FELDSPAR-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 provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lh erzolites commonly have foliated fabrics and spongy rims around clinop yroxene, and many contain accessory feldspar. The feldspar occurs in r eaction zones adjacent to spinel and orthopyroxene (where it appears t o have been formed by the reaction: spl+opx+cpx+fluid-->fs+ol) and les s commonly as thin, irregular veins. The feldspars have variable compo sitions but are generally alkali-rich; their K2O content ranges from 0 .3 to 11.2% and is much higher than in plagioclase from orogenic lherz olites (usually <0.1% K2O). The temperature range for the Hamar-Daban xenolith suite (950-1010 degrees C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The f eldspar-bearing lherzolites yield equilibration temperatures similar t o or slightly lower than feldspar-free ones. The majority of the Hamar -Daban lherzolites are fertile and clinopyroxene-rich, as for most oth er occurrences in the Baikal region. Trace element compositions of sel ected xenoliths and their clinopyroxenes were determined by TCP-MS, IN AA and proton microprobe. Feldspar-bearing xeno liths are enriched in alkalies indicating that feldspar formation is associated with additio n of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately dep leted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate longterm depletion in incompatible elements similar to unmetasomatised xenolith s from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enri chment in alkalies appear to be recent phenomena related to infiltrati on of an alkali-rich, H2O-poor fluid into spinel peridotites.-