PROVENANCE OF A DEFORMED CONGLOMERATE IN THE MESOZOIC MCHUGH COMPLEX MÉLANGE, ANCHORAGE QUADRANGLE, ALASKA

The McHugh complex is a Mesozoic mélange unit located south of the Border Ranges fault in south-central Alaska and includes km-scale blocks of argillite, conglomerate, pillow basalt, and chert. This study focuses on McHugh conglomerate composition at the type locality (Beluga Point). The average clast composition (n = 4,568 clasts) is 45% argillite, 20% volcanic-metavolcanic (including porphyritic greenstone, vesicular andesite and basalt, pyroclastic breccia, and minor tuff), 15% chert, 7% granite, 3% limestone, 2% lithic sandstone, and less than 1% each of quartzite, schist, gneiss, and gabbro. Clasts are rounded to angular and the average clast size is 4-5 cm; maximum clasts are over 1 m. Granite, limestone, and basalt form the largest clasts. The conglomerate is matrix supported and poorly sorted but beds are obscured by shear zones, brittle faults, hydrothermal veins, and low grade metamorphism. The observable textures of the conglomerate indicate deposition by sediment gravity flows proximal to source area(s). The absence of paleoflow data and the possibility for margin-parallel transport along the Border Ranges fault make it difficult to pinpoint source areas but, the clast composition is consistent with a dissected arc provenance. Possible sediment source areas are the Late Triassic-Jurassic Talkeetna arc and/or the Late Jurassic Chitina arc, which cross-cut the Wrangellia composite terrane on the north side of the Border Ranges fault. Representative granite and volcanic clasts exhibit arc-like geochemistry, consistent with this interpretation. Also, a limestone clast yields Late Paleozoic fossils that are correlated with rocks of the Wrangellia terrane near the Chitina arc and Early Cretaceous radiolarian were recovered from a chert block nearby in the McHugh (Nelson et al., 1987; USGS Circular 998). These data indicate that the McHugh conglomerate was deposited outboard of an uplifted Jurassic arc, in a remnant forearc or trench-slope setting. The deposits were then accreted to southern Alaska and incorporated into an accretionary prism. One implication of this model is that the McHugh conglomerate can provide a link between accreted terranes across the Border Ranges fault. This will be tested with age control on McHugh deposition and matching of potential source rocks with clast types.