U-PB AND HF ISOTOPE ANALYSIS OF DETRITAL ZIRCONS IN QUARTZITES OF THE BANKS ISLAND ASSEMBLAGE TO IDENTIFY POTENTIAL SOURCE TERRANES AND ORIGINS

We report new U-Pb and Hf isotope data for quartzites of the Banks Island assemblage sampled along the coast of British Columbia to better constrain the origin of these rocks. The Banks Island assemblage is a highly metamorphosed sequence of quartzite, marble, and metapelitic schist situated between the Alexander and Wrangellia terranes, both of which consist of Paleozoic arc-type rocks. Correlations between the Banks Island assemblage and the Alexander, Wrangellia, and Yukon-Tanana terranes have been proposed based on geologic observations. However, limited geochronologic data has previously been available for quantitative comparisons between this assemblage and surrounding terranes.

Quartzites of the Banks Island assemblage can be divided into 3 categories based on zircon U-Pb ages: samples containing mostly <400 Ma grains, samples with grains predominantly between 400-450 Ma, and samples containing only grains >450 Ma. The samples containing the youngest age population (300-350 Ma) overlap with U-Pb ages from the Wrangellia terrane, suggesting possible correlations between Wrangellia and the Banks Island assemblage. The 2 sets of samples with grains >400 Ma overlap well with ages of 410-480 Ma in the Alexander terrane. The set of samples containing grains >450 Ma also contains a population of ~800 Ma grains not seen in other samples. These ages are possibly derived from an Arctic source. All 3 sets of samples contain a number of grains ranging from 1.0-3.0 Ga, though the set with <400 Ma grains contains far fewer older grains.

Each set of samples displays a distinct Hf isotopic signature. Grains in the youngest set display positive ε_Hf values ranging from 0 to +15, consistent with juvenile values seen in the Wrangellia terrane. The set of samples with 400-450 Ma grains contains 2 groups of Hf values, one with positive values from 0 to +13 and the other with negative values from -8 to -28. These values contrast with those in the third set of samples, which contains one group of Hf values ranging from +10 to -7. Taken together, the U-Pb and Hf data indicate that the Banks Island assemblage may be a facies of the Alexander terrane that also received input from a continental source. The differences between each set of samples also indicate a potential shift in provenance at ~450 Ma, possibly from Arctic sources to Laurentian sources.