STRONTIUM, NEODYMIUM, AND LEAD ISOTOPIC EVIDENCE FOR THE INTERACTION OF POSTSUBDUCTION ASTHENOSPHERIC POTASSIC MAFIC MAGMAS OF THE HIGHWOOD-MOUNTAINS, MONTANA, USA, WITH ANCIENT WYOMING CRATON LITHOSPHERIC MANTLE

The Eocene potassic mafic rocks of the Highwood Mountains in Montana, USA, share many petrographic, major element, and trace element charact eristics with potassic rocks erupted in Recent arcs, including Italy, Indonesia, and western Mexico. However, isotopic compositions of the H ighwood samples (radiogenic Sr-87/Sr-86 of 0.707 to 0.709, unradiogeni c epsilon(Nd) of -11 to -20, unradiogenic Pb-206/Pb-204 of 16 to 18) a re very different from those of their more modern counterparts, and, a s for most other magmas emplaced into the Archean/Proterozoic Wyoming Province, must reflect the influence of ancient, geochemically extreme lithologies in their petrogenesis. The most primitive Highwood minett es and leucitites (8-14 wt% MgO) have high K2O (4.6 to 8.2 wt%) and Ba (2000-5000 ppm), yet are relatively depleted in TiO2, Nb, and Ta. Alt hough the Highwood magmas ascended through thick Precambrian crust, th eir very high trace element contents coupled with their primitive comp ositions indicate that crustal assimilation was negligible. Instead, i t is proposed that the distinctive isotopic and trace element characte ristics of the Highwood magmas were acquired by assimilation of lithos pheric mantle by ascending asthenospheric melts. Alternative models su ggesting derivation of these and other Wyoming Province magmas by dire ct melting of lithospheric mantle are rejected on the basis of thermal constraints and the extreme isotopic compositions of mantle xenoliths , including a glimmerite-veined harzburgite, sampled by one of the Hig hwood minettes. The isotopic and trace element systematics can be mode led by mixing one or more ancient metasomatized mantle components with a dominantly asthenospheric component that has epsilon(Nd) near or gr eater than zero (as observed for many Wyoming Province kimberlitic aln oitic magmas and for Recent potassic are magmas that have not traverse d ancient lithosphere). The voluminous Eocene mafic magmatism througho ut central Montana may have been triggered by foundering and southwest ward rollback of the subducted, low-angle Farallon plate as convergenc e slowed. By analogy with their occurrence in modern arcs, potassic ma gmas could have been generated by decompression melting within convect ively upwelling portions of phlogopite-bearing asthenospheric wedge th at had been metasomatized by earlier (Cretaceous) slab-derived fluids. It is possible that their ascent to the surface was facilitated by pr eferential channeling into pre-existing vein networks, resulting in en hanced assimilation of ancient, isotopically extreme, mica-pyroxene-ri ch metasomes. The rare, younger (28-2 Ma) lamproitic magmas of the pro vince likely reflect a larger contribution from veined lithospheric ma ntle than is evident in the minettes. Nevertheless, we propose that an important component in their petrogenesis is asthenospheric mantle mo dified by subduction-related, potassic metasomatism that preceded thei r eruption by 50-80 Ma.