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Summary Full report
Scalp Hill
Rec. Number:581File Number:  
ESCC:EO
Locality Type:Crags, Inland exposure Status:
Grid Reference: X26353745 Centroid
County: Tyrone District:
Period:Ordovician
Stages:
Lithostrat:Tyrone Volcanic Group
Site Description
Highlights:
Scalp Hill consists of a number of knolly outcrops of gabbro and closely associated dolerite and granitic intrusive rocks belonging to the Tyrone Plutonic Complex (Ordovician). Examples of mineral layering, a phenomenon seen comparatively rarely within the Tyrone Plutonic Complex, are well documented at this site.
Introduction:
Scalp Hill consists of a series of rocky knoll-like outcrops in semi-enclosed rough pasture ~3.5km northeast of Carrickmore in Co Tyrone. Vehicular access is via a farm track north from Devesky Road leading to a farmyard and thereafter by foot across fields and rough pasture.
Portlock's early geological map of the area, (1843) included all rocks in the vicinity of Scalp Hill within the category "metamorphic rocks of hornblendic type".
On the first edition of the one-inch to the mile scale geological map (Sheet 34, Pomeroy, Geological Survey of Ireland, 1877) rocks at Scalp Hill were referred to as "Hornblendic & Pyroxenic Rocks" and were part of the "Metamorphic and Igneous Rocks". In the explanatory memoir which accompanied the map (Nolan, 1878) the rocks at "the Scalp" were described as being pyroxenitic with an augitic variety prevalent and sometimes "developed in large crystals of hypersthene, glittering with the semi-metallic lustre so characteristic of that mineral". This early description contains no indication of the possible origins or the geological context in which these rocks were formed.
Hartley (1933) produced the earliest detailed geological map of the central Tyrone Ordovician volcanic \ plutonic terrane. On Hartley's lithostratigraphic map, Scalp Hill shown as gabbro and included within a plutonic subgroup of what Hartley referred to as the Tyrone Igneous Series.
There was renewed research interest in the Scalp Hill locality (Cobbing, 1969) following the resurvey and publication of the 1:50 000 second edition of Sheet 34 (Pomeroy, Geological Survey of Northern Ireland, 1979).
The stratigraphy of central Tyrone was revised following the resurvey and publication of the second editions of Sheet 34 (Pomeroy) (1979) and Sheet 26 (Draperstown) (1995). Hartley's Tyrone Igneous Series is now subdivided into two major components, the Tyrone Plutonic Group and the Tyrone Volcanic Group. The rocks at Scalp were assigned to the Tyrone Plutonic Group and have been interpreted as being part of an ophiolite complex (Hutton et al., 1985). On a regional scale, they form part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck et al., 1992) and have broad lithological correlatives in the Girvan-Ballintrae ophiolite in Scotland and at Clew Bay in the west of Ireland.
Description:
Olivine gabbro forms the greater part of a series of scoured outcrops at Scalp Hill. These gabbros in turn form part of a much more extensive tract lying directly to the southeast of the central Tyrone metamorphic inlier (Cornvanaghan Formation).
Like the gabbros elsewhere in the Tyrone Plutonic Group, the rocks at Scalp are extensively amphibolitized. Hornblende replaced most of the original pyroxenes, although some relics of augite? and hypersthene remain as well as olivine.
Scalp Hill is most notable for the presence of mineral layering in the gabbro. Locally, gabbro is differentiated into compositionally different bands ranging in thickness from cm to m scale. The larger scale banding is due to textural and compositional variation whereas the fine-scale banding (up to~5cm thick) is the result of mineralogical banding and the alternation of mafic rich and feldspar rich bands. This banding traverses the hill in a north-south orientation and dips at between 30 and 80 towards the east.
Cobbing (1969) noted that banding in the gabbros is arranged in such a way that concentrations of the thin monomineralic bands alternate with groups of the thicker gabbroic bands. Within the monomineralic groups, mafic banding predominates over felsic banding. There appears to be is no regular relationship between dark and light bands and they are usually intermingled haphazardly.
Cobbing (1969) also noted areas at Scalp where normal gabbro grades into a strongly deformed rock best described as a hornblende schist. These rocks have well developed planar and linear fabrics and can be seen in an area ~60m south of the trigonometric point on Scalp Hill. The schistosity is parallel to the mineral layering in the surrounding gabbro indicating that it formed in response to the same forces which rotated the mineral layering in the gabbro to its present steep orientation. Locally, the hornblende schist is intensely deformed with five distinct phases of folding documented within one small area of the Scalp outcrop (Cobbing, 1969).
Other rock types of note at Scalp Hill include a series of northeast to southwest trending hornblende bearing acid porphyrite dykes and undeformed basic dykes. Thin section petrography of the dykes (Merriman and Hards, 2000) indicates that these are porphyritic dacite (a fine-grained variety of granodiorite) with 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 plagioclases (<4mm) which have compositions in the oligoclase-albite range. Primary biotite (1.0mm) is fresh but forms less than 1% of the rock. The hornblende commonly forms clusters of small prisms (0.5mm) and may represent mafic xenolithic material. The groundmass is a mosaic of small (<0.1mm) anhedral quartz and alkali feldspar grains, here and there showing a relict flowage orientation.
Importance:
Scalp Hill is a site of national importance which contains the best exposed and documented example of mineral banding and layering in gabbros of the Tyrone Plutonic Group. The site also provides excellent opportunities to study textural and mineralogical variations within the gabbros and complex folding in hornblende schist which formed as a result of extreme deformation of the gabbro.
Understanding the petrography and structure of these gabbros is key to the wider understanding of the Tyrone Ophiolite and perhaps to unravelling the sequence of Caledonian events which led to the final accretion of the mid Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane.
Interpretation:
The gabbros and associated rock types on Scalp Hill (Tyrone Plutonic Complex) are components of an ophiolitic suite, remnants of oceanic crustal material which formed during Ordovician times at the northern margin of the Iapetus Ocean. These rocks are broadly analogous to ophiolitic associations at Girvan-Ballintrae in Scotland and at Clew Bay in County Mayo (Bluck et al., 1992). The contact relationship between the gabbro (Tyrone Plutonic Complex) and the metamorphic inlier is poorly exposed and has been subject of discussion in the scientific literature (Cobbing, 1964; Cobbing et al., 1965). The more recent opinion is that the ophiolitic rocks were obducted onto the Laurentian continental margin as the Iapetus Ocean closed by northwards directed subduction from the late Ordovician onwards. The present day location of the ophiolite fragment is also believed to be the result of a major component of transpressional during a later phase of terrane accretion.
Conclusions:
Scalp Hill is a site of national and possibly international significance. It offers access to high quality outcrop of layered gabbros of the Tyrone Plutonic Group and an opportunity to study an important component of the Tyrone Ophiolite.
Notes:

For information and references on the other site within the Tyrone Basic Plutonic Complex see the following site:

Key Site 580 - Black Rock
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 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 Key Site 594 - Cushendun
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

Keywords
Minerals:
Rocks:Gabbro, Horneblende schist
FossilGroups:
Fossil List:
Products:
Structures:Caledonian, mineral layering, ophiolite
Relations:
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Measurements
Length: Width: Height: 
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Access
Approach:Vehicular access is via a farm track north from Devesky Road leading to a farmyard and thereafter by foot across fields and rough pasture.
Restrictions: 
Planning: 
Management:Access to the site is across fenced pasture. The site is remote and does not appear to be under any immediate threat from development.
Outcrop is extensive at this locality but is prone to overgrowth by gorse and lichen. The site has become notably more overgrown in recent years and there has been a deterioration and reduction of exposure since the site was mapped by the Geological Survey of Northern Ireland in the 1960's.
The best examples of mineral layering and deformation in the hornblende schists occur in relatively restricted areas at Scalp. In order to preserve these, consideration should be given to restricting or embargoing hammering and sampling at the site.
The rocks of the Tyrone Plutonic Group are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration attention in the future.
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Uses
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References

Map(s):

GEOLOGICAL SURVEY OF IRELAND, 1877. 1:63,360 Geological Sheet 34 (Pomeroy). / Dublin, Ordnance Survey for Geological Survey of Ireland. / GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 / (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). / GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1979. Pomeroy, Northern Ireland / Sheet 34. Solid. 1:50,000. (Southampton: Ordnance Survey for the Geological / Survey of Northern Ireland). / GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland / Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: / British Geological Survey).

Map No: 
Rec Type ESCR report Recorder:  
Enterer: E M Porter
Updates: 2 Nov 2004 / 14 APR 2001 / 10 MAR 2001 / 04 MAR 2001
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