Lithosphere asthenosphere interaction using Cenozoic volcanism of northern Madagascar as an example
From mid Cretaceous until today the Indian Ocean is a place of exceptional volcanism. A volcanism which appears to have been generated from large and small scale mantle plumes, originated in the sub-lithospheric mantle. The ascent of plumes to the base at the lithosphere is an important dynamic process in the interior of the Earth. Mantle Plumes are considered to be responsible for the formation of igneous provinces on continents and oceanic basins and additionally are thought to be the trigger for continental rifting (McKenzie and Bickle, 1988; Zeyen et al., 1997)
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 (Fo88-90) and brown Al-, Ti-rich, subcalcic magnesian augite (XMg~0.9) occur in the basanites, whereas phenocrysts of Ti-magnetite, green ferroaugite (XMg~0.4), K-feldspar, and plagioclase (An70-80) 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.
Buchwaldt, R.; Tucker, R. D., Dymek, R. F, Criss, R. E. (2003): Geochemistry and Petrology of a Miocene Trachyte – Basanite suite from Mt. Tsaratanana, northern Madagascar- Geological Society of America, 2003 annual meeting, Abstracts with Programs - Geological Society of America, 34, no. 7, 182.