MAKING MIDDLE CRUST; A COMPARISON OF JUXTAPOSED OCEANIC AND CONTINENTAL MARGIN PLUTONS IN THE WESTERN ALASKA RANGE

Middle- to upper-crust plutonic sequences exposed in the Alaska-Aleutian Range batholith (ARB) are associated with two long-lived (≥55 My) episodes. The earliest, from ca. 210-150 Ma, is the intrusive core of the Jurassic intra-oceanic Talkeetna arc, which was progressively accreted to the North American margin during the first half of the younger, mostly Cretaceous magmatic stage, lasting from ca. 110 to 55 Ma. The combination of these assemblages, formed in intra-oceanic and continental margin settings, respectively, have important constraints for the mechanisms and timing of middle crust formation and the generation of new continental crust.

Both episodes have similar mineral assemblages, approximated seismic velocity consistent with global middle crust, and demonstrate similar compositional evolution, over a similar duration. For example, both show increases in ratios of more- to less-incompatible elements (e.g. La/Yb, Nb/Yb and Sr/Y) after the first ca. 40 My, and maximum εHf(t) and εNd(t) for both series gradually decreases over the duration of each. However, there are important contrasts within, and between each series. For example, an apparent along-arc variation in the role of amphibole cumulates is noted for the Jurassic assemblage. The starting and ending compositions for the Jurassic and Cretaceous vary significantly, despite their similar relative evolution paths. Within the Cretaceous, a significant εHf(t) pull-down suggests melting of a distinct enriched source, despite little elemental contrast with more-depleted contemporaneous plutonic rocks. This is interpreted as melting of mineralogically similar lower-crustal rocks of contrasting isotopic composition, likely tracing the tectonic suture margin between Jurassic arc and North America.

Both systems suggest an increasing role of partially melting mafic crust in the presence of amphibole, but produce distinct compositions, reflecting different starting conditions and tectonic settings. However, only younger Cretaceous rocks have trace element traits consistent with estimated global bulk continental middle crust (BCMC), suggesting that although crustal processing in oceanic settings can yield middle crust mineralogically similar to BCMC, thicker preexisting crust is required to produce BCMC within the modern geotherm.