Early Palaeozoic metamorphic history of the Midland Valley, Southern Uplands–Longford-Down massif and the Lake District, British Isles

Abstract

A model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

Geochemistry and potential correlation of Silurian (Telychian) metabentonites from Ireland and SW Scotland

“…All the ARD, KB samples, and NI samples have experienced prehnite-pumpellyite grade metamorphism (Oliver et al 1984), equivalent to anchizone metamorphism (Frey and Robinson 1999), resulting in the almost complete conversion of smectite to illite. Sample KB02 from Kirkcudbright was described by Merriman and Roberts (1990) as sample BRS289.…”

The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland

“…see Molyneux, 1987). These rocks are highly disrupted by soft-sediment deformation structures and are of significantly lower metamorphic grade compared to the Silurian rocks in the adjacent coastal sections (Oliver et al 1984). This suggests that these rocks may have formed in a very shallow part of the thrust wedge where the effects of down-slope, gravity-induced deformation were dominant.…”

The Siluro-Devonian evolution of the southern Midland Valley of Scotland

“…Rocks from the Llandovery/Wenlock inliers were analysed using XRD (Oliver et al 1984) and some con- The study showed the rocks regional metamorphic state lies between the zeolite and the prehnite-pumpellyite facies, that is, weak burial metamorphism. In addition, Colour Alteration Indices of conodonts from near Girvan and the Pentland Hills indicate temperatures up to 150 °C.…”