The 50 Ma granodiorite of the eastern Gulf of Alaska: Melting in an accretionary prism in the forearc

Abstract

The generation of granitic rocks by melting of flyschoid sediments in an accretionary prism is addressed in this study of 50 Ma silicic igneous rocks in the Gulf of Alaska, near Cordova, Alaska. Plutons of relatively homogeneous biotite and biotite‐hornblende granodiorite and minor tonalite and granite are scattered through the Paleocene and Eocene Orca Group. Two masses of cointrusive gabbro and minor dacite dikes also were intruded here. The Orca Group consists of flysch, (quartzofeldspathic graywacke and argillite of turbidite or deep‐sea fan origin) and of minor basaltic rocks and pelagic sedimentary rocks. The Orca represents the youngest and structurally lowest part of a late Cretaceous to Eocene composite accretionary prism that extends for 2100 km along the Gulf of Alaska. The plutons are 5–150 km2 in plan and represent less than perhaps 5% of the total volume of this part of the prism. These granitic rocks are unusual in that they were emplaced in a forearc environment during the last stages of deformation of the prism. The three intrusive bodies chosen for study (the McKinley Peak, Rude River, and Sheep Bay plutons) show a range of chemical and initial isotopic compositions (SiO2 = 66.3–71.3%, Na2O = 2.8–3.6%, K2O = 1.8–3.0%, εNd = +2.1 to −3.3, 87Sr/86Sr = 0.7051–0.7067, 206Pb/204Pb = 19.04–19.20, 207Pb/204Pb = 15.60–15.66, and 208Pb/204Pb = 38.59–38.85). Relative to the other two plutons, the McKinley Peak pluton generally shows slightly lower K2O, higher Al2O3, higher εNd, and lower 87Sr/86Sr ratios. All three plutons, however, have similar, well‐defined minor and trace element abundances characterized by relative enrichment in light rare earth elements and depletion in high field strength elements. The granodiorites and flysch of the Orca Group show overlapping elemental and isotopic compositions. The only clearly defined chemical differences between the flysch and the granodiorites are weak negative Eu anomalies in the granodiorites and slightly lower Ca and higher Na contents in the flysch. The Nd and Sr isotopic compositions of the Rude River and Sheep Bay plutons completely overlap those of the flysch. The McKinley Peak pluton, however, shows discretely higher εNd slightly lower 87Sr/86Sr values than those of the flysch. The Pb isotopic compositions of the flysch and the Rude River pluton also overlap, but Pb of the other two plutons is slightly less radiogenic. Our chemical data, modeling, and comparison with Conrad et al.'s (1988) melting experiments of graywacke indicate that the granodiorite orginated by large fractions (65–90%) of melting of the Orca Group graywacke and argillite. Plagioclase, pyroxene(s), and biotite(?) were residual to melting at about 850°–950°C and at low H2O activities. The distinct chemical and isotopic compositions of the McKinley Peak pluton probably result from variations in the character of the flysch at depth in the prism, rather than from mixing between melts of the flysch and mafic magmas injected into the prism itself. However, basaltic magmas were injected into the accretionary pile, as evidenced by the coeval gabbroic plutons, and these apparently provided the heat necessary for crustal melting. The mafic magmas probably originated from the subjacent oceanic Kula plate. We suggest that the subducting Kula “plate” consisted of several small plates, much as the modern Juan de Fuca and nearby smaller plates lie at the margin of the Pacific plate. Basaltic magmas produced along the boundaries of such small plates were injected for more than 12 m.y., first into the Orca Group deep‐sea fans and later into the accretionary prism. Accretionary prisms have been an important, but little discussed, source of granitic magmas since Archean time. Their emplacement as complete sections of lower to upper crust means that any basaltic magma coming up from the mantle will impinge upon and tend to melt them. Furthermore, many prisms, especially those bearing high proportions of quartzofeldspathic graywacke, are fertile in granitic melts. These Alaskan granodiorites do not fit into the alphabetical classification of Australian workers. Being melts of sedimentary rocks, they should have S‐type character. Because the source flysch is quartzofeldspathic and of arc origin, however, the granodiorite shows I‐type character. Our results also highlight a problem with Pearce et al.'s (1984) and Harris et al.'s (1986) purportedly tectonic‐discriminant plots for granitic rocks. These diagrams classify our granodiorites as “volcanic arc granite” and reflect their source rocks rather than their tectonic environment of origin.