Ane K. Engvik

Graphite formation in deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten-Vesterålen Complex, northern Norway. Regionally distributed graphite zones are hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite, quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69-74) and/or clinopyroxene (En33-53Fs1-14Wo44-53). Although graphite is usually described in pelitic rocks or as vein deposits in the granulite facies rocks, we document graphite in assemblage with metamorphic orthopyroxene.Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810-835 °C and 0.73-0.77 GPa; Zr-in-rutile thermometry 726-854°C. COH-fluids stabilise graphite and orthopyroxene; high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u. indicate importance of fluids during metamorphism.Stable isotopic δ13Cgraphite in the graphite schist is -38 to -17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = -8.7‰ to -9.5‰ and heavier values for δ13Cgraphite = -11.5‰ to -8.9‰. δ18Ocalcite for marble shows lighter values ranging -15.4‰ to -7.5‰ (Engvik et al. 2023). We interpret the graphite origin as organic carbon accumulated in sediments contemporaneous with the Early Proterozoic global Lomagundi-Jatuli isotopic excursion, while an isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.The high-ordered graphite (< modality 39%) and biotite with a strong-preferred orientation defines the well-developed foliation. Increased graphite content resulted in high-conductivity zones with a contrast to the host low-conductive crust (Rodinov et al. 2013; Engvik et al. 2021). Enrichment of graphite resulted in zones with strong schistosity and a sharp strain gradient towards host massive granulite gneiss. The presence of graphite causes strain localisation in the granulite facies crust, reducing crustal strength and may thereby influence continental architecture and evolution of collision zones.References:Engvik AK et al. (2023) Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway). Minerals 13(10), 1279Engvik AK et al. (2021) The control of shear-zone development and electric conductivity by graphite in granulite: An example from the Proterozoic Lofoten-Vesterålen Complex of northern Norway. Terra Nova, https://doi.org/10.111/ter.12545Rodinov A et al. (2013) Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey at Langøya in Vesterålen, Nordland. NGU Report 2013.044

The tectonic status, the mode of Caledonian deformation and the transport distance of the Precambrian units on the Fennoscandian Shield that constitute the so-called tectonic windows within the realm of the Scandinavian Caledonian nappe pile have long been a subject of dispute. It has been argued for several hundred kilometres of eastward transport for some of these tectonic units, whereas parautochthonous to autochthonous positions have also been suggested. In this contribution, geophysical data have been studied to shed light on this controversy. A dominant regional NNW–SSE to north–south structural grain of Precambrian origin extends beneath the Caledonian nappes. Indeed, similar structural patterns appear in several of the tectonic windows within the orogen. However, magnetic anomalies in southern Norway and the Nordland area show an apparent c. 90° counterclockwise rotation into the Caledonian trend along a line from Nordfjord in western Norway to Senja in northern Norway, whereas the anomalies in Finnmark (northernmost Norway), seem to be rotated clockwise along a line from Kvænangen to Porsangerfjorden. We suggest that this pattern extending through the Caledonides likely represents a boundary between two crustal domains with different Caledonian reworking. The eastern domain demonstrates less extensive modification during the Caledonian continent–continent collision. Some thrusts within basement windows in the eastern domain accommodated transport of a few tens of kilometres, revealing a parautochthonous status for most of these windows. Interpretations of aeromagnetic and gravity data have shown that the Caledonian nappes occurring along detachments extend to a maximum depth of c. 9 km below sea-level. The thickest nappe piles occur along the late Caledonian detachments. The eastern basement domain with minor Caledonian deformation has a thinner nappe stack (up to 4–5 km below). Our combined interpretations also show the need to modify the tectonostratigraphy of the northern Scandinavian Caledonides. The Lower Laksefjord Nappe in Finnmark, for instance, is interpreted as a parautochthonous window into the Precambrian basement and is not part of the Laksefjord Nappe Complex that has been transported hundreds of kilometres.

Fluid infiltration into Proterozoic and Early Palaeozoic dry, orthopyroxene-bearing granitoids and gneisses in Dronning Maud Land, Antarctica, has caused changes to rock appearance, mineralogy, and rock chemistry. The main mineralogical changes are the replacement of orthopyroxene by hornblende and biotite, ilmenite by titanite, and various changes in feldspar structure and composition. Geochemically, these processes resulted in general gains of Si, mostly of Al, and marginally of K and Na but losses of Fe, Mg, Ti, Ca, and P. The isotopic oxygen composition (δ¹⁸O_SMOW = 6.0‰–9.9‰) is in accordance with that of the magmatic precursor, both for the host rock and infiltrating fluid. U-Pb isotopes in zircon of the altered and unaltered syenite to quartz-monzonite indicate a primary crystallization age of 520.2 ± 1.0 Ma, while titanite defines alteration at 485.5 ± 1.4 Ma. Two sets of gneiss samples yield a Rb-Sr age of 517 ± 6 Ma and a Sm-Nd age of 536 ± 23 Ma. The initial Sr and Nd isotopic ratios suggest derivation of the gneisses from a relatively juvenile source but with a very strong metasomatic effect that introduced radiogenic Sr into the system. The granitoid data indicate instead a derivation from Mid-Proterozoic crust, probably with additions of mantle components.

Sillimanite-bearing gneisses in the Romsdal region of the Western Gneiss Region (south Norway) have been investigated to document the presence, formation, composition and petrological evolution of the sillimanite-bearing assemblages. Sillimanite is found in augen gneiss, as nodular gneiss, and in well-foliated sillimanite–mica gneiss. Lenses and layers of eclogite occur within the gneiss units. The sillimanite-bearing gneisses are heterogranular and dominated by quartz, plagioclase (An_29–41), K-feldspar and biotite (Mg# = 0.48–0.58; Ti = 0.16–0.36 a.p.f.u.), with variable amounts of white mica (Si = 6.1–6.3). K-feldspar occurs as porphyroclasts in augen gneiss, and garnet constitutes resorbed porphyroblasts. Garnet (Alm_46–56Sps_24–36Prp₁₀₋₂₀Grs_4–6; Mg# = 0.22–0.29) shows rimward-decreasing Mg#, together with a smaller grossular decrease and a marked spessartine increase up to Sps₃₆. The foliation is defined by crystal-preferred-orientation micas, elongation of shape-preferred-orientation coarse K-feldspar phenocrysts and a modal banding of phases. Sillimanite occurs as coarse orientation-parallel matrix porphyroblasts, as finer grains and as fibrolitic aggregates. Quartz constitutes coarser elongated grains and monomineralic rods. Pseudosection modelling suggests that the peak-metamorphic mineral assemblage of garnet–sillimanite–feldspar–biotite–quartz–ilmenite–liquid equilibrated at temperatures up to 750 °C and pressures of 0.6 GPa. Subsequent retrogression consumed garnet. Mineral replacement and melt crystallization involved sillimanite, white mica, K-feldspar and quartz. The results document a metamorphic retrogression of the sillimanite gneisses in accordance with the presence of remnants of eclogites and high-pressure granulites in this northwestern part of the Western Gneiss Region.

Central Dronning Maud Land (cDML) is part of the late Mesoproterozoic Maud Belt in East Antarctica, which was metamorphosed and deformed during the Ediacaran–Cambrian Gondwana assembly. Here we study high-pressure (HP) mafic rocks in Gjelsvikfjella, cDML, which occur as lenses and pods transposed in highly strained, upper amphibolite-facies gneisses. We present a P–T–t history for the HP rocks based on mineral assemblages, reaction textures and new U–Pb zircon data. Relics that indicate an early HP granulite-facies stage have been identified in anhydrous garnet–clinopyroxene rocks. The peak-pressure assemblage was plagioclase-free and contained garnet, titanite, clinopyroxene and quartz. The HP assemblage has been extensively overprinted by lower-pressure phases and exhibits a variety of symplectite and corona textures that record the post-peak-pressure evolution of the rocks. Decompression and heating in the granulite-facies field resulted in the replacement of titanite by ilmenite–clinopyroxene symplectite, formation of clinopyroxene–plagioclase intergrowths and resorption of garnet by plagioclase–clinopyroxene kelyphite. Formation of late orthopyroxene in symplectites and kelyphites demonstrates that the P–T evolution entered the medium-pressure granulite-facies field. The peak metamorphic stage was followed by retrograde cooling into the amphibolite facies. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicate Mesoproterozoic protolith ages (1150–1000 Ma) and Ediacaran–Cambrian metamorphic reworking at ca. 568 and ca. 514 Ma. The HP granulites were formed and exhumed during a clockwise P–T evolution related to continental collision during Gondwana amalgamation, followed by post-collisional extension and orogenic collapse.

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (69–74) and/or clinopyroxene (En_33–53Fs_1–14Wo_44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ¹³C_graphite in the graphite schist is −38 to −17‰; δ¹³C_calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ¹³C_calcite = −8.7‰ to −9.5‰ and heavier values for δ¹³C_graphite = −11.5‰ to −8.9‰. δ¹⁸O_calcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

The Kongsberg and Bamble lithotectonic domains of SE‐Norway are known as classical Precambrian high‐grade metamorphic terrains. The area has undergone extensive metasomatism with formation of albitites and scapolite‐rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low‐grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite‐pumpellyite and zeolite facies. Structurally, the low‐grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low‐grade minerals are part of larger fracture systems. The low‐grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K‐feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low‐grade minerals occur. As albite, K‐feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low‐grade minerals, they probably belong to the low‐grade assemblage and some of the albitization in the region presumably occurred at low‐grade conditions. Alteration of olivine (Fo69) at low‐grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low‐grade assemblages in south‐western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar‐Ar dating of K‐feldspar from the low‐grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low‐grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro‐fractures and porosity in the adjacent minerals that enhance fluid flow and low‐grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the opening of the larger, horizontally oriented fracture systems needed to transport the fluid over a distance of several kilometres was also the results of displacive reactions remains unknown. The low‐grade metamorphism and metasomatism formed in the shoulder of the Oslo rift and may have contributed to its uplift.

Tafoni are a type of cavernous weathering that is found in a variety of rock types and locations around the world. Tafoni have been documented in a number of climatic zones ranging from hot and cold deserts to moist coastal environments. Despite the widespread distribution of tafoni, the major processes controlling tafoni weathering are not well understood and are still a matter of discussion. This study addresses the frequent distribution of well-developed tafoni in the cold, arid environment of the inland mountain range of central Dronning Maud Land, Antarctica. The aim is to document and characterize the nature of tafoni present in Gjelsvikfjella (2 degrees E) eastward to Filchnerfjella (8 degrees E) and to discuss formation processes. The cavities occur in groups and are typically spherical to oval shaped. They range in diameter and depth from 1 dm up to 1.5 m. The cold, arid environment of this region favors mechanical weathering mechanisms such as freeze-thaw actions and wind abrasion. Furthermore, the structural, textural, and mineralogical properties of the parent rock can potentially have a strong control on weathering and cavity development. Observed tafoni are typically formed in massive granitoid intrusives and granitic gneisses and migmatites. Chemical dissolution of pyroxene to iddingsite and radiation from rare earth element-bearing accessory minerals cause microfracturing, which facilitates freeze-thaw actions and accordingly enhances the weathering.