CHULITNA: SPACE ALIEN OR NOT?

Previous literature has described much of the Chulitna region as allochthonous terranes accreted to Wrangellia and rafted north to collide with North America in the Late Cretaceous. Based on our mapping, petrology, and new radiometric ages, we suggest instead that Chulitna may be part of Wrangellia terrane and can be correlated with upper Paleozoic to Mesozoic rocks of the east-central Alaska Range and western Yukon Territory, with a complex Cretaceous and younger structural history accompanied by two major intrusive episodes. Our compositional data suggest the following changes in stratigraphic and tectonic interpretations for the Chulitna region: 1. Much of what was previously mapped as Late Triassic redbeds is better described as a red volcanic wacke that compositionally resembles calc–alkaline arc volcanic rock. We believe this unit consists of an older, largely volcaniclastic and volcanic portion that may extend into the Permian and a younger, Late Triassic, commonly white quartz conglomerate that may interfinger with a fossiliferous, near-shore marine calc-sandstone. Both members broadly resemble the upper Paleozoic to lower Triassic marine sedimentary and andesitic volcanic unit observed below the Nikolai basalt to the east. 2. Trace-element chemistry of Triassic basalt in the Chulitna region correlates with that of extensional Late Triassic Nikolai greenstone mapped throughout southern Alaska and contrasts strongly with Stikinia terrane rocks of similar age. Consequently, the limestone–basalt unit in the Chulitna region is best described as a variant of the Wrangellia Nikolai basalt. 3. Volcanic rocks spatially associated with Late Devonian chert and locally with serpentinite fault slivers have the chemical characteristics of volcanic arc-related igneous rocks and are neither mafic/ultramafic dominated nor possess MORB-like compositions. We thus reject the suggestions that this sequence is an ophiolite and that it is a unique rock package in southcentral Alaska. 4. Devonian volcanics and tuffs in the previously mapped West Fork and Broad Pass terranes may be the same time–stratigraphic unit. In summary, our evidence contradicts the assertion that Chulitna is a ‘unique terrane' but is compatible with the hypothesis that it represents a variant of the Wrangellia terrane of eastern Alaska.