Processes and components contributing to the formation of arc volcanic rocks: evidence from the western Aleutians

Processes and components contributing to the formation of arc volcanic rocks: evidence from the western Aleutians
Yogodzinski, G. M.
Cornell University, Ithaca, NY
256 p.
Rasmuson Library: ALASKA QE511.46 Y64 1993a (microfiche)
The Western Aleutians have been constructed by $\sim$43 million years of magmatism and convergence in an oceanic, strike-slip setting. Early development of the western arc (43-15 Ma) appears more similar to rift-related tectonics and tholeiitic magmatism in the western Bering Sea than to contemporaneous subduction-related processes in the central and eastern Aleutians. This is clearest on Attu Island, where early magmatism was dominated by chemically MORB-like basalts. These are interpreted to have formed within arc-adjacent, transtensional rifts, wherein the overriding plate of the subduction zone is stretched in response to tectonic adjustments following the switch to highly oblique convergence after plate reorganization 43 Ma ago. In the modern Western Aleutian growth phase (15 Ma-present), small volumes of strongly calcalkaline andesite and dacite (FeO*/MgO $\sim$ 1.2 at 65% SiO$\sb2)$ erupted in the Near and Komandorsky islands (15-5 Ma) and at Piip Volcano $(<$1 Ma). These crystal-rich, amphibole-bearing rocks, have the arc trace element signature (high K/La, Th/Ta), but Pb, Sr, and Nd isotopes of ocean ridge basalts. At Piip Volcano $\rm\sp{87}Sr/\sp{86}Sr$ =.70255-.70280, $\varepsilon$Nd = 10.1-11.3, $\rm\sp{206}Ph/\sp{204}Pb<18.2.$ Sediment did not significantly contribute to the subduction component in these rocks. It is concluded that slab melting has been important in the formation of all the Miocene and younger volcanic rocks in the Western Aleutians, and that the mantle source beneath the region is chemically similar to that which produces ocean ridge basalts. Magnesian andesites (not Mg-basalts) have been the important parental magmas in the Western Aleutians since the Middle Miocene. This implies that shallow, relatively small percentage melting and/or extended melt-mantle interaction are important in the broadly transpressional, Western Aleutian tectonic regime. Among mantle-derived magmas in the Aleutians, there may be continuous variation from primitive basalts (high MgO, FeO*, and CaO) to primitive basaltic and magnesian andesites (high SiO$\rm sb2,\ Al\sb2O\sb3,$ and Na$\sb2$O).
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)