U-TH-U AGES FROM KLAMATH RIVER DETRITAL ZIRCONS USING LA-ICP-MS

One hundred zircons panned from the river‘s mouth at Klamath, CA yielded 78 acceptable ages, where acceptance depends on concordance, how measured uncertainties in ²⁰⁶Pb/²³⁸U agree with those expected from counting statistics, and absence of inclusions. Holes 32 microns in diameter were drilled by excimer laser in the cores of polished grains during 1 minute analyses, and data were reduced relative to zircon standard Temora, and NIST 610 glass. The agreement of the measured age of zircon standard R33 of 422.7+/-4.7 Ma (+/-2 s.e. or 1.1%) for 9 of 11 grains and the known age of 419 Ma indicates uncertainty on an individual unknown analysis of Mesozoic age is better than +/- 2 Ma.

Ages of unknowns ranged from 131 to 177 Ma, a remarkably tight age range. Three age populations are separated by distinct gaps. The oldest population of 4 grains yields an age of 177.0+/-0.5 Ma. The middle population (151-169 Ma; n=55) contains multiple subpopulations with the mode at 159 Ma, but with a distinct older peak at 169 Ma. The youngest population (131-143 Ma; n=19) has a mode of 137 Ma and it too contains subpopulations. The middle population has much larger range in and higher average U than the youngest population but the atomic Th/U of both are the same (~0.5).

The sand accumulated at the mouth of the Klamath River is gleaned from across the Klamath Mts, the Cascades, and potentially the Coast Ranges (where sample collected). These results match well the published zircons ages from the Jurassic and Cretaceous plutons of the Klamath Mts. Most plutons in the central western Klamath Mts are 150-160 Ma, and the Ironside Mt Batholith is 169 Ma. Plutons in the Central Metamorphic belt are 130-140 Ma. These are more mafic on average than those plutons further west, which could account for their lower U contents. No 177 Ma plutons are known, however dikes in the Western Hayfork Terrane are of this age. What has not been sampled are young zircons from the Cascades or Coast Ranges, zircons from metamorphosed argillaceous rocks (Th/U<0.01) or Paleozoic and older zircons of the eastern Klamaths even though the river cuts aerially-extensive units such as the Jurassic Galice Formation that is known to contain Precambrian zircons.