The Genesis of the Cornubian Batholith (South-West England): the example of the Carnmenellis Pluton

The origin of the Carnmenellis granite is discussed in the lightof new petrographic and chemical data which suggest that thepresent mineral assemblage comprises refractory, magmatic, subsolidusand hydrothermal phases. The present homogeneity of the plutonresults from subsolidus to pervasive hydrothermal reworkinginvolving selective leaching of alkalis and Fe-Mg and an increasein Li, F, B, and Rb, which were introduced, or redistributed,through high temperature hydrothermal circulation. The compositionsof the primary minerals were re-equilibrated with respect tothese water-rock interactions. Rb-Sr systematics demonstratethat whole rock samples show lower ages than the minerals (260and 285 Ma, respectively). The perturbation in the former isotopedistribution is in agreement with the chemical re-equilibrationof the mica with external Rb-rich and Sr-depleted solutions.The distribution of rare-earths reveal that most reside in afew accessory phases, viz. monazite, zircon, and apatite; theirnear-liquidus fractionation is responsible for the loweringof the bulk REE content compared to pelitic source materialand for the present REE pattern of the pluton. Conditions of magma generation by partial melting of cordierite-sillimanite-spinelpelitic gneisses were estimated to be around 800?C and 5 kbwith water content in the magma of about 4 wt. per cent. H₂Osaturation was reached late during the ascent of the magma,making possible the crystallization of muscovite from the residualmelt. The highly evolved peraluminous composition of the plutonis not explained by a simple magmatic differentiation. Comparedto the Carnmenellis pluton, the Land's End massif of similarmineralogy appears less evolved, either because of less subsolidusreworking or deeper structural emplacement.