Petrology and comparative thermal and mechanical histories of clasts in breccia 62236.

This breccia contains large lithic fragments of troctolite, norite and anorthosite. The mafic phases olivine, inverted pigeonite and augite fill interstitial areas between large plagioclases and appear to be cumulate phases with extensive adcumulus growth. Pyroxene homogeneity indicates that cation exchange during cooling was limited to approx 1 mm. Primary augite and pigeonite both contain 30 mu m-wide lamellae of the other along '001'. Pigeonite inverted to orthopyroxene without retaining any crystallographic orientation and subsequently exsolved fine lamellae of augite of (100). TEM profiles show an increase of approx 5% wollastonite in augite within 0.5 mu m of the interface, suggesting that short- range cation exchange continued to T = or < 500oC. The entire sample has undergone heterogeneous shock deformation. Shock melting of the troctolite clast suggests P of 200-300 kbar, and basal twinning in augite from the norite clasts suggests P of 50-300 kbar. The present evidence indicates that 62236 contains parts of a slowly cooled microlayered adcumulate that has been heterogeneously shocked several times and combined into breccia.-P.Br.