Graphite in granulite - characterization, origin, role of fluids and consequences for rheology
Authors: Engvik Ane K. , Gautneb Håvard , Mørkved Pål Tore , Knezevic Janja , Erambert Muriel , Austrheim Håkon 
Published: March 2022

&lt;p&gt;In a combined geological, petrological and isotopic study from the Lofoten-Vester&amp;#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&lt;sub&gt;47-93&lt;/sub&gt;An&lt;sub&gt;5-52&lt;/sub&gt;), pyroxenes, biotite (Mg# = 0.67-0.91; Ti &lt; 0.66 a.p.f.u.), and K-feldspar (Ab&lt;sub&gt;1-8&lt;/sub&gt;Kfs&lt;sub&gt;92-99&lt;/sub&gt;) or perthite (Ab&lt;sub&gt;35-64&lt;/sub&gt;An&lt;sub&gt;3&lt;/sub&gt;Kfs&lt;sub&gt;50-62&lt;/sub&gt;) are additional major phases. Pyroxene is present either as orthopyroxene (En&lt;sub&gt;69-74&lt;/sub&gt;Fs&lt;sub&gt;26-29&lt;/sub&gt;; Mg#=0.70-0.74), as clinopyroxene (En&lt;sub&gt;33-53&lt;/sub&gt;Fs&lt;sub&gt;1-14&lt;/sub&gt;Wo&lt;sub&gt;44-53&lt;/sub&gt;; 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 &amp;#176;C and 0.73-0.77 GPa. Zr-in-rutile also supports a temperature of formation of 740-870&amp;#176;C.&lt;/p&gt;&lt;p&gt;Stable isotopic &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C in graphite schist shows values from -38 to -17&amp;#8240; while &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values of marbles range from +3&amp;#8240; to +10&amp;#8240;. Mixed graphitic and calcite carbon samples give lighter values for the calcite (&lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;calcite&lt;/sub&gt; = -8.65&amp;#8240; to -9.52&amp;#8240;) and heavier values for graphite (&lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;grapite&lt;/sub&gt; = -11.50&amp;#8240; to -8.88&amp;#8240;) compared to the &amp;#8220;pure&amp;#8221; samples. &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O for marble shows relatively light values for calcite ranging from -15.44&amp;#8240; to -7.53&amp;#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.&lt;/p&gt;&lt;p&gt;From petrography and mineral composition, we deduce the reaction equations producing and consuming H&lt;sub&gt;2&lt;/sub&gt;O- and CO&lt;sub&gt;2&lt;/sub&gt;-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.&lt;/p&gt;&lt;p&gt;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 &amp;#8220;flakes&amp;#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.&lt;/p&gt;