A suite of nine garnet peridotites and eleven spinel (picrochromite) peridotite xenoliths from the Lashaine volcano in northern Tanzania have been studied petrographically and by electron microprobe techniques. The primary assemblages ol+opx+cpx+ga, ol+opx+ga, ol+opx+cpx, ol+opx+ga+ sp, ol+opx+cpx+sp, ol+opx+sp, and ol+cpx+sp have been identified. With some exceptions the compositions of primary ol, opx, cpx, ga, and sp are the same in both ga and sp peridotite groups. Olivines are Ni-rich,

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... low Ca forsterites (Fo92); orthopyroxenes are low Ca, A1, Cr, Ti, Mn enstatites (^WoiEn92Fs7); clinopyroxenes are low A1, Ti, Mn chromediopsides (M/\l044En52Fs4, A1203 two weight percent); garnets are chromepyropes (^Py75Almi3Gri2); and the spinels are Fe, A1 picrochromites (^Mg.7Fe.4Cri.4A1.5O4). The primary garnet lherzolite assemblages are stable at ^1100°C and 50kb., equivalent to a depth of M50km. Ga and sp coexist in some samples and the ga and sp assemblages apparently have formed at essenti¬ ally the same P-T conditions from different bulk compositions. The Lashaine spinels are distinct from the highly aluminous variety common in spinel peridotite inclusions in alkali basalts, and the Lashaine spinel peridotites are not the low P equivalents of the garnet perido¬ tites . Two types of secondary processes are evident in the Lashaine xeno-1iths. 1. Reaction rims around garnet illustrate the reaction ol+ga+Al opx + A1 cpx + Al-sp that reflects the transition from ga peridotite to sp peridotite. 2. In several of the ga and sp peridotites there is evidence of partial melting. At some grain boundaries, particularly those involving cpx, local melting has occurred and the melt has quenched to produce the assemblage ol+opx+cpx+sp+ glass (K-rich)+ph1ogopite. These secondary phases are texturally and compositionally distinct from the primary peridotite phases, and their compositions allow consideration of the nature of the liquids produced by small degrees of partial melting and of the physical environment under which they formed. Reid, Ridley, Donaldson, Brown and Dawson