GEOCHEMISTRY AND PETROLOGY OF A MIOCENE TRACHYTE-BASANITE SUITE FROM MT. TSARATANANA, NORTHERN MADAGASCAR

Mt. Tsaratanana, the highest peak in north-central Madagascar (~2876 m), is one of several Cenozoic eruptive centers that extend from the Seychelles south to the Comores Islands. Miocene volcanism (~10 Ma) is of peraluminous type and consists of less abundant basanitic flows and more abundant phonolitic and trachytic tuffs and flows. Phenocrysts of olivine (Fo_88-90) and brown Al-, Ti-rich, subcalcic magnesian augite (X_Mg~0.9) occur in the basanites, whereas phenocrysts of Ti-magnetite, green ferroaugite (X_Mg~0.4), K-feldspar, and plagioclase (An_70-80) occur within the phonolitic and trachytic suite. For the entire suite, SiO₂ ranges from 44 to 66 with a large “Daly Gap” between 47-56 wt %, which is reminiscent of features found in volcanic rocks on many oceanic islands worldwide. In the basanites, as SiO₂ increases, Cr, Ni, MgO and TiO₂ contents decrease whereas Al₂O₃ and incompatible elements increase, indicating fractionation of mainly clinopyroxene, feldspar, amphibole, and Ti-magnetite. In the phonolites and trachytes, however, concentrations of Al₂O₃ and incompatible elements decrease but CaO/TiO₂ increases with increasing SiO₂, which we interpret as the result of increasing degrees of melting. Primitive mantle normalized trace element patterns are typical of ocean island basalts, showing characteristic hump patterns with high relative concentrations of Nb and Ta and pronounced negative anomalies of Ba, Sr, and Ti.  These patterns are are similar to those from the type localities of HIMU basalts, and also resemble patterns for basalts from the Comore hot spot trail.

Basanites, trachytes and phonolites have a restricted range in ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd (0.701-0.703 and 0.5127-0.5129, respectively), and d¹⁸O values are between 5.2 and 6.9 ‰.  Two basanites samples have average Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb=19.53, ²⁰⁷Pb/²⁰⁴Pb=15.66 and ²⁰⁸Pb/²⁰⁴Pb=39.14, indicating a significant HIMU mantle component, consistent with trace element data.

The observed variations suggest that the Mt. Tsaratanana magmas may have originated from a sub-lithospheric HIMU reservoir similarly to other basaltic magmas generated throughout the Comores hot spot trace. The Sr- and O-isotope variations observed in trachytes and phonolites point to contamination of the magmas by only small amount of crustal materials (<5%).