MORB melt metasomatism and deserpentinization in the peridotitic member of Variscan ophiolite: an example of the Braszowice\xe2\x80\x93Brze\xc5\xbanica serpentinites (SW Poland)

[摘要] The Variscan Braszowice-Brze\xc5\xbanica Massif (SW Poland) consists of gabbros and serpentinized peridotites with gabbro veins. Antigorite serpentinites form the western part of the Massif, whereas tremolite peridotites, tremolite serpentinites and lizardite-chrysotile serpentinites are found at the contact with granite intrusion in the east. Sparse relics of clinopyroxene, olivine and chromite were studied within the antigorite serpentinites. Clinopyroxene I (Mg# 90.9-93.47, 1.92-3.80 wt. % Al2O3) occurs in the neighbourhood of gabbro veins. Its REE patterns are similar to those of clinopyroxene from the mid-ocean ridge gabbros. Clinopyroxene II (Mg# 96.0-97.0) is Al-poor (? 0.10 wt. % Al2O3). Olivine I (Fo = 90.1-92.3) contains 0.32-0.50 wt. % NiO, whereas olivine II (Fo = 86.0-91.2) is Ni-poor (0.01-0.25 wt. % NiO) and contains micrometric magnetite intergrowths. Chromite I (Cr# 44.9-54.0, Mg# 45.0-52.1, < 0.17 wt. % TiO2) is associated with olivine I and clinopyroxene I, whereas chromite II (Cr# 43.2-51.4, Mg# = 34.6-47.7, 0.49-0.74 wt. % TiO2) occurs in serpentinites penetrated by gabbro veins. The serpentinites of the Braszowice-Brze\xc5\xbanica Massif were formed supposedly immediately below the paleo-Moho in the ocean-spreading setting. Chemistry of clinopyroxene I from antigorite serpentinites resembles, in terms of major elements and REE, clinopyroxenes that originate due to MORB-like melt percolation through abyssal peridotites. The coexisting olivine I with both chromite I and II supposedly shared a similar origin. Composition of chromite suggests the back-arc setting of the Braszowice-Brze\xc5\xbanica Massif. Clinopyroxene II and olivine II have major-element compositions indicative of metamorphic origin at expense of serpentine ± magnetite (deserpentinization). The deserpentinization assemblage occurring in serpentinites (antigorite-olivine-clinopyroxene) was formed probably under low-grade metamorphic conditions. The tremolite-bearing rocks record the thermal metamorphism by granite intrusion.

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