The genesis of Variscan (Hercynian) plutonic rocks: Inferences from Sr, Pb, and O studies on the Maladeta igneous complex, central Pyrenees (Spain)

Abstract

The Maladeta plutonic complex formed during the latest stages of the Variscan orogeny. It was emplaced into the Paleozoic sedimentary sequence of the Pyrenees. The eastern part, investigated in the present study, consists of an early intrusion of cumulate gabbronorites followed in order of emplacement by the main biotite-hornblende granodiorite, which was itself intruded by two small stocks of two-mica cordierite granite. An ⁸⁷Rb-⁸⁷Sr isochron dates the granodiorite at 277±7 m.y. with an initial (⁸⁷Sr/⁸⁶Sr)₀ ratio of 0.7117±3. Gabbroic rocks have lower strontium initial ratios, down to 0.7092, while those of granite range from that of the granodiorite up to about 0.715. The three rock types have distinctive δ¹⁸O values: 8.7 to 9.6 for the gabbronorites, 9.4 to 10.4 for the granodiorites and 10.3 to 11.8 for the granites. Lead isotopic compositions of rocks and feldspars are all radiogenic. Feldspars give consistent Pb model ages around 280 m.y., with μ and κ values of about 9.7 and 4.05, respectively. No pristine, mantle-derived magma was found among the investigated samples and the rocks cannot be related to one another by any simple mechanism of fractional crystallization. Some type of mixing process involving two end members seems to be required: a high-¹⁸O, high-⁸⁷Sr material that is clearly of crustal origin, and a lower-¹⁸O, lower-⁸⁷Sr end member derived from the mantle. Examination of various mixing models does not support magma mixing nor assimilation of crustal rocks by a mafic magma. The most acceptable model involves melting at different levels of a vertically-zoned source in the continental crust; this source was formed by mixing between mantle-derived magmas and crustal metasediments. This material was apparently thickened, tectonically downwarped and partially melted. None of the Maladeta magma-types appear to have been derived at a consuming plate boundary.

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