Petrology of an ophiolitic cumulate sequence from Pindos, Greece

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Abstract
The ophiolitic sequence which crops out along the Aspropotamos Valley, Northern Pindos, Greece is composed from the bottom to the top of cumulates, dolerites, basaltic lavas, upper pillow lavas with basaltic/andesitic composition, and scarce basaltic dykes. The intrusive sequence, which is the subject of the present paper, exhibits magmatic layering more pronounced at the bottom than at the top where isotropic gabbros occur; they grade into the overlying dolerites. Troctolites with rare ultramafites prevail in the lower section and olivine gabbros in the upper section; at the top two‐pyroxene gabbros appear. The rocks are mainly adcumulates and mesocumulates with subordinate heteradcumulates. The cumulus phases separated in the order: olivine and Cr‐spinel, plagioclase, clinopyroxene, orthopyroxene. Olivine, plagioclase and pyroxenes frequently exhibit adeumulus overgrowth. Intercumulus phases may be plagioclase, clinopyroxene, orthopyroxene, pale brown amphibole and magnetite. Where pore material is present, it is composed of plagioclase, clinopyroxene, orthopyroxene, hornblende and ores. Cr‐spinel occurs mainly at the bottom of the sequence (Cr2O3 between 30·5 and 39·8 per cent), while magnetite appears as a very rare phase in the upper section. Olivine, orthopyroxene, clinopyroxene exhibit slight cryptic variation (Mg × 100/(Mg + Fe) in the range 90–79, 90–70, 93–72 respectively). The investigated dolerites are non‐cumulus rocks where clinopyroxene may be more magnesian than in the uppermost gabbros. The cumulate sequence and dolerites underwent variable but generally slight spilitization, in contrast to the overlying lavas. The sequence was generated through crystal accumulation probably from periodic pulses of tholeiitic magma; newly injected magma batches mixing with magma fractions already differentiated in the magma chamber. The high fluid pressure evidenced by the fluid inclusions in plagioclase and the whole chemical trend of the cumulate sequence are consistent with a genesis above a subduction zone, as already hypothesized for the overlying lavas.