Erling J. Krogh Ravna

Kyanite eclogite thermobarometry and evidence for thrusting of UHP over HP metamorphic rocks, Nordøyane, Western Gneiss Region, Norway

Abstract Quantitative estimates of metamorphic conditions are combined with previously reported structural analysis to develop the thermotectonic evolution of two separate lithotectonic units metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. A structurally higher plate, regionally correlated with the Blåhø Nappe, contains kyanite eclogites associated with microdiamond-bearing kyanite-garnet-graphite gneiss on the north coast of Fjørtoft and correlated with kyanite eclogites at Nogva, Flemsøya. The kyanite eclogites, containing the assemblage kyanite- garnet-omphacite-coesite (now polycrystalline quartz pseudomorphs) ± zoisite ± phengite, yield conditions of 820 °C and 34-39 kbar at Fjørtoft and of 820 °C and 30-36 kbar at Nogva, best characterized by two recently recalibrated geothermobarometers. The conditions at Fjørtoft overlap the diamond-graphite phase boundary and represent the first quantitative petrologic determination of UHP diamond-forming conditions in crustal rocks outside the Dabie Mountains, China and the Kokchetav Massif, Kazahkstan. In a structurally lower plate, eclogitized mylonite with the assemblage kyanite-garnet-omphacite-quartz-oligoclase, produced from the mid-Proterozoic Haram Gabbro that has intruded diorite country rock, yields 780 °C and 18 kbar. This result agrees with other normal HP estimates by Mørk (1985) from partially to completely eclogitized gabbro in the same unit on Flemsøya. We propose that the UHP plate reached a maximum depth of 125 km and then experienced 65 km of exhumation during top-southeast thrusting that brought it into contact with the HP plate. Following this, both plates were exhumed together until reaching a depth of 37 km where they experienced extensive amphibolite-facies re-equilibration and top-west or left-lateral shearing. Temporal details of these histories were determined by monazite U-Th-Pb geochronology described in a companion paper

Distribution of Fe2+ and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer

Abstract Multiple regression analysis of a compilation of the Fe2+–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln KD(Fe2+/Mg)Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln KD(Fe2+/Mg)Grt–Hbl–P–T compositional relationship (r2=0.93): T (° C )= 1504+1784(X Ca Grt +X Mn Grt ) ln K D ( Fe 2+ / Mg ) Grt – Hbl +0.720 −273 where K D ( Fe 2+ / Mg ) Grt – Hbl = ( Fe 2+ / Mg ) Grt ( Fe 2+ / Mg ) M 1– M 3 Hbl X Ca Grt = Ca Ca + Mn + Fe 2+ + Mg in garnet X Mn Grt = Mn Ca + Mn + Fe 2+ + Mg in garnet Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.

First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides

The first evidence for ultrahigh-pressure ( P ) metamorphism in the Greenland Caledonides is reported from kyanite eclogites and associated host gneisses on an island in Jokelbugt. Polycrystalline quartz inclusions in garnet and omphacite exhibit palisade quartz rims that are a diagnostic feature of quartz pseudomorphs after coesite, thus providing textural evidence for ultrahigh- P conditions. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + quartz and/or coesite ± phengite confirms the microstructural interpretation of ultrahigh- P metamorphism. Peak pressure and temperature conditions (∼972 °C at 3.6 GPa) are well within the coesite stability field. The host gneisses are more retrograded than the kyanite eclogites and only record high- P conditions of 2.5 GPa at 826 °C; however, garnet contains polycrystalline quartz inclusions with radial fractures, suggesting that the gneisses also were subject to ultrahigh- P conditions. The presence of high- and ultrahigh- P metamorphism along the Laurentian and Baltica margins, the high temperatures recorded by the ultrahigh- P terranes in Greenland and Norway, and the absence of mantle peridotites in Greenland suggest that crustal thickening may have played an important role in the formation of an extensive orogenic root in the Greenland and Scandinavian Caledonides.

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.

Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature (P–T) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

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.

Reply to comment by C. Miller and J. Konzett on “First evidence for ultrahigh‐pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the eastern Alps”

Received 27 June 2005; revised 26 September 2005; accepted 19 October 2005; published 9 December 2005.Citation: Jana´k, M., N. Froitzheim, M. Vrabec, and E. J. KroghRavna (2005), Reply to comment by C. Miller and J. Konzett on‘‘First evidence for ultrahigh-pressure metamorphism of eclogitesin Pohorje, Slovenia: Tracing deep continental subductionin the eastern Alps,’’ Tectonics, 24, TC6011, doi:10.1029/2005TC001875.

Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite

Carbonatites (sensu stricto) are igneous rocks typically associated with continental rifts, being emplaced at relatively shallow crustal levels or as extrusive rocks. Some carbonatites are, however, related to subduction and lithospheric collision zones, but so far no carbonatite has been reported from ultrahigh-pressure (UHP) metamorphic terranes. In this study, we present detailed petrological and geochemical data on carbonatites from the Tromsø Nappe—a UHP metamorphic terrane in the Scandinavian Caledonides. Massive to weakly foliated silicate-rich carbonate rocks, comprising the high-P mineral assemblage of Mg–Fe-calcite 6 Fe-dolomiteþgarnetþomphacitic clinopyroxeneþphlogopiteþ apatiteþ rutileþ ilmenite, are inferred to be carbonatites. They show apparent intrusive relationships to eclogite, garnet pyroxenite, garnet–mica gneiss, foliated calc-silicate marble and massive marble. Large grains of omphacitic pyroxene and megacrysts (up to 5 cm across) of Cr-diopside in the carbonatite contain rods of phlogopite oriented parallel to the c-axis, the density of rods being highest in the central part of the megacrysts. Garnet contains numerous inclusions of all the other phases of the carbonatite, and, in places, composite polyphase inclusions. Zircon, monazite and allanite are common accessory phases. Locally, veins of silicate-poor carbonatite (up to 10 cm across) occur. Extensive fenitization by K-rich fluids, with enrichment in phlogopite along contacts between carbonatite and silicate country rocks, is common. Primitive mantle-normalized incompatible element patterns for the carbonatite document a strong enrichment of light rare earth elements, Ba and Rb, and negative anomalies in Th, Nb, Ta, Zr and Hf. The carbon and oxygen isotope compositions of the carbonatite are distinctly different from those of the spatially associated calc-silicate marble, but also from mantle-derived carbonatites elsewhere. Neodymium and Sr isotope data coupled with the trace element distribution indicate a similarity of the Tromsø carbonatite to orogenic (off-craton) carbonatites rather than to anorogenic (on-craton) ones. U–Pb dating of relatively U-rich prismatic, oscillatory-zoned zircon gives an age of 454 5 6 1 1 Ma. We suggest that VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2403 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 12, 2403–2428 doi: 10.1093/petrology/egy016 Advance Access Publication Date: 27 February 2018