SUBDUCTION-RELATED MAGMATISM OF LATE ORDOVICIAN AGE IN EASTERN ENGLAND

Deep boreholes show that plutonic and volcanic igneous rocks comprise an important component of the Caledonian basement in eastern England. The isotopic compositions of these rocks reveal that many of them are of late Ordovician age (440-460 Ma), and their geochemical composition s suggest calc-alkaline affinities. The intermediate (diorite-tonalite ) plutonic rocks are associated with a prominent northwest-southeast t rending belt of aeromagnetic anomalies extending from Derby to St Ives , Hunts., which is interpreted to work the plutonic core of a calc-alk aline magmatic arc. It is inferred that this arc was generated by the subduction of oceanic lithosphere, possibly from the Tornquist Sea, in a south or southwest direction beneath the Midlands Microcraton in la te Ordovician times. The age and geochemical composition of concealed Ordovician volcanic rocks in eastern England, and hypabyssal intrusion s of the Midlands Minor Intrusive Suite in central England, is compati ble with such a hypothesis.