COMPOSITE ORDOVICIAN LAMPROPHYRE (SPESSARTITE) INTRUSIONS AROUND THE MIDLANDS MICROCRATON IN CENTRAL BRITAIN

Lamprophyre sills and dykes of Ordovician age were emplaced within Cam brian-Lower Ordovician sedimentary rocks around the northern margins o f the Midlands Microcraton. The intrusions show internal mineralogical and chemical variations indicating emplacement as multiple intrusions of co-magmatic pulses. The chemical characteristics of the lamprophyr e magmas indicate formation by small-degree volatile-rich partial melt ing of lithospheric mantle enriched and modified by Lower Palaeozoic s ubduction (Th/Ta 5.3-11.6, La/Ta 29-82.3), together with a contributio n from within-plate mantle source (Zr/Yc. 6) and/or mineralogically he terogeneous lithosphere, followed by varying degrees of fractional cry stallization during uprise.