Rock and age relationships within the Talkeetna forearc subduction complex in the Nelchina area, Southern Alaska

Abstract

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 crystalline basement of the Talkeetna volcanic arc is in contact with the mélange of its associated accretionary complex along the Border Ranges fault. A new zircon U-Pb age of an amphibolite from the Talkeetna arc mid-crustal basement just north of the fault is 188.9 ± 2.2 Ma, coincident with previously published dates from the mafic section of the arc. A new amphibole ⁴⁰Ar/³⁹Ar age from the same outcrop yields a plateau age of 182.6 ± 1.3 Ma, reflecting cooling/exhumation of this part of the arc. The mélange south of the arc and the Border Ranges fault, known as the McHugh Complex, comprises sheared metasedimentary rocks, 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 block formed in an intraplate setting. New whole-rock ⁴⁰Ar/³⁹Ar analyses of two pillow-lava samples yielded irregular plateaus with an approximate age of 60 Ma, which we interpret to be largely reset due to reheating. Hypabyssal dikes crosscut the mélange, as well as younger accretionary prism deposits 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 the McHugh Complex formation and subsequent hydrothermal alteration to pre-55 Ma. We suggest that the pillow lava was originally part of a Triassic (or earlier) seamount that was decapitated and incorporated into the mélange as the oceanic plate entered the subduction zone. The pillow lava subsequently underwent extensive hydrothermal alteration that almost completely reset its age during the ridge subduction event. We further posit that the Talkeetna volcanic arc and its associated accretionary prism sediments were in their current configuration during the ca. 55 Ma plutonism that was common throughout southern Alaska.