Geochemistry and tectonic significance of the Lower Carboniferous Cockermouth lavas, Cumbria

Abstract

SUMMARY Dinantian volcanism in the Cockermouth area was associated with the initiation of the Northumberland Basin and was related to crustal attenuation resulting from lithospheric stretching. The sequence consists mainly of basalts of tholeiitic (quartz-normative) affinity, with minor tholeiitic andesites. The basaltic magmas were generated by variable degrees of partial melting of homogeneous mantle source rocks followed by fractional crystallization and final equilibration at high crustal levels. The tholeiitic andesites probably indicate the establishment of high level magma chambers. The intrusive basalts of Little Mell Fell are confirmed on chemical grounds as likely outliers of the Cockermouth lavas. There are also close compositional similarities with Dinantian lavas in Derbyshire and south and central Scotland, and with the Whin Sill magma type, indicating that changing tectonic conditions during the Carboniferous are not reflected in magma compositions. There is no evidence in magma chemistry that the Iapetus suture underlies the Northumberland Basin.