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

Abstract

The origin of the Carnmenellis granite is discussed in the light of new petrographic and chemical data which suggest that the present mineral assemblage comprises refractory, magmatic, subsolidus and hydrothermal phases. The present homogeneity of the pluton results from subsolidus to pervasive hydrothermal reworking involving selective leaching of alkalis and Fe-Mg and an increase in Li, F, B, and Rb, which were introduced, or redistributed, through high temperature hydrothermal circulation. The compositions of the primary minerals were re-equilibrated with respect to these water-rock interactions. Rb-Sr systematics demonstrate that whole rock samples show lower ages than the minerals (260 and 285 Ma, respectively). The perturbation in the former isotope distribution is in agreement with the chemical re-equilibration of the mica with external Rb-rich and Sr-depleted solutions. The distribution of rare-earths reveal that most reside in a few accessory phases, viz. monazite, zircon, and apatite; their near-liquidus fractionation is responsible for the lowering of the bulk REE content compared to pelitic source material and for the present REE pattern of the pluton. Conditions of magma generation by partial melting of cordierite-sillimanite-spinel pelitic gneisses were estimated to be around 800°C and 5 kb with water content in the magma of about 4 wt. per cent. H2O saturation was reached late during the ascent of the magma, making possible the crystallization of muscovite from the residual melt. The highly evolved peraluminous composition of the pluton is not explained by a simple magmatic differentiation. Compared to the Carnmenellis pluton, the Land's End massif of similar mineralogy appears less evolved, either because of less subsolidus reworking or deeper structural emplacement.