EVIDENCE FOR HIGH PRESSURE GARNET PYROXENITES IN THE CONTINENTAL LITHOSPHERE

Garnets containing oriented arrays of diopside, rutile and Mg-rich ilmenite inclusions are fairly common in magnesian garnet pyroxenite xenoliths from the Mir kimberlite, Russia. Six pyroxenites we studied in detail have garnet containing arrays of diopside alone, ilmenite + rutile or diopside + rutile. We interpret these arrays to reflect exsolution of high pressure garnet components following emplacement into the continental lithosphere. The arrays with diopside alone require a majoritic component in the original garnet, and presumably the other arrays also reflect a high pressure substitution in garnet (such as NaTi(MAl)_-1, NaSi(MAl)_-1 or MTiAl_-2 where M=Fe²⁺, Ca or Mg). However, present compositions of garnets, enstatites and diopsides in these pyroxenites show that the minerals last equilibrated at relatively low pressures of 2.1-3.7 GPa and temperatures of 680-820 C. These equilibration conditions overlap the range (P=2.7-4.2 GPa, T=790-950 C) for relatively low temperature granular lherzolites from Mir. The exsolved minerals indicate that some of the original pyroxenite garnets had significant amounts of Si substituting in the octahedral site prior to exsolution. The majoritic substitution becomes important at pressures greater than 5 GPa, suggesting that the garnets and, presumably the associated pyroxene, originally formed at depths of more than 150 km. These minerals may have been transported in crystal-melt suspensions to relatively shallow depths where their minerals reequilibrated. Similar garnet exsolution textures were described from xenoliths in African kimberlites, and pyroxenites with a high pressure ancestory may be a common feature of mantle lithospheres beneath cratons.