Plagioclase microtextures and their importance for magma chamber dynamics - examples from Less, Hellas and Teide, Canary Islands

摘要

Lavas throughout the volcanic stratigraphy of the Mid-Miocene volcanic field of Lesvos, Aegean Sea and the Quaternary Teide volcanic complex, Canary Islands Archipelago, display inhomogeneous plagioclase populations and phenocryst re-sorption microtextures interpreted as indicative of magma mixing. For Lesvos plagioclase zoning characteristics studied by No-marski differential interference contrast imaging and electron microprobe techniques indicate plagioclase resorbed mantles and/or cores and dissolution surfaces associated with jumps in An content of more than 10 mol%. These features suggest rather frequent episodes of mixing with more mafic magma. Lesvos plagioclase crystals display also numerous saw-tooth dissolution interfaces often intercepting oscillatory-zoned crystal portions indicative of repeated transfer of crystals across borders of transient thermal and compositional gradients in the chamber formed by incomplete magma mixing. With one exception (Skalohorion lavas), rim compositions in all plagioclase populations show less than 10 mol percent An content variation indicating their final equilibration with a relatively homogenous hybrid liquid. All these features allude to the presence of small, mobile, subvolcanic chambers frequently fed by more mafic magma. Physical mixing of magmas of usually not extremely contrasting compositions is followed by chemical (diffusive) mixing and formation of rather homogenous hybrids. Similarly applied techniques for Teide volcano indicate plagioclase populations with complex zoning patterns, including oscillatory and patchy zoning, often singular dissolution interfaces associated with An jumps more than 10 mol percent and multiple saw-tooth dissolution interfaces with less than 10 mol percent An jumps. These features suggest repeated heating episodes but rarer mixing with mafic magma. Plagioclase crystals with sieved cores due to decompression and thick alkali-feldspar mantles and rims indicate crystal ascent and extended residence and re-equilibration in the felsic layer of the chamber. All these features allude to the presence of a large, stagnant, bipolarly-zoned subvolcanic chamber in Teide with infrequent influxes of juvenile mafic liquid resulting in catastrophic density inversion and magma mixing shortly before or during the explosion in the upper conduit. Magma mixing processes in both volcanic systems are confirmed by the parabolic non-random distributions of incompatible trace element ratios.