Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

Abstract

The main phase (∼400 Ma) emplacement of the central and northern part of the reversely zoned Galway Granite was incremental by progressive northward marginal dyke injection and stoping of the 470–467 Ma Connemara metagabbro-gneiss country rock. The space was provided by the synchronous ESE-opening, along the strike of the country rocks, of extensional fractures generated successively northward by a releasing bend in the sinistrally moving Skird Rocks Fault or an equivalent Galway Bay Fault. This fault is a prolongation of the Antrim–Galway (a splay off the Highland Boundary Fault) and Southern Upland Faults. The ESE-strike of the spalled-off rocks controlled the resultant ESE-elongated shape of the batholith. The magma pulses (∼5–30 m in thickness) were progressively more fractionated towards the northern margin so that the coarse Porphyritic (or Megacrystic) Granite (GP; technically granodiorite) in the centre was followed outwards by finer grained, drier and more siliceous granite, until the movements opening the fractures ceased and the magma became too viscous to intrude. ‘Out-of-sequence’ pulses of more basic diorite-granodiorite (including the Mingling–Mixing Zone) and late main phase, more acid, coarse but Aphyric Granite, into the centre of the batholith, complicated the outward fractionation scheme. The outward expansion, caused by the intrusions into the centre, caused a foliation and flattening of cognate xenoliths within the partly crystallised northern marginal granite and in the Mingling–Mixing Zone to the south.Late phase (∼380 Ma) central intrusions of the newly-discovered aphyric Shannapheasteen Finegrained Granite (technically granodiorite), the Knock, the Lurgan and the Costello Murvey Granites, all more siliceous and less dense than the GP, were emplaced by pushing up the already solid and jointed GP along marginal faults. This concentration of lighter granites plus compression shown in thrusting, caused overall fault uplift of the Central Block of the Galway batholith so that the originally deepest part of the GP is exposed where there is the most late phase granite. Chemical analyses show the main and late phase magmas, including late dykes, were very similar, with repetition of the same fractionation except that the late phase magmas were drier and more quickly cooled, giving finer grained rocks.

The Neoproterozoic to Mid-Devonian evolution of Scotland: a review and unresolved issues

“…The Newry Granite problems have already been highlighted. The Siluro-Devonian Galway Granites are north of the OBF; the largest intrusion, the Main Batholith, straddles the Skird Rocks Fault (SRF) (either an extension of the Southern Uplands Fault (Feely et al 2006;Leake 2006) or a southern splay of the Highland Boundary Fault (Ryan et al 1995)). The Galway Granites mostly meet the criteria for belonging to the Northern Granites.…”

Subglacial hydrology and drumlin sediments in connemara, western ireland

“…Connemara, a region of west County Galway, western Ireland ( Fig. 1), is underlain by Dalradian granites and associated metamorphic rocks (Tanner et al 1997;Leake 2006). Across the region (around 240 km 2 ), bedrock is commonly exposed at the surface with only isolated patches of glacigenic sediment.…”

Long-lived granite-related molybdenite mineralization at Connemara, western Irish Caledonides

“…Ireland, Galway, 1997) indicate a similar source of granite magmas, and therefore the emplacement of the main Galway Granite may be related to prolonged asthenospheric flow following slab break-off. The ascent of granite magma was facilitated by extensional fractures associated with a releasing bend on the sinistrally moving Skird Rocks Fault (Leake, 2006). The final stages of magmatic activity in the Galway Granite (e.g.…”

“…The petrology, geochemistry and field relationships of the central block granites has been described in detail by the following: El-Desouky, Feely & Mohr (1996), Crowley & Feely (1997), Baxter et al (2005), Feely et al (2006) and Leake (2006). These studies present unequivocal evidence for several phases of granite emplacement.…”

“…Two major faults, the NNE-trending Shannawona Fault and the NW-trending Barna Fault, divide the Granite into Friedrich et al 1999) Oughterard Granite of east Connemara is also shown. Geology adapted from Townend (1966), Leake & Tanner (1994), Pracht et al (2004), Feely et al (2006) and Leake (2006). SFZ -Shannowona Fault Zone; BFZ -Barna Fault Zone; SRF -Skird Rocks Fault.…”