TIKCHIK TERRANE (SW ALASKA) RECORDS PENNSYLVANIAN-EARLY PERMIAN COLLISION OF OCEANIC ARC WITH CONTINENTAL FAREWELL TERRANE

The Tikchik terrane in southwestern Alaska consists of two contrasting rock assemblages, unconformably overlain by Middle Permian and younger clastic basin strata. Beneath the unconformity, a volcanic complex of andesitic to dacitic volcanic, volcaniclastic and hypabyssal intrusive rocks in the west is thrust to the northeast over a structural complex of boudinaged sandstone and chert in a pervasively sheared shale matrix. The igneous rocks are moderately LREE-enriched and have relatively enriched LIL elements and depleted Nb and Ta, characteristic of subduction-related magmatic rocks. These rocks locally have thin interbedded carbonate sands which contain latest Devonian-earliest Mississippian conodonts. A dacitic intrusive rock within the complex yielded a U-Pb zircon age of 317.7 + 0.6 Ma (earliest Pennsylvanian), a T_DM model age of 406 Ma, and εNd of +6.72, indicative of an intraoceanic volcanic arc. The broken formation-melange complex has a pervasive southwest-dipping extensional fabric and sparse folds overturned to the northeast. Radiolarian ages from chert blocks range from Early Paleozoic to Late Triassic. Composite detrital zircon data show a prominent peak at 570 Ma, a broad hump of peaks from 765-1965 Ma, and a broad isolated peak at 2730 Ma; a few samples show a youngest peak at 430 Ma (Silurian). The detrital zircon signature strongly resembles that of the adjacent Farewell terrane and we presume the clastic components of the boudinaged sedimentary complex are derived from the Farewell terrane. Farewell terrane is exposed 120 km to the northeast, separated by Cretaceous sedimentary cover. Cretaceous plutons in the intervening area have slightly negative εNd values to within about 30 km of the Tikchik terrane, suggesting continental Farewell terrane basement rocks underlie that area.

The Tikchik terrane represents a NE-vergent Pennsylvanian-Permian collision complex, consisting of an amalgamated intraoceanic arc emplaced northeastward in Pennsylvanian-Early Permian time over an accretionary wedge composed of distal Farewell clastic and interbedded ocean basin (chert) sedimentary rocks, on the southwestern edge (present coordinates) of the Farewell terrane. This collision may be the causative agent of the Permian Browns Fork orogeny described elsewhere in the Farewell terrane.