DETRITAL ZIRCON-BASED PROTOLITH AGES, AFFINITIES, AND TECTONIC INTERPRETATIONS OF TWO GEOLOGIC “BLACK HOLES” IN ALASKA

A number of bedrock mapping units in Alaska could be called geologic black holes. These unnamed, undated tracts of deformed metamorphic rocks, which have only catch-all map abbreviations such as “MzPzs,” have been ignored or misinterpreted in models of the tectonic assembly of Alaska. Detrital zircons (DZs) elucidate their provenance, depositional age, correlation (or not) with better-known rocks, and tectonic context. DZ data from the problematic rocks are compared with baseline data from Alaska’s Mesozoic turbidite basins (e.g., Kuskokwim, Kahiltna) and older terranes (e.g., Farewell, Yukon-Tanana). We highlight findings from two tracts that had been assigned to the Peninsular terrane. (1) Unit Jm (Anchorage quad. near Hatcher Pass in the Talkeetna Mtns.) is a 5 by 20 km belt of schist that, on scant evidence, was thought to have a Jurassic depositional age and to be Peninsular terrane basement. DZs show age peaks at 76, 198, 346, and 1828 Ma, which, together with metamorphic white-mica ages, bracket deposition between about 75 and 61 Ma. The DZ age distribution is like that of the Valdez Group of the Chugach terrane. We infer that the schist of Hatcher Pass represents subducted Valdez Group that was exhumed in the forearc region—probably by transtension or extension—from beneath the Peninsular terrane. (2) Unit MzPzm (Lake Clark quad.) is a previously undated 10 by 10 km metamorphic complex that crops out along a poorly delineated boundary between arc rocks of the Peninsular terrane and Jurassic to Lower Cretaceous turbidites (Koksetna River or Kahiltna). Strongly tectonized biotite schist yielded DZ and apatite fission-track ages that bracket deposition between about 94 and 49 Ma. The DZ age distribution, with peaks at 97, 185, 335, and 988 Ma, is comparable to that of nearby Upper Cretaceous Kuskokwim Group. The metamorphic rocks were considered to be the Peninsular terrane’s Jurassic or older basement but instead appear to be a part of the Kuskokwim basin that, shortly after deposition in a retroarc foreland-basin inboard of the Peninsular terrane, was thrust beneath the Peninsular terrane. The mechanism of exhumation is unknown. The two metamorphic belts thus record events linked to Late Cretaceous to Paleogene subduction and backarc shortening along the southern Alaska margin, and are not basement to the Mesozoic arc.