ISOTOPICALLY DEPLETED MIOCENE ADAKITES IN THE CASCADIA REAR-ARC AND THEIR IMPLICATIONS FOR MANTLE FLOW

Burch Mountain (BM) and Eagle Rock (ER) are two small andesitic intrusions dated at 11 Ma (Tabor et al., 1987) and located ~100 km behind the Cascade arc volcanic front near Wenatchee, WA. Both stocks classify as low-silica adakites with Sr > 1000 ppm (1125-2360), La/Yb > 10 (16.8-24.3), high Mg#’s (65-70) and moderate Cr (110-150 ppm). They have Ta-Nb depletions characteristic of arc magmas but their Sr-Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.70229 – 0.70249; εNd_i = +7.1 to +10.2) are the most depleted of any Cascade arc samples reported to date. Both stocks share these chemical traits, but appear to represent separate magmas as they have different phenocryst assemblages (BM = amph only; ER = cpx + plag). ER amphiboles have reaction rims that we attribute to water loss during magma ascent and their width (6-8 microns) is consistent with ascent through the upper ~5 km of crust in less than 10 days (Rutherford & Hill, 1993), a rate (~5x10^-3 m/sec) typical for intermediate magmas (Rutherford, 2008).

We propose that these stocks are the products of decompression melting of mantle wedge that had been metasomatized by slab melts. Allowing for downdip wedge flow, this metasomatism likely occurred significantly (>10 m.y.) before the stocks were emplaced, and probably closer to the volcanic front. Upwelling mantle in this rear-arc setting may have driven extension and facilitated rapid magma ascent with minimal fractionation or crustal contamination. Notably, these stocks were emplaced during an interval of magmatic quiescence within the main arc at this latitude (~15-5 Ma, Mullen et al., 2018) but whether that was a factor in this rear-arc activity is unclear. We are unaware of other rear-arc adakites in the Washington Cascades but without trace element analyses such rocks could easily be overlooked.