IDENTIFYING RUTILE- AND APATITE-BEARING METASOMATIZED LITHOSPHERIC MANTLE INVOLVEMENT IN CENOZOIC CONTINENTAL MAGMATISM, SOUTHWESTERN NORTH AMERICA

The accessory minerals rutile and apatite are rare or absent in the convecting upper mantle, but occur in shallow, colder, metasomatized continental lithospheric mantle (CLM) where they serve as carrier phases for the trace elements Ta (rutile) and Th (apatite). Because both minerals are near-solidus phases and are eliminated during partial mantle melting, the relative abundances of rutile and apatite should control the high field strength element abundances of incipient mantle melts and provide a means of identifying the contribution to mafic continental magmatism by rutile- and/or apatite-bearing metasomatized CLM. To test this possibility, we investigated published Ta and Th abundance data from ~2,000 whole rock samples of mafic to intermediate composition Cenozoic volcanic rocks in southwestern North America. Roughly half of the volcanic rocks in this compilation have Ta/Th values that are similar to those characteristic either of island arc volcanic rocks (≤0.2) or ocean island and mid-ocean ridge basalts (≥0.6). The remaining samples have variable Ta and Th contents and Ta/Th intermediate between ~0.2 and ~0.5 that vary independently of volcanic rock bulk compositions. The "intermediate Ta/Th" volcanic rocks also have relatively uniform initial ⁸⁷Sr/⁸⁶Sr (0.706 to 0.708), despite variable Nd isotopic compositions that correlate with lithospheric age. The variable Ta/Th ratios for these rocks, combined with their Nd isotopic compositions, are consistent with the derivation of their parental melts from rutile- and/or apatite-bearing CLM. This conclusion supports arguments that the CLM beneath much of southwestern North America was metasomatized by upwelling, Sr-rich but Nd-poor, fluids aqueous fluids generated by the dehydration of oceanic lithosphere and overlying sediments during low-angle subduction beneath this region in the Late Cretaceous to Paleogene.