DETRITAL ZIRCON AGES FROM THE CALAVERAS COMPLEX, WESTERN METAMORPHIC BELT (WMB), CALIFORNIA: IMPLICATIONS FOR MESOZOIC TECTONICS AND CONTINENTAL GROWTH

Southern portions of the WMB, Sierra Nevada, California are composed of four major complexes including from east to west: the Shoo Fly Formation, the Calaveras Complex, the Sullivan Creek Terrane, and the Foothills Terrane. The Calaveras Complex has been previously interpreted to be a Permo-Triassic accretionary wedge that along its western margin was juxtaposed across the Sonora Fault with the volcanic rich Sullivan Creek Terrane. Unpublished mapping by Bhattacharyya and Paterson indicate that from east to west units in this part of the Calaveras Complex are: (1) a package of quartzites and phyllites, (2) a unit dominated by banded chert, (3) an argillite/siltone unit with local pebbly mudstones, chert lenses, and Permian to early Triassic limestone blocks, and (4) a sedimentary package of quartzites, phyllites, marble, pebbly mudstones and local chert sandwiched between large volcanic belts. One marble layer surrounded by phyllite and siltstone in the western part of #4 preserves Late Triassic fossils. We find no evidence for the existence of a terrane bounding fault between these rock packages. We collected seven samples from the different clastic units of the Calaveras Complex. Four of these had too few or zircons too small to date. However we successfully completed ICP-MS U/Pb dating of detrital zircons in three samples, two from the easternmost package of quartzite/phyllite and a third from the western package near one of the volcanic belts. Minimum age peaks of ~159 and ~150 Ma occurred in the former and ~170 Ma in the latter with Paleozoic zircons and older peaks at ~1100, ~1760, ~2800 Ma in samples as well. These zircon ages indicate that much of what has been mapped as Calaveras Complex in the southern WMB is Jurassic and thus younger than the enclosed Permo-Triassic fossiliferous marble blocks, and surprisingly similar in age to the Mariposa Formation in the Foothills Terrane. It also confirms that clastic units with both Jurassic, older Paleozoic and Precambrian zircons occur in the Calaveras Complex. No ages are yet available from the banded chert-rich units leaving open the question of whether Jurassic rock packages are getting tectonically mixed in with older chert-rich units.