RT Journal Article
SR Electronic
T1 The petrology and structure of Dalradian metabasaltic dykes of Jura: implications for early Dalradian evolution
JF Scottish Journal of Geology
FD Geological Society of London
SP 257
OP 270
DO 10.1144/sjg20020257
VO 20
IS 2
A1 Graham, Colin M.
A1 Borradaile, Graham J.
YR 1984
UL http://sjg.lyellcollection.org/content/20/2/257.abstract
AB Massive tholeiitic metadolerite dykes cutting Dalradian metasediments of the Jura Quartzite formation of Jura and Islay are comparable in major and trace element composition to other Dalradian metabasic rocks of the SW Highlands. Trace element geochemistry indicates that these dykes are usually less evolved than most of the overlying metabasaltic sills and volcanics in mainland Argyllshire, consistent with their likely role as feeders to the sills and volcanics. Metamorphic assemblages in the Jura dykes and in greenschist facies metabasic rocks elsewhere in the SW Highlands are identical, and the metamorphic fluid phase behaviour in both groups of rocks appears to have been similar. Reconstruction of pre-tectonic dyke geometries suggests that the dykes were intruded perpendicular to bedding as a NW–SE group responding to NE–SW crustal tension (contemporaneously with extrusion of the volcanic rocks). This evidence, together with that of sedimentary-basin and syndepositional-fault geometries, may indicate changes in the direction of relative crustal tension from NE–SW to NW–SE and back to NE–SW during oblique or strike-slip (transtensional) rifting of a subsiding continental margin during Argyll Group times. The dominance of sill intrusion relative to dyke intrusion in SW Highland metabasaltic rocks is analogous to the intrusive behaviour of basic magma in modern deep rifted sedimentary basins such as the present day Gulf of California. Expulsion of water from adjacent sediments and establishment of large-scale hydrothermal systems may explain the abundance of post-crystallisation spilitic alteration of SW Highland metabasaltic rocks.© 1984 Scottish Journal of Geology