Geological Society, London, Special Publications

Crenulation-slip development in a Caledonian zone in NW Ireland: evidence for a multi-stage movement history

D. M. Chew, J. S. Daly, M. J. Flowerdew, M. J. Kennedy and L. M. Page

Geological Society, London, Special Publications 2004; v. 224; p. 337-352 doi:10.1144/GSL.SP.2004.224.01.21

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© 2004 Geological Society of London -slip development in a Caledonian in NW Ireland: evidence for a multi-stage movement history

D. M. CHEW 1, J. S. DALY 2, M. J. FLOWERDEW 3, M. J. KENNEDY 2 & L. M. PAGE 4,5 1D4partement de Min&alogie, Universit~ de GenOve, Rue des Marafchers 13, CH-1205 GenOve, Switzerland (e-maik david, chew@terre, unige, ch) 2Department of , University College Dublin, Belfield, Dublin 4, Ireland 3British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK 4Laboratory of Isotope Geology, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands 5present address: Department of Geology, Lund University, SOlvegatan 13, 223 62 Lund, Sweden

Abstract: In Scotland and Ireland, a Laurentian sequence, the Dalradian Supergroup, was deformed during the c. 470-460 Ma Grampian , resulting in the formation of crustal-scale recumbent . In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasedi- ments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian acting as a d6collement-like slip surface in both cases. As the orientation of these slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers. 4°Ar-39Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 _+ 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at c. 400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulation- slip morphologies combined with geochronology can thus be used to constrain the reacti- vation history of a major crustal-scale shear zone.

Deforming materials are seldom isotropic, and deformation (Ramsay & Graham 1970). In hence anisotropy plays an important role in par- order to maintain the initial thickness of the titioning strain in shear zones. Common shear zone and thus preserve a simple shear examples of anisotropy encountered in mid- deformation path, crenulation slip has been crustal shear zones include planar elements such interpreted to compensate for this normal dis- as sedimentary layering or foliations. Such placement component (Dennis & Secor 1987). planar anisotropies should act as d6collement- This paper presents structural and 4°Ar-39Ar like surfaces during shear deformation when and Rb-Sr isotopic data from a major shear zone they are suitably orientated (i.e. subparallel to within the Caledonides of NW Ireland, the Fair the shear plane). Renewed movement along Head-Clew Bay Line. Detailed field mapping pre-existing foliations (foliation reactivation) is has demonstrated that the main regional folia- thus likely to be a feature of many mid-crustal tion developed within Dalradian Supergroup shear zones. metasediments adjacent to the Fair Head-Clew Kinematic models have been used (e.g. Bay Line is used as a slip surface within this Dennis & Secor 1987) to predict the structural shear zone, where it is accompanied by crenula- features produced when slip occurs along folia- tion slip. The age of the main regional foliation tion surfaces which are oblique to the walls of a is also well constrained by previous geochrono- shear zone. Oblique slip produces a displace- logical studies based immediately outside this ment component normal to the zone wall, which shear zone (e.g. Flowerdew et al. 2000; Chew et is inconsistent with plane strain, simple shear al. 2003), where it is unaffected by later

From: ALSOP,G. I., HOLDSWORTH,R. E., MCCAFFREY,K. J.W. & HAND,M. (eds) 2004. FlowProcesses in Faults and Shear Zones. Geological Society, London, Special Publications, 224, 337-352. 0305-8719/$15.00 © The Geological Society of London 2004. 338 D.M. CHEW ETAL.

Faults: _------Magnetic .... 50 km Pettigoe lineament: I N W Major Caledonian Granites ~'~"b / Silurian Cj/ Donegal :,7,'~ ...... ,...i, • ¢,, 9" , " i.,!,.,,~,',"~ 40rdovician J v ,~.,.~ / .,+,s /" (. a,ra ,an Su e oroup ,,.~. ~" :, Slishwood Division !

[i[II]i[Ii Tyrone Central Inlier .... Palaeoproterozoic basement (Annagh Gneiss Complex) ~'* ...... ~..

Achill Mountain,, '"Mayo:• ]~r~..-.j.,..C-"f~I] c. 400 Ma /,-," c. 448 Ma _~u~-----/ ~j-~.7-/.¢~_./ ...t-tg.t _ @44" Fig. 3 ~','.*.'.~'.! - Sense of shear and timJ~ -°--'~'~~~ouu.t." " - " /4/of strike-slip movement a ^_~.~.~:~ :,, Mayo /~ the Fair Head- Clew Ba,~ CUI II I~1 I I~/~1 ~NN~ Sense of overthrusting ol Grampian (c. 470 Ma - 460 Ma) nappes adjacen a the Fair Head - Clew Bay ......

Fig. 1. (a) Regional geology of NW Ireland displaying localities referred to in the text. (b) Location map of Ireland within the Caledonides. deformation (i.e. shearing). Isotopic dating of The Dalradian Supergroup in NW Ireland both the crenulation-slip fabrics and the reacti- (with the exception of the allochthonous Con- vated foliation within the shear zone enable nemara ) is bounded to the SE by the Fair individual phases of reactivation to be con- Head-Clew Bay Line (Fig. 1). This structure is strained temporally. The crenulation morpholo- believed to be equivalent to the Highland gies predicted by the model described above can Boundary Fault of Scotland and the Baie thus be used to identify not only shear sense on Verte-Brompton Line of Newfoundland and as a major crustal-scale shear zone, but also to such is a significant lineament within the Cale- establish the timing of movement. donides. It is believed to represent the original collisional between the deformed and metamorphosed Laurentian margin sequences Geological significance of the Fair and the outboard oceanic arc (Dewey & Shack- leton 1984). Head-Clew Bay Line The Fair Head-Clew Bay Line itself is defined In NW Ireland, a Laurentian passive margin by a conspicuous magnetic lineament (Max & sequence, the Neoproterozoic-Cambrian Dalra- Riddihough 1975) from northeastern Ireland dian Supergroup (Fig. 1), was deformed during (Fig. 1) to the north shore of Clew Bay on the the c. 470460 Ma Grampian orogeny (Friedrich western Irish coast (Fig. 2a), whereas the main et al. 1999a, b; Flowerdew et al. 2000). This oro- surface expression of the Fair Head-Clew Bay genic episode is believed to be related to the Line is a fault zone which in general lies about collision of the Laurentian margin with an out- 10 km to the south of the magnetic lineament board oceanic arc and associated forearc ophio- (Fig. 1). Both are often collectively referred to as lite (Dewey & Shackleton 1984), which resulted the Fair Head-Clew Bay Line (Ryan et al. 1995). in metamorphism and the production of crustal- Throughout most of the southeastern margin scale recumbent nappes within the Dalradian of the Dalradian Supergroup in Scotland and sequence. Ireland, ductile structures related to movement REACTIVATION OF A CALEDONIAN SHEAR ZONE 339

a v ~ 0 50 kml .#,, Sense of shear and timing N ~ ~-~~ '///of strike-slip movement along/ ~~ "~ F~ulls --'~~ "r the Fair Head - Clew Bay Line ~ Attitudeof the main foliation ($2) n the NW Mayo n ier

c~~ ~rg~'oup " "~" - ~~ ~.2_.~..~V ~L~..... L~ZJ i-1.7~. -' ='-7 ~ .~-~ .I .... ~ Silurian ~ Slmshwooo / " " "-" " ~ ~ Dvson r~---~" Y ;', ' !?~+~';'J,.'K" . .'." J/'. "",, ~ Annagn I' I ~/ (:'. ':!"~'~'.'.::'.'." ;J".'; 1 ClewBay ~ Gneiss . ). • • ...... -~;-~...-..~ l bomplex t.,ompFex

Fig. 2. (a) Regional geology of Co. Mayo displaying localities referred to in the text. (b) North-south cross- section (X-Y) across the NW Mayo inlier displaying major F 2 folds. (c) NW-SE cross section across the Central Ox Mountains inlier adapted from MacDermot et al. (1996) and displaying major F3 folds.

along the Highland Boundary Fault or the Fair above the Annagh Gneiss Complex, the D 2 Head-Clew Bay Line are rarely exposed. nappes are upward-facing (Fig. 2b); to the south However, in Dalradian Supergroup metasedi- of this 'root zone' recumbent D2 folds face south ments on Achill Island in NW Mayo and in the (Fig. 2b; Kennedy 1980) and are rotated into a Central Ox Mountains (Fig. 1), ductile struc- downward-facing orientation approaching the tures which can be related to strike-slip motion southern margin of the inlier (Fig. 2b; Chew along the Fair Head-Clew Bay Line (FHCBL) 2003). The production of downward-facing D 2 are clearly observed. folds and the associated $2 strike-swing (Fig. 2a) in the southern part of the inlier has been attrib- uted to either later modification of the 0 2 nappe Evidence for strike-slip motion along the pile by an east-west dextral shear zone running FHCBL in NW Mayo (South Achiil) along the north margin of Clew Bay (Sanderson et al. 1980; Chew 2003), or a steep zone of D 2 Achill Island is situated on the southern margin dextral transpression contemporaneous with the of the NW Mayo inlier (Figs 1 & 2a). The NW development of flat-lying D 2 nappes to the north Mayo inlier preserves an excellently exposed (Harris 1993, 1995). transect from Laurentian basement, the Annagh The model of Sanderson et al. (1980) assumed Gneiss Complex (Daly 1996), through presumed that the $2 nappe fabric was progressively para-autochthonous (Winchester 1992) Lau- rotated into the proposed shear zone rather than rentian cover, the Dalradian Supergroup, to out- new fabrics forming. However, detailed board oceanic elements located immediately to mapping of the southern part of Achill Island the south of the FHCBL (e.g. the Clew Bay (South Achill) and the island of Achill Beg Complex; Williams et al. 1994). (Fig. 3) on the southern margin of the inlier Polyphase deformation is pervasive through- demonstrates that the $2 nappe fabric is modi- out the Dalradian Supergroup outcrop in fied by D3 structures consistent with dextral Ireland and Scotland. In the NW Mayo inlier, a strike-slip displacement. The northern bound- D~ deformation event is responsible for the bulk ary of the shear zone to the north is not sharply of the high-strain observed, and is associated defined, but D3 structures gradually become less with the development of tectonic slides and abundant to the north of Ashleam Bay in South locally-developed isoclinal F] folds (Kennedy Achill (Fig. 3). Two discrete elements have been 1980). The D2 deformation event in the NW recognized in the D3 deformation episode, Mayo inlier is the main nappe-forming phase. asymmetrical buckle folds with axial planes anti- The D 2 nappes in general plunge gently east and clockwise to the $2 foliation, and extensional 'root' in the basement core, the Annagh Gneiss crenulation cleavages which cut the $2 foliation Complex (Fig. 2b). Adjacent to, and directly in a clockwise sense. 340 D.M. CHEW ETAL.

97 6 8 /- 69 70 ~~71 ....i-- 72 84 r ~ "~-3-"~ .... \ \ \ AP Ashleam Bay AQdJ,~ <. ~2-~-~ / 85

do C.,__.-,-''~- so AP /APdm --

~6

• BY : ;......

CV

' ...... 17 .... ~...... Stratigraphy • C , . li~ .... : NC ,. __. / • , f " - .c, " , )/ ~5 E Psammite / 8 Member (ABps) ~" Banded Phyllite South Achill rn Member (ABph) Beg Formation Calcareous Pelite [/I/111~/t Member (ABc) p~///j Achill Beq Fault

• - ,:~, o , ,~ c, c .o , ~:, c, South Carrowgarve Formation (SC) Achill Island Ooghnadarve Formation (OD) North Carrowgarve £ Formation (NC) (5 ~, Clagqan Ray Claggan Volcanic ::c :c ;,'V a",~,?, t': C I ::.,v.i:v~, ~CV Cove Schist Formation (CE) ;3< Corraun Formation (CN) Bunnafahy Derreen Schist Formation (BY) Member (APds) Derreen Marble Ashleam Head ...... Member (APdm) Formation (AP) Slide..... At!anhc L)qve Schist Formation (AT) Ashleam Bridge Ashleam Bridge Do!omitic Formation (AQ) c: Member (AQd) 5. Ashleam Bay Formation (AS) Slide

Portnahally Formation (PO)

Lithologies & structure ,_,£_0 Beddingwith Microconglomerate .' -';,*;*'-. ~- Inverted Quartzite ~-rr-, $2 foliation ~'~' - . 60 .i: :i Psammite ~ $2 foliation • ,> ~:; Psammitic wacke z8 R----~4e Plunge of F2 :;. 1 ::::;::~: Semipelite Scale • "-' ""-~ H'~--..~27 Plunge of F3 fold .... Pelite / phyllite ~-.-~ Shear band orientation 1 0 250 500 750 t 000 2000 m Dolomite / limestone Metabasite ~ X ...... F2 synform Gdd references correspond to ldsh National Grid square L O Serpentinite olistoliths ...... "~ ...... F4 antiform etc, ~'lSample location for -~*"~'~-- Fault .,,.,... S Slide "" k"~"m'~isoto ic datin

Fig. 3. Geological map of South Achill and Achill Beg. REACTIVATION OF A CALEDONIAN SHEAR ZONE 341

a) ...... _~ ...... :~ ..[ ' -~'< -:::::,2[~...... ::::-::: :lillYif......

--'-:.7.5.:..... 2L: ...... L~ ...... "...... " ...... :...... ¢ ~,

• / Pre-existing foliation Shear zone watt / \

b) / \

• ...... ~~ ~"~':'~ ~---..~... "~"~"----~:_~...--":p~:~~. ~ ~J-'-'~-~ y-~-~.t,[..''C ...... ~ ...... :.>., _~ ~_~2.- -~'~: ~~-> .....~ ...... ~ ...... _~ ..... ":~,!:-..:...... :: "-.

L j> .~:~-~%zT~S~:~-~,-c.::.:~.... ~"-..~.'~:~?.'~."~.JJ"*'-~ ~-...... -.....--~.~ "'~ ...... i~...... ~" 2 ~- ...... -~-~.~ ...... ----~ -..-.~ - ...... 21--'JI-.I- ...... "_ :~:-~: .... -~.'~:~::~: :2: ...... ;i...... :::?~: ..: ...... / ~:.~-'-~"~-.2"~ '" - ~ ': ..... , ~ *": ...... / ..--'--- "--~.~ ~.~p'~ :-- "~--"~..~':..'. ...-. :..:.~,b-.:;::-::..: .-:.,:,:~.---...'"~.-~..." ...... /~ -'~.-22 _._it .--.- ...... ~--~,: ':.:::>-'::"-:: :.-'.:~'~\. " ----"5~ ......

Fig. 4. Angular relationships predicted between the shear zone wall and the foliation-slip and crenulation-slip surfaces. (a) Reverse-slip . (b) Normal-slip crenulations. Reprinted from Journal of , 9, Dennis & Secor: A model for the development of crenulations in shear zones with applications from the southern Appalachian Piedmont, pp. 809-817. Copyright 1987, with permission from Elsevier.

Crenulation-slip morphologies produced shear, the $3 foliation which initiates anticlock- by oblique foliation-slip wise to $2 is progressively brought into para- llelism (Fig. 5a). This is particularly evident The kinematic models of Dennis & Secor (1987, where there is a large contrast 1990) predict the crenulation morphologies that across a bedding surface. Relatively rigid psam- develop in order to compensate for the displace- mitic layers respond to D 3 shear by folding with ment component of foliation slip normal to the $3 usually oblique to $2. If the $3 foliation con- shear zone wall. In a dextral shear zone, when tinues out into an adjacent graphitic pelite layer, the pre-existing foliation is at an acute, clock- then commonly the $3 foliation swings clockwise wise angle to the shear zone wall (Fig. 4a), into parallelism with $2 and hence the weak movement away from the shear zone wall due to graphitic pelite layers are accommodating the oblique foliation slip is compensated by reverse- bulk of the displacement. The weak graphitic slip crenulations (RSC), which transfer slip up to pelite layers also often display evidence of slip 'higher' foliation planes. When the slipping foli- along the $2 foliation surfaces (Fig. 5b). On a ation is at an acute anticlockwise angle to the vertical surface, the $3 foliation commonly shear zone wall in a dextral shear zone (Fig. 4b), rotates into the vertical parallel to $2, with a movement normal to the shear zone wall is com- down-to-the-south shear sense. pensated by normal-slip crenulations (NSC).

Extensional crenulation cleavages (normal- Asymmetrical buckle folds (reverse-slip slip crenulations) crenulations) Extensional crenulation cleavages (Platt & The most common crenulation-slip morpholo- Vissers 1980) are relatively common within gies in South Achill and Achill Beg are asym- pelitic lithologies in the South Achill sequence. metrical buckle folds (Fig. 5a). The strike of the They make an angle of approximately 29 ° (Fig. F3 axial planes makes an angle of approximately 6b) with the $2 foliation in a clockwise direction, 27 ° with the strike of the $2 foliation in an anti- and consistently give a dextral sense of shear clockwise direction (Fig. 6a). F3 fold axes plunge (Fig. 5c). Identical in style to the normal-slip moderately to the NE (Figs 3, 6a). The largest F3 crenulations of Dennis & Secor (1987), they are folds have wavelengths of only a few metres, and believed to be broadly contemporaneous with typically the smaller F3 folds can be observed to the F3 asymmetrical buckle folds based on the 'root' in the S 2 foliation surface. These features absence of apparent overprinting relationships. are typical of the reverse-slip crenulations of The orientation of the earlier $2 foliation con- Dennis & Secor (1987). With progressive dextral trols the morphology of the later crenulations 342 D.M. CHEW ETAL.

,.....;...... ~;~:::::77,~..!7.-~-~!~ i...... , ...... ::...... ::~.~..: ...... ::;.~...~...... ~2,, : ~",: '~~~": ~~'t " "~ "

: ,...... Z~,!:,:.~-":: ....--~... :,.::t-. ~::~.::::.. ;::~:, i ~e ~;:~[~J": 6"~,~.-~,4-~"~

.,~ .... ,~ ...... ~t::~.', . .. " "~, ..... !~.~,'.I ' *~..:' ,; • " .. ~<':~~..:.;....~,~.'y~',~,~,~~~.,~.,

i ~.-----:-..27:.,~.'-.-~_:~ -' 7 .-

i~ , ~ .~--< ~,,~ ..

, ~" .... , ," ..... $2 ..

~. ,,' Ate,' .,, , • : : '~ , ' ,* •

Fig. 5. (a) Asymmetrical buckle folds (F3), interpreted as reverse slip crenulations (RSC). Later rotation of $3 is due to progressive dextral shear. Hammer 40 cm long. South Achill Dalradian [L69329495]. (b) Graphitic pelite illustrating d6collement-like slip along the $2 foliation surface. Plane-polarized light, scale bar 1000 IJm. South Achill Dalradian [L69039540]. (e) D3 dextral shear bands cutting the $2 foliation, interpreted as normal slip crenulations (NSC). Hammer 40 cm long. South Achill Dalradian [L69029544]. (d) Sinistral extensional crenulations affecting the composite $2/$3 foliation, interpreted as normal slip crenulations (NSC). Coin 2.2 cm in diameter. Central Ox Mountains Dalradian [G323026]. (e) Photograph of a polished rock slice used for 4°Ar-39Ar in situ laserprobe dating of muscovite defining both the $2 and $3 foliations. Sample DC-79, scale bar 1000 tJm. South Achill Dalradian [L69089511].

(e.g. RSC vs. NSC). On a vertical surface, the expected to lie between the axial planes of the extensional shear bands give a down-to-the- asymmetrical buckle folds (Fig. 6a) and the south shear sense. extensional shear bands (Fig. 6b). At localities which display both RSC and NSC fabrics, the dominant slip foliation ($2) modified by the RSC Orientation of the D 3 dextral shear zone makes a very small clockwise angle with the $2 From the angular relationships proposed by foliation affected by the NSC, similar to the Dennis & Secor (1987), the shear zone wall is geometry predicted by Fig. 4. However, on a REACTIVATION OF A CALEDONIAN SHEAR ZONE 343 (b)

(foliation-slip surface) "" ~......

, n=58 (Poles to $3 foliation, South Achill) Largest petal (120°): 27% of all values ~x mean pole to $3 foliation Petal width: 3 °, n = 48 + n=18 (Plunge of F3 fold axes, South Achill)

0 (d) (

I . n=133 (Poles to $2 foliation, South Achill) . n=102 (Poles to S2/S3 foliation, Ox Mountains) C~mean pole to $2 foliation mean pole to S2/$3 foliation + mean pole to 25 sinistral extensional crenulation cleavage readings

Fig. 6. (a) Stereographic plot of the orientation of RSC-related structures in South Achill (F3 fold hinges and poles to the $3 foliation) along with the mean orientation of the $2 foliation. (b) Rose diagram illustrating the mean orientation of NSC-related structures in South Achill (dextral shears measured on horizontal surfaces). The mean orientation of the $2 foliation is also illustrated. (e) Stereographic plot of the orientation of the pre- existing foliation-slip surface in South Achill (poles to the $2 foliation). (d) Stereographic plot of the orientation of the pre-existing foliation-slip surface in the Central Ox Mountains (poles to the composite $2/$3 foliation), along with the mean orientation of NSC-related structures (sinistral extensional crenulation cleavages). regional scale this relationship is not apparent would therefore be an approximately east-west and hence the dominant slip foliation ($2) data trending, sub-vertical structure, similar to the for both the RSC and NSC sets are presented geometry proposed by Sanderson et al. (1980). together (Fig. 6c). Combining data from both No L 3 elongation lineations have been observed horizontal and vertical surfaces, the shear zone and hence although shear sense has been 344 D.M. CHEW ETAL. determined reliably from the crenulation-slip Glennawoo Slide and the Callow Shear Zone morphologies on both horizontal and vertical (Taylor 1969). In the Callow Loughs region surfaces, the exact shear direction remains (Fig. 7), the Lough Talt and Glennawoo Slides uncertain. may be adequately represented as discrete shear zones, but the Callow Shear Zone is significantly wider (more than 750 m across strike) and is Evidence for strike-slip motion along the regarded as a substantial belt. Both the FHCBL in the Central Ox Mountains Glennawoo Slide and the Callow Shear Zone display abundant evidence of sinistral extensional The Central Ox Mountains inlier consists of a crenulation cleavage development (Fig. 7). sequence of Dalradian metasediments (Long & Max 1977; Alsop & Jones 1991) intruded by a Caledonian granite, the Ox Mountains granodi- Extensional crenulation cleavages (normal- orite (Fig. 2a, c). The intrusion age of the Ox slip crenuIations) Mountain granodiorite has proved controversial in the past (e.g. Kennan 1997), and it is discussed In the Callow Loughs region, the main foliation in detail later. This granodiorite is intruded into is close to the vertical and trends NE (Figs 6d the core of a significant upright D3 antiform & 7). It is regarded as a composite $2/$3 fabric as which trends NE-SW, subparallel to the length $2 and $3 are usually coplanar (Jones 1989; Mac- of the inlier (Fig. 2c; Taylor 1969). High strain Dermot et al. 1996) and the $3 fabric can only be zones are well developed on the limbs of the conclusively identified when F3 folds are main D 3 antiformal structure, are parallel to the present. F3 folds plunge gently to the NE and vertical, axial planar $3 fabric and kinematic verge towards the Ox Mountains granodiorite indicators such as rotated porphyroblasts and (Fig. 7). Extensional crenulation cleavages make extensional crenulation cleavages display abun- an angle of approximately 28 ° (Fig. 6d) with the dant evidence for sinistral shear (e.g. Hutton & main (composite $2/$3 foliation) in an anticlock- Dewey 1986; Hutton 1987; Jones 1989; McCaf- wise direction, and consistently give a sinistral frey 1992, 1994). These shear zones have been sense of shear (Fig. 5d). On a vertical surface, regarded as contemporaneous with the develop- the extensional shear bands give a down to the ment of the main D3 antiform and the Central south shear sense. Ox Mountains has thus been regarded a trans- The composite ($2/$3) main foliation is usually pressive sinistral shear zone during D3 (Hutton defined by muscovite, chlorite and equigranular & Dewey 1986; Hutton 1987; Jones 1989; quartz grains with interlobate grain boundaries. McCaffrey 1992, 1994). These quartz grains are typically of the order of The Ox Mountains granodiorite itself also dis- 100-200 pm in diameter and display undulose plays abundant evidence of sinistral strike-slip extinction. MP3 porphyroblasts of albite and deformation. The main solid-state foliation is almandine garnet overgrow the main foliation subvertical, strikes NE-SW and is accompanied and staurolite is locally developed (MacDermot by a stretching which plunges gently to et al. 1996). Along the extensional crenulation the NE or SW (McCaffrey 1992, 1994). NNE cleavage planes, quartz has undergone signifi- trending sinistral S-C fabrics are commonly well cant dynamic recrystallization. It is conspicu- developed within the granodiorite and are sub- ously finer grained (10-20 pm) than that parallel to the asymmetrical extensional crenu- defining the composite $2/$3 foliation, and has a lation cleavages developed in the country rock weak shape-preferred orientation (aspect ratios (Hutton & Dewey 1986; McCaffrey 1992, 1994). of up to 3:1). Additionally, phyllosilicates within The Ox Mountains granodiorite has thus been the shear bands (chlorite and muscovite) are regarded as being emplaced synkinematically significantlyfiner grained than those defining the with respect to D3 sinistral transpressive defor- $2/$3 foliation. The extensional crenulation mation in the country rock (Hutton & Dewey cleavage planes are short and anastomosing, 1986; Jones 1989; McCaffrey 1992, 1994). commonly rooting in the pre-existing composite $2/$3 foliation, and are very similar in morphol- ogy to the normal-slip crenulations (NSC) of High strain zones (tectonic slides) in the Dennis & Secor (1987). Central Ox Mountains Three high strain zones are particularly well Isotopic dating of crenulation-slip surfaces developed on the SE flank of the Central Ox Many deformed rocks contain evidence (e.g. Mountains inlier - the Lough Talt Slide, the multiple fabrics) for having experienced more REACTIVATION OF A CALEDONIAN SHEAR ZONE 345

i:{::~:77Z77 7y.U;:-7,7?~ ~:7-~IZ., 7-77:~::~ ...... ~ I":' "1 ...... e X/ ..... i X 1 "'e "\ / ..... l \/.."" .,.~'\ "" / 1 .o.~...... ,. c---., t..--,.. !..--,--. ;..7::., ~ . / / i"" I%\ .....: ::--\ .... i ..+:I.,:-\ ...... i/ '~. ~Ii~/\/"-\ 1"-,./\, * --+h .4 // I 1 ...... t ~ " .,,,~ s", ...... ~,/ I //~].~a d A ~ ,,5/ ~'~,~' D, " ~ h P" 1 "# ,~s # / / / . I- ~#" / 1 I / o~.

/ i / /- / I _~U / "" /~ / Z ..:'~ / / / / • UM./ / A // "%~" / , / i7 ~i j

.,#'l .<,,,~ ~:: ~ .... (><; ..,o;,.... ~ r!~.> ii' # ,, 41 / Callow Loi ',%. Lower / . It, / : / # /

Ut. /. ....~' / /,. / CGp il / / )/

/ /,/

/" .Qa/' /" // / /, ../ Structure \~ • Lithologies ~4~-" Bedding ..... Conglomerate -~°+ Bedding with way-up "-'=";' Main Ca,"f~ntfemus ~ M~ Sa~sto~e Quartzite 60 foliation (composite Unconformity **~.~ Form~l~ (MO) Psammite 78 Psammitic wacke 60 Member~'CGw~ CkJony.qm~an ~--*'~ L3 lineation ~ Form;!ho,: ,'C(;) Semipelite ~-~3;" F3 (with ) M,.ube, ~c::~r,, ...... ~ c:alto. &,ea~ zorJe Petite / phytlite ...... :..:--- ...... Shear band orientation .~ ~ I UpperU~mo~an Metabasite 7"~a Shear band orientation =~ ~ Forrr~don(UL) F3 synform (with dip) ~ ~ ca~w F~mat~ (CA) Granodiodte

"~---- Fault ~ .1 Format~ (LL) ~* "" ~ Shear zone ~ G/os,,m,,JoS'ide ~...... Scale I ...... ;'~' ' t r rn(m~ "(tr '~ , UM) E~II~=7,717;7 III I '"'~ or~! . ~ location for N...... , ,UMiv,L. 1 ...... 0 250 500 750 ~000m isot( pic dating Grid refererces correspond to Irish " L_J LeckeeQ~artz~te Forn~f~m~ (LQ) National Grid square G

Fig. 7. Geological map of the Callow Loughs area in the Central Ox Mountains. 346 D.M. CHEW ETAL. than one deformation event. The timing of heating plateaux from the same samples range growth of multiple fabrics can be constrained by from 463-457 Ma (_+ 4 Ma) (Chew et al. 2003). dating fabric-forming minerals which crystallize Two Rb-Sr $2 muscovite ages of 460 _+ 7 Ma and below their closure temperatures (e.g. Cliff 461 _+ 7 Ma have been obtained from the low 1985). One of the most common fabric-forming greenschist facies Clew Bay Complex and prob- minerals is muscovite, and Rb-Sr dating and ably record crystallization (Chew et al. 2003). As 4°Ar-39Ar laserprobe dating of multiple gener- this outboard terrane (Figs 2 & 3) is in structural ations of muscovite has been employed in many continuity with the Dalradian Supergroup studies (e.g. Mt~ller et aL 1999). (Chew 2003), it is thought likely that the c. Both the Rb-Sr and Ar-Ar systems have 460 Ma Rb-Sr $2 muscovite ages for the South shown that muscovite grown below its closure Achill Dalradian also record crystallization temperature during deformation (e.g. in during the Grampian orogeny, particularly as ) may record the age of neocrystalliza- the peak metamorphic temperature in South tion (e.g. Dunlap 1997; Freeman et al. 1997, Achill is unlikely to have exceeded 450 °C Midller et al. 1999). Additionally, samples con- (Chew et al. 2003). The marked similarity with taining earlier generations of white mica (e.g. as the $2 muscovite 4°Ar-39Ar step heating data porphyroclasts or older foliations) record the may imply that the Ar-Ar system is recording crystallization age of these early fabrics where either rapid cooling or crystallization, despite they have not been rejuvenated by later defor- growing c. 50 to 100 °C above its closure tem- mation (West & Lux 1993; Freeman et al. 1997, perature (c. 350-400 °C; Wijbrans & McDougall Mtiller et al. 1999). 1988). The possibility of extraneous Ar contami- In this study, fabric-forming muscovite has nation is discussed in detail later. been dated using the 4°Ar-39Ar and Rb-Sr One 4°Ar-39Ar hornblende age of 467 + 3 Ma systems in order to constrain both the age of and one Rb-Sr muscovite age of 472 + 8 Ma shearing and the age of the reactivated foliation- have been obtained from the composite $2/$3 slip surface within samples that have undergone foliation in the Dalradian of Central Ox Moun- shear-related deformation along the FHCBL. tains inlier (Flowerdew et al. 2000). Younger The age of the pre-existing foliation which is 4°Ar-39Ar and Rb-Sr ages (429-410 Ma) have exploited by shearing along the FHCBL is also been strongly influenced by the intrusion of the well constrained by previous geochronological Ox Mountains granodiorite. Similar Grampian studies based immediately outside of this shear (c. 470-460) ages have also been obtained from zone (e.g. Flowerdew et al. 2000; Chew et al. Dalradian rocks in the NE Ox Mountains (Fig. 2003), where it is unaffected by later defor- 1). Here Dalradian rocks are interleaved during mation. Thus the age of this foliation in unde- D 3 with the Slishwood Division, a unit of psam- formed samples and in samples where it has mitic gneisses which has experienced late Pre- been reactivated due to subsequent shearing can cambrian granulite-facies metamorphism be compared. (Sanders et al. 1987). Three 4°Ar-39Ar horn- blende ages and six Rb-Sr muscovite ages defin- ing the $3 foliation within the interleaved Pre-existing constraints on the age of the Dalradian range from 470-446 Ma. The older $3 mineral ages are indistinguishable from the foliation-slip fabric in both inliers older ages (472 + 7 Ma, 467 + 3 Ma) obtained Recent geochronological studies in the Dalra- from the composite $2/$3 fabric in the Central dian of NW Ireland (Flowerdew et al. 2000; Ox Mountains inlier (Flowerdew et al. 2000). Chew et al. 2003) has shown that main foliation development in both South Achill and the Central Ox Mountains inlier is Grampian (c. Sampling 470-460 Ma) in age in samples which are unaf- In this study we present isotopic data from two fected by later shearing along the FHCBL. Dalradian samples from two localities. DC-79 is Knowledge of the age of this main foliation in a semipelite from South Achill (Fig. 3) in which samples unaffected by later deformation is muscovite defining both the local $2 and $3 foli- essential, as it enables us to assess whether the ations was dated in situ by the 4°Ar-39Ar laser- age of the reactivated foliation is partially reset probe spot fusion method. DC-03/02-1 is a when it is rejuvenated by later shearing along semipelite from the Central Ox mountains inlier the FHCBL. (Fig. 7) from which a bulk mineral separate of Four Rb-Sr $2 muscovite ages from the Dal- muscovite defining the main (composite $2/$3) radian of South Achill range from 460-458 Ma foliation was analysed by the Rb-Sr method. (+ 7 Ma), and four 4°Ar-39Ar $2 muscovite step- The whole rock was also analysed. REACTIVATION OF A CALEDONIAN SHEAR ZONE 347

Table 1. 4°Ar-S9Ar spot fusion data

Spot Laser 36Ar(a) 37Ar(Ca) 38Ar(C1) 39Ar(K) 4°Ar(r) Age(Ma) _+2G 4°Ar(r) 39Ar(k) power (%) (%)

Sample DC-79, $2 and $3 ms. Dalradian, South Achill (L69089511). 2 (82) Fusion 0.0002 0.0000 0.0001 0.1262 9.0407 453.4 3.4 99.2 7.1 4 ($2) Fusion 0.0002 0.0266 0.0000 0.0320 2.2206 441.3 10.1 97.3 1.8 6 ($2) Fusion 0.0000 0.0299 0.0002 0.0209 1.5135 457.8 14.5 99.4 1.2 10 ($2) Fusion 0.0010 0.0208 0.0000 0.5635 40.1131 450.7 1.3 99.3 31.7 11 ($2) Fusion 0.0004 0.0308 0.0004 0.1893 13.5803 453.8 3.5 99.1 10.7 1 ($3) Fusion 0.0001 0.0000 0.0000 0.1492 10.5535 448.2 2.8 99.7 8.4 3 ($3) Fusion 0.0000 0.0000 0.0001 0.0972 6.8772 448.2 3.5 99.8 5.5 5 ($3) Fusion 0.0001 0.0345 0.0001 0.1325 9.3329 446.6 3.6 99.8 7.5 7 ($3) Fusion 0.0013 0.0511 0.0002 0.0683 4.7528 441.7 5.5 92.4 3.8 8 ($3) Fusion 0.0002 0.0371 0.0000 0.0891 6.2796 446.9 4.8 99.0 5.0 9 ($3) Fusion 0.0003 0.0178 0.0000 0.3099 22.0392 450.4 1.9 99.6 17.4 Weighted average* (S 2 spots): 451 e 2 Ma (2cy) Weighted average ($3 spots): 448 e 3 Ma (2~) J value: 0.003987 e 0.5% (1~)

*Weighted averages calculated using ISOPLOT (Ludwig 1999) and use the 2cy error associated with each analysis.

Analytical methods single collector VG Micromass 30 mass spec- trometer at the Department of Geology, Uni- 4°Ar-39Ar spot fusion analyses were carried out versity College Dublin. During the course of using the VULKAAN argon laserprobe analysis, NBS SRM 987 gave 87Sr/86Sr ratios of (Wijbrans et al. 1995) at the Vrije Universiteit in 0.71027 + 5 (n = 8, 2cy) and NBS SRM 607 Amsterdam. Samples were irradiated at the yielded 87Rb/s6Sr ratios of 8.005 + 13 (n = 7, 2c0. HPPIF facility in the high flux research reactor Sr blanks averaged 1.5 ng and are not significant. at Petten, Netherlands. Polished slices were 2~ analytical uncertainties of 1.5 % for 87Rb/86Sr interspersed between the A1 tablets containing and tabulated values (Table 2) for 87Sr/86Sr the flux monitor DRA-1 sanidine (24.99 + ratios were used in age calculations which 0.07 Ma; Wijbrans et al. 1995) prior to irradi- employed a value of 0.0142 Ga -1 for the 87Rb ation. Four flux monitors were used to construct decay constant (Steiger and J~iger 1977). a O-curve with a 0.5% error (lc 0. Samples were analysed within six months of irradiation to min- imize the interference effects produced by radio- Constraining dextral shear in NW Mayo active decay after irradiation. The analytical (South Achill) procedure is described in detail by Wijbrans et al. (1995) and is outlined below. Samples were New $3 mica growth is not commonly associated heated using a continuous 18 W argon ion laser with the D 3 deformation event in South Achill, (454.5-514.5 nm wavelength). For spot fusion but locally $3 muscovite is found overgrowing a experiments, several short laser pulses (0.1 s) crenulated $2 fabric in the hinges of asymmetri- excavated a pit approximately 30 microns in cal buckle folds (reverse-slip crenulations diameter, surrounded by a crater of melt associated with D 3 dextral shear). One polished material. The Ar released was cleaned with slice from the hinge of an F 3 fold was selected for Fe-V-Zr getters (250 °C), prior to analysis on a in situ 4°Ar-39Ar laserprobe spot fusion analyses MAP-215/50 mass spectrometer. Data reduction (Table 1). Sample DC-79 is semipelitic, with an was carried out using in-house software, ArAr- $2 foliation defined by muscovite and minor CALC V20. Blank intensities were measured chlorite and epidote. Individual muscovite every 3-5 sample runs and mass fractionation grains are typically around 500 pm long and was corrected for by regular measurement of 50 pm wide, but both the $2 and $3 fabrics are shots of clean air argon. composed of seams of white mica which are typi- For Rb-Sr analyses, standard ion exchange cally several grains in width. Typically $3 seams methods were used for chemical separation of are approximately 150 pm across (Fig. 5e), elements. Samples were loaded on tantalum fil- whereas $2 lithons are larger and may be up aments and were analysed on a semi-automated to 500 pm wide (Fig. 5e). Chlorite grains are 348 D.M. CHEW ETAL. typically similar in size to the muscovite grains significantly higher closure temperatures (e.g. (c. 500 jam long and 50 ~tm wide) and the largest Arnaud & Kelley 1995; Sherlock & Arnaud epidote needles observed are 200 pm long. The 1999). This disparity is not observed in South $2 foliation is overgrown by MP2 plagioclase Achill as both the Rb-Sr and 4°Ar-39Ar ages for which is augened by the seams of $3 muscovite. undeformed $2 muscovite ages cluster at c. The calcic component in some of the Ar analy- 460 Ma (Chew et al. 2003) and are mutually ses (Table 1) is probably derived from the within error. The $3 muscovite ages are tempo- epidote, as the MP2 plagioclase is essentially rally distinct, and are clearly post-Grampian pure albite. Muscovite defining the $2 foliation is (c. 470-460 Ma) in age. typically more celadonite-rich and paragonite- poor than the later ($3) fabric (Chew et al. 2003). In situ 4°Ar-39Ar laserprobe dating of the $3 Constraining sinistral shear in the Central muscovite seams yields a weighted mean age of Ox Mountains 448 + 3 Ma, whereas the older, crenulated $2 muscovite seams yield a weighted mean age of The Central Ox Mountains displays extensional 451 + 2 Ma (Table 1). The 4°Ar-39Ar system is crenulation cleavage development superim- only reliably recording the youngest defor- posed on what is believed to be a pre-existing mation fabrics present, as undeformed $2 mus- Grampian ($2/$3) foliation. However, whereas covite from South Achill yields consistent c. dextral shear in South Achill was constrained by 460 Ma Rb-Sr and 4°Ar-39Ar ages (Chew et al. dating both the pre-existing foliation-slip 2003). The 448 + 3 Ma age for muscovite within surface and the later crenulation-slip surfaces the $3 crenulation-slip fabric is interpreted as a within the same sample, this strategy has proved crystallization age based on the low-greenschist impossible in the Central Ox Mountains. Dalra- facies assemblages observed in the $3 crenula- dian metasediments in the Central Ox Moun- tion seams as detailed above. tains display only limited growth of extremely The possibility of extraneous argon cannot be fine-grained muscovite on extensional crenula- ruled out, particularly in a high-pressure, low tion cleavage surfaces, which is not sufficient for temperature terrane such as South Achill. isotopic dating. However, pegmatites and Excess argon (argon with 4°Ar/36Ar ratios which granite sheets associated with the Ox Mountains differ from the modern atmospheric ratio of granodiorite are intruded into the Dalradian 295.5) has been documented in white mica from metasediments, and in common with the Ox several high-pressure, low-temperature Mountains granodiorite, the pegmatites and (e.g. Arnaud & Kelley 1995; Sherlock & Kelley granite sheets were emplaced synkinematically 2002). The presence of excess argon may be with respect to sinistral deformation in the evaluated by using an inverse isochron corre- country rocks (McCaffrey 1992, 1994). Sinistral lation diagram (a plot of 36Ar/4°Ar versus shearing in the Central Ox Mountains can thus 39Ar/4°Ar) as the incorporation of excess argon be constrained by dating pegmatite crystalliza- will result in the intercept of the isochron on the tion. The age of the foliation surface that is ordinate axis deviating from the modern atmos- affected by sinistral extensional crenulation pheric 4°Ar/36Ar ratio of 295.5. However, K-rich cleavages in the Central Ox Mountains has phases (such as white mica) can produce large until now remained uncertain, as previous quantities of radiogenic 4°Ar, and hence the data geochronological studies in the inlier (e.g. Flow- often cluster close to the 39Ar/4°Ar axis. The erdew et al. 2000) were undertaken on foliated presence of excess argon is therefore difficult to samples that were not affected by later defor- assess, as the intercept with the 36Ar/a°Ar axis is mation. Whereas the reactivated foliation poorly constrained. This is the case with the surface is believed to be the regional $2/$3 com- South Achill Ar data. The presence of excess posite foliation based on detailed field mapping argon has also been documented in samples (Fig. 7), it may have developed contemporane- which yield intercepts within error of the ously with sinistral shear development in the modern 4°Ar/36Ar atmospheric ratio on an Central Ox Mountains, as the timing relation- inverse isochron correlation diagram (Sherlock ship between mylonitic foliation development & Arnaud 1999). and cross-cutting shear bands can be often diffi- Studies that have documented the presence of cult to establish (e.g. Lister & Snoke 1984). excess argon in white mica in high-temperature, The age of the pre-existing foliation has been low-pressure terranes instead are based on constrained by a Rb-Sr muscovite-whole rock 4°Ar-39Ar phengite ages that are significantly age from a semi-pelitic schist sample displaying a older than either the corresponding Rb-Sr pervasive sinistral extensional crenulation cleav- phengite ages or other geochronometers with age (Fig. 5d). This age of 448 + 9 Ma (Table 2) is REACTIVATION OF A CALEDONIAN SHEAR ZONE 349

Table 2. Rb-Sr geochronology Sample Locality and Textural Mineral Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr+ 2(y 87Sr/86Sr(i) Age + 2~ Irish National relationship (Ma) Grid Ref. DC-03/02-131 Lismoran Mainfoliation muscovite 338.77 126.60 7.79 0.776157_+ 56 0.72641 448 + 9 (G323026) (composite SJS3) whole rock 77.8571 141.07 1.60 0.736629+ 58

Table 3. Rb-Sr geochronology of Ox Mountains pegmatites from Flowerdew et al. (2000)

Sample Irish National Sample description Minerals dated Age _+ 2~ Grid Ref. (Ma)

37 G270003 Dalradian-hosted pegmatite coarse igneous muscovite-K feldspar 392 + 6 38 M167963 Dalradian-hosted pegmatite coarse igneous muscovite-plagioclase 400 + 6 39 G198956 Dalradian-hosted pegmatite coarse igneous muscovite-K feldspar 402 + 6 40 G242069 Ox Mountains granodiorite coarse igneous muscovite-K feldspar 400 + 6 37 G270003 Dalradian-hosted pegmatite recrystallized muscovite-K feldspar 381 + 6 40 G242069 Ox Mountains granodiorite recrystallized muscovite-K feldspar 384 + 6 41 G198956 Dalradian-hosted pegmatite recrystallized muscovite-plagioclase 384 + 5

slightly younger than the c. 470-460 Ma age esti- obtained from the Ox Mountains pegmatite mates for the Grampian $3 foliation in the suite (see above), the Ox Mountains granodior- Central and NE Ox Mountains inliers (Flow- ite has yielded c. 480 Ma Rb-Sr whole rock erdew et al. 2000). However, it is broadly coinci- isochrons (Pankhurst et al. 1976; Max et al. dent with most of the mineral cooling age data 1976). The old Rb-Sr ages suggest that granite from the Central and NE Ox Mountains inliers emplacement and therefore sinistral strike-slip which cluster at or around 460-450 Ma (Flow- deformation occurred either before or very early erdew et al. 2000), and the reactivated foliation in the Grampian orogeny. surface is thus thought to represent the regional However several lines of evidence mitigate composite Grampian $2/$3 foliation. against a c. 480 Ma intrusion age. Most of the Ox The pre-existing foliation surface is markedly Mountains granodiorite Rb-Sr whole rock data older than the age constraints for sinistral exten- are characterized by low 87Rb/S6Sr values, typi- sional crenulation cleavage development in the cally less than 2. Rb-Sr whole rock isochrons Central Ox Mountains inlier. Sinistral shearing characterized by low SYRb/86Sr values have also is constrained by four Rb-Sr muscovite-feldspar been obtained from other Caledonian granites, ages from pegmatites which range from and these too yield c. 480 Ma intrusion ages even 402-392 Ma (Table 3) which are interpreted as though the intrusion is demonstrably younger recording igneous crystallization (Flowerdew et (c. 400 Ma) by independent evidence (Kennan al. 2000). Late sinistral shearing has recrystal- 1997). Additionally, unpublished U-Pb multi- lized coarse magmatic muscovite within both the grain zircon data suggest a c. 415 Ma intrusion Ox Mountains pegmatite suite and the Ox age for the Ox Mountains granodiorite (Mac- Mountains granodiorite, yielding three Rb-Sr Dermot et al., 1996). This is consistent with the muscovite-feldspar ages of between 385 and youngest mineral cooling ages (c. 410 Ma) 381 Ma (Table 3; Flowerdew et al. 2000). obtained from the Central Ox Mountains inlier close to the Ox Mountains granodiorite which are likely to be due to thermal resetting, and a c. Discussion on the intrusion age of the Ox 400 Ma emplacement age for the pegmatite suite (Flowerdew et al. 2000); a c. 400 Ma intrusion age Mountains granodiorite for the Ox Mountains granodiorite is more likely. Sinistral shearing along the FHCBL in the Central Ox Mountains is effectively constrained Earlier movements along the FHCBL by the age of intrusion of the Ox Mountains gra- nodiorite and its presumed coeval pegmatite This study documents two examples of post- suite. However, in contrast to the c. 400 Ma age Grampian strike-slip movement along the 350 D.M. CHEW ETAL.

FHCBL. However, there is evidence for earlier non-coaxial deformation. Careful analysis of the stages of movement along the FHCBL during resulting crenulation morphologies combined the Grampian orogeny, illustrating further how with isotopic dating yields a more complete this important crustal-scale shear zone has had a understanding of the reactivation history of long and complicated history of movement. In major crustal-scale shear zones. Tyrone, the D30magh Thrust has translated inverted Dalradian rocks towards the ESE D.M.C. gratefully acknowledges a Forbairt Basic (Fig. 1) over Arenig-Llanvirn shales of the Research Grant and a UCD Research Doctoral Tyrone volcanics (Alsop & Hutton 1993). In Scholarship. Barry Long is thanked for many fruitful southern Donegal and the NE Ox Mountains discussions about Dalradian geology, and the Central Ox Mountains in particular. Sarah Sherlock and inlier, Dalradian rocks were thrust to the SE Grahame Oliver are thanked for their careful and con- over granulite-facies basement of the Slishwood structive reviews, which significantly improved this Division (Fig. 1) along the Lough Derg Slide paper. (Alsop 1991) and the North Ox Mountains Slide (Flowerdew 1998/9) respectively. Tectonic jux- taposition (D3) of the Dalradian and Slishwood References Division is likely to have occurred between 470 ALSOP, G.I. 1991. Gravitational collapse and extension and 460 Ma based on 4°Ar-39Ar, Rb-Sr and along a mid-crustal detachment: the Lough Derg Sm-Nd mineral ages (Flowerdew et al. 2000). Slide, northwest Ireland. Geological Magazine, Thus in Donegal, the NE Ox Mountains and 128, 345-354. Tyrone, the earliest constrained phase of move- ALSOP, G.I. & HuTroN, D.H.W. 1993. Major southeast- ment (c. 470-460 Ma) along the Fair Head-Clew directed Caledonian thrusting and folding in the Bay Line involves the translation of the main Dalradian rocks of mid-Ulster: implications for Dalradian nappes over outboard terranes to the Caledonian and mid-crustal shear zones. SE. Geological Magazine, 130, 233-244. ALSOP, G.I. & JONES, C.S. 1991. A review and corre- lation of Dalradian stratigraphy in the Ox Moun- Conclusions tains and southern Donegal, Ireland. Irish Journal of Earth Sciences, 11, 99-112. The Fair Head-Clew Bay Line has been shown ARNAUD, N.O. & KELLEY, S.P. 1995. Evidence for to have been reactivated several times. It was excess argon during high pressure metamorphism originally active as a ductile thrust during the in the Dora Maira Massif (western Alps, Italy), (c. 470-460 Ma) Grampian orogeny, where Dal- using an ultraviolet laser ablation microprobe radian nappes adjacent to the Fair Head-Clew 4°Ar-39Ar technique. Contributions to Mineral- Bay Line were thrust over outboard terranes to ogy and Petrology, 121, 1-11. CHEW, D.M. 2003. Structural and stratigraphic the SE. relationships across the continuation of the High- The Dalradian metasediments have under- land Boundary Fault in western Ireland. Geo- gone two separate phases of post-Grampian logical Magazine, 140, 73-85. strike-slip movement adjacent to the Fair CHEW, D.M., DALY, J.S., PAGE, L.M. & KENNEDY, M.J. Head-Clew Bay Line. These two phases of 2003. Grampian orogenesis and the development movement have produced reverse-slip and of blueschist-facies metamorphism in western normal-slip crenulations which modified the Ireland. Journal of the Geological Society, earlier Grampian nappe fabrics, and tilted the London, 160, 911-924. initially recumbent Grampian nappes into a ver- CLIFF, R.A. 1985. Isotopic dating in metamorphic tical orientation (Fig. 2b, c). belts. Journal of the Geological Society, London, 142, 97-110. The development of the reverse-slip and DALY,J.S. 1996. Pre-Caledonian history of the Annagh normal-slip crenulations produced by these two Gneiss Complex, north-western Ireland, and discrete phases of strike-slip movement has correlation with Laurentia-Baltica. Irish Journal been constrained by isotopic dating. Dextral dis- of Earth Sciences, 15, 5-18. placement along the Fair Head-Clew Bay Line DENNIS, A.J. & SECOR, D.T. 1987. A model for the in the NW Mayo inlier is constrained to development of crenulations in shear zones with c. 448 Ma, whereas sinistral displacement along applications from the southern Appalachian the Fair Head-Clew Bay Line in the Ox Moun- Piedmont. Journal of Structural Geology, 9, tains inlier is constrained to c. 400 Ma based on 809-817. DENNIS, A.J. & SECOR, D.T. 1990. On resolving shear previously published pegmatite intrusion ages. direction in foliated rocks deformed by simple Major crustal-scale shear zones may therefore shear. Geological Society of America Bulletin, have a long and complicated history of move- 102, 1257-1267. ment, in which pre-existing planar anisotropies DEWEY, J.E & SHACKLETON,R.M. 1984. A model for (e.g. foliations) act as slip surfaces during later the evolution of the Grampian tract in the early REACTIVATION OF A CALEDONIAN SHEAR ZONE 351

Caledonides and Appalachians. Nature, 312, Microsoft Excel. Berkeley Geochronology Center 115-121. Special Publication, la, 49 pp. DUNLAP, W.J. 1997. Neocrystallization or cooling? MACDERMOT, C.V., LONG, C.B. & HARNEY, S. 1996. 4°Ar/39Ar ages of white micas from low-grade Geology of Sligo-Leitrim: A geological description mylonites. Chemical Geology, 143, 181-203. of Sligo, Leitrim, and adjoining parts of Cavan, FLOWERDEW, M.J. 1998/9. Tonalite bodies and base- Fermanagh, Mayo and Roscommon, to accom- ment-cover relationships in the North-eastern Ox pany the Bedrock Geology 1:100 000 Scale Map Mountains Inlier, north-western Ireland. Irish Series, Sheet 7, Sligo-Leitrim. Geological Survey Journal of Earth Sciences, 17, 71-82. of Ireland. FLOWERDEW, M.J., DALY,J.S., GUISE, P.G. & REX, D.C. MAX, M.D. & RIDDIHOUGH,R.R 1975. Continuation of 2000. Isotopic dating of overthrusting, collapse the Highland Boundary fault in Ireland. Geology, and related granitoid intrusion in the Grampian 3, 206-210. , northwestern Ireland. Geological MAX, M.D., LONG, C.B. & MONET, J. 1976. The geo- Magazine, 137, 419-435. logical age and setting of the Ox Mountains Gra- FREEMAN, S.R., INGER, S., BUTLER, R.W.H. & CLIFF, nodiorite. Bulletin of the Geologial Survey of R.A. 1997. Dating deformation using Rb-Sr in Ireland, 2, 27-5. white mica: greenschist facies deformation ages MCCAEFREY, K.J.W. 1992. Igneous emplacement in a from the Entrelor shear zone, Italian Alps. transpressive shear zone: Ox Mountains igneous Tectonics, 16, 57-76. complex. Journal of the Geological Society, FRIEDRICH, A.M., BOWRING, S.A., MARTIN, M.W. & London, 149, 221-235. HODGES, K.V. 1999a. Short-lived continental MCCAFFREY, K.J.W. 1994. Magmatic and solid state magmatic arc at Connemara, western Ireland deformation partitioning in the Ox Mountains Caledonides: implications for the age of the granodiorite. Geological Magazine, 131, 639-652. Grampian orogeny. Geology, 27, 27-30. MOLLER, W., DALLMEYER, R.D., NEUBAUER, E ~: FRIEDRICH, A.M., HODGES, K.V., BOWRING, S.A. & THONI, M. 1999. Deformation-induced resetting MARTIN, M.W. 1999b. Geochronological con- of Rb/Sr and 4°Ar/39Ar mineral systems in a low- straints on the magmatic, metamorphic and grade, polymetamorphic terrane (Eastern Alps, thermal evolution of the Connemara Cale- Austria). Journal of the Geological Society, donides, western Ireland. Journal of the Geo- London, 156, 261-278. logical Society, London, 156, 1217-1230. PANKHURST, R.J., ANDREWS, J.R., PHILLIPS, W.E.A., HARRIS, D.H.M. 1993. The Caledonian evolution of SANDERS, I.S. t~ TAYLOR, W.E.G. 1976. Age and the Laurentian margin in western Ireland. Journal structural setting of the Slieve Gamph Igneous of the Geological Society, London, 150, 66%672. Complex, Co. Mayo, Ireland. Journal of the Geo- HARRIS, D.H.M. 1995. Caledonian transpressional logical Society, London, 132, 327-36. terrane accretion along the Laurentian margin in PLATF, J.P. ~z VISSERS, R.L. 1980. Extensional struc- Co. Mayo, Ireland. Journal of the Geological tures in anisotropic rocks. Journal of Structural Society, London, 152, 797-806. Geology, 2, 397-410. HUrl'ON, D.H.W. 1987. Strike-slip terranes and a RAMSAY, J.G. 8z GRAHAM, R.H. 1970. Strain variation model for the evolution of the British and Irish in shear belts. Canadian Journal of Earth Sciences, Caledonides. Geological Magazine, 124, 405-425. 7, 786-813. HurroN, D.H.W. & DEWEY, J.E 1986. Palaeozoic RYAN, ED., MOPER,N.J., SNYDER,D.B., ENGLAND, R.W. terrane accretion in the Irish Caledonides. & HurroN, D.H.W. 1995. The Antrim-Galway Tectonics, 5, 1115-1124. Line: a resolution of the Highland Border fault JONES, C.S. 1989. The structure and kinematics of the enigma of the Caledonides of Britain and Ireland. Ox Mountains, western Ireland; a mid-crustal Geological Magazine, 132, 171-184. transcurrent shear-zone. Unpublished Ph.D. SANDERS, I.S., DALY, J.S. & DAVIES, G.R. 1987. Late thesis, University of Durham. Proterozoic high-pressure granulite facies meta- KENNAN, P.M. 1997. Granite: a singular rock. In: Occa- morphism in the north-east Ox inlier, north-west sional papers in Irish science and technology, 15. Ireland. Journal of Metamorphic Geology, 5, Royal Dublin Society, 16. 69-85. KENNEDY, M.J. 1980. Serpentinite-bearing m61ange in SANDERSON, D.J., ANDREWS, J.R., PHILLIPS,W.E.A. & the Dalradian of County Mayo and its significance HcrroN, D.H.W. 1980. Deformation studies in the in the development of the Dalradian basin. Irish Caledonides. Journal of the Geological Journal of Earth Sciences of the Royal Dublin Society, London, 137, 289-302. Society, 3, 117-126. SHERLOCK, S.C. t~z ARNAUD,N.O. 1999. Flat plateau and LISTER, G.S. & SNOKE, A.W. 1984. S-C mylonites. impossible isochrons: Apparent 4°Ar-a9Ar geo- Journal of Structural Geology, 6, 617-638. chronology in a high-pressure terrain. Geochimica LONG, C.B. & MAX, M.D. 1977. Metamorphic rocks in et Cosmochimica Acta, 63, 2835-2838. the SW Ox Mountains Inlier, Ireland; their struc- SHERLOCK, S.C. & KELLEY, S.P. 2002. Excess argon tural compartmentation and place in the Cale- evolution in HP-LT rocks: a UVLAMP study of donian orogen. Journal of the Geological Society, phengite and K-free minerals, NW Turkey. London, 133, 413-432. Chemical Geology, 182, 619-636. LUDWIG, K.R. 1999. Users Manual for Isoplot/Ex, STEIGER, R.H. & JAGER, E. 1977. Subcommission on Version 2.10: a geochronological toolkit for geochronology: convention on the use of decay 352 D.M. CHEW ETAL.

constants in geo- and cosmochronology. Earth WIJBRANS, J.R., PRINGLE, M.S., KOPPERS, A.A.E & and Planetary Science Letters, 36, 359-362. SCHEEVERS, R. 1995. Argon geochronology of TAYLOR, W.E.G. 1969. The structural geology of the small samples using the Vulkaan argon laser- Dalradian rocks of Slieve Gamph, Cos. Mayo and probe. Proceedings of the Royal Netherlands Sligo, western Ireland. Geologische Rundschau, Academy of Arts and Sciences, 98, 185-219. 57, 564-588. WILLIAMS, D.M., HARKIN, J., ARMSTRONG, H.A. & WEST, D.P.W. & LUX, D.R. 1993. Dating mylonitc HIGGS, K.T. 1994. A late Caledonian mglange in deformation by the 4°Ar-39Ar method: an Ireland: implications for tectonic models. Journal example from the Norumbega Fault Zone, Maine. of the Geological Society, London, 151, 307-314. Earth and Planetary Science Letters, 120, 221-237. WINCHESTER, J.A. 1992. Comment .on 'Exotic meta- WIJBRANS, J.R. & McDOUGALL, I. 1988. Metamorphic morphic terranes in the Caledonides: Tectonic evolution of the Attic Cycladic Metamorphic Belt history of the Dalradian block, Scotland'. on Naxos (Cyclades, Greece) utilizing 4°Ar/39Ar Geology, 20, 764. age spectrum measurements. Journal of Meta- morphic Geology, 6, 571-594.