Plagioclase-bearing Iherzolite xenoliths in alkali basalts from Hamar-Daban, Southern Baikal region, Russia

Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia (Ionov et al., submitted) provide sampling of the mantle close to the axis of the Baikal rift. The xenoliths from the Tumusun and Margasan vocanic centres and two lava flows in the Margasan river valley (Fig. 1) are spinel Iherzolites that commonly have foliated fabrics and spongy rims around clinopyroxene (cpx), and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to resorbed spinel and orthopyroxene (where it appears to have been formed by the reaction: spl + opx + cpx = fs + ol) and less commonly as thin, irregular veins. The temperature range for the Hamar-Daban xenolith suite (950-1010°C and a single value of 880°C) is more restricted than those for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing Iherzolites yield T estimates similar to or slightly lower than feldspar-free ones. Xenoliths from a lava outcrop near Slyudyanka at Lake Baikal (Fig. 1) yield high T values (>1120°C) and show mineral zoning indicating a major heating event. No amphibole or mica was found in the samples studied. The majority of the Hamar-Daban Iherzolites are fertile and cpx-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. All xenoliths analysed are enriched in alkalies (Fig. 2). Most xenoliths and their clinopyroxenes have contents of heavy REE, Sr and Y common for fertile or moderately depleted mantle peridotites but are depleted in light REE, Nb, Ta, Th and Ba and show minor negative Ti-Zr-Hf anomalies (Fig. 2). Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of cpx indicate long-term depletion (probably about 2 Ga) in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal (Ionov et al., 1992). The feldspars have variable compositions but are generally alkali-rich, some are extremely rich in potassium. Bulk-rock enrichment in Na, K and Rb suggests the presence of about 1% of feldspar, an estimate consistent with petrographic observations. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H20-poor fluid into spinel peridotites. Estimates of crustal thicknesses in the region (40-45 km) indicate pressures of about 12 kbar just below the crust/mantle boundary, i.e. higher than the upper limit of 10-11 kbar for plagioclase stability inferred for fertile peridotites at ~1000°C (Green and Hibberson, 1970). The alkali-rich