LATE CADOMIAN INTERMEDIATE MINOR INTRUSIONS OF GUERNSEY, CHANNEL ISLANDS: THE MICRODIORITE GROUP

One of the distinctive features of the geology of Guernsey is the presence of numerous dykes, with a wide range of compositions, which were emplaced at various times during the evolution of the island (Roach et al., 1991). Various groups of what are here referred
to as the older dykes are variably deformed, are metamorphosed (greenschist facies or higher) and predate the emplacement of the Cadomian Northern Igneous Complex (N.I.C.). These older dykes are restricted to the Southern Metamorphic Complex (S.M.C.). The most
abundant examples of the older dykes are the tholeiitic to subalkaline metadolerites of the Vazon Dyke Swarm (Lees and Roach, 1987). In contrast, the younger dykes, which are either contemporaneous with or postdate the emplacement of the N.I.C., exhibit a low or very low
grade of metamorphism, involving only partial alteration of the primary minerals. A table of the dyke sequence is given by Lees and Roach (1987). Three main groups of dykes can be shown to postdate all components of the N.I.C. : (1) the microdiorite group; (2) the albite
dolerite group; (3) the lamprophyre group; in order of decreasing age. The albite dolerites, of alkaline basalt affinity and with a prehnitepumpellyite metamorphic assemblage, have been described by Lees et al. (1989). The present communication is concerned with the microdiorite group, examples of which occur within both the N.I.C. and S.M.C. These dykes are characterized by primary brownish amphibole, zoned plagioclase, a porphyritic texture, and a broad east-northeast to westsouth-west trend. Their field relationships, petrography and variable alteration are briefly described. Major, and trace element data are presented to demonstrate their calc-alkaline character. The dykes are considered to represent a very late phase of calc-alkaline magmatism on Guernsey within the late Precambrian-early Palaeozoic Cadomian orogenic cycle rather than being related to a subsequent geotectonic regime.