Characterisation, origin, petrophysical properties and the role of fluids during high-grade metamorphism of graphite (Lofoten-Vesterålen Complex, Norway)

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 (En_69-74) and/or clinopyroxene (En_33-53Fs_1-14Wo_44-53). 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 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 -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), 1279

Engvik 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.12545

Rodinov A et al. (2013) Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey at Langøya in Vesterålen, Nordland. NGU Report 2013.044