STRUCTURE OF THE MASSIF DU TSARATANANA MAGMA CHAMBER, NORTH MADAGASCAR, BASED ON MAJOR- AND MINOR-ELEMENT ZONING IN TERNARY FELDSPARS

Microtextures and zoning patterns in sanidine and plagioclase phenocrysts in two chemically homogeneous Cenozoic volcanic units (~17 Ma) from North Madagascar were investigated to constrain magma-chamber dynamics prior to eruption.  Feldspar crystals within the underlying trachytic and overlying phonolitic lavas show extensive development of oscillatory zoning, patchy zoning, and resorption surfaces.  The trachytes are characterized by a large number of phenocrysts (~30 modal %) that range up to ~2 mm across, whereas the phonolites contain only a few small phenocrysts (<5 modal %, <0.5 mm).

In the trachytes, there is a change upward from two-feldspar assemblages in samples near the base to one-feldspar assemblages near the top.  In addition, sanidine compositions change rather systematically from Ab_40-50An_0-10Or_60-50 (at the base) to Ab_50-70An_0-10Or_50-30 (in the middle) to Ab_70-50An_5-15Or_50-30 (at the top), and plagioclase compositions change from Ab_65-70An_35-30Or_5-20 (base) to Ab_10-20An_10-25Or_10-20 (middle);  plagioclase is absent from the uppermost samples.  In the phonolites, only sanidine is present, which shows a more-restricted compositional range from Ab_45-60An_5-10Or_40-60 (base) to Ab_60-80An_0-10Or_20-40 (top).  The Or content of the sanidine decreases gradually and regularly with decreasing An content, and the An content of the feldspars with the lowest Or tends to scatter.  The FeO content of the feldspars in the trachytes shows a large variation (0.1 - 0.8 wt%), where feldspars in the phonolites typically contain less FeO (0.1-0.2 wt%).  All feldspars have low concentrations of Ti, Sr, and Ba (less than ~0.04 wt%).

The trachytic and phonolitic feldspars define parallel trends in the An-Ab-Or ternary system, which are typical for undisturbed fractional crystallization under constant P_H2O resulting in a shift towards Ab-rich compositions.  However, the changeover from a two-feldspar assemblage to a one-feldspar assemblage in the trachytic rocks requires that the liquid at one point was located within the cotectic-like region of the sanidine + liquid field.  At that point, the anorthoclase or oligoclase were resorbed to produce sanidine, which then continued to crystallize by itself.  These observations indicate crystal dispersion within a magma chamber without large-scale mixing and overturn.