THE ORIGIN AND METAMORPHISM OF THE TLIKAKILA COMPLEX, LAKE CLARK NATIONAL PARK, ALASKA: IMPLICATIONS FOR THE COLLISION OF THE TALKEETNA ARC/PENINSULAR TERRANE WITH ALASKA DURING THE MIDDLE JURASSIC

The Tlikakila complex is a NE-striking belt of metamorphosed and highly deformed mafic, ultramafic, and clastic sedimentary rocks exposed near the northern margin of the Peninsular terrane in Lake Clark National Park, Alaska. The Tlikakila complex has been postulated to be either part of the Talkeetna arc in the Peninsula terrane or part of an ophiolite complex that formed between the Peninsula terrane and the previously accreted terranes in southern Alaska. We used trace element and Nd isotope geochemistry to address the origins of metabasite, metagabbro, and amphibolites interpreted to have basaltic or gabbroic protoliths. These rocks have low Y, Zr, and Ti/V similar to other island arc-related volcanic rocks. Nd isotope ratios calculated at 215 Ma yielded eNd of +2.5 to +5.5, with one sample at +9.3, all in rocks with mafic protoliths. A serpentinite with an ultramafic protolith has eNd of +8.3. Initial Sr for all these rocks ranged from 0.7042 to 0.7074. The elevated Sr ratios relative to eNd likely reflect either seawater interaction or alteration during metamorphism. Nd isotope ratios are similar to those from the arc rocks of the Wrangellia terrane (eNd +1.0 to +7.3) and are consistent with an island arc origin.

A key question regarding the Peninsular terrane is the timing of its collision with southern Alaska. We used 40Ar/39Ar thermochronology on biotite to determine the age of greenschist facies metamorphism of three metasedimentary rocks from the Tlikakila complex. The deformation and metamorphism is inferred to have occurred during the collision. The weighted mean ages are 176.3 ± 0.6 Ma, 177.3 ± 0.8 Ma, and 178.2 ± 0.8 Ma. These ages are younger than recent U/Pb ages from plutonic rocks in the arc in the Chugach Mountains (184-199 Ma; Rioux et al., 2002). If Talkeetna arc magmatism ceased around 185 Ma, then our data suggest the collision may have occurred shortly thereafter at 176-178 Ma.

Taken together, these data suggest that the Tlikakila complex may have originated as part of the Talkeetna arc. Metamorphism of these rocks may have occurred during deformation associated with the arc-continent collision at 176-178 Ma. The Tlikakila complex would therefore represent the northernmost exposures of the Peninsula terrane in southwestern Alaska and may provide additional constraints on the history of the collision.