THE FELSIC PLUTONIC CORE OF THE WESTERN TALKEETNA ISLAND ARC CRUSTAL SECTION, ALASKA: ITS FORMATION AND IMPLICATIONS FOR CRUSTAL GROWTH ALONG CONTINENTAL MARGINS

The accretion of island arc crust is believed to be a major contributor to the growth of continents. A particularly important question in arc evolution is the origin of felsic plutonic rocks in island arcs. Felsic rocks represent the nucleus of continents, yet there is no clear consensus on how these rocks originate. The Jurassic Talkeetna island arc in south-central Alaska is an exhumed and titled arc section where middle and upper crust lithologies are now exposed at the surface, providing us with the rare opportunity to directly model processes responsible for the formation of the felsic core of an island arc.

Studies in the eastern Talkeetna arc (Talkeetna and Chugach Mountains) indicate that the arc crustal section comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends, each group of rocks forming by processes of fractional crystallization and accumulation. In contrast, two chemically distinct groups can be defined in the western Talkeetna arc (Lower Cook Inlet region). Compositional group I (n = 75; 43.0-76.1 wt % SiO₂) shares many of the same trends as rocks from the eastern arc: relatively low K at a given SiO₂ and flat REE patterns ([La/Yb]_N < 5; average 2.6) where REE abundances increase with increasing SiO₂. Compositional group II (n = 21; 56.1-73.6 wt % SiO₂) shows trends of LREE enrichment and HREE depletion ([La/Yb]_N > 5; average 7.5), where REE abundances decrease with increasing SiO₂. The most silicic members of this group exhibit concave-up patterns of HREE depletion.

Of particular interest is the formation of the voluminous felsic core of the western arc, exposed over more than 4800 km² of the region. Major element and REE modeling indicate the majority of these rocks formed through fractional crystallization and accumulation (compositional group I). In addition, modeling results for compositional group II rocks indicate that the observed range of intermediate-felsic compositions can be produced by variable mixing of an andesitic parent liquid (presumably formed by fractional crystallization) with a felsic end-member magma (formed by ~30% partial melting of mafic crust). This study provides important insights into the range of processes responsible for the formation of the felsic core of island arcs and potentially the nucleus of continental crust.