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

<p>In a combined geological, petrological and isotopic study from the Lofoten-Vester&#229;len 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 &#176;C and 0.73-0.77 GPa. Zr-in-rutile also supports a temperature of formation of 740-870&#176;C.</p><p>Stable isotopic <em>&#948;</em>¹³C in graphite schist shows values from -38 to -17&#8240; while <em>&#948;</em>¹³C values of marbles range from +3&#8240; to +10&#8240;. Mixed graphitic and calcite carbon samples give lighter values for the calcite (<em>&#948;</em>¹³C_calcite = -8.65&#8240; to -9.52&#8240;) and heavier values for graphite (<em>&#948;</em>¹³C_grapite = -11.50&#8240; to -8.88&#8240;) compared to the &#8220;pure&#8221; samples. <em>&#948;</em>¹⁸O for marble shows relatively light values for calcite ranging from -15.44&#8240; to -7.53&#8240; 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.</p><p>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.</p><p>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 &#8220;flakes&#8221; (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.</p>