| Site Description |
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| Highlights: |
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| The site is a quarried exposure of porphyritic dacite (fine-grained granodiorite) that is chemically related to other late Caledonian acid intrusions in Northern Ireland. Age dates for the intrusion indicate emplacement during early Ordovician times. Geochemistry indicates magma generation at the base of the Earth's crust, probably in the roots of the Caledonian orogenic belt. |
| Introduction: |
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| This site is in a quarry located ~2km northwest of Cushendun in County Antrim. |
| The first edition of the Cushendall (Sheet 14) one-inch to the mile geological map (Geological Survey of Ireland, 1886) shows an intrusive body of "quartziferous porphyry", emplaced in the schist country rocks. In the accompanying explanatory memoir, Symes and McHenry (1886) described the porphyry as having a gradational and foliated southwest boundary. The central and northwest portions are described as being more massive and intrusive looking. The rock is described as highly feldspathic with large crystals of orthoclase feldspar in a plagioclase, quartz and mica matrix. |
| Part of the intrusion is marked on the resurveyed second edition of Ballycastle (Sheet 8) one-inch to the mile geological map (Geological Survey of Northern Ireland, 1963). The rock type is referred to as a porphyritic microgranite in the accompanying memoir (Wilson and Robbie, 1966). |
| Resurvey of the Cushendall map has as yet not been undertaken. A detailed account of the intrusion and surrounding country rock was however reported by Harrison and Wilson (1978). This report describes the geological context, petrography, chemistry and petrogenesis, of what is considered to be a granodiorite intrusion chemically related to other late post-tectonic Caledonian acid intrusions from the United Kingdom. The report also contains details of dykes associated with the Cushendun intrusion. Geochemistry would appear to indicate magma generation at the base of the Earth's crust, probably in the roots of the Caledonian orogenic belt. A Rb/Sr date of 496±128Ma and a K/Ar date of 462±6 indicate emplacement significantly earlier than most Caledonian post-tectonic acid intrusions in Northern Ireland. The emplacement date appears to equate more closely with the Craigballyharky and Craigbardahessiagh complexes of County Tyrone. |
| Description: |
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| The best location to view the Cushendun intrusion is in a quarry that may be entered via a track off the Cushleake Road. The Dalradian country rock is dark albite-biotite-schist that forms part of the Ranabay Formation, Southern Highland Group (Geological Survey of Northern Ireland, 1997). These rocks can be seen in outcrops in the fields leading up to the main quarry workings. The contact between country rock and the Cushendun intrusion is not exposed at the site but can be seen nearby in a section of the Milltown Burn at [3235 4345]. |
| Within the main quarry, the rock appears reddish orange with phenocrysts of feldspar (aligned in places) in a fine-grained groundmass. The rock contains rounded xenoliths of schistose country rock. Networks of quartz rich and feldspar rich veinlets cut the rock in places. Faults and joints are inclined steeply to the southwest giving the rock a sheeted appearance. |
| Thin section examination of the rock shows that it is composed of phenocrysts of feldspar (40%) and quartz (4%) in a fine-grained groundmass (40%) comprising recrystallised quartz and feldspar. Most of the feldspar phenocrysts are zoned plagioclase (<4 mm) with compositions in the oligoclase-albite range. Primary biotite (1.0 mm) is fresh but forms less than 1% of the rock. Hornblende commonly forms clusters of small prisms (0.5 mm) and may represent mafic xenolithic material. The groundmass is a mosaic of small (<0.1 mm) anhedral quartz and alkali feldspar grains, here and there showing a relict flowage orientation. This rock can be classified as a porphyritic dacite (i.e. a fine-grained variety of granodiorite) (Merriman and Hards, 2000). |
| Importance: |
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| The Cushendun intrusion, occurring as it does within the Dalradian of northeast Antrim, provides vital information relating to the origin of magma and the timing of intrusive events during the Caledonian Orogeny. |
| Interpretation: |
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| Geochemistry indicates magma generation at the base of the Earth's crust, probably in the roots of the Caledonian orogenic belt. A Rb/Sr date of 496±128Ma and a K/Ar date of 462±6 suggest an intrusion that is significantly older than most Caledonian post-tectonic acid intrusions including those of the Newry Igneous Complex. Timing does however equate to that of the Craigballyharky and Craigbardahessiagh complexes of County Tyrone. |
| Conclusions: |
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| The Cushendun intrusion occurs along strike from similar intrusions in Counties Londonderry and Tyrone and is chemically related to late post-tectonic Caledonian acid intrusions elsewhere in the United Kingdom. |
| The geochemistry of the intrusion indicates that the magma was generation at the base of the Earth's crust, probably in the roots of the Caledonian orogenic belt. A Rb/Sr date of 496±128Ma and a K/Ar date of 462±6 indicate that the intrusion is significantly older than most Caledonian post-tectonic acid intrusions. |
| Notes: |
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For information and references on sites within the Tyrone Basic Plutonic Complex see the following sites: |
| Key Site 580 - Black Rock Key Site 581 - Scalp Hill |
| For information and references on sites within the Tyrone Volcanic Group see the following sites: |
| Key Site 582 - Mveela More Key Site 583 - Creggan Key Site 584 - Copney Hill Key Site 585 - Beaghbeg Key Site 586 - Bonnety Bush, Altihaskey Key Site 587 - Tullybrick Key Site 588 - Stuhanleanantawey Burn |
| For information and references on other Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: |
| Key Site 589 - Cashel Rock Key Site 590 - Craigballyharky and Craigbardahessiagh Key Site 591 - Craiganawork, Pomeroy Key Site 592 - Tintagh, Slieve Gallion Key Site 593 - Loughmacrory |
| For general information and references on the Caledonian Igneous Complexes of N. Ireland, and for directions to other regional sites, see the following site: |
| Key Site 595 - Introduction to the Caledonian Igneous Complexes of N. Ireland |
| For more information on the geology of the area around Cushendun, see the following sites: |
| Key Site 164 - Cushendun Bay and Rock Port Key Site 307 - Devonian - Bay Hotel-Cave House Tunnel, Cushendun Key Site 308 - Devonian - Cave House (South Of), Cushendun Key Site 1146 - Devonian - Cross Slieve Group; Cushendun To Port Obe |