MEGA-XENOCRYSTS IN ALKALI OLIVINE BASALTS - FRAGMENTS OF DISRUPTED MANTLE ASSEMBLAGES

Megacrysts of augite (34), feldspar (12), kaersutite (10), olivine (8) , and biotite (1) from eight alkali basalt localities in the western U .S. and Mexico have been analyzed by electron microprobe and wet chemi cal techniques. These crystals range from 1 to 10 cm, are free of incl usions, and are unzoned. Such crystals cannot have grown from the basa lt host, as that would require unreasonably large growth and diffusion rates. In addition, many of the megacrysts have fragmented or irregul ar edges in contact with the host basalt. With these constraints, such unzoned megacrysts must be xenocrysts acquired along the ascent path of the basalt host. Their composition and large size indicate that man y of the megacrysts could be derived from disaggregated gabbroic (oliv ine, augite, plagioclase, kaersutite, biotite), pyroxenitic (augite), wehrlitic (olivine and augite), and syenitic (anorthoclase) dikes or p egmatites.