DETRITAL ZIRCON GEOCHRONOLOGY FROM A CLASTIC UNIT OVERLYING THE JARBO GAP OPHIOLITE, CENTRAL BELT, SIERRA NEVADA

The Jarbo Gap Ophiolite, located in the Sierra Nevada north of Oroville, is comprised of greenschist-facies mafic and ultramafic rock underlain by high-pressure amphibolite and overlain by a thin cover of pelagic sedimentary and volcanic rock. It represents one of the northernmost exposures of the Central Belt, a north-south trending composite of Triassic-Jurassic arc, ophiolitic, and subduction complex rocks exposed in the northern Sierra Nevada. The Jarbo Gap Ophiolite is bounded to the south by the Big Bend Fault, a northern continuation of the Bear Mountains Fault Zone. Immediately on the other side of the fault is a hemipelagic, siliceous sedimentary unit with mélange horizons that has been interpreted as depositionally overlying the ophiolite and correlated with the Clipper Gap and Colfax Formations of the Fiddle Creek Complex. Existing radiolarian identification from Fiddle Creek units indicate these rocks are Mid-Triassic through Early-Jurassic in age.

Here we present detrital zircon data from this clastic unit overlying the Jarbo Gap Ophiolite. The detrital zircon sample was processed using a 70%-110% concordance filter (n=235). The detrital age spectrum shows approximately 75% Proterozoic zircons, with only 10% Mesozoic in age. A maximum depositional age (MDA) of 162 Ma was determined using the average of the three youngest concordant zircon ages. The detrital age spectrum contains peaks at about 200-250 Ma, 300-350 Ma, 1000-1100 Ma, 1400-1450 Ma, 1750-1900 Ma, and 2550-2650 Ma.

It is not clear which sedimentary units in the Sierra Nevada or Klamath are most correlative to these rocks. The Jarbo Gap clastic rocks are older than the Mariposa Formation near Lake Don Pedro, which is reported to have an MDA ca. 152 Ma, and the Galice Formation, which is as young as 151 Ma. Additionally, the Jarbo Gap clastic rocks contain a substantial peak between 1800-1900 Ma which is absent in the Mariposa Formation and boast a high ratio of Proterozoic to Phanerozoic zircon ages, disparate from the Galice Formation. However, the large Proterozoic zircon population and approximately 162 Ma MDA found in the Jarbo Gap clastic rock is similar to published data from the Rattlesnake Creek Terrane of the Western Klamath.