Late Caledonian dyke‐swarms in Southern Scotland: New field, petrological and geochemical data for the Wigtown Peninsula, Galloway

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Abstract
Several hundred, dominantly NE‐trending dykes invade Silurian flysch sediments of the Wigtown Peninsula. The most basic dykes are K‐rich biotite‐lamprophyres, hornblende‐lamprophyres and appinites ('diorites' of previous maps). More acidic (qz and cm normative) dykes include restricted andesitic compositions ('porphyritcs', c. 57 per cent SiO2) and a continuum from dacitic to rhyolitic compositions ('acid porphyrites' and 'porphyries', c. 63–71 per cent SiO2). The relative abundance of basic to intermediate + acidic dykes, based on examination of 180 thin sections and 70 new whole‐rock analyses, is approximately 7:1. The biotite‐ and hornblende‐lamprophyres are distinct in chemistry and distribution, biotite‐lamprophyres being confined to the south of the Peninsula. Neither type has any compositional equivalent among the plutonic rocks of the area. Porphyrite and porphyry dykes do not represent simple lamprophyre fractionates, but may be derived from at least two (intermediate and acidic) parent magmas, which may represent crustally‐contaminated lamprophyre or entirely separate crustal melts. Emplacement of all dyke compositions was apparently episodic through Wenlock to early Devonian times (c. 425‐395 Ma). Early dykes post‐date the main deformation (D1) but predate minor (D2) folding, both D1 and D2 being related to thrusting. On the current accretionary model for the Southern Uplands, this implies that dyke emplacement commenced before the end of accretion in a compressional tectonic regime—scarcely consistent with the moderate (up to 6 per cent) crustal extension represented by the dykes. The geochemistry of the lamprophyres requires a deep mantle origin several hundred kilometres behind any subduction zone. This suggests that the 'lapetus suture', at the time of dyke emplacement, was much farther south than its currently inferred position and places the Southern Uplands in a back‐arc situation.

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