Olivier Bolle

Derivation of the 1.0-0.9 Ga ferro-potassic A-type granitoids of southern Norway by extreme differentiation from basic magmas

Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0–0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svofjell, Handeland-Tveit, Aseral, Lyngdal gabbronorites) that occur close to the Mandal-Ustaoset Line (MUL) of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50 wt.% SiO 2) to granites (77 wt.% SiO2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930 Ma, the HBG suite displays a narrower range in ISr (0.7027–0.7056) than in eNd(t) (+1.97 down to −4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929 ± 47 Ma is given by a Rb-Sr isochron on the Holum granite (Sri = 0.7046 ± 0.0006, MSWD = 1.7). Geothermobarometers indicate a low pressure of emplacement (1.3–2.7 kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005–1054 ◦ C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930 Ma anorthosite–mangerite–charnockite suite (AMC suite) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat.

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

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 ( 87 Sr/ 86 Sr0.93 Ga = ca. 0.704–0.707, eNd(0.93 Ga) =+ 3. 6d own to+1.2); (2) evolved jotunites that differentiated from the primitive jotunites and constitute the starting components of dyke-scale fractionation trends ( 87 Sr/ 86 Sr0.93 Ga = ca. 0.705–0.713, eNd(0.93 Ga) =+ 0. 4d own to−2.0); and (3) felsic plutons ( 87 Sr/ 86 Sr0.93 Ga = ca. 0.707–0.723, eNd(0.93 Ga) =+ 1. 4d own 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 87 Sr/ 86 Sr0.93 Ga ratios and (negative) eNd(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.

Magnetic fabric and deformation in charnockitic igneous rocks of the Bjerkreim-Sokndal layered intrusion (Rogaland, Southwest Norway)

The Bjerkreim‐Sokndal intrusion (BKSK) belongs to the Proterozoic Rogaland igneous complex of Southwest Norway. The BKSK displays an anorthosite to mangerite cumulate series, folded into a syncline, in the core of which crop out massive acidic rocks of quartz mangerite to charnockite composition. We present a study which focuses on the acidic rocks, in which we use the low-field anisotropy of magnetic susceptibility (AMS) technique. Systematic microscope examination of the samples collected at 148 AMS stations, and considerations based on mineral intrinsic magnetic susceptibilities reveal magnetic mineralogy and anisotropy controlled by magnetite. Macro- to micro-scale petrofabrics and magnetic fabrics indicate that the acidic rocks, which intruded after deposition of the underlying cumulate series, were also aAected by the syn- to post-magmatic folding event which gave rise to the BKSK syncline. In absence of any recognized plate-scale regional stress field, it is suggested that this folding results from a gravity-induced subsidence linked to the final stage of diapiric emplacement of huge neighbouring anorthosite plutons. This model is mainly supported by the structural pattern in the BKSK, which reveals a convergent flow

The Tellnes ilmenite deposit (Rogaland, South Norway): magnetic and petrofabric evidence for emplacement of a Ti-enriched noritic crystal mush in a fracture zone

Abstract The Tellnes ilmenite deposit, a world class titanium deposit, occurs in the Ana–Sira anorthosite (Rogaland anorthosite province, South Norway). It is mainly made up of an ilmenite-rich norite that has been previously interpreted as injected in a crystal mush state, in a weakness zone of the enclosing anorthosite. This emplacement mechanism has produced a faint orientation in the ore due to the flow of the mush. The internal flow structure of the orebody is studied here using the low-field anisotropy of magnetic susceptibility (AMS) method. Partial anhysteretic remanent magnetization (pAARM) indicates that coarse magnetite is the main mineral responsible for the magnetic fabric. Parallelism of the magnetic fabric with the shape-preferred orientation of the ore-forming minerals is checked using image analysis (IA) from oriented sections (intercept method). Interpretation of the AMS data verified by pAARM and IA, provides information on the magmatic foliation and lineation. Emplacement flow of the ilmenite norite crystal mush occurred in the direction of the orebody, parallel to its walls, and with an average SE 18° plunge. The feeder zone was likely situated below a network of veins on the SE end of the orebody. The sickle-shaped outcrop of the deposit suggests a transcurrent, dextral opening of a WNW–ESE-striking weakness zone across the anorthosite pluton.

Anisotropie de la susceptibilité magnétique dans une intrusion composite de la suite charnockitique: l'apophyse du massif stratiforme de Bjerkreim-Sokndal (Rogaland, Norvège méridionale)

Deformation in the Bjerkreim-Sokndal layered intrusion apophysis was developed continuously from magmatic to solid state. The tectonic setting dictating these deformations was dominated by anorthosite diapirism. An anisotropy of magnetic susceptibility (AMS) study has been performed in the northern part of this apophysis. It shows that titanomagnetite orientation is governing the magnetic fabrics. It confirms the foliation structures visible in the field and, moreover, clarifies the linear structures. The AMS scalar parameters specify the deformation symmetry and put forward a correlation between the intensity of the deformation which is observed in the field and that revealed by AMS.