Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo 92), low Ca, Al, Cr, Ti, Mn,enstatite (Wo 1En 93Fs 7), low Al, Ti, Mn chrome diopside (WO 44En 52Fs 4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.