MAGMATIC MODIFICATION OF THE UPPERMOST MANTLE BENEATH THE BASIN AND RANGE TO COLORADO PLATEAU TRANSITION ZONE - EVIDENCE FROM XENOLITHS, WIKIEUP, ARIZONA

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range pro vince. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing g abbro and pyroxenite xenoliths from this locality are interpreted to h ave formed by crystal fractionation processes from parent alkali basal ts similar to the Wikieup host basalt. Mineral and whole rock composit ions show consistent trends of increasing incompatible element content s (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible elemen t contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagiocl ase peridotite to the host basalt composition. These compositional tre nds are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mant le peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metaso matized spinel peridotite to wehrlite, and the highest degrees metasom atized it to plagioclase peridotite. Whole rock compositions and clino pyroxene, plagioclase, and whole rock isotopic data suggest that the i nfiltrating magmas were the same as those from which the gabbros and p yroxenites crystallized, and that they were alkalic in composition, si milar to the Wikieup host alkali olivine basalts. Relatively uniform N d-143/Nd-144 for the mineral separates and whole rocks in spite of the significantly wide range in their Sm-147/Nd-144 (0.71-0.23 in clinopy roxene) suggests that the Wikieup xenoliths including gabbro, pyroxeni te, peridotite, wehrlite, and plagioclase peridotite. are all relative ly young rocks formed or metasomatized by a relatively recent magmatic episode.