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Magmatism |
1 School of Earth, Environmental and Physical Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
2 Department of Earth Sciences, The Jane Herdman Laboratories, University of Liverpool, Brownlow Street, Liverpool L69 3BX, UK
The rocks of the Poortown Quarry, 3 km east of Peel, Isle of Man, are shown to comprise a complex series of sills of pyroxene-rich dolerite, plagioclase-rich dolerite and plagioclase-phyric andesite intruded into Manx Group deep-water marine sedimentary rocks of Arenig age. They have suffered early Devonian deformation and greenschist facies metamorphism, together with later alteration and faulting. The pyroxene-rich dolerite has the composition of a Mg-rich basalt relatively enriched in Fe, Cr and Ni. It contains up to 60% augite and is likely to have been produced by fractionation in a high-level magma chamber before intrusion into its present position. Some of the pyroxene grains have more primitive (higher Mg, Cr and lower Ti), partly resorbed cores which supports a multi-stage history for this magma.
The sills cover a range of compositions from Mg-rich basalt to calc-alkaline basaltic andesite and the geochemistry of the more immobile elements suggests a calc-alkaline volcanic arc origin in an active continental margin environment. Although the age of the Poortown Complex is poorly constrained, a tentative comparison is made with the lower part of the Borrowdale Volcanic Group of the English Lake District.
