JURASSIC TO EARLY TERTIARY PLUTONIC SUITES OF THE NORTHERN TALKEETNA MOUNTAINS, ALASKA

The northern Talkeetna Mountains preserve a complex magmatic history which crosscuts the Peninsular and Wrangellia terranes and Jurassic to Cretaceous flysch of the Kahiltna overlap assemblage to their northwest. This magmatic history includes discrete episodes of intermediate to felsic intrusion in Jurassic, Late Cretaceous and Early Tertiary time. New geologic mapping, petrology, and Ar^40/39geochronology are helping resolve some of this complexity.

Plutons of quartz diorite, tonalite and granodiorite composition, previously mapped as part of the Peninsular terrane, have Late Jurassic (~146-154 Ma) K-Ar cooling ages (Csejtey and others, 1978). Muscovite from a quartz vein cross cutting one of the tonalite bodies has an Ar^40/39age of 156.5 Ma. These ages make it difficult to include these rocks as part of any batholithic root to the Talkeetna Formation island arc, which is Late Triassic(?) to Early Jurassic in age. Metabasalts and gabbros from the Late Triassic Nikolai Greenstone in Wrangellia, immediately adjacent to the tonalite/granodiorite plutons, show a strong thermal overprint at ~ 155-160 Ma (Ar^40/39). A hornblende andesite dike (156.8 Ma by Ar^40/39) crosscuts late Paleozoic metasediments below the Nikolai, also suggesting that the Peninsular and Wrangellia terranes shared a common history by Late Jurassic time.

Cretaceous and Tertiary granitic rocks in the area have previously been grouped together as part of a loosely defined "Alaska Range-Talkeetna Mountains" belt. Recent geochronologic studies suggest that Cretaceous magmatic activity is restricted in both time (~68-78 Ma) and space (Valdez Creek, Iron Ck, Willow Ck), but includes compositions from diorite to granite and crosscuts both the Wrangellia and Peninsular terranes. Stocks of equigranular hornblende-biotite granodiorite to granite commonly intrude late Jurassic to Cretaceous flysch in the northern Talkeetna Mountains south and east of Broad Pass. These have yielded only Late Paleocene (57-59 Ma) argon ages, which are similar to those from the McKinley suite of granites in the Alaska Range. The youngest plutonic activity documented thus far in the northern Talkeetna Mountains is a fine-grained felsic stock from Gold Hill in the Maclaren metamorphic belt, with an Ar^40/39 age of ~50 Ma.