Lithosphere asthenosphere interaction using Cenozoic volcanism of northern Madagascar as an example

The Indian Ocean is of particular interest for this relation because it has provided the impetus for recent plume impact models to explain the formation of large igneous provinces and their association to sea floor spreading (Richards et al., 1989) The Indian plate had interacted with mantle plumes on at least three occasions. Madagascar inhabits a key position to understand the plume activity and its relationship to sea-floor spreading within the Indian Ocean, because accounts of all three plume impacts can be found within the Malagasy rock record. To understand the dynamics for these volcanics we carried out chemical and isotopic investigations on the Tsaratanana-Ankaizina volcanics of the Massif du Tsaratanana. 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 wt % to 66 wt % 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 δ¹⁸O 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%). The role of lithosphere-asthenosphere interaction is presently under investigation.