 (see Fig. 4). The D2 event produced a pervasive schistosity (S2) in the Dalradian lithologies. The most common observable folds are tight D3 structures. These formed at the peak of metamorphism. The final major phase D4 produced broad open E-W trending upright structures most notably the Connemara Antiform. The gabbros are post-D2 (in the N they are syn D2 and immediately prior to D3) but pre- to syn-D3. The younger acid magmas appear to overlap with D3. Melting in the migmatitic metasediments also spanned the D3 events. Minor D3 folds in competent unmelted strata are sometimes distorted and disrupted by the partial melts. It is suggested that D3 may have been triggered by the cycle of arc magmatism. The youngest major structure in Connemara is the Mannin Thrust exposed in the western part of the complex. It occurred between ~470 and 462 Ma the latter age being the age of the intrusion of the Oughterard Granite. The amphibolite facies rocks of the metamorphic complex comprise the upper thrust section while greenschist facies meta-rhyolites of lower Ordovician age (~475Ma) are present in the lower section of the flat lying Mannin Thrust.

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Figure 4. Summary of the deformational features and structural setting of the Connemara Metamorphic Complex. See Fig 2 above for location of NS Crossection. After Friedrich & Hodges (2016). Metamorphism The rocks of the complex have been subjected to amphibolite facies metamorphism with grade increasing from north (garnet zone) to south (upper sillimanite zone) through a series of E-W trending metamorphic zones (Fig 1). The metamorphic history is one of an early garnet-staurolite +/- kyanite regional metamorphism (M2 Barrovian metamorphism) essentially related to D2, which is overprinted by a syn-D3 metamorphic event (M3) which is at sillimanite grade in the southern half of the

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complex and is related to the arc magmatism i.e. Intrusion of the synorogenic gabbros (amphibolites) and granites (orthogneisses). Andalusite and cordierite growth are related to these magmatic events. This history implies that an early high-pressure metamorphism (P= ~5kb, T= 650-750oC) was followed by a later metamorphism concomitant with a reduction of pressure (P~3kb; T~650oC)- (Barrovian followed by Buchan style metamorphism see fig 5).

Figure 5. P-T diagram showing the PT ranges for different types of metamorphic

regimes. Approximate stability fields of the three Al2SiO5 polymorphs are also shown.

Connemara has both Barrovian (M2) and Buchan (M3)style metamorphic regimes

typically ranging from greenschist to amphibolite facies.

Allocthonous Connemara: the anomalous position of the Connemara Metamorphic Complex.

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Fig. 6. Terrane map showing distribution of the Dalradian of Scotland and Ireland. Note the anomalous position of Connemara’s Dalradian rocks.

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The position of the Connemara Metamorphic Complex. The Connemara metamorphic complex is flanked to the north and south by low-grade greenschist facies lower Palaeozoic sediments and volcanics. The contact between it and the lower Ordovician rocks of the South Connemara Group is faulted i.e. Skerd Rocks Fault, while Silurian strata rest unconformably along its northern margin. The position of the Connemara metamorphic complex is different because it lies to the south of the Highland Boundary Fault. Elsewhere in Ireland and in Scotland Dalradian lithologies occur on the north-western side of this fault which is a major crustal discontinuity extending from Scotland through to the west of Ireland (Fig. 6). It was considered to have been thrust southwards from the main Dalradian terrane into its present position however, Friedrich and Hodges(2016) and Dewey and Ryan (2016) show that the CMC formed in situ (see Fig 8 below). Plate tectonic reconstructions. Figure 7a below shows the Earth's surface and relevant tectonic plates and their movement from ~500Ma to ~400 Ma. NW Ireland and Great Britain are at the leading edge of Laurentia and they moved closer with time to Baltica and Avalonia. The latter is where the SE part of Ireland and Great Britain lay until suturing began to occur ~425Ma (Fig 7b). The Dalradian metasediments that comprise the northern belt of the Connemara Metamorphic Complex were laid down at the SE edge of Laurentia by 510 Ma. The ocean in between Laurentia, Baltica and Avalonia is called the Iapetus Ocean and the closure of this ocean took place ~ 425-400 Ma. The suture line is called the Iapetus Suture. Between c. 475 and 465 Ma the metagabbros followed by granite orthogneisses were emplaced into the Dalradian metasediments. Their tholeiitic/high alumina basalt magma (gabbros) to calc-alkaline magma (granite gneisses) geochemistry reflect a twin series of magmatic rocks that originated in a continental magmatic arc along the southeast side of Laurentia. Northwestward subduction of oceanic crust was underway at this time. Prior to this subduction was in a SE direction, the change in direction to NW directed subduction occurred during the Grampian orogeny (~470-465Ma) –see Fig 8 below. This setting is akin to the east coast of north and south America where oceanic crust has been subducted under continental crust with the generation of continental magmatic arc complexes.

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Figure 7a. Cartoon showing distribution of relevant tectonic plates at ~500Ma. Position of Connemara is highlighted with the red star. The Precambrian/Cambrian sedimentary rocks e.g. sandstones, limestones and mudstones, and basaltic lava flows had formed at the leading edge of Laurentia (including Connemara) by now. These were the protoliths that formed the marbles, quartzites, pelitic schists and amphibolites (not to be mistaken with the rocks of the MGGS) that form the metamorphic rocks of the Connemara Metamorphic Complex during the Grampian Orogeny (~475-463Ma)

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Figure 7b. Cartoon showing distribution of tectonic plates after suturing took place. At this time Ireland (including Connemara) and Great Britain had sutured with the death of the Iapetus ocean. Suturing started at ~425Ma and was marked by major strike slip faulting and granite emplacements e.g. Galway Granites. Finally, A series of diagrams showing the making of the CMC during the Grampian Orogeny What follows are four diagrams (Fig 8, 9, 10 and 11) based on, and adapted from Friedrich and Hodges (2016). They bring together all the elements relating to Plate Tectonics (Fig 8), Deformation and Metamorphism (Fig. 9), Pressure, Temperature, Metamorphism and Time (Fig 10 and 11).

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Fig 8. Newly published schematic profiles after Friedrich & Hodges(2016) of the island arc-continent collision (Grampian Orogeny) at five time intervals.

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Figure 9. Fig 8 above with the major deformation and metamorphic events linked to the plate tectonic stages.

Figure 10. P-T-t paths for Connemara –adapted from Friedrich and Hodges (2016)

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Figure 11. Linking Deformation, Metamorphism, Temperature and Time- sapphire formation from Feely et al., (2017). Selected references. Chew, D.M. and Strachan, R. A. 2014 The Laurentian Caledonides of Scotland and Ireland. Geol. Soc., Lond. Special Publication 390, 45-91. Cliff, R.A., Yardley, B.W.D. and Bussy, F.R. 1996 U-Pb and Rb-Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland. J. Geol. Soc. London, V153, 109-120. Dewey, J and Ryan, P.D. 2016 Connemara: its position and role in the Grampian OrogenyCan. J. Earth Sci. 53: 1246–1257 Feely, M., Coleman, D., Baxter, S. & Miller, B. 2003. U-Pb zircon geochronology of the Galway Granite, Connemara, Ireland: Implications for the timing of late Caledonian tectonic and magmatic events and for correlations with Acadian plutonism in New England. Atlantic Geology, 39,175-184. Feely, M., Leake, B. E., Baxter, S., Hunt, J. and Mohr, P. 2006 A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland. Dublin. Geological Survey of Ireland.

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Feely, M., Leake, B.E., Costanzo, A., Cassidy, P. and Walsh B 2017 Sapphire

occurrences in Connemara: field and mineralogical descriptions from an erratic,and from bedrock pelitic xenoliths in the Grampian Metagabbro-Gneiss Suite. Irish Journal of Earth Sciences 35 45-54.

Feely, M., Selby, D., Conliffe, J. & Judge, M. 2007. Re-Os geochronology and fluid

inclusion microthermometry of molybdenite mineralization in the late-Caledonian Omey Granite, western Ireland. Applied Earth Science, 116 (3), 143-149.

Feely, M., Selby, D., Hunt, J. and Conliffe, J. 2010 Long-lived granite related

molybdenite mineralization at Connemara, western Irish Caledonides. Geological Magazine 147, 886-894.

Friedrich, A.M., Bowring, S.A., Martin, M.W. & Hodges, K.V. 1999. Short-lived continental magmatic arc at Connemara, western Irish Caledonides: Implications for the age of the Grampian orogeny. Geology, v.27; no.1; p. 27-30. Friedrich, A.M. & Hodges, K.V. 2016. Geological significance of 40Ar/39Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions. Can. J. Earth Sci. 53: 1258-1278. Jagger, M.D., Max, M.D., Aftalion, M. and Leake, B.E. 1988. U-Pb zircon ages of basic rocks and gneisses intruded into the Dalradian rocks of Cashel, Connemara, Western Ireland. J. Geol. Soc. Lond. V 154: 645-648. Kennan, P.S., Feely, M. & Mohr, P.M. 1987. The age of the Oughterard Granite, Connemara, Ireland. Geol. Jour. 22, 273-280. Kennan, P.S., Feely, M. & Mohr, P.M. 1989. Reply on the age of the Oughterard Granite, Connemara, Ireland. Geol. Jour., 24, 223-225. Leake, B.E., Tanner, P.W.G. and Senior, A. 1975. The composition and origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland. J. Petrology, 16, Part 2, 237-277. Leake, B.E., Tanner, P.W.G. and Senior, A. 1981. The geology of Connemara, 1:63360 Geological map, University of Glasgow. Leake, B.E. and Tanner, P.W.G. 1994. The Geology of the Dalradian and Associated Rocks of Connemara, Western Ireland. Royal Irish Academy Dublin, Special Publication. Lees, A. & Feely, M. 2016. The Connemara Eastern Boundary fault: a review and assessment using new evidence. Irish J. Earth Sciences, 34, 1-25.

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Leggo, P.J., Compston, W. and Leake, B.E. 1966. The geochronology of the Connemara granites and its bearing on the antiquity of the Dalradian series. Q.J.G.S. Lond.122, 91-188. Max, M.D. 1985. Connemara Marble and the industry based upon it. GSI Report Series RS/85/2 pp.32. Mohr, P. and Feely, M. 1990. Connemara 1990- Report on a discussion meeting on the Geology of Connemara, Western Ireland. Terra Nova. 516-518. Pidgeon, R.T. 1969. Zircon U-Pb ages from the Galway Granite and the Dalradian, Connemara, Ireland. Scott. J. Geol. 5, 375-392. Tanner, P.W.G. 1990. The structural age of the Connemara gabbros, western Ireland. J. Geol. Soc. Lond. 147. 599-602. Tanner, P.W.G. and Shackleton, R.M. 1979. Structure and stratigraphy of the Dalradian rocks of Bennabeola, Connemara, Eire. In; Harris et al (eds) The Caledonides of the British Isles Reviewed. Geol. Soc. Lond. Spec. Pub. 8, 243-256. Tanner, P.W.G., Dempster, T.J. and Dickin,A.P. 1989. Time of docking of the Connemara terrane with the Delaney Dome Formation, western Ireland. J. Geol. Soc. Lond. 146 389-392. Trelaor, P.J. 1982. The stratigraphy and structure of the rocks of the Lissoughter area, Connemara. Proc. R. Ir. Acad. 82.B. 83-107. Wellings, S . A. 1998. Timing of deformation associated with the syn-tectonic Dawros - Currywongaun - Doughruagh Complex, NW Connemara, western Ireland. Jour. Geol. Soc. London, V 155, 25-37. Yardley, B.W.D., Barber, J.P. and Gray, J.R. 1987. The metamorphism of the Dalradian rocks of Western Ireland and its relation to the tectonic setting. Phil. Trans. R. Soc. Lond. A321: 243-270.

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Localities to be visited during the field trip to the Connemara Metamorphic Complex:

Letterfrack

Figure 7

Omey

N

10 km

Metagabbro-GneissSuite (475-465Ma)

Galway Granites~400Ma

Oughterard Granite~462 Ma.

RoundstoneCarboniferousLimestone

Kylemore

twelve bens

CorribL.

Ireland

Quartzitemarbleschistsedimentsvolcanics

}} volcanics

(~470 Ma)

~600Ma

470-420Ma

~325 Ma

Simplified Geology Map of Connemara.(approximate ages for the different rock units are given in millions of years(Ma).)

1

5

4

2 3

Fieldtrip Locations�

The Connemara Metamorphic Complex Locality 1. Claggan quarry. Staurolite zone Kylemore Formation metasediments (Middle Dalradian). Andalusite + muscovite + quartz +/- tourmaline pegmatite veins. Pinitized cordierite poikiloblasts are also present in pelites. Locality 2. Cur Mountain. The rocks by the parking area are Ballynakill Formation pelites with interbedded pebbly bands. They are in the staurolite to sillimanite transition zone. Walking northwards Lakes Marble Fm. rocks are encountered, especially the highly distinctive Upper Marble Member displaying spectacular D3 folds and then the older Massive Amphibolite (with interbedded gritty quartzites). Locality 3. Lough Derryclare. Before visiting the lakeshore, outcrops of Bennabeola Quartzite Fm by the road display asymmetric F3 folds in quartzite.

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By the shore of L. Derryclare an old Connemara Marble quarry is encountered. Rocks exposed are those of the Green Marble Member of the Connemara Marble Fm. The marbles show banding and folding and display a variety of colours - note the jade green (serpentine rich) variety commonly used in the jewellery trade. In addition to serpentine, calcite, dolomite, tremolite, diopside, forsterite, chlorite, graphite and quartz are present in varying proportions in these "marbles". The lakeshore also exposes outcrops of F3 folded schists of the Barnanoraun Fm. Locality 4. Lough Nahasleam - migmatized metasediments (i.e. migmatites) of the Cashel Formation (Middle Dalradian) resulting from partial melting at ~ 750o C. The onset of melting was triggered by basic to acid magmatism represented by the metagabbros and granite gneisses (see Locality 5). Garnets (~3cm across), cordierite and sillimanite (var. fibrolite) are visible in the pelites here. The leucosomes are composed of plagioclase and quartz. The exposures are dominated by D3 fabrics. Locality 5. S. of L. Nahasleam. Potash-feldspar orthogneisses (~468 Ma) south of the main migmatite belt. These are similar in appearance to the Errisbeg Townland Granite of the much younger Galway Granite (~410-380Ma) exposed further south.

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PART 2

The Galway Granite Batholith.

The Granite Face of South Connemara, western Ireland

Geological setting of the Galway Granite:

The Galway Granite is a late-Caledonian calc-alkaline batholith emplaced at between

~410 Ma and 380Ma (Leggo et al., 1966; Pidgeon, 1969; Feely et al. 2003; Selby et al.,

2004; Feely et. al., 2006 & 2010) into the ~475-463 Ma Metagabbro-Gneiss Suite to the

north (Leake 1989; Leake and Tanner, 1994; Friedrich et al., 1999), and into Lower

Ordovician greenschist facies rocks (the South Connemara Group) to the south (McKie

and Burke 1955; Williams et al., 1988). The batholith emplacement postdates the Skerd

Rocks Fault that Leake (1978) considers to be a splay of the Southern Uplands Fault and

to have strongly influenced it’s siting. The Granite extends for several kilometers beneath

the Carboniferous rocks of the Galway Bay area, as indicated on gravity and

aeromagnetic maps (Murphy, 1952; Max et al. 1983; Madden, 1987). The long axis of

the batholith is oriented WNW-ESE and is oblique to the E-W strike of the Skerd Rocks

Fault. Two major faults, the NNE-trending Shannawona Fault (SF) and the NW-trending

Barna Fault (BF), define the boundaries between the western, central and eastern blocks

(see regional map below –Fig 1) in the batholith.

The western and eastern blocks expose lithologies that range from granodiorite through

granite to alkali granite (Leake, 1978, and references therein). The petrology,

geochemistry and field relationships of the central block granites has been described in

detail by the following: Feely and Madden (1986, 1987 and 1988), Whitworth and Feely

(1989, 1994), Feely et al., (1989, 1991), El Desouky et al., (1996), Crowley and Feely

(1997), Graham et al., (2000), Baxter and Feely (2002), Baxter et al., (2005) and Pracht

et al., (2004). These studies present unequivocal evidence for several phases of granite

emplacement. Interactions between coeval diorite and granite magmas (e.g. Mingling and

Mixing Zone – MMZ) are concomitant with the development of pure flattening fabrics

and are succeeded by emplacement of a suite of generally unfoliated granites intruded in

a brittle fracture regime. Thus, intergranite temporal relationships in this part of the

batholith demonstrate that the Megacrystic Granite was emplaced first along with the

MMZ Granodiorite and its enclaves of coeval diorite magma. Baxter et al., (2005)

interpret fabrics within the Megacrystic Granite and MMZ Granodiorite to reflect

ballooning processes operating in successive magma batches (e.g. Megacrystic Granite

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and MMZ Granodiorite) at the emplacement level. These earlier granite batches were

stoped by the later granite intrusions such as the Costelloe Murvey Granite (CMG-Fig 2)

during a brittle fracture regime (Crowley and Feely, 1997). The central block therefore

exposes a juxtaposition of earlier deeper level granites with late-stage higher level

granites (Fig 3)

Disseminated and vein mineralisation in the Galway Granite:

Disseminated and quartz vein-hosted molybdenite mineralisation occurs throughout the

Galway Granite (Max and Talbot 1986; Derham 1986; Derham and Feely 1988; Feely

and Hoegelsberger 1991; Gallagher et al. 1992; Feely et al., 2010). The most notable

occurrences are at the western end of the batholith i.e. at Mace Head and Murvey At

Mace Head, molybdenite bearing quartz veins (~ 5 - 30 cm thick) trend NE-SW their

orientation controlled by early jointing in the host granite (Derham 1986; Max and

Talbot 1986). Additional vein minerals include chalcopyrite, pyrite, magnetite and

muscovite. Mineralised and altered granite extends over an area of 2km2 and in

addition to the vein mineralisation, quartz-magnetite pods and an intrusive K-feldspar

breccia (Derham and Feely 1988) also occur. At Murvey, a highly evolved

leucogranite contains molybdenite intergrown with muscovite. Quartz veins in the area

also contain Mo-mineralisation. Gallagher et al. (1992) interpreted the Mo-

mineralisation in both areas, using geochemical, fluid inclusion and stable isotope (O,

H, S and C) data, as having been produced by highly fractionated granite magma.

In addition to the Mo-Cu mineralisation a suite of polymineralic veins that commonly

strike E-W occurs throughout the granite (O'Reilly et al. 1997). They are vertical to

steeply dipping veins, <22 cm thick and fill late strike-slip faults and fractures. They

contain quartz ± fluorite ± calcite ± barite ± galena ± chalcopyrite ± pyrite with minor

chlorite/clay. Wallrock alteration occurs up to 1 m from the veins and is characterised

by sericitization of plagioclase, strain-free quartz, muscovite, fluorite, pyrite, galena

and chalcopyrite. Brecciated fragments of wallrock occur in several veins. The veins

commonly exhibit comb-quartz and vugs of several cm. in longest dimension. Drusy

quartz crystals often exhibit growth zones. Each vein typically shows two generations

of mineral deposition, with ore mineralisation (galena and chalcopyrite) usually

associated with, or later than, the second, vug infilling generation. Fluorite and barite

joint coatings overgrow earlier hematite coatings.

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!

Locali'es*to*be*visited*

1*

2*3* 4*

5*

Figure*1:*Regional*Geology*Map*of*the*Galway*Granite*

Figure'2'

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!

2"

3" 4"

Figure"2:"Geology"of"the"Costelloe"Area"" Locali9es"

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Figure 3. Schematic diagram showing the spatial and temporal distribution of Connemara’s

late-Caledonian granites. O = Omey Granite, L = Letterfrack Granite, I = Inish Granite, R=

Roundstone Granite, CG = Carna Granite + Roundstone Murvey Granite, GG = Main

Galway Granite (Megacrystic Granite, Errisbeg Townland Granite, MMZ Granodiorite,

Lough Lurgan Granite and Kilkieran Murvey Granite), ShG = Shannapheasteen Granite and

CMG = Costelloe Murvey Granite. OG is the Oughterard Granite. (after Feely et al., 2010)

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Selected References: Baxter, S., Graham, N.T., Feely, M., Reavy, R.J. and Dewey, J.F. 2005. Fabric studies of the Galway Granite, Connemara, Ireland. Geological Magazine 142, 81-95. Baxter, S. and Feely, M. 2002. Magma mixing and mingling textures in granitoids: examples from the Galway Granite, Connemara, Ireland. Mineralogy and Petrology, 76, 63-74. Conliffe, J. & Feely, M. (2010) Fluid inclusions in Irish granite quartz: monitors of fluids trapped in the onshore Irish Massif. Transactions of the Royal Society of Edinburgh: Earth Sciences. 101 (1), pp 53-66 Crowley Q. & Feely M. 1997. New perspectives on the order and style of granite emplacement in the Galway Batholith, western Ireland. Geol. Mag. in press. Derham J.M. and Feely M. 1988. A K-feldspar breccia from the Mo-Cu stockwork deposit in the Galway Granite, west of Ireland. J. Geol. Soc. London, 145, 661-667. El Desouky M., Feely M. and Mohr P. 1996. Diorite-granite magma mingling and mixing along the axis of the Galway Granite batholith, Ireland. J. Geol. Soc. London, 153, 361-374. Feely, M., Coleman, D., Baxter, S. & Miller, B. 2003. U-Pb zircon geochronology of the Galway Granite, Connemara, Ireland: Implications for the timing of late Caledonian tectonic and magmatic events and for correlations with Acadian plutonism in New England. Atlantic Geology, 39,175-184. Feely, M and Hoegelsberger, H. 1991. Preliminary fluid inclusion studies of the Mace Head Mo-Cu deposit in the Galway Granite. Irish Jour.of Earth Sciences, 11,1-10. Feely, M., Leake, B., Baxter, S., Hunt, J., & Mohr, P. (2006). A geological guide to the granites of the Galway Batholith, Connemara, western Ireland. Geological Survey of Ireland, Field guides Series. 70pp. ISBN 1-899702-56-3 Feely, M. and Madden, J.S. 1986. A quantitative regional gamma-ray survey on the Galway Granite, western Ireland. In: Andrew, C.J., Crowe, R.W.A., Finlay, S., Pennell, W.M. and Pyne, J.F. (eds) Geology and genesis of mineral deposits in Ireland. Irish Assoc. Econ. Geol., 195-200. Feely, M. and Madden, J.S. 1987. The spatial distribution of K, U, Th and surface heat production in the Galway Granite, Connemara, western Ireland. Irish Journal of Earth Science, 8, 155-164. Feely, M. and Madden, J.S. 1988. Trace element variation in the leucogranites within the main Galway Granite, Connemara, western Ireland. Min. Mag. 52. 139-146. Feely, M., McCabe, E. & Kunzendorf, H. 1991. The evolution of REE profiles in the Galway Granite, western Ireland. Irish Journal of Earth Science, 11, 71-89. Feely, M., McCabe, E. & Williams, C.T. 1989. U, Th and REE bearing accessory minerals in a high heat production (HHP) leucogranite within the Galway Granite, western Ireland. Trans. Inst. Min. Met. Section B.98. B27-B32. Feely, M., Selby, D., Conliffe, J. & Judge, M. (2007). Re-Os geochronology and fluid inclusion microthermometry of molybdenite mineralisation in the late-Caledonian Omey Granite, western Ireland. Applied Earth Science, (Trans. IMM. B) V 116, No3 143-149. Feely, M., Selby, D., Hunt, J. & Conliffe, J. 2010. Long-lived granite related molybdenite mineralization at Connemara, western Irish Caledonides. Geol Mag. 147 (6) 886-894. Gallagher, V., Feely, M., Hoegelsberger, H., Jenkin, G.R.T. & Fallick, A.E.1992. Geological, fluid inclusion and stable isotope studies of Mo mineralization, Galway Granite, Ireland. Mineralium Deposita 27, 314-325. Graham, N.T., Feely, M. and Callaghan, B. 2000. Plagioclase-rich microgranular inclusions from the late-Caledonian Galway Granite, Connemara, Ireland. Mineralogical Magazine V.64 (1), 113-120. Leake B.E. 1974. The crystallisation history and mechanism of emplacement of the western part of the Galway Granite, Connemara, western Ireland. Min. Mag., 39, 498-513.

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Leake, B.E. 1978. Granite emplacement: the granites of Ireland and their origin. In: Crustal Evolution in Northwest Britain and Adjacent Regions (D.R. Bowes and B.E. Leake, eds.). Geological Journal, Special Issue, 10, 221-248. Madden, J.S. 1987. Gamma-ray spectrometric studies of the Main Galway Granite, Connemara, west of Ireland. Unpublished PhD thesis, National University of Ireland. Max M.D. and Talbot V. 1986 Molybdenum concentrations in the western end of the Galway Granite and their structural setting. In: Andrew C.J., Crowe R.W.A., Finlay S., Pennell W.M. and Pyne J.F. (eds) Geology and genesis of mineral deposits in Ireland. Irish Assoc. Econ. Geol., 177-185. McKie, D. & Burke, K. 1955. The geology of the islands of south Connemara. Geological Magazine, 92, 487-498. Menuge, J.F., Feely, M. & O'Reilly, C. 1997. Origin and granite alteration effects of hydrothermal fluid: isotopic evidence from fluorite veins, Co. Galway, Ireland. Mineralium Deposita.v. 32, pp. 34-43. Mohr, P. 2003. Late magmatism of the Galway Granite Batholith: 1. Dacite Dikes. Irish Journal of Earth Sciences 21, 71-104. O’Connor, P.J., Hoegelsberger, H., Feely, M. & Rex, D, C. 1993. Fluid inclusion studies, REE chemistry and age of hydrothermal fluorite mineralization from western Ireland: a link with continental rifting. Trans. Inst. for Mining and Metallurgy. V. 102, Sec. B. 141-148. O'Reilly, C., Jenkin, G.R.T., Feely, M., Alderton, D.H.M. & Fallick, A.E. 1997. A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland. Contributions to Mineralogy & Petrology. Pidgeon R.T. 1969. Zircon U-Pb ages from the Galway granite and the Dalradian, Connemara, Ireland. Scott. J. Geol., 5, 375-392. Pracht, M., Lees, A., Leake, B., Feely, M., Long, B., Morris, J., McConnell, B. 2004 Geology of Galway Bay: A geological description to accompany the Bedrock Geology 1:100,000 Scale Map Series, Sheet 14, Galway Bay. Geological Survey of Ireland.pp76. Selby, D., Creaser, R.A. and Feely, M. 2004. Accurate Re-Os molybdenite dates from the Galway Granite, Ireland. A Critical Comment to: Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation. Geochemical Journal.v35, 29-35. Whitworth, M.P. and Feely, M. 1989. The geochemistry of selected pegmatites and their host granites from the Galway Granite, western Ireland. Irish Journal of Earth Science, 10, 89-97. Whitworth, M.P. and Feely, M. 1994. The compositional range of magmatic Mn-garnets in the Galway Granite, Connemara, Ireland. Mineralogical Magazine, 58, 163-168. Williams, D.M., Armstrong, M.A. & Harper, D.A.T. 1988. The age of the South Connemara Group, Ireland, and its relationship to the Southern Uplands Zone of Scotland and Ireland. Scottish Journal of Geology, 24, 279-287.

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Localities to be visited during the field trip: The Galway Granite Batholith. (localities are marked on Figures 1 & 2 above ) Locality 1. Glentrasna. The northern contact of the Galway Granite (410-380Ma; Re-Os & U-Pb determinations Feely et al., 2010) with the rocks of the older Connemara Metamorphic Complex is exposed along the road to Glentrasna. Metagabbros (massive amphibolite ~475Ma) and orthogneisses (~466Ma granite magmatism) are exposed at this locality in contact with the later Galway Granite (~400Ma). Here the Galway Granite has a strong flattening fabric at the contact, which can be traced southwards away from the contact, with decreasing intensity, for several hundred metres. Locality 2. Costelloe Road cutting. This cutting exposes the magma-mingling zone (U-Pb: ~400Ma). Hybrid granodiorite displays Microdiorite Enclaves, Microgranular Plagioclase “enclaves” and Rapakivi Feldspars. Hornblende prisms are clearly visible in the granodiorite. Molybdenite and chalcopyrite occur on joint surfaces and in quartz veins. A Molybdenite Re-Os age determination from here yielded an age of 380Ma. Locality 3. Martello Tower coastal section Microdiorite and granite mingling relationships are continuously exposed for ~750m along the coastline north of the early 19th century Martello Tower. These towers were nicknamed “bulldogs” by the French, and were built by the English around 1804 to defend the British Empire against Napoleon Bonaparte’s heavy cannons, an invasion that never materialised. These squat circular forts derive their name from Cape Mortella in Corsica where a tower of this kind was captured by the English fleet in 1794. It took 1400 English troops two and a half hours to capture 33 French troops who held their ground within the tower: so impressed were the English by this structure that they built 200 of them worldwide, 40 of them in Ireland. This example here at Rossaveel is built of Carboniferous Limestone. Now back to the geology!! Three main lithologies can be recognised in these exposures, which are part of the ~400Ma magma mixing and mingling zone: dark grey microgranular enclaves (MME), a highly variable medium- to fine-grained hybrid granodiorite, and a pinkish grey medium-grained granite. The granite typically contains quartz (~28%), potassium feldspar (~43%) plagioclase (~27%) and biotite (~3%). The hybrid is very variable but a typical modal abundance indicates the presence of quartz (~20%), potassium feldspar (~17%), plagioclase (~50%), biotite (~10%), hornblende (~3%) and accessory sphene, apatite and magnetite. The enclaves contain phenocrysts of plagioclase (An30–40) set in a fine grained (<1mm) groundmass of plagioclase (46–54%), hornblende and biotite (22–25%), quartz (15–18%), potassium feldspar (4–14%) and accessory sphene, acicular apatite and zircon. Notable in some of the enclaves, is the presence of many sphene–plagioclase ocelli (~2–10mm across), a texture indicative of magma mixing and mingling. This texture consists of sphene crystals typically, but not always, in ophitic relationship with calcic plagioclase laths, within a zone of plagioclase ± quartz ± K-feldspar, which is notably devoid of biotite and hornblende. Locality 4. Costelloe Leucogranite Quarry. The quarry is located in the centre of the ~380Ma (U-Pb) Costelloe Murvey Granite (CMG). This granite is a member of a suite of leucogranites collectively termed the Murvey Granite. It is a High Heat Production

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(HHP) Granite (15-20ppm U, 40-50ppm Th & ~4.5wt% K). The CMG has a roughly circular surface area of approximately 30km2 and displays, shallow (~15o) outward dipping, chilled marginal contacts and crosscutting relationships with all surrounding granite types. It is a light pink, medium to coarse-grained alkali granite containing quartz, orthoclase, albite (~An5) and biotite. NNE trending vertical hydrothermal veins contain quartz, fluorite, calcite, barite, galena, chalcopyrite and pyrite. A NNE trending dolerite dyke (~233 Ma Ar/Ar) intrudes (note the chilled margins) the granite and is in turn cut by the fluorite-calcite-sulphide veins. Extensive alteration of both granite and dolerite is spatially related to these veins. Aplites and pegmatites, some containing spessartine (cherry red Mn - rich garnet), are also present in the quarry. Locality 5. Megacrystic Granite –Oughterard Road. Roadside exposures display fine examples of a ~400 foliated mafic coarse porphyritic granodiorite. The megacrysts of orthoclase are aligned and enclaves also form part of this flattening fabric. This granodiorite contrasts markedly with the marginal megacrystic granite encountered at locality 1. Prof. (emeritus) Martin Feely, Earth and Ocean Sciences, NUI,Galway. May 2018.