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Petrogenesis of jotunitic and acidic members of an AMC suite (Rogaland anorthosite province, SW Norway): A Sr and Nd isotopic assessment

par Bolle, Olivier;Demaiffe, Daniel ;Duchesne, Jean-Clair
Référence Precambrian research, 124, 2-4, page (185-214)
Publication Publié, 2003-07

Article révisé par les pairs

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Résumé :

Sr and Nd isotopic data from the anorthosite-mangerite-charnockite (AMC) suite of the Late-Sveconorwegian (ca. 0.93 Ga) Rogaland anorthosite province of SW Norway are discussed. The study focuses on new data obtained from three distinct rock occurrences: (1) primitive (=MgO-rich and K2O-poor) jotunites that represent the parental magmas of leuconorite and mafic plutons (87Sr/86Sr0.93Ga = ca. 0.704-0.707, εNd (0.93 Ga) = +3.6 down to +1.2); (2) evolved jotunites that differentiated from the primitive jotunites and constitute the starting components of dyke-scale fractionation trends (87Sr/86Sr0.93Ga = ca. 0.705-0.713, εNd (0.93 Ga) = +0.4 down to -2.0); and (3) felsic plutons (87Sr/86Sr0.93Ga = ca. 0.707-0.723, εNd (0.93 Ga) = +1.4 down to -1.7). A comparison of the Sr and Nd isotopic compositions with available geochemical data (major and trace elements) is also made. Fractionation of the primitive to the evolved jotunites, below the intrusion level of the anorthosite province, is shown to have involved crustal contamination. A case of fractional crystallization and simultaneous contamination/assimilation is substantiated in the felsic cap of a layered intrusion (the Bjerkreim-Sokndal intrusion). It is further proposed that the Sr and Nd isotopic compositions of the various felsic occurrences result from fractional crystallization of primitive jotunitic melts, with or without contamination/assimilation; this strengthens the possibility for large volumes of silica-rich magmas to have been produced by fractionation, through evolved jotunites, of primitive jotunites. The entire range in Sr and Nd isotopic compositions of the Rogaland anorthosite province is interpreted as reflecting a mixing of crustal contaminants, having variable 87Sr/86Sr0.93Ga ratios and (negative) εNd (0.93 Ga) values, with a more isotopically primitive source. The crustal end members of that mixing array are represented by moderately to strongly LILE-enriched high-grade gneisses from the Pre-Sveconorwegian basement of southernmost Norway, that have a crustal history extending back to 1.5-1.9 Ga in the surroundings of the anorthosite province. The primitive end member is either an unusual LREE-depleted and Rb-enriched component, possibly corresponding to an originally depleted source subsequently modified by metasomatic fluids, or a more classical depleted component; it corresponds to a gabbronoritic rock of the deep crust, as shown by recent experimental data, with a quite short crustal residence time (<0.4-0.2 Ga). If that mafic end member is the lower granulitic crust itself, this would imply the presence in the deep continental crust of southwesternmost Norway of a crustal material strongly different from the 1.7- to 1.9-Gyr-old, moderately LILE-enriched component that is supposed to constitute the largest volume of the present deep continental crust in southern Norway. © 2003 Elsevier Science B.V. All rights reserved.