Crust and mantle contributions to granite genesis; an example from the Variscan batholith of Corsica, France, studied by trace-element and Nd-Sr-O-isotope systematics

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doi: 10.1016/0009-2541(94)90186-4
Authors:Cocherie, A.; Rossi, P.; Fouillac, A. M.; Vidal, P.
Author Affiliations:Primary:
B.R.G.M., Service Géologique National, Orléans, France
C.N.R.S., Clermont-Ferrand, France
Volume Title:Chemical Geology
Source:Chemical Geology, 115(3-4), p.173-211. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1994
Note:In English. Includes appendices. 107 refs.; illus. incl. 9 tables, geol. sketch map
Summary:The Corsican Batholith, formed during the Variscan Orogeny, was studied with particular emphasis on the respective roles played by crustal- and mantle-derived material in granite genesis. Major- and trace-element data identify: the various types of magma and their genetic relationships; the magmatic processes that gave rise to the observed rocks; and the primitive liquids. O-Sr-Nd-isotope systematics constrain the origin of primitive liquids. Trace-element models provide data on the various source materials and their possible geodynamic setting. The evolution of two main calc-alkaline plutonic associations is compared: an older high-Mg-K association (U1) with both ultrapotassic mafic and silicic rocks, the latter ranging from monzonite to leucosyenogranite, and a younger calc-alkaline composite association (U2) involving a mafic cumulate sequence and granodiorite to leucomonzogranite. Both types of granite are accompanied by mafic units that were non-cogenetic with surrounding granitic melts. Trace-element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the U1 Mg-K granite by fractional crystallization, amphibole and titanite playing a major role, which, with increasing crystallization, gave decreasing REE contents without a strong negative Eu anomaly. The U2 calc-alkaline suite was formed by fractional crystallization involving feldspar and LREE-rich minerals, i.e. monazite, from a liquid of monzogranitic composition. Field and petrographical data identify magma mingling in both U1 and U2, between mafic and silicic rocks, but geochemical data only indicate how magma-mixing processes led to U2 granodiorite. Geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions: the high-Mg-K monzodiorite melt was formed after partial melting of a 30% non-modal batch of granulite-facies metamorphic protolith (low pH2O), but the calc-alkaline monzogranite was formed by the same process in an amphibolite-facies source of similar composition (higher pH2O). The primitive granite magmas of both associations show the same crustal characteristics, i.e.: Sri = 0.706-0.707; εNd(t) = -4.3 to -2.2 and δ18O = +7 to +8 per mil. This provides evidence that the composition of both U1 and U2 granite melts probably was controlled by physico-chemical fusion rather than protolith-composition parameters. The U1 ultrapotassic mafic rock is thought to be of mantle origin, i.e. a deep source containing phlogopite (±garnet). Zone refining led to a significant increase of incompatible trace-element contents during ascent of the magma. Mantle-crust interaction lowered the La/Yb and 143Nd/144Nd ratios, but the 87Sr/86Sr ratio increased. Nevertheless, interaction with associated U1 Mg-K granite was not important and the crustal component is thought to be different from the associated granitic magma. In U2, the mafic cumulate sequence, clearly of mantle origin, has E-MORB characteristics and seems the result of 10% non-modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet. Some interaction occurred with the associated granitic melt. The E-MORB character of the mafic rock indicates that a large part of the batholith, and at least the U2 magmatic association, was generated under post-collisional extensional conditions.
Subjects:Alkaline earth metals; Amphibolite facies; Basalts; Batholiths; Calc-alkalic composition; Crust; Cumulates; Extension tectonics; Facies; Felsic composition; Fractional crystallization; Gabbros; Genesis; Geodynamics; Gneisses; Granites; Granitic composition; Granodiorites; Granulite facies; Igneous rocks; Intrusions; Isotope ratios; Isotopes; Mafic composition; Magmas; Major elements; Mantle; Melts; Metals; Metamorphic rocks; Metasedimentary rocks; Mid-ocean ridge basalts; Mixing; Monzodiorite; Nd-144/Nd-143; Neodymium; O-18/O-16; Orogeny; Oxygen; Paragneiss; Partial melting; Peridotites; Plutonic rocks; Protoliths; Rare earths; Siliceous composition; Sr-87/Sr-86; Stable isotopes; Strontium; Tectonics; Tectonophysics; Trace elements; Troctolite; Ultramafics; Variscan Orogeny; Volcanic rocks; Corsica; Europe; France; Western Europe; Corsican Batholith; Leucomonzogranite
Abstract Numbers:95M/1822
Record ID:1994040759
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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