Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

• Published in: Minerals
• Main Authors: Ane K. Engvik, Håvard Gautneb, Pål Tore Mørkved, Janja Knežević Solberg, Muriel Erambert
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2023
• Subjects: C-isotope; fluid; granulite; graphite; Vesterålen; Mineralogy; QE351-399.2; Lofoten; Northern Norway
• Online Access: https://doi.org/10.3390/min13101279; https://doaj.org/article/de460379b168461b8abb4e17b6ccdef5

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 (<modality 39%), 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 33–53 Fs 1–14 Wo 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 δ 13 C graphite in the graphite schist is −38 to −17‰; δ 13 C calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ 13 C calcite = −8.7‰ to −9.5‰ and heavier values for δ 13 C graphite = −11.5‰ to −8.9‰. δ 18 O 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.