Abstract

Major- and trace-element mass-balance calculations were used to determine the bulk composition of the Jurassic Talkeetna island arc (Talkeetna Formation and Border Ranges mafic and ultramafic complex) of south-central Alaska. A representative section of the arc was chosen near Tonsina, Alaska, where a cross section of the arc exposes 18% ultramafic cumulate rocks (dunites, wehrlites, and pyroxenites), 16% lower gabbroic (garnet- and spinel-bearing) rocks, 34% upper gabbroic to tonalitic rocks, and 32% volcanic rocks (tuffs and flows). Mass-balance calculations for the Talkeetna arc yield a high-Mg basaltic bulk composition with REE abundances approximately 8x chondrites and La/Yb ∼1.8. The bulk composition is high in mafic components (MgO = 11.3 wt. %, Ni = 247 ppm, Cr = 723 ppm) and is compatible with derivation by partial melting of a mantle-wedge source. It is too mafic and too poor in Al2O3 (15 wt. %) to have been derived by partial melting of subducted oceanic crust. High-alumina basaltic compositions are present within the arc as fractionates after crystallization of dunite and pyroxenite; however, they do not represent primary magmas parental to the arc. This finding supports the idea that the mass flux of magma from mantle to crust in island arc regions is high Mg, not high Al, in composition. The arc is stratified with respect to composition and density, with a dense ultramafic to mafic cumulate lower crust, a mafic to intermediate plutonic middle crust, and a mafic to silicic plutonic and extrusive upper crust. The presence of a highly dense, deformed, ultramafic, and garnet-bearing lower crust in this arc suggests that lower crustal delamination may be a viable mechanism for removing a highly mafic component from island arc crust.

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