Graphite and quartz petrofabrics: Examples from the Ediacaran black quartzites of the Ossa-Morena Zone (SW Iberia)

•First lattice-preferred orientation data presented for naturally deformed graphite.•Graphite concentration constrains quartz intracrystalline plasticity partitioning.•Quartz intracrystalline slip systems relate to syn-metamorphic strain gradients.•Graphite facilitated rock deformation and likely increased electrical conductivity.

We study with the Electron Back-Scattered Diffraction (EBSD) technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts, a common lithotype in Ediacaran supracrustals of the west European Cadomian orogen. Currently they are black quartzites with a planolinear tectonite structure. Microscopically they present a sub-mm-scale alternation of coarse- and fine-grained, dynamically recrystallized quartz bands. We attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non-coaxial strain regimes and moderate to elevated temperatures (400-650 °C). Lattice-preferred orientations of quartz [c] and < a > axes are geometrically related to the external reference frame provided by foliations and lineations. We also identify the involvement of (0001), {r}, {m}, and {m}[c] quartz intracrystalline slip systems. Deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm- to cm-thick in adjacent coarse- and fine-grained bands. Medium- to high-T plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite-facies metamorphism of the country rocks.We also present the first specific study published so far on natural deformation graphite lattice-preferred orientation. Graphite inclusions (as well as those of mica) exhibit mineral shape fabrics that suggest operation of (0001) slip. However, EBSD measurements also record fabrics suggestive of {m} slip. In spite of a rather small volumetric proportion, graphite spatial organization at increased shear strains facilitated ductile deformation. If a graphite network is established in the rock, it can potentially increase rock electrical conductivity, thus accounting for mid and lower crust anomalous electric conductivity.

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