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 2 are 5-150 km 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%, Na20 = 2.8-3.6%, K20 = 1.8-3.0%, eNa = +2.1 to --3.3, 87 86 206 204 207 204 Sr/ Sr = 0.7051-0.7067, Pb/ Pb = 19.04-19.20, Pb/ Pb = 15.60-15.66, and 2øspb/204pb = 38.59-38.85). Relative to the other two plutons, the McKinley Peak pluton generally shows slightly lower K20, higher A120 3, higher eNa, 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 eNa and slightly lower 87 Sr/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 850ø-950øC and at low H20 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...

Origines des enrichissements en biotite dans les granites de type S :

“…FeO* : tout le fer est considéré comme étant du Fe Sources des données de granitoïdes : , , Barbero and Villaseca (1992), Barker et al (1992), Chappell and White (1992), , Collins et al (1989), Crawford and Windley (1990), Crisci et al (1979), McKenzie and Clarke (1975), Neiva (1981), Nockolds (1954), Pankhurst et al (1992), Sawyer (1987). Sources des données des verres ejqjérimentaux à partir de pélite, greywacke et psammite (T<925°C): Gardien et al (1995), Grant (2004), Montel and Vielzeuf (1997), Patifio Douce and , Patino , Patino Douce and Johnston (1991), , Stevens et al (1997), Vielzeuf and Holloway (1988).…”

Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera

“…The tonalities are believed to have been the primary source of heat for development of an inverted metamorphic gradient that extends westward as far as the megalineament (Himmelberg et al, 1991). Extensive Eocene magmatism forming part of the Coast batholith took place from about 56--48Ma (Barker and Arth, 1990;Snee, unpub, data), approximately 20-50 km east of the gold belt.…”

“…Relaxation along this shear zone, during a shift from orthogonal to more oblique convergence and resulting strike-slip motion, is hypothesised as having led to increased permeability and widespread fluid migration. A belt of tonalitic plutons were intruded approximately 5 km east of the megalineament between 68-61 Ma (Barker et al, 1986;Wood et al, 1991). The tonalities are believed to have been the primary source of heat for development of an inverted metamorphic gradient that extends westward as far as the megalineament (Himmelberg et al, 1991).…”

“…Farallon and Kula plate subduction below southeastern and south-central Alaska led to the development of the 72-48 Ma Coast and 74-55 Ma Alaska Range continental margin batholiths (Barker et al, 1986;Reed et al, 1983). Gold veining, R. J. GOLDFARB ETAL.…”

“…Late Jurassic and Cretaceous flysch, melange, and minor mafic volcanic rocks of the Chugach terrane were accreted to and subducted below the southern Alaska superterrane by the latest Cretaceous (Plafker, 1987 (Dusel-Bacon, 1991). The Cretaceous to Eocene (72-48 Ma) Coast batholith, the largest continental-margin batholith in the world, occurs inboard of the Jurassic and Cretaceous flysch basin in southeastern Alaska (Barker et al, 1986). Magmatic activity forming the Alaska-Aleutian Range batholith occurred inboard of the flysch sequence in the Alaska Range of south-central Alaska between 74-55 Ma and in the Aleutian arc of southwest Alaska between 42-21 Ma (Reed et al, 1983).…”