DETRITAL ZIRCON U-PB ANALYSIS OF PRE-MISSISSIPPIAN METASEDIMENTARY BASEMENT STRATA, NORTH SLOPE, ALASKA: EVIDENCE OF A CALEDONIAN CONNECTION

The continental Arctic Alaska terrane (ATT) consists of deformed Proterozoic and lower Paleozoic metasedimentary strata that are intruded by Devonian granitic rocks and unconformably overlain by platformal carbonate and clastic rocks of the Mississippian to Triassic Ellesmerian sequence. This succession differs from the Canadian Arctic platform of northwestern Laurentia by (1) the apparent absence of Precambrian crystalline rocks, (2) Early Devonian, instead of the Late Devonian-Early Mississippian (Ellesmerian) tectonism found in Arctic Canada, (3) Middle Devonian granitic intrusions not present in autochthonous Arctic Canada, and (4) Paleozoic and early Mesozoic fossil assemblages having mixed Siberian and North American affinities in Arctic Alaska. These observations suggest that AAT probably originated at a location distant from its present position adjacent to Arctic Canada prior to the middle Mesozoic. To try to constrain the origin of AAT, 100 detrital zircon grains each from a lower Paleozoic sandstone from the Barrow area, a Silurian sandstone from the Lisburne Peninsula, and locally derived post-tectonic Lower Mississippian covering clastic strata from both areas have been analyzed using LA-ICPMS techniques. The two Barrow samples display a broad spectrum of ages between 1.0 to 2.1 Ga and 2.8 Ga, including peaks at 1.0-1.2 (Grenvillian) and 1.5-1.7 Ga. Most Paleozoic zircons form peaks at 390 and 440 Ma. In contrast, the two Lisburne Peninsula samples yielded a spectrum of ages ranging from 350 to 1750 Ma, with a large peak at 475-600 Ma. Both the Barrow and Lisburne Peninsula groups contain populations of 1.5 Ga zircons, which are thought to be absent in sandstones derived from Laurentian sources. These results suggest that the provenance of early Paleozoic AAT sandstones was non-Laurentian but nonetheless contained Grenvillian-age source rocks. The two age spectra are echoed in detrital zircon results from overlying middle Paleozoic and Mesozoic strata, suggesting they might represent distinct pre-Mississippian domains (terranes?) that are broadly distributed in Arctic Alaska. The ages suggest the sediments were derived from source regions in Baltica, providing evidence that the Early Devonian tectonism of AAT may represent an extension of Caledonian (Scandian) deformation in northern Alaska.