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.

Plate 3 depicts three major belts distinguished by their style and age of metamorphism: the orthotectonic and paratec-tonic zones of the Caledonides and the Hercynides. Age of metamorphism, shown on the inset map (Plate 3), in part reflects the areal distribution of these zones and refers to the timing of the main phase or peak of metamorphism. The orthotectonic zone is, therefore, shown to be metamorphosed during the earliest Ordovician although recrystaHization was almost certainly taking place from the late or even mid-Cambrian to the Silurian. In the paratec-tonic belt the main metamorphism is shown as occurring in the period Lower Ordovician-to-Silurian although recrystahization may well have continued into the Lower Devonian. The Lower Devonian rocks within the central and southern Caledonides have suffered folding and weak metamorphism normally before Middle Devonian times and have been differentiated on the map.The southern limit of significant Caledonian deformation is normally taken as the northern edge of the Midland Block (F. W. Dunning, pers. comm.). However, it is believed that weak Caledonian metamorphism affected rocks south of this line. Similarly, recrystaHization associated with the Hercynian extended well to the north of the so-called Hercynian front, the northern limit of strong deformation (F. W. Dunning, pers. comm.). There is, therefore, some doubt as to whether the main recrystaHization in pre-Carboniferous rocks of South Wales and the English Midlands relates to the late-Caledonian or to the Hercynian.

SummaryIllite crystallinity determinations on Palaeozoic pelitic rocks, whose stratigraphic range runs from Lower Cambrian to Westphalian, indicate that anchimetamorphism has affected both the Lower and Upper Palaeozoic sequences. Two metamorphic episodes are in evidence, with the earlier, Caledonian, being of slightly higher grade. The higher anchizone crystallinity values are recorded from the Fishguard area in which mineral assemblages of the prehnite-pumpellyite facies have recently been recognized in basic igneous rocks. The later metamorphic episode has affected rocks to the south of the Variscan front. Here crystallinity values are low anchizone but some straddle the boundary with the diagenetic state. The Pembroke coalfield lies in this southern area and has coals largely of anthracite rank with volatile matter contents of between 10.1 and 5% Grade of metamorphism as indicated by crystallinity and by coal rank data from the Pembroke coalfield shows anomalous results to that described from the main South Wales coalfield. A neo-formed 2M illite from the Variscan spaced cleavage is described with analytical and X-ray diffraction data.

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