THERMAL AND IMPACT METAMORPHISM ON THE HED PARENT ASTEROID

The bulk texture and composition of four monomict eucrites, five polym ict eucrites and one howardite, as well as those of 16 separated clast s and lithological units from these samples were analyzed by optical a nd scanning electron microscopy and by electron microprobe. Bulk chemi cal compositions were obtained by INAA. The monomict eucrites Stannern , Millbillillie, Camel Donga, and Juvinas are recrystallized monomict breccias that probably originate from brecciated crater floors or ejec ta blocks. The texture of igneous clasts from Juvinas can be explained by interaction of impact and igneous activity that led to disturbance of magma crystallization. Due to the presence of lithic clasts with h ighly variable chemical compositions and the occurrence of both equili brated and unequilibrated pyroxenes, the monomict eucrite Pasamonte is redescribed as a polymict eucrite. Three clasts of impact-related lit hologies in the polymict eucrite Pasamonte and the howardite EET 87503 contain considerable amounts of chondritic projectile contaminations. The textures of the investigated meteorites reflect a complex post-ig neous history dominated by the multistage thermal and impact metamorph ism. The chronological sequence of thermal and impact events comprises up to six evolutionary phases. Phase I represents crystallization of primary magmas that led to the formation of unequilibrated basalts and other igneous rocks. Phase II represents slow subsolidus cooling or a period of reheating during which pyroxene equilibrated. Phases III an d V represent periods of impact brecciation during which the rocks wer e brecciated in situ or, in the case of polymict HED breccias, mixed w ith various other rock types. During Phases IV and VI the breccias suf fered annealing and recrystallization due to the thermal metamorphism. The thermal events that caused recrystallization and equilibration of HED lithologies were active prior, during, and after the formation of impact breccias, indicating that the thermal input by impact might be responsible for thermal overprinting.