Magma mixing in the South Leicestershire diorite : evidence from an Ordovician pluton at Croft Quarry

At Croft Quarry, exposures of a pluton belonging to the South Leicestershire Diorite suite have revealed a complex history of multiple intrusion. Soon after emplacement of the main-stage quartz-diorite, the partially crystallised pluton received an influx of magma which became dispersed and is now seen as partially assimilated dioritic xenoliths. A much later episode of intrusion occurred when the pluton had cooled sufficiently to be capable of fracturing. It resulted in a spectacular swarm of synplutonic quartz-diorite/tonalite sheets with contacts indicating that the host quartz-diorite was locally remobilized, disrupting and net-veining the later sheets. These features are typical of ‘magma mixing’ phenomena, and suggest an underlying process that may account for some of the geochemical and petrographical variations previously noted within the South Leicestershire diorites.