PROVENANCE AND BASIN DEVELOPMENT DURING THE EARLY MESOZOIC EVOLUTION OF THE CHULITNA TERRANE: IMPLICATIONS FOR STIKINIA BACKARC RELICTS IN THE ALASKA RANGE SUTURE ZONE?

Of the dozens of tectonostratigraphic terranes that make up the North American Cordillera, one of the more enigmatic and least understood is the Chulitna terrane of south-central Alaska. Mesozoic lithologic components of the Chulitna terrane consist largely of a two-part succession that includes (1) Triassic red-bed units, limestone, and basalt, and (2) Jurassic siliciclastic strata. Initial studies of the Chulitna terrane have cited it as a type example of a suspect crustal fragment that developed in an intraoceanic setting distinct from the North American Cordillera. However new provenance data presented here support a link with peri-Laurentian cratonic elements of the Cordilleran Intermontane belt by as early as Triassic time.

LA-ICPMS analyses of detrital zircon grains (n=577) from Triassic red-bed units of the Chulitna terrane reveal a bulk U-Pb age distribution of Precambrian–Mesozoic grains (Mz-8%–Pz-77%–Pc-15%). Phanerozoic age peaks occur between 205–215, 315–365, and 410–430 Ma and Precambrian age grains cluster near 1.1, 1.7, and 1.9 Ga with the oldest ages near 2.5, and 2.9 Ga. Maximum depositional ages support a Late Triassic (Norian) age for these strata. Framework modes from Upper Triassic sandstone samples reveal a relative abundance of quartz (Q-33%–F-10%–L-57%), plagioclase (Qm-22%–P-76%–K-2%), and lithic volcanic fragments (Lv-80%– Lm-6%–Ls-14%) suggestive of detrital contributions from both arc and recycled orogen source areas.

Relative abundances of Mississippian–Devonian age zircons are interpreted here to reflect detrital contributions from inboard magmatic provinces of the Cordilleran Intermontane belt (e.g. Yukon Tanana terrane and Stikinia). Early Mesozoic magmatism associated with Stikinia (e.g. Stuhini arc) make it a potential source candidate for Late Triassic detrital zircons in the Chulitna terrane. One alternative model proposed here involves a link between Late Triassic red-bed units, limestone, and basalt of the Chulitna terrane and coeval backarc rifting, sedimentation, and volcanism associated with Stikinia. In this scenario, the Chulitna terrane would represent a displaced relict of a Late Triassic backarc basin that developed in close proximity to northwestern Stikinia and the Yukon Tanana terrane.