Continental margin volcanism at sites of spreading ridge subduction: Examples from southern Alaska and western California

Abstract

Episodes of spreading ridge subduction occurred during late Oligocene to early Miocene time along western California and during late Paleocene to early Eocene time along southern Alaska. In each case, ridge subduction and subsequent slab window formation has had a profound influence on continental margin magmatism. Foremost, in each setting there was a hiatus in arc magmatism following ridge subduction, followed by the onset of volcanism in zones of local extension within arc-front, forearc, and accretionary prism settings. These near-trench volcanic rocks are distinctive from adjacent arc rocks and represent unique episodes of continental margin magmatism. The western California volcanic rocks were erupted into several local extensional basins and form discrete volcanic centers within each basin. The southern Alaska volcanic rocks, which form the Caribou Creek volcanic field, were erupted in a broad zone of extension that trended orthogonally to the continental margin. Basalts from the western California volcanic centers and the Caribou Creek volcanic field are tholeiitic and have depleted Nd and Sr isotope compositions with ɛ Nd( t) as high as + 9.3 and + 10.9 and 87Sr/ 86Sr( t) as low as 0.70258 and 0.70278, respectively. These basalts are unique because they are the most geochemically depleted basalts yet documented along the continental margin of the northern Cordillera. The basalts of each group also have high Ti contents (TiO 2 above 1.5%) and low ratios of fluid-mobile and other large ion lithophile elements compared to high field strength elements. For example, Ba/Ta ratios among the basalts range from about 40 to 600, with most samples below 270, while arc basalts typically have Ba/Ta ratios greater than 450. The basalts also have Th/Yb and Ta/Yb ratios in the range of mid-ocean-ridge basalts and do not exhibit the typical enrichment in Th/Yb that characterizes arc basalts. The basaltic andesites through dacites and rhyolites have more enriched isotope compositions, with ɛ Nd( t) and 87Sr/ 86Sr( t) ranging from + 6.3 to − 3.2 and 0.70390 to 0.71131, respectively, among the western California samples and from + 10.1 to + 7.3 and 0.70299 to 0.70413, respectively, in the southern Alaska Caribou Creek samples. Compared to the basalts, the intermediate to acidic samples of each group also show enrichment in the light rare earth and large ion lithophile elements (e.g., Ba, Rb, Th, K). These data indicate that assimilation of crustal rocks was important in the evolution of each volcanic suite, although in the case of the Caribou Creek volcanic field, the crustal rocks are of oceanic island arc affinity and did not impart a strong geochemical enrichment to the magmas. The depleted Nd and Sr isotope and trace element compositions, coupled with the relatively high Ti content and low Ba/Ta and Th/Yb ratios among the basalts, indicates that the basalt magmas of each group formed by a low-degree of partial melting from a depleted mantle source without a strong flux of fluid-mobile large ion lithophile elements as would occur above an actively dewatering subducted slab. This petrogenesis is consistent with a slab window model in which parental magmas were derived by decompression melting of suboceanic (sub-slab) mantle that upwelled into the opening that formed beneath each continental margin following spreading ridge subduction.