The sheared porphyrite dykes of South Galloway

Abstract

Summary A group of more than 100 dykes, mainly porphyrites with some microdiorites, quartz-diorites, and lamprophyres, trends east-north-east from the Solway coast between Creetown and Gatehouse-of-Fleet; the dykes are intruded along the strike of the closely folded Lower Palaeozoic sediments, the belt in which they are emplaced being some eight miles in length and four miles wide. Most of the dykes possess directional textures, formerly described as foliation; these textures are partly of primary origin but mainly due to shearing. The shearing has taken place on vertical surfaces which lie either parallel to the length of the dykes or make small acute angles with their walls. A feature shown by most of the dyke-rocks is the development of numerous flakes of white mica within their fabrics; in the sheared dykes the micas are oriented along the surfaces of shear. The mica-forming materials may have been derived from an internal source in the zone of shearing, or as emanations from subjacent granite masses. The relationships between the shear directions, transverse joints, and quartz veins which cross the dykes are analysed, and experimental work due to Reidel is cited in support of the suggested origin of the structures in a belt of shearing. The attitude of the shear surfaces in relation to the ellipsoids of stress and strain is also examined. The dykes are considered to fill tear (or transcurrent) fractures, on which lateral movement recurred after the emplacement of the igneous bodies. The implications of this mechanism in respect of a wider area are indicated.

Structural history of the Silurian rocks of the coast west of Gatehouse, Kirkcudbrightshire

“…Of 21 dykes recorded by Blyth (1949), two are lamprophyres, one a microdiorite, one a quartz-diorite, and the balance are porphyrites. Of the concordant sheets two are felsite and the other porphyrite (Pi.…”

“…Caledonian igneous intrusions The igenous intrusions of the area have already been studied in detail by Blyth (1949), both from a petrographical and from a structural standpoint, and it remains only to relate the injection and subsequent deformation of these rocks to more recent knowledge of the deformational history of the host rocks.…”

“…The Caledonian dykes have been dealt with exhaustively by Blyth (1949), and the present account will only place the phases of their intrusion and subsequent deformation in context within the whole tectonic history. STRATIGRAPHY The rocks which outcrop in the area comprise a monotonous and unfossiliferous sequence of fine-grained sandstones of the greywacke kindred, interbedded with cleaved siltstones and mudstones, and containing occasional horizons of red rocks.…”

The stratigraphy and structure of the Whithorn area of Wigtownshire, Scotland

“…A group of dykes containing varying proportions of quartz, acidic felspars and ferro-magnesian minerals have been classified as porphyrites and assorted lamprophyres by Blyth (1949), Read (1926) and Reynolds (1931). The Caledonian age of the dykes has been assessed on geographical and penological affinities to the Caledonian plutonic bodies,* and on their absence from post-Silurian strata.…”

‘Lamprophyre’ minor intrusions of Colonsay – a comment

“…Berger's (1971) study offelsic dykes is less relevant than numerous, partly structural, papers on mafic dykes equivalent to those of Colonsay (e.g. Reynolds, 1931;Blyth, 1949;Rust, 1965;French, 1978;Smith, 1979;Leake & Cooper, 1983), or indeed than the one published study on the Colonsay plugs themselves (Reynolds, 1936). Borradaile's omission of all this work has broader implications, notably the question of how many igneous suites (i.e.…”