科研之友
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摘要
The ultramafic member of the Variscan Sleza Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1-olivine (Fo 90.2-91.0%, NiO 0.38-0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2-olivine (Fo 90.5-91.5, NiO 0.32-0.44wt.%), olivine with magnetite inclusions (Fo 87.1-92.5, NiO 0.01-0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1-93.2, Al2O3 3.00-4.00 wt.%, Cr2O3 1.00-1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2-94.1, Al2O3 2.20-3.20 wt.% and Cr2O3 0.80-1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8-92.7, NiO 0.20-0.38 wt.%) and clinopyroxene (Mg#= 96.0-98.1, Al2O3 0.00-1.00 wt.% and Cr2O3 0.20-0.60 wt%). The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Sleza ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre.
- 出版日期2016
