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

Albarede, F. and Dupuis, C. and Taylor, H. P., Jr. (1980) ¹⁸O/¹⁶O evidence for non-cogenetic magmas associated in a 300 Ma old concentric pluton at Ploumanac'h (Brittany, France). Journal of the Geological Society, 137 (5). pp. 641-647. ISSN 0016-7649. doi:10.1144/gsjgs.137.5.0641. https://resolver.caltech.edu/CaltechAUTHORS:20221012-498020200.1

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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.


Item Type:Article
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https://doi.org/10.1144/gsjgs.137.5.0641DOIArticle
ORCID:
AuthorORCID
Albarede, F.0000-0003-1994-1428
Alternate Title:18O/16O evidence for non-cogenetic magmas associated in a 300 Ma old concentric pluton at Ploumanac'h (Brittany, France)
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.
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences3290
Issue or Number:5
DOI:10.1144/gsjgs.137.5.0641
Record Number:CaltechAUTHORS:20221012-498020200.1
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20221012-498020200.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117374
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Oct 2022 22:53
Last Modified:12 Oct 2022 22:53

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