PERIDOTITE-PYROXENITE RELATIONSHIPS IN OROGENIC PERIDOTITES FROM LIVERPOOL LAND, EASTERN GREENLAND CALEDONIDES: CHEMICAL METASOMATISM OR MECHANICAL MIXING?

Orogenic peridotite lenses occur within the Liverpool Land Eclogite Terrane (LLET) of the Eastern Greenland Caledonides. The LLET was metamorphosed under high to ultrahigh pressure conditions ≈ 400 Ma years ago during the Scandian collision of Baltica and Laurentia. Variably-serpentinized garnet (grt) harzburgite and dunite are host to numerous parallel, cm- to mm-thick layers of grt pyroxenite. Major- and trace- element compositions define the familiar trend of increasing SiO₂, Al₂O₃, CaO, constant FeO and an increase, then decrease in Cr₂O₃, with decreasing MgO. This trend is classically considered the result of chemical refertilization of depleted dunite/harzburgite as pyroxenite melts intruded into and interacted with the host peridotite producing “lherzolitic” hybrid rocks. However, inconsistent P-T-t results and other observations suggest that mechanical mixing also played a role in producing such hybrids. Most samples contain high-Cr grts heterogeneously mixed with Cr-poor grts, both of which show core-to-rim increases in Cr. Normally the growth of Cr-rich grts at the expense of Cr-spinel indicates increasing pressure, but standard geothermobarometry on low-Cr grts gives higher T-P (907°C, 38.7 kbar) than high-Cr grts (801°C, 30.5 kbar). Sm-Nd geochronology is similarly contradictory with darker, Cr-rich grt older than lighter grt in one garnet pyroxenite, but younger in another. Evidence for extreme shearing, including parallelism of the pyroxenite veins (some composed of grt ± cpx ± opx trains one mineral wide), asymmetric Cr profiles across fractured (?) grt grains, and mixed dark and light grts in single thin sections suggest physical mixing occurred as the pyroxenites were stretched, thinned and brought into parallelism by shear. A logical extrapolation to extreme shearing would produce peridotite with isolated, large, mechanically strong grt in a finer-grained recrystallized ol±opx±cpx matrix. Mixing refractory, presumably older, Cr-rich grts in the peridotite with younger Cr-poor grts from the pyroxenite complicates determining the thermobarometry, chronology and geochemical evolution of hybrid peridotite/pyroxenites. Mechanical mixing through shear may be a more important process in the mantle than generally recognized.