MAGMA MIXING INDICATED BY FELDSPAR ZONATION PATTERNS IN MIOCENE RHYOLITIC AND TRACHYTIC FLOWS FROM THE MONANGORIVO VOLCANIC SEQUENCE, NORTHERN MADAGASCAR
These three varieties of felsic lava each crystallized a distinct type of feldspar, with Trachyte 1 containing albite-oligoclase (~An1-25Or1-15), Trachyte 2 containing anorthoclase (~An2-25Or18-35), and the Rhyolite containing a more-sodic anorthoclase (~An1-15Or10-25). About 25% of the feldspar phenocrysts contain evidence for magma mixing in the form of partial to severe dissolution-resorption rims, distinct zones having drastically different compositions, and overgrowths on formerly resorbed crystals. Four major types of zoning have been recognized, including a normal type, a reverse type, and two complexly zoned types. The feldspars with normal and reverse zonation show only minor compositional variation between adjacent zones (ΔAb ~7%), whereas the complexly zoned types show compositional differences between zones of up to 18% Ab and 20 % Or, and are commonly associated with an internal dissolution surface. Complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movement within the magma chamber and across compositional boundaries between magma batches. A multiple step-cycle model, involving growth and transport of a crystal into another magma batch and its return to the original host magma is suggested by the data.