Abstract

An oxygen isotope study has been carried out on rocks and minerals from the 3 units which comprise the concentric Hercynian pluton of Ploumanac'h (Brittany, France). Lack of extensive low-temperature water/rock interactions enabled 3 non-cogenetic pulses of magma to be identified: (1) an early mantle-derived basaltic magma of alkaline affinity (δ18O ∼ 6); (2) biotite granites (δ18O ∼ 9) which form the bulk of the intrusion; and (3) a ring dyke of late, two mica-cordierite granite (δ18O = 12.3) that is clearly of crustal derivation. The δ18O data, together with published 87Sr/86Sr results, suggest that the last 2 (granitic) pulses were generated in the continental crust. These results are not compatible with any models of closed-system fractional crystallization of a single parent magma-type, although they probably can be fitted into a 'mixing' model involving combined assimilation/fractional crystallization of continental crustal material by basaltic magma. A more likely explanation of the biotite granites is melting of amphibolites or granulites of the lower crust (possibly derived from volcano-detrital parent rocks), with the two-mica granites forming by melting of shallower, more pelitic metasedimentary rocks. The position of the Ploumanac'h pluton in a linear arrangement of related intrusions, together with the clear-cut structural control of its emplacement, are compatible with a model of melting at different levels in the crust and upper mantle, either associated with a mantle plume, or by a pressure release near the tip of a propagating lithospheric crack.

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