Asthenosphere-derived mafic alkaline magmas which are erupted in continental tectonic settings have to traverse several 10s of km of overlying sub-continental lithospheric mantle on their way to the surface. The geodynamic setting of the magmatic activity may, to some extent, dictate the nature of the interaction which takes place between the
alkaline magmas and the lithospheric mantle. Here we discuss two "end-member" examples of such interaction:
(1) the formation of amphibole-rich veins and dykelets, through which alkaline magmas traversed the depleted mantle lithosphere. Only regions of the lithospheric mantle directly adjacent to the veins have been affected by the passage of melts; (2) the formation of Cr-diopside-rich patches and veins, devoid of amphibole, where the magma in effect soaks into the overlying depleted mantle. This metasomatism affects much larger volumes of the lithospheric mantle. The two styles may be related to the geodynamic setting of the magmatism.
An example of the first style of metasomatism is found in veined ultramafic xenoliths from Pliocene alkali basalts of the Eastern Transylvanian Basin, Romania (Vaselli et al., 1995), where the tectonic setting is a back-arc extensional basin. The xenoliths are dominantly protogranular to porphyroclastic LREE-depleted spinel herzolites, which occasionally contain interstitial amphiboles. These interstitial amphiboles are LREE-depleted and have low K, Nb, Ti and Zr, most probably because of strong re-equilibration with their depleted peridotite host. 87Sr/86Sr ratios of the common lherzolites vary from 0.7018 to 0.7039; 143Nd/144Nd ratios range from 0.51355 to 0.51304. Hydrous lithologies are mainly amphibole pyroxenites and amphibole-rich veins and selvages; the latter cut the peridotites and the former occur as discrete xenoliths, interpreted as having been formed by the break-up of slightly larger veins. The amphibole veins from the Romanian xenoliths are strongly LREE-enriched and have high K, Ti, Zr and Nb concentrations. They also
have higher 87Sr/86Sr and lower 143Nd/144Nd than the
peridotites and completely overlap the field of the host alkali basalts. We interpret the veins as products of the passage of asthenosphere-derived mafic alkaline magmas through narrow conduits in the lithospheric mantle. The effects of a vein on the enclosing peridotite have been investigated and are shown to be very limited, although some enrichment in LREE and in Sr and Nd isotope ratios can be discerned. The "amphibole vein metasomatism" style of enrichment may be due to brittle extension associated with basin formation, which has focussed the flow of the magmas into cracks in the lithospheric mantle.
In contrast, most of the volcanoes in the uplifted Massif Central region of France contain peridotite xenoliths which are devoid of amphibole. Seismic tomographic studies reveal that a mantle plume is impinging on the base of the lithosphere in this region (Wilson et al., 1996). The locality of Ray Pic (Zangana, 1996) contains both deformed and undeformed spinel peridotite xenoliths; the more deformed peridotites tend to yield lower equilibration temperatures (<945°C), are LREE-depleted, have low 87Sr/86Sr ratios, low Pb isotope ratios and high 143Nd/144Nd. They are derived from a region of depleted mantle which probably forms the cool Mechanical Boundary Layer immediately beneath the continental crust. The less deformed xenoliths, however, yield higher equlibration temperatures (>945°C), are LREE-enriched, have high 87Sr/86Sr ratios, high Pb isotope ratios and low 143Nd/144Nd. These trend towards the field for Tertiary asthenosphere-derived mafic alkaline magmas of Europe. We interpret these xenoliths as part of the recently-accreted hot Thermal Boundary Layer, which has been fluxed by passage of asthenospheric melts. However, instead of the formation of narrow veins throughout the lithosphere, the melts have soaked into the TBL and reacted widely with the depleted harzburgitic mantle, forming cryptic enrichment. This "harzburgite sponge" style of metasomatism may occur where the volume of available alkaline magma is relatively large, or where extension is not as strongly developed, or where a mantle plume is interacting with the base of the lithosphere.
Vaselli, O., Downes, H., Thirlwall, M., Dobosi, G., Coradossi, N., Seghedi, I., Szakacs, A. & Vannucci, R., J. Petrology 36, 23-53 (1995).
Wilson, M., Granet, M. & Achauer, U., Earth Planet. Sci. Lett. (1996, in press).
Zangana, N., Unpub. Ph.D. thesis, University of London, 305 pp. (1996).