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Published October 1980 | public
Journal Article

¹⁸O/¹⁶O evidence for non-cogenetic magmas associated in a 300 Ma old concentric pluton at Ploumanac'h (Brittany, France)


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.

Additional Information

Thanks are due to C. Dupuy for SiO2 determinations in the samples and to S. Fourcade and P. Vidal for fruitful discussions. This paper is Contribution No. 3290, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.

Additional details

August 22, 2023
October 24, 2023