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Petrogenetic implications from biotite compositional variations in the Cornubian granite batholith

Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK

Lithium oxide contents of biotites in some Cornubian granites which have been studied by electron microprobe are estimated from their SiO₂ and F contents using samples formerly analysed by XRF and wet chemistry. Better formulae which include Li show that most of these biotites are lithian siderophyllites/lithian Al-annites although some Dartmoor samples, especially those in the basic microgranite (ME) enclaves, are closer to Al-annites. Dartmoor samples are slightly more basic and poorer in Al^vi than those from the Isles of Scilly and Carnmenellis granites agreeing with the broad differences in the host rocks. Biotites in volcanic clasts from the New Red Sandstone are significantly different in having much higher Si, Ti, Mg, F and Y and lower Al^vi contents than those of the granite biotites: these are lithian Fe-Mg biotites. These clasts are believed to be the volcanic equivalents of an evolving Dartmoor magma (Awad et al., 1996). If so, their biotites reflect the composition of the granite biotites at the time of magma emplacement. They subsequently evolved as they equilibrated with the cooling granite to give the present biotite compositions of the Dartmoor granites.

KEYWORDS: lithium oxide, biotite, granite, volcanic clast