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Regional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low-grade metamorphism whilst in the southern region the very low-grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High-Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late-stage structural up-doming of the higher grade areas over the roof of the batholith.

ABSTRACT. The north coast of Cornwall, from Bude to Newquay, provides a continuous section through a major Variscan fold/thrust complex. Illite crystallinity studies have revealed a transition from diagenesis in the north to greenschist facies metamorphism in the south in the Upper Palaeozoic succession. More detailed studies of mineral assemblages in both metabasites and pelitic rocks support the regional pattern of metamorphism indicated by illite crystallinity, and show that locally in the Tintagel district, the grade of metamorphism may have reached middle to upper greenschist facies. An attempt to correlate the above data with temperatures (108-985~ derived from O-isotope geothermometers is made. Interpretation of the metamorphic data presented helps to emphasize the tectonic importance of the major structures seen in the fold/thrust complex. KEVWORDS: diagenesis, greenschist facies, illite, Variscan, Cornwall, England.THE geology of south-west England is dominated by a largely clastic sequence of Upper Palaeozoic sediments that has suffered a polyphase deforrnational history as a result of Variscan orogenic activity. In this context a study of the extent to which these sediments have been altered by burial and tectonism may help elucidate the complex pattern of structures in south-west England.The coastal section between Newquay and Bude ( fig. 1) has been chosen to demonstrate the degree of low-temperature alteration as it displays a continuous section through a major Variscan fold/ thrust complex. Here the rocks show both the greatest range in age (Lower Devonian to Upper Carboniferous) and variability in structural style. Brazier et aL (1979) Copyright the Mineralogical SocietyDevonian and Lower Carboniferous rocks in the Tintagel area and lower grades, anchizone to diagenetic zone for Upper Carboniferous rocks to the north. Descriptions of greenscbist facies mineral assemblages in the Tintagel area (Robinson and Read, 1981) support earlier observations (Tilley, 1925;Phillips, 1928) and confirm that the highest grade metamorphic effects occurred in that area. A more detailed study using similar techniques is made here and isotopic geothermometers are also used to provide more detailed information which is interpreted in the light of recent structural and tectonic syntheses.Layer silicate mineralogy and crystallinity determinations. XRD analysis of 122 samples shows that throughout the Devonian slates the < 2/tm~phyllo-silicate assemblage is dominated by a 10 A illite/ muscovite phase and chlorite with minor occurrences of pyrophyllite ( fig. 2). Chlorite-free assemblages are found in black, Lower Carboniferous slates, but the greatest variety of assemblage is found in the Upper Carboniferous, Crackington and Bude Formations. The uniformity in mineralogy throughout rocks of Devonian age suggests that there is a low-temperature metamorphic influence, with rocks in the illite-chlorite zone of Velde (1977), and indicating that temperatures must have exceeded 200~ This contrasts sharply with the very rapi...

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