CLINOPYROXENES FROM SPACE-ASSOCIATED AND TIME-ASSOCIATED WITHIN-PLATE DOMINANT AND SUBDUCTION-RELATED VARISCAN BASIC ROCKS (TAZEKKA, MOROCCO)

A study has been made of clinopyroxene phenocrysts from "within-plate dominant" and "subduction-related" basic lavas, which are within the same volcanic pile of the Lower Namurian volcanic complex of Tazekka (Eastern Morocco). Twenty-one clinopyroxene phenocrysts (113 probe analyses in total) have been analysed from two thin sections of the "within-plate" lava sample, and 17 clinopyroxene phenocrysts (82 probe analyses in total) have been analysed from two thin sections of the "subduction-related" lava sample. Petrographic and microprobe data on clinopyroxene phenocrysts from the two lavas indicate specific textural and chemical features according to the source, namely: relatively low pressure and high temperature crystallisation conditions for the sector-zoned crystals from the basalt of the first group, although such a sector-zoned growth in clinopyroxenes is usually considered as evidence for a continental within-plate affinity of host-rocks; intermediate pressure and relatively low temperature crystallisation conditions for homogeneous crystals from the basalt of the second group. Thus distinct P-T conditions for equilibrium crystallisation of the pyroxenes in each host-magma are proposed. Clinopyroxene affinities observed in specific discriminative diagrams are in agreement with trace element geochemistry of the host-lavas, i.e. "continental within-plate" features for clinopyroxenes of the group 1 host-rock and "subduction-related" features for clinopyroxenes of the group 2 host-rock, which often exhibit extreme parameters highlighting the problem of petrogenesis of these basic lavas in an unusual continental crust subduction-related environment.