Pyroxenite xenoliths found in Vitim picrite basalt tuffs are derived from several melt systems with variable geochemical characteristics: basaltic (black pyroxenite suite), anatectic (Cr-diopside veins) and hybrid high-T (green and dark green websterites). Each group defines compositional trends of descending T (and P) supposed to be formed in a (vertical) vein system. Their time of formation is suggested to be close to the age of the eruption event, because Quaternary volcanoes carry pyroxenites totally different in composition.
The majority of pyroxenites have signs of crystallization in moving systems:
(1) evident and pulsing Pb anomalies (loss of sulphides or sulphide liquid);
(2) unequilibrated associations in REE;
(3) several generations of grains with crystallographic shapes and signs of crushing and filling empty space with later minerals due to the successive pulsation of melt within the same channels;
(4) decreasing grain sizes of minerals across some veins;
(5) abundance in gaseous cavities sometimes filled with later minerals (Ilm and Phl);
(6) complicated inflections in REE patterns;
(7) high-T contact associations, represented by orthopyroxenites with Phl, and Ilm for the black pyroxenite suite.
The main process regulating pyroxenite compositions is fractional crystallization (FC) or assimilation-with-fractionation (AFC) (Powell, 1984). Assimilation fractionation ratio is lower for the garnet-free black megacrystalline suite formed from basaltic magmas (>0.1) and higher for Grt-bearing pyroxenites and green high-T websterites. Volatile-enriched hybrid Grt pyroxenites as well as the ferriferous group of Cr-diopside pyroxenites reveal the highest assimilation ratio. The proportion between assimilated and precipitated volumes of Cpx and Grt may determine the shape of REE and PM patterns and the changes in characteristic ratios (Zr/Hf, Gd/Yb, La/Sm etc.). Modeling of AFC processes with natural compositions of minerals and rocks as well as chromatographic percolation (Navon and Stolper, 1987) may explain the observed features in REE patterns of natural lherzolites and pyroxenites (V shape, U shape in LREE and minima in Ce or Sm, Eu)
All basalts from the plateau have REE patterns constituted from two concave branches inflected in Eu suggesting melting of Grt lherzolites or mixing with anatexic melts. Reconstructed with Kd's melts, parent for the pyroxenite suites have concave or convex HREE part (melting of Grt) and convex LREE for black suite (fractionation of Cpx). Basalts show mainly two convex branches, melting of Grt and Spl (high - Cr-Al basalts) as well as magmas generated high-T green and black Ga-pyroxenites which reveal TR characteristics close to basalts except for HFSE. Black giant-grained pyroxenites represent less contaminated material than erupted basalts and are referred to initial stage of the formation of feeding system. More evolved melts forming black Phl-, Ilm pyroxenites differ from basalts. Melts forming transitional grey pyroxenites as well as intergranular lherzolite liquids are Ta-Nb enriched. Mixing of about 95% lherzolite-derived melt or anatectic melts with basaltic melts and additionally some Amph and Phl (source for HFSE) matches the compositions of Vitim basalts and changes the isotopic ratio of Sr from 0.7045 to 0.7039.
The model of volcanism in Vitim suggests intrusion of deep-seated basalts at first to the asthenospheric level (near 100km), creation of high-temperature transitional melts and feeding system and then rising of hybrid and basaltic melts to the upper levels as separate streams forming individual sites of partial melting in different depths in a relatively hydrous mantle accompanied by heating and drying of the lithosphere, mixing with anatectic melts and eruption of basalts, forming flood plateau. In Pliocene - Quaternary times basaltic melts were derived from asthenosphere and again passed through the lithospheric mantle as more local streams. Percolation of melts through the lherzolite column in Miocene and Quaternary totally change their structure, forming an asthenospheric jet, relatively depleted intermediate part and enriched upper level, concentrated intergranular melts.
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Navon, O., Stolper, E., J. Geol. 95, 285-307 (1987).
Powell, R., J. Geol. Soc. Lond. 141, 447-452 (1984).
Fig. 1: REE and TRE spider diagram normalized to C1 (Evensen et al., 1978) and primitive mantle (McDonough et al., 1991) for Vitim plateau basalts (dashed fields) and various mantle melt reconstructed by Kd's (Hart and Dunn, 1993) from the composition of clinopyroxenes.