THE EASTERN HAYFORK REVISITED: SINGLE-GRAIN DETRITAL ZIRCON GEOCHRONOLOGY IN THE CENTRAL KLAMATH MOUNTAINS

The Eastern Hayfork terrane in the Klamath Mountains, northern California, has been described as an accretionary mélange and/ or broken formation with numerous likely ties to the Klamath Mountains. Many workers have proposed that the sediments of the Eastern Hayfork and adjacent North Fork terranes are likely correlative or at least contemporary based largely on sediment characteristics, lithology of mélange blocks, and fossil ages. New SHRIMP-RG analyses from Eastern Hayfork and North Fork samples illustrate the utility of suites of single-grain detrital zircon ages for testing these types of correlations. Eastern Hayfork samples adjacent to the Hayfork Summit-Salmon River segment of the North Fork terrane indicate a purely cratonic signature, with a surprising lack of Klamath ages. Cumulative probability plots show peaks near ~1800 Ma, ~1950 Ma, and ~2250 Ma for all three Eastern Hayfork samples analyzed. No grains younger than Precambrian were observed in these samples. In contrast to these findings, the North Fork sample analyzed shows numerous Mesozoic ages (with a peak at 189 Ma) in addition to the Precambrian and older suite. There are numerous explanations for the observed difference between the two terranes, including: (1) The North Fork sample is from the Sawyers Bar area of the terrane, which is significantly further north than the Eastern Hayfork samples analyzed, thus the difference is recording north-south variations in sediment source. (2) The Eastern Hayfork terrane had a different sediment source, and possible paleogeographic origin, than the North Fork terrane. (3) The sediments of the North Fork and Eastern Hayfork terranes are not correlative and may represent different depositional settings in the same convergent-margin system. Future work on samples from adjacent portions of these two terranes is crucial for distinguishing between these and other explanations for the differences.