RETROGRADED ECLOGITE-FACIES PSEUDOTACHYLITES AS DEEP-CRUSTAL PALEOSEISMIC FAULTS WITHIN CONTINENTAL BASEMENT OF LOFOTEN, NORTH NORWAY

Field observations and electron microprobe analyses indicate that pseudotachylites exposed on the Lofoten island of Flakstadøy, north Norway, represent rare examples of deep-crustal paleoseismic faults. The pseudotachylites are restricted in occurrence to the margins of rare eclogite-facies shear zones that cut granulite-facies continental basement rocks. The pseudotachylite veins mostly are sharply truncated by the eclogite shears but some have been sheared and folded into them, implying pre- to syn-kinematic injection. Both the eclogite shear zones and the pseudotachylites were variably retrograded during Caledonian amphibolite-facies metamorphism. Electronmicroprobe analysis and back-scatter electron imaging document that the pseudotachylite matrix comprises microlites of garnet(Gr25-30, Py15-19, and Al54-58), orthopyroxene(En61-64), low-Na clinopyroxene(Jd6), amphibole (ferroan pargasite), with or without K-feldspar, quartz, biotite, and various Fe- and Fe-Ti opaques. The cogenetic field association and pre- to syn-kinematic timing requires that the pseudotachylites followed the same retrograde metamorphic pathway (from high- to moderate-pressure conditions) as did the coarser-grained, less-retrograded eclogite shears. This is documented by: distinct, shared mineral chemical patterns; omphacite replacement by clusters or symplectites of low-Na clinopyroxene (Jd6) and oligoclase/andesine (An20-36); and kyanite, orthopyroxene, Na-Ca clinopyroxene inclusions in garnet. Textures preserved in the pseudotachylite matrix imply crystallization directly from the frictional melt. For example: dendritic garnets, similar in appearance, size, and composition to those from eclogite pseudotachylites of the Bergen Arcs and Ålesund, may reflect rapid (likely in terms of tens of seconds) crystallization; and distinct fining of grains toward the margins of the pseudotachylite veins are quenching textures. Preservation of these pseudotachylite textures combined with the maintenance of the microlitic grain size implies that fluids were not abundant during the amphibolite-facies retrogression. We interpret the Flakstadøy pseudotachylites to reflect amphibolite-facies retrogression of eclogite-facies pseudotachylites that had formed as deep-crustal level (~50 km depths) paleoseismic faults.