ROCK AND AGE RELATIONSHIPS OF THE TALKEETNA FOREARC SUBDUCTION COMPLEX IN THE NELCHINA AREA, SOUTHERN ALASKA

Subduction-zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. In south-central Alaska, a remarkably well-preserved exposure of subduction-related outcrops is located at the foot of Nelchina glacier. Here, the contact between crystalline basement of the Talkeetna volcanic arc and the mélange of its related accretionary complex is exposed. This contact is known as the Border Ranges fault. A new zircon U-Pb age of an amphibolite from the Talkeetna arc mid-crustal basement near the fault is 188.9 ±2.2 Ma, which coincides with previous dates from arc rocks. A new hornblende 40Ar/39Ar age from the same outcrop yields a plateau age of 181.1 ±1.3 Ma, indicating uplift of this part of the arc. The mélange south of the Border Ranges fault, known as the McHugh Complex, comprises generally deformed argillite, metavolcanic rocks, and chert, and in the Nelchina area it includes a roughly 100-m-diameter block of pillow lavas that are undeformed but altered. Detailed compositional data show that the pillow lava formed in an intraplate setting, indicating it is unrelated to Talkeetna arc volcanism. New whole-rock 40Ar/39Ar analyses of two pillow-lava samples yielded irregular plateaus with a general age of ca. 60 Ma. Dikes of dacitic composition crosscut the mélange in the area and provide a new zircon U-Pb age of 53.0 ± 0.9 Ma, which coincides with ages of near-trench plutonism across southern Alaska. This plutonism has been ascribed to subduction of a spreading ridge that migrated eastward along the southern Alaska margin. These new ages constrain McHugh Complex formation and subsequent hydrothermal alteration to pre-55 Mya. We suggest that the pillow lava was originally part of a Triassic (or earlier) seamount that was scraped off in the subduction zone during Talkeetna arc accretion, and underwent extensive hydrothermal alteration that almost completely reset its age during the later ridge subduction event.