Search Results (2)

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_33–53Fs_1–14Wo_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 δ¹³C_graphite in the graphite schist is −38 to −17‰; δ¹³C_calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ¹³C_calcite = −8.7‰ to −9.5‰ and heavier values for δ¹³C_graphite = −11.5‰ to −8.9‰. δ¹⁸O_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. Full article

Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island. Full article