MAGMA MIXING INDICATED BY FELDSPAR ZONATION PATTERNS IN MIOCENE RHYOLITIC AND TRACHYTIC FLOWS FROM THE MONANGORIVO VOLCANIC SEQUENCE, NORTHERN MADAGASCAR

The Miocene (~10 Ma) rocks of the Monangorivo volcanic sequence are part of the northern Malagasy Volcanic Field, which comprises several Cenozoic eruptive centers extending from the Seychelles south to the Comoros Islands that are thought to be part of a hot-spot trail. The volcanism observed along the Monangorivo River is of peraluminous type, and consists of less-abundant basanitic flows and more-abundant flows of rhyolite and two types of trachyte. Major element compositional trends and phase relations for the Monangorivo rocks display a fairly continuous trend, suggesting a differentiation sequence between two end-member compositions. The trace-element spectrum, however, indicates three compositional groups, as follows: least-evolved Trachyte 1 (SiO2 65–67 wt%, Zr 562–615 ppm, Nb 97–112 ppm, Zr/Nb=5.79, Rb 55–68 ppm); more-evolved Trachyte 2 (SiO2 68–70 wt%, Zr 1127–1229 ppm, Nb 163–190 ppm, Zr/Nb=6.88, Rb 96–103 ppm); and evolved Rhyolite (SiO2 69–71 wt%, Zr 993–1012 ppm, Nb 99–105 ppm, Zr/Nb 10.22; Rb 113-150 ppm).

These three varieties of felsic lava each crystallized a distinct type of feldspar, with Trachyte 1 containing albite-oligoclase (~An_1-25Or_1-15), Trachyte 2 containing anorthoclase (~An_2-25Or_18-35), and the Rhyolite containing a more-sodic anorthoclase (~An_1-15Or_10-25). 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.