Evidence for high silicic melt circulation and metasomatic events in the mantle beneath alkaline provinces: the Na-Fe-augitic green-core pyroxenes in the Tertiary alkali basalts of the Cantal massif (French Massif Central)

摘要

Na-Fe augitic green-core pyroxenes (hereafter called GCPX) are common in the silica-undersaturated basaltic rocks of many magmatic alkaline provinces. In the Cantal massif, green-core pyroxenes occur in nearly all the Supracantalian basalts (9.5 to 2 Ma), in contrast to the Infracantalian basalts (13 to 9.5 Ma) where they are rarely observed. An electron microprobe study demonstrates that the GCPX crystallized from evolved melts at intermediate to high pressures. Both Supra- and Infracantalian basalts show major and trace-element compositions similar to those of Ocean Island Basalts (OIB). However, the Supracantalian basalts exhibit an additional enrichment in Nb and Ta which we ascribe to a metasomatic event in the magma source. This enrichment could be related to the remobilization of metasomatic Ta and Nb-rich oxides located on grain boundaries in mantle peridotites (Bodinier et al., 1996). The hypothesis of the percolation of Si and K-rich metasomatic melts through the mantle is invoked to explain the deposition of these Ta-Nb oxides. On the basis of mineralogical and geochemical arguments, we suggest a relationship between these melts, the timing of the percolation and the formation of GCPX observed in Cantal basalts. We propose that GCPX crystallized in a melt differentiated by "percolative fractional crystallization" (PFC - Harte et al., 1993), a process which would account for the similar major-element composition of the GCPX and pyroxenes observed in the phonolites of the Cantal massif. Amphibolitic siliceous metasomatic veins in peridotite xenoliths recently described by Wulff-Pedersen et at. (1999) contain augites similar in composition to the GCPX in Cantal basalts, in addition to Nb-rich oxides. This confirms the genetic relationship between metasomatic melts, GCPX and Nb-rich oxide parageneses. GCPX would therefore seem to be linked to a metasomatic event which took place both/either during and/or between the two main stages of basaltic outpouring, and is consequently contemporaneous with the volcanism. We suggest that this process may represent a common mechanism in many alkaline provinces, where asthenospheric upwelling generates silicic metasomatizing liquids by PFC. This metasomatic process is inferred to deposit accessory phases within the lithospheric mantle, a process followed by the subsequent inheritance of this metasomatic imprint by the younger basalts.