U-PB GEOCHRONOLOGY AND HF ISOTOPE ANALYSIS OF DETRITAL AND IGNEOUS ZIRCONS FROM THE FOREST KERR-MESS CREEKS AREA: THE PALEOZOIC ORIGINS OF STIKINIA, BRITISH COLUMBIA

Stikinia is a Late Devonian –Middle Jurassic juvenile oceanic arc system formed on an east dipping subduction zone in eastern Panthalassa. The terrane is currently bound by the Cache Creek terrane to the east, Yukon-Tanana terrane to the north and west, and Wrangellia and Alexander terranes to the west. The Paleozoic arc facies of Stikinia, referred to as the Stikine assemblage, preserves a submarine succession of tholeiitic island arc flows, breccias, and related intrusions and epiclastic rocks. The best exposures of the Stikine assemblage are in the vicinity of the drainages of Forest Kerr, More, and Mess creeks, in the Iskut River area, northwestern British Columbia. Existing geochemical data for Stikine assemblage igneous rocks has led to interpretations of a primitive arc formed near the North American margin, but without sufficient interaction with cratonic material, although field relations do indicate a potential depositional contact with the adjacent Yukon-Tanana terrane. However, the nature of the basement rocks, and the timing of interactions with other proximal terranes during deposition of the Stikine assemblage remains uncertain.

We report new and updated U-Pb geochronology coupled with new Hf isotopic data on Carboniferous to earliest Permian igneous and sedimentary rocks from the Forest Kerr- Mess creeks area. Resampling of igneous zircons from the Forest Kerr (351.1 ± 2.8 Ma) and More Creek plutons (350.5 ± 1.5 Ma) provide robust ages and highly variable epsilon Hf values (+14 to –30). Detrital zircon samples from metasedimentary rocks and tuffaceous sandstones yield ages from 370-290 Ma with a range of epsilon Hf values from +15 to +5. New igneous ages for the oldest known plutons indicate an earliest Carboniferous rather than Late Devonian initiation of bimodal arc activity in Stikinia. Furthermore, the variable Hf values from these plutonic rocks indicate a significant input from crustal material similar to that observed in the Yukon-Tanana terrane. Thus arc initiation in Stikinia may have occurred during latest Devonian to earliest Carboniferous time on a Yukon-Tanana/North American substrate which was either thin or rapidly exhausted during magmagenesis, leading to a prominent juvenile signature until the arc foundered in the early Permian, and in the younger Mesozoic arcs constructed thereon.