Kåre Kullerud

Cl-scapolite, Cl-amphibole, and plagioclase equilibria in ductile shear zones at Nusfjord, Lofoten, Norway: implications for fluid compositional evolution during fluid-mineral interaction in the deep crust

Abstract Cl-rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived Cl-bearing fluid in a gabbroanorthosite of the Flakstadoy Basic Complex, Lofoten, Norway. Amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals (orthopyroxene, clinopyroxene, biotite, and ilmenite) and plagioclase and internally in the grains of primary igneous plagioclase. The secondary minerals show large compositional variations on thin-section scale. The Cl content of scapolite varies between 0.3 and 0.95 apfu (atoms per formula unit), whereas amphibole shows Cl concentrations from 0 to 1.5 apfu. The primary igneous plagioclase (An50-An60) underwent extensive recrystallization and alteration during the fluid–rock interactions. Secondary plagioclase shows compositions in the range from An20 to An55. In general, plagioclase did not equilibrate with the fluid phase during alteration, due to the sluggishness of the cation exchange reactions between plagioclase and fluid. Occasionally, however, equilibrium among plagioclase, scapolite, and the fluid phase was attained. The compositional variations of amphibole, scapolite, and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios aCl−/a(CO3)2− and aCl−/aOH−. The large compositional variations of the minerals on thin-section scale thus indicate steep gradients in the fluid activity ratios aCl−/a(CO3)2− and aCl−/aOH−. The activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain-boundary fluid during the formation of hydrous silicates. Scapolite and the most Cl-enriched amphibole formed in equilibrium with an evolved fluid phase, enriched in Cl and CO2.

Pillow lavas as protoliths for eclogites: evidence from a late Precambrian-Cambrian continental margin, Seve Nappes, Scandinavian Caledonides

Pillow lavas containing an eclogitic mineral assemblage, locally with glaucophane/crossite, are described from the Tsäkkok Lens within the Seve Nappes, north-central Scandinavian Caledonides. Critical primary relationships to the host metasedimentary rocks which are dominated by marble and quartz-garnet-phengite schist are preserved. It is argued that these clastic rocks, and the ongoing igneous activity in the Tsäkkok Lens, are late Precambrian to Cambrian in age and that these rocks were deposited/emplaced along the outermost part of a continental margin related to the continent Baltica.Variations in the whole-rock chemistry of the eclogitic rocks within the Tsäkkok Lens suggest that most elements were mobile during post-extrusive/intrusive evolution. Only Ti, P and Y show significant magmatic trends in element vs Zr plots. Discriminant diagrams employing the elements Ti, P, Zr, Y, Nb and Cr suggest a tholeiitic protolith with a tendency towards enriched mid-ocean ridge basalt (E-MORB) affinity characterized by a relatively low Zr/Nb ratio. A similarity to metabasic rocks occurring in other Seve thrust sheets but lacking evidence for high-P metamorphism is apparent. A common origin for all these rocks is inferred. The Tsäkkok eclogites formed by ‘in situ’ high-P metamorphism and provide further evidence for subduction of continental crustal material down to mantle depths. This subduction event has been related to collision of Baltica with an outboard are system during the late Cambrian to early Ordovician.

Chlorine, titanium and barium-rich biotites: factors controlling biotite composition and the implications for garnet-biotite geothermometry

Barium-, Cl- and Ti-rich biotite occurs together with garnet, plagioclase and amphibole within narrow shear zones in 1800 Ma old noritic granulites in the Flakstadøy Basic Complex, Lofoten, north Norway. The granulite facies assemblage, plagioclase, clinopyroxene, orthopyroxene, biotite and ilmenite, was replaced by an amphibolite facies mineral assemblage including Ba-, Cl- and Ti-rich biotite during ductile deformation. Biotite shows complex compositional variations with respect to the contents of Ba, K, Cl, Ti, Al, Fe, Mg and Si. There are correlations between Si, AlIV, K, Ba and Cl and between AlVI and Ti. Titanium and Cl are uncorrelated. The Fe and Mg are correlated to both Cl and Ti. Multivariate analysis shows that most of the compositional variation of biotite can be described by two exchange reactions. This indicates that most of the variation in biotite composition was controlled by two chemical variables of the system. The content of the first exchange component (Ti1.0 Fe0.6 Al-1.1VIMg-0.8) in biotite can be related to the original distribution of Ti-bearing minerals in the igneous protolith. The content of the second exchange component (Al0.4IVFe0.8 Ba0.5 Cl1.0 Si-0.4 Mg-1.0 K-0.5 OH-1.0) is related to compositional variations of an externally derived Ba- and Cl-bearing fluid in equilibrium with biotite.The initially low Cl-content of the externally derived fluid was increasing during bioite forming reactions, because OH was preferentially incorporated, relative to Cl, into biotite. Continued hydration/chloridisation reactions resulted in a gradual consumption of the free fluid phase, resulting in local fluid-absent conditions. The composition of biotite reflects the composition of the last fluid in equilibrium with the mineral, i.e. the composition of the fluid immediately before the grain boundaries were fluid-undersaturated. Thus, the variations in biotite composition reflect how the fluid was gradually consumed throughout the shear zone rock. The correlations between Fe, Mg, Ba, K and Cl can be attributed to differences between the structure of the crystal lattices and the sizes of the cation sites of OH-phlogopite and Cl-annite. The dependency of the Fe/Mg ratios of biotite on the Cl-and Ti-content has a strong effect on the Fe−Mg partitioning between biotite and garnet. The relationship between lnKD, XTiBtand XClBtcan be expressed by the regression equation: lnK′D=-1.82+2.60XTiBt+5.67XClBt

Early Caledonian tectonothermal evolution in outboard terranes, central Scandinavian Caledonides: new constraints from U-Pb zircon dates

Early Caledonian deformation in an outboard terrane (Storfjället) in the central Scandinavian Caledonides is constrained to the time-period Arenig to Caradoc on thebasis of U-Pb zircon age determinations. A trondhjemite clast in a conglomerate belonging to a supracrustal sequence that was affected by the early deformational event yields an age of 489+10–5Ma. A granite intruded after this event defines an age of 445+24–6 Ma. The clast is similar in age to a trondhjemite intrusion in a contiguous terrane (Gjersvik) dated at 483+5–3 Ma and the granite age is in agreement with a Rb–Sr whole-rock isochron of 438 ± 6 Ma determined in a previous study. The three new ages provide further evidence for the separation of igneous events in outboard terranes in the Scandinavian Caledonides into older (Tremadoc to Arenig) and younger (Late Ordovician to Early Silurian) episodes. More critically, they argue against simple correlation of early Caledonian deformational events in outboard terranes with the Late Cambrian to Early Ordovician tectonometamorphic history along the margin of the continent Baltica (the so-called Finnmarkian orogenic phase). This age disparity provides support for significant spatial separation of the Storfjället terrane from Baltica during the early–mid Ordovician and an influence of the Taconic orogenic event on this outboard terrane is proposed.

Occurrence and origin of Cl-rich amphibole and biotite in the Earth’s crust — implications for fluid composition and evolution

Analyses of Cl-bearing amphiboles and biotites from more than 20 occurrences around the world have been reviewed. The Cl-content of amphibole ranges up to about 6 wt%, while the most Cl-enriched biotite contains about 7 wt% Cl. For the individual occurrences of amphibole and biotite, systematic compositional variations, correlated to the Cl-contents of the minerals can be observed. It is argued that these variations were governed by variations in the fluid activity ratio aCl-/aOH- during mineral growth. The most Cl-enriched amphiboles and biotites formed in equilibrium with highly saline solutions.

Oriented inclusions in apatite in a post-UHP fluid-mediated regime (Tromsø Nappe, Norway)

We report pyrrhotite, anhydrite and dolomite crystal rods in fluorapatite occurring in silicate-bearing carbonate rocks associated with UH P eclogites in the Tromso Nappe of the Scandinavian Caledonides in Norway. The apatite-rich rock (up to 10 vol. %) is composed of Mg-rich calcite-dolomite exsolutions, almandine-grossular garnet, low-jadeite clinopyroxene, magnesiohornblende, phlogopite, and accessory minerals represented mainly by zircon, Fe-Ti oxides and allanite. Fluorapatite occurring as euhedral crystals in the carbonate matrix and as inclusions in garnet and clinopyroxene shows up to 45 mol. % of the hydroxylapatite component, traces of CO 3 2− , probably CN − and small amounts of the britholite and ellestadite components. Pyrrhotite occurs as crystallographically oriented rods parallel to the c axis of the host hydroxyl-bearing fluorapatite either as a dense trellis or in the form of scarce inclusions. Precipitation of pyrrhotite in the fluorapatite was probably facilitated by a volatile sulphur phase ( e.g ., H 2 S), which was enclosed within the apatite nano-channels and interacted with Fe in apatite. Anhydrite and dolomite rods have also been identified in the apatite, pointing to the presence of HCO 3 − in the fluids. The anhydrite is also trapped by exsolved dolomite from calcite in the carbonate matrix. Crystallisation of anhydrite, and probably also the associated pyrrhotite, at about 550–650°C was deduced from calcite–dolomite thermometry. At these amphibolite-facies, post-UH P conditions rapid pyrrhotite precipitation in the host apatite is presumed. Relaxation of the fluorapatite structure in the a -axis direction during decompression facilitated the formation of the oriented inclusions in apatite.

Results of U–Pb (LA–ICPMS) dating of detrital zircons from metaterrigenous rocks of the basement of the North Kara basin

The U–Pb (LA–ICPMS) age was established for detrital zircons from the oldest, intensely deformed metasedimentary complexes without reliable fossils’ record, which represent the folded basement of the North Kara basin and are exposed on Bol’shevik and Troinoi islands (Severnaya Zemlya and Izvestii TSIK archipelagoes, respectively). Our data suggest the common evolution of the Cambrian–Ordovician sequences exposed on the islands of both archipelagoes.

Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., ∼50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5–2.8 GPa and ∼650 °C. Lu-Hf geochronology yields 399 ± 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to ∼90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of ∼90 km, and (2) it may be exhumed during ongoing collision and crustal shortening.

Solid solution between potassic alkali amphiboles from the silica-rich Kvaløya lamproite, West Troms Basement Complex, northern Norway

Alkali amphibole of rare compositions occurs as a rock-forming mineral in a high-Si phlogopite lamproite from Kvaloya, northern Norway. The amphibole typically occurs as small grains forming irregular and rosette-shaped aggregates in a matrix dominated by Fe-rich K-feldspar and quartz. Amphibole shows compositions ranging between the three limiting compositions: A : A,B ( K 1.01 Na 1.99 ) C ( Na 0.26 Mg 1.58 Mn 0.03 Fe 2 + 0.91 Fe 3 + 1.6 Ti 0.47 □ 0.13 ) T Si 8 O 22 W [ F 0.97 O 1.03 ] B : A,B ( KNa 2 ) C ( Na 0.04 Mg 1.04 Mn 0.22 Fe 2 + 0.65 Fe 3 + 2.07 Ti 0.25 □ 0.7 ) T Si 8 O 22 W [ F 0.68 Cl 0.01 O 0.13 ( OH ) 1.18 ] C : A,B ( K 0.9 Na 2.1 ) C ( Na 0.04 Mg 3.54 Mn 0.02 Fe 2 + 0.28 Fe 3 + 1.04 Ti 0.03 □ 0.02 ) T Si 8 O 22 W [ F 1.34 O 0.06 ( OH ) 0.6 ] Composition C shows significant content of fluoro-potassic-magnesio-arfvedsonite, while composition A is a Fe 2+ , Fe 3+ and C Na rich variety of potassic-obertiite. Composition B is characterized by an exceptional high value of C □. It is emphasized that the presence of C □ and C Na in amphibole needs to be confirmed by other methods. The relationship between W O 2− , C □,Ti 4+ and Fe 3+ of amphibole can be expressed by the following exchange operators, choosing potassic-magnesio-arfvedsonite [KNa 2 (Mg 4 Fe 3+ )Si 8 O 22 (OH) 2 ] as the additive component: Ti 4 + Mg 2 + − 1 H + − 2 Fe 3 + Mg 2 + − 1 H + − 1 Ti 4 + □ Mg 2 + − 2 Fe 3 + 2 □ Mg 2 + − 3 The two first exchange operators result in deprotonation of OH, while the two others result in the formation of vacancies on the C sites. The presence of amphibole both in the lamproite and in the adjacent fenitized granite suggests that the mineral formed during reactions between rock and fluids derived from the volatile-rich lamproite magma. Possibly, amphibole core (composition A and B), formed in equilibrium with the fluid phase during crystallization of the melt, while amphibole rim (composition C) formed during subsequent mineral-fluid reactions. Presence of hematite in the lamproite matrix in addition to oxo-amphibole indicates that the rock formed during highly oxidizing conditions.

Unique Accessory Ti-Ba-P Mineralization in the Kvalöya Ultrapotassic Dike, Northern Norway

Unusual ultrapotassic dikes were recently found on the Kvalöya Island in Northern Norway. The dikes crosscutting granites 1.8 Ga in age are 0.1–1.0 m thick and consist of phlogopite phenocrysts in a fine-grained groundmass of K-magnesioarfvedsonite, orthoclase, apatite, and secondary chlorite. According to the composition of the rock-forming minerals (4.5–6.0 wt % K2O and 0.7–3.5 wt % TiO2 in magnesioarfved-sonite, 1.6–3.6 wt % FeO in orthoclase, 9.2–10.7 wt % Al2O3 and 2.1–2.6 wt % TiO2 in phlogopite) and its bulk chemical composition (K/Na = 2.3–2.9, K/Al = 1.0–1.2, (Na + K)/Al = 1.4–1.7, Mg# V = 65–73, (La/Yb)n = 100–140, 3.2–4.0 wt % TiO2, 0.55–1.47 wt % BaO, 2.5–3.0 wt % P2O5, 2650–3000 ppm Zr, 900–1260 ppm REE total, 2300–2500 ppm Sr), the rock corresponds to lamproite of the transitional type. The unique chemical composition of the rock resulted in uncommon Ti-Ba-P accessory mineralization, including baotite Ba4(Ti,Nb)8Si4O28Cl (up to 5 vol %), Sr-apatite (5–7 vol %), and previously unknown Na-Mg-Ba phosphate. Baotite forms anhedral elongated and isometric grains 10–500 μm in size. It is characterized by low Nb (0.03–0.05 f.c.); admixtures of K (0.04–0.12 f.c.) and Sr (0.04–0.07) replacing Ba and Fe (0.01–0.03 f.c.); and Al (0.03–0.04 f.c.) substituting Ti. Euhedral elongated zonal apatite crystals are extremely enriched in SrO (8–12 wt %) and REE2O3 + Y2O3 (6–9 wt %) in the marginal zone. Na-Mg-Ba phosphate occurs as prismatic grains 10–100 μm in size. The atomic ratio of its major cations Na: Mg: Ba: P ∼ 2: 1: 1: 2 corresponds to the conventional formula Na2MgBa(PO4)2; the mineral contains Sr, Mn, Fe, Ca, Si, and Al admixtures.