Graphite in granulite - characterization, origin, role of fluids and consequences for rheology

In a combined geological, petrological and isotopic study from the Lofoten-Vesterålen Complex, Norway, graphite is documented formed in the deep Proterozoic crust. Graphite schist is hosted in sequences of banded gneisses dominated by orthopyroxene-bearing quartzofeldspatic gneiss, interlayered with horizons of marble, calcsilicates and amphibolite. The schist displays a strong foliation and has a major content of graphite up to a modality of 39%. Quartz and plagioclase (Ab_47-93An_5-52), pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.), and K-feldspar (Ab_1-8Kfs_92-99) or perthite (Ab_35-64An₃Kfs_50-62) are additional major phases. Pyroxene is present either as orthopyroxene (En_69-74Fs_26-29; Mg#=0.70-0.74), as clinopyroxene (En_33-53Fs_1-14Wo_44-53; Mg#=0.70-0.97), or both. Pseudosection modeling of the plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage constrains its stability field to pressure-temperature conditions of 810-835 °C and 0.73-0.77 GPa. Zr-in-rutile also supports a temperature of formation of 740-870°C.

Stable isotopic δ¹³C in graphite schist shows values from -38 to -17‰ while δ¹³C values of marbles range from +3‰ to +10‰. Mixed graphitic and calcite carbon samples give lighter values for the calcite (δ¹³C_calcite = -8.65‰ to -9.52‰) and heavier values for graphite (δ¹³C_grapite = -11.50‰ to -8.88‰) compared to the “pure” samples. δ¹⁸O for marble shows relatively light values for calcite ranging from -15.44‰ to -7.53‰ reflecting metamorphic and hydrothermal processes. From the stable C-isotopes we interpret the graphite origin as organic carbon accumulated in sediments contemporaneous with the Early Proterozoic global Lomagundi-Jatuli isotopic excursion.

From petrography and mineral composition, we deduce the reaction equations producing and consuming H₂O- and CO₂-fluids leading to the stabilisation of graphite and orthopyroxene. The high Mg#-ratio of biotite and pyroxenes is an indication of metasomatism, and together with a high Cl-content of apatite up to 2 a.p.f.u. show the importance of fluids during the high-grade formation of graphite.

The enrichment of graphite resulted in zones with strong schistosity and a sharp strain gradient towards host massive granulite gneiss; High-ordered graphite occurs as euhedral “flakes” (i.e., flake graphite) of fine- to medium grain size, with a strong preferred crystal orientation forming the well-developed foliation together with the crystal preferred orientation of biotite. The presence of graphite reduces crustal strength and causes strain localisation in the granulite facies crust.