Susan M. Debari

High-Mg, low-Al bulk composition of the Talkeetna island arc, Alaska: Implications for primary magmas and the nature of arc crust

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 Al 2 O 3 (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.

Ultramafic Xenoliths from Adagdak Volcano, Adak, Aleutian Islands, Alaska: Deformed Igneous Cumulates from the Moho of an Island Arc

Deformed ultramafic xenoliths with basaltic salvages occur in a postglacial ejecta deposit from Adagdak volcano on Adak Island in the central Aleutian Island Arc. These dunites, wehrlites, clinopyroxenites, and amphibolites are interpreted to be cumulates from primitive arc magmas that formed in magma chambers beneath the crust-mantle boundary. The primitive arc magmas are similar in composition to the xenoliths host basalt (11.8% MgO). Trace element and Nd and Sr isotopic data are consistent with derivation of these xenoliths from the same parent as other Aleutian calc-alkaline volcanic rocks. The peridotite xenoliths contain olivine (F086.6 to 83.2), clinopyroxene (FS4.1En46.4Wo49.5 to FE5.2En46.6Wo48.2), Cr-Al spinel (19-30%

Subduction erosion of the Jurassic Talkeetna-Bonanza arc and the Mesozoic accretionary tectonics of western North America

Cr_{2}O_{3}

Vertical Stratification of Composition, Density, and Inferred Magmatic Processes in Exposed Arc Crustal Sections

and 30-42%

The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

Al_{2}O_{3}

Using mineral geochemistry to decipher slab, mantle, and crustal input in the generation of high-Mg andesites and basaltic andesites from the northern Cascade Arc

), and secondary pargasitic amphibole that increase in FeO content as the percentage of clinopyroxene increases. These xenoliths form the necessary residuum to produce the high-Al basalt predicted in an open system mixing model for Aleutian calc-alkaline volcanism. The recrystallized textures of the peridotites, along with high equilibration temperatures (> 1000°C), suggest that the upper mantle asthenospheric flow regime reaches the crust-mantle boundary region.