Comparative major and trace element geochemistry of gabbroic and volcanic rock sequences, Montgenèvre ophiolite, Western Alps

Abstract

The Montgenevre ophiolite complex is one of the best preserved in the Alps. Ultramafic, and basaltic rocks occur as distinct tectonic units. Fifty-four representative samples from the and the volcanic rocks have been analysed. The sequence exhibits a very wide compositional range from troctolite to evolved ferrogabbro and albititic rocks. The volcanic sequence, which is mostly composed of pillowed basalts, is characterized by its rather small compositional range. Both the and volcanic sequences show typical tholeiitic differentiation trends. The results are discussed using the following variation diagrams: MgO versus Ti0₂, Ni versus Ti0₂, Ti/1000 versus V, Sc versus Ni, Zr versus Y and Zr versus Zr/Y. In the first three, the gabbros and basalts obviously show distinctive differentation trends, whereas a similar trend is observed in the last three. This observation is interpreted as reflecting crystallization of comparable melts under different P, T and oxygen fugacity conditions. The early stage of differentiation is characterized by fractionation of olivine, spinel, clinopyroxene and plagioclase within a rather closed system. Part of the crystallized as troctolites and olivine-rich-gabbros, and partly as clinopyroxene gabbros. There is no evidence that part of the deep-seated gabbroic melt has escaped to higher levels to crystallize as basalts. During crystallization of clinopyroxene gabbros, Fe and Ti are drastically enriched in the residual melt. Within the same system, and probably at deeper crustal levels (≥ 2 kb), ferrogabbros could crystallize from this residual melt. The appearance of amphibole, however, indicates infiltration of H₂O and possibly also an increase in oxygen fugacity. In the final stage of differentiation, a small volume of remaining melt, enriched in Si0₂, Na₂O and incompatible elements, crystallized as albititic dikes within the sequence. The basalts exhibit characteristics clearly indicating a magmatic origin comparable to MORB. Major and trace element data from the volcanic and plutonic rocks sugggest an independent magmatic evolution for the and basaltic sequences. This conclusion is supported by field relations that show basaltic dikes to crosscut the sequence.