THE ROLE OF INTERMEDIATE TO FELSIC PLUTONISM IN THE ACCRETED TALKEETNA ARC, SOUTH CENTRAL ALASKA

Seismic profiles and rare exposures of arc middle crust suggest that large volumes of intermediate plutons may be present in modern arc settings (e.g. Suyehiro et al., 1996; Kawate and Arima, 1998). However, studies of intrusive processes in active arcs are fundamentally limited by the lack of exposed plutonic units, and as a result, accreted arc sections such as the Talkeetna and Kohistan arcs are essential for characterizing the role of intermediate to felsic magmatism in arc genesis.

Plutonic rocks in the classic Talkeetna arc section, exposed in the Chugach Mountains of south central Alaska, range in composition from 45–77 wt. % SiO₂ and define a low- to medium-K calc-alkaline trend. Chemically similar intermediate to felsic plutonic rocks intrude the volcanic carapace of the arc in the Talkeetna Mountains and may represent an important but previously unidentified section of evolved arc crust. This interpretation is supported by trace element analyses that show typical arc signatures, including flat REE slopes, for both sections.

Our ongoing U/Pb zircon geochronology places further constraints on the relationship between the Talkeetna arc and the Talkeetna Mountains plutons. Ages from the Talkeetna arc range from 201–181 Ma. Talkeetna Mountain plutonic ages overlap this period of magmatism and record continued magmatic activity from 184–163 Ma.

These results are consistent with a cogenetic relationship between the Talkeetna arc and the Talkeetna Mountain plutonic suite, and suggest that intermediate plutonism plays an important role in island arc growth and evolution. The exposure of a nearly complete arc cross section in the Talkeetna arc will constrain models of petrogenesis for intermediate to felsic plutonic rocks.