Late Caledonian Dyke-Swarms in Southern Scotland: A Regional Zone of Primitive K-Rich Lamprophyres and Associated Vents

• Published in: The Journal of geology Vol. 94; no. 4; pp. 505 - 522
• Main Authors: Rock, N. M. S., Gaskarth, J. W., Rundle, C. C.
• Format: Journal Article
• Language: English
• Published: University of Chicago Press 01-07-1986
• Subjects: Geology; Highlands; Igneous rocks; Lava; Magma; Paradoxes; Plutons; Rocks; Subduction; Tectonics
• ISSN: 0022-1376; 1537-5269
• DOI: 10.1086/629054

Siluro-Devonian magmatism in the Southern Uplands (Scotland) and Northern Ireland includes a hitherto neglected regional SW-NE zone, c. 10 km wide and >300 km long, of calc-alkaline lamprophyre dykes. Lamprophyres were locally emplaced contemporaneously with small (<400 X 200 m) subvolcanic vents of greywacke/mudstone agglomerate, themselves intruded by kersantites, primitive biotite-bearing olivine-basalts, igneous breccias, and tuffisites. The lamprophyres and basalts are among the most primitive ($mg 65-81\%; Ni \leq 300; Cr \leq 1000 ppm$) K-rich ($\leq 6\% K_{2}O$) and Ba-rich ($\leq 9000 ppm Ba$) of British Caledonian igneous rocks, and among the few candidates for primary magmas. They possibly comprise at least three components: (a) a partial melt of (?metasomatized) mantle peridotite from >100 km depths; (b) a contribution from subducted (?sedimentary) material; and (c) volatile components added metasomatically in the crust or mantle during ascent. Rb-Sr isochron and new K-Ar biotite/hornblende ages for unfoliated lamprophyres range from 395-418 Ma, supporting field evidence for prolonged, episodic, dyke emplacement. Post-orogenic lamprophyres are among the youngest Caledonian igneous rocks. Late orogenic, unfoliated lamprophyres closely postdated, while syn-orogenic, crushed, and foliated lamprophyres accompanied the latest (lundgreni zone) greywacke sedimentation and folding. Collectively, these dykes present several paradoxes for the current accretionary prism model, relating the Southern Uplands to subduction along the "Iapetus suture": (1) The dykes imply substantial extension at a high angle to the assumed suture. (2) Volcanic, subvolcanic, and plutonic magmatism is juxtaposed in both space and time. (3) The lamprophyres are too K-rich, and of excessively deep mantle source, for their close proximity to the assumed suture trace. (4) Unlike contemporaneous lavas, they reveal no known chemical zonation but show similar chemistry from northern England (south of the assumed suture) to the Western and Northern Isles of Scotland (700 km north of the suture).