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 characteristics with potassic rocks erupted in Recent arcs, including Italy, Indonesia, and western Mexico. However, isotopic compositions of the Highwood samples (radiogenic Sr-87/Sr-86 of 0.707 to 0.709, unradiogenic epsilon(Nd) of -11 to -20, unradiogenic Pb-206/Pb-204 of 16 to 18) are very different from those of their more modern counterparts, and, as 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 minettes 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. Although the Highwood magmas ascended through thick Precambrian crust, their very high trace element contents coupled with their primitive compositions indicate that crustal assimilation was negligible. Instead, it is proposed that the distinctive isotopic and trace element characteristics of the Highwood magmas were acquired by assimilation of lithospheric mantle by ascending asthenospheric melts. Alternative models suggesting derivation of these and other Wyoming Province magmas by direct 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 Highwood minettes. The isotopic and trace element systematics can be modeled by mixing one or more ancient metasomatized mantle components with a dominantly asthenospheric component that has epsilon(Nd) near or greater than zero (as observed for many Wyoming Province kimberlitic alnoitic magmas and for Recent potassic are magmas that have not traversed ancient lithosphere). The voluminous Eocene mafic magmatism throughout central Montana may have been triggered by foundering and southwestward rollback of the subducted, low-angle Farallon plate as convergence slowed. By analogy with their occurrence in modern arcs, potassic magmas could have been generated by decompression melting within convectively upwelling portions of phlogopite-bearing asthenospheric wedge that had been metasomatized by earlier (Cretaceous) slab-derived fluids. It is possible that their ascent to the surface was facilitated by preferential channeling into pre-existing vein networks, resulting in enhanced assimilation of ancient, isotopically extreme, mica-pyroxene-rich metasomes. The rare, younger (28-2 Ma) lamproitic magmas of the province likely reflect a larger contribution from veined lithospheric mantle than is evident in the minettes. Nevertheless, we propose that an important component in their petrogenesis is asthenospheric mantle modified by subduction-related, potassic metasomatism that preceded their eruption by 50-80 Ma.