PETROGRAPHIC TEXTURE AND VITRINITE REFLECTANCE VARIATION ON THE ALSTON BLOCK, NORTH-EAST ENGLAND

Abstract

Summary A detailed study of the petrography and vitrinite reflectance of coals has been used to elucidate patterns of heat flow on the Alston Block. Four petrographic/rank vitrinite groups have been distinguished: I. Non-thermally metamorphosed with a vitrinite reflectance below 1.80% (approximately). II. Thermally metamorphosed and showing a granular mosaic. III. Thermally metamorphosed with no granular mosaic. IV. Non-thermally metamorphosed with reflectances in excess of 1.80%. Transitional vitrinites are also described. Certain features of the Group II vitrinites suggest a coking coal rank prior to Whin Sill intrusion and the Group II – Group III transition has been interpreted as a pre-Whin 1.60% isoreflectance contour. This isoreflectance surface has been traced laterally across the block and is shown to dome across the cupolas of the Weardale Granite, indicating that the pre-Whin Sill basement heat flow was channelled through the basement granite stock. Temperatures, calculated by the method of Karweil (1973), for this coalification interval were probably in excess of 185°C at the Great Limestone horizon around the cupola margins. This indicates that some form of rejuvenation of the Weardale Granite occurred at depth, probably in response to the Hercynian orogeny. The intrusion of the Whin Sill (295 ± 6 Ma) produced the Group II and Group III vitrinites and caused an exponential increase in vitrinite reflectance towards its contacts. Graphitisation has occurred immediately adjacent to the Whin Sill in certain localities where pre-Whin Sill rank and intrusion temperatures were favourable. The abnormally wide zone of natural coke above the Whin Sill is attributed to the high background temperatures present in the Alston Block during intrusion. A detailed lateral survey of vitrinite reflectance at the horizon between the Great and Little Limestones has revealed both the dependence of this parameter upon the Whin Sill thickness and stratigraphic horizon in the central part of the block, and a spatial relationship between vitrinite reflectance and the Weardale Granite subcrop around the block margins. The post-Whin Sill mineralisation has had no discernable effect on vitrinite reflectance.