Slockenray Coast

Geological Conservation Review site | GCR #2411 | Igneous Petrology | Ordovician Igneous

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Summary

The Ballantrae Complex, of Ordovician age, consists of three belts of pillow lavas and associated sediments, separated from each other by ultramafic intrusive rocks. The Complex shows many of the features of an obducted ophiolite although geochemical studies indicate that the various igneous rocks were generated in a variety of tectonic settings and have since been tectonically juxtaposed. The Pinbain Block is the most northerly of the lava belts in the Ballantrae Complex and is well-exposed along the coast at Slockenray. In the southern part of the site conglomerates are interbedded with cherts and black shales. The shales show soft-sediment deformation structures and it is thought that the conglomerate bodies, which contain boulders up to several metres in length are olistostromes. The variety of the clasts, representing almost all the known rock types found in the Complex, shows that the ophiolite was being actively eroded at the time the conglomerates were accumulating. Further north, the section exposes spilitic lavas and volcanogenic sediments. The latter show upwards coarsening sequences which record the build up of a hyalotuff delta formed in front of advancing lava flows in shallow water conditions.

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https://earthwise.bgs.ac.uk/index.php/Pinbain_Block_-_an_ex…'s_Dyke_[NX_1347_9113]:_Gabbro_pegmatite_(Figure_25.17)

Locality 13. Bonney's Dyke [NX 1347 9113]: Gabbro pegmatite. Pinbain Block - an excursion - From: Lawson, J.D. and Weedon, D.S. (editors). 1992. Geological excursions around Glasgow & Girvan. Glasgow : Geological Society of Glasgow

Bonney's Dyke was named by Balsillie (1932) after the famous petrologist T.G.Bonney. The feature is not a dyke but a steeply dipping, sill-like sheet intruded into serpentinite, comprising a coarse gabbro pegmatite of altered diallage and feldspar, most of the latter being replaced by white prehnite and pectolite. The following points should be noted: the sheet thins towards the west: it is sinistrally shifted by a number of faults and as such provides an excellent strain marker within the serpentinite: it has a hydrogrossular northern margin and contains rafts of serpentinite. The serpentinite to the immediate south of the sheet contains large crystals of diallage: to the immediate north is a norite with enstatite crystals enclosed in plagioclase; the norite is only visible at low tide.

https://earthwise.bgs.ac.uk/index.php/Girvan_and_Ballantrae…'s_Dyke_and_Pinbain_Beach:_gabbro_pegmatite,_Balcreuchan_Group_melange_and_breccia

Girvan and Ballantrae - an excursion. By P Stone. Excursion 8. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. (Keyworth, Nottingham: British Geological Survey.)