GEOCHRONOLOGY OF THE FRANCISCAN SOUTH FORK MOUNTAIN SCHIST IN THE YOLLA BOLLY AREA, NORTHERN CALIFORNIA

We have determined SHRIMP-RG U-Pb ages on 285 detrital zircon grains from 7 samples from the Franciscan Eastern belt (EB). The youngest clusters of zircon ages place upper brackets on the depositional ages of the protoliths of EB units at the sampled localities as follows: South Fork Mtn Schist (SFMS), ≤135 Ma; Valentine Springs Fm (VSF), ≤120 Ma; Yolla Bolly terrane (YBT), ≤111 Ma. Ages of subsequent accretion and metamorphism must be younger.

Three new step-heat Ar/Ar ages on pure separates of metamorphic white mica from the SFMS clustered tightly at 121 Ma. About 38 K-Ar and Ar/Ar total gas (not step heat) ages have previously been reported on SFMS whole rock (WR) samples and cluster loosely around 120 Ma. However, some WR ages deviate markedly. These deviations show little correlation with location, structural position, etc. Reconsideration of these data suggests that the entire ≈330-km-long outcrop belt of the SFMS is characterized by a single, strikingly consistent argon cooling age of approximately 121 Ma. Older ages appear to be unreliable.

The volume of Franciscan rocks that are older than the SFMS is very small (e.g., high-grade blocks, Skaggs Spring schist), whereas the volume that is slightly younger is large (e.g., VSF, YBT). This suggests that the accretion of the SFMS marks a transition from predominately nonaccretionary to accretionary conditions in the Franciscan subduction zone. The SFMS protolith was predominately mudstone, whereas younger units contain substantial metagraywacke, so there may have been a simultaneous transition in sedimentary facies. The well-known c. 140-120 Ma magmatic lull in the Sierra Nevada also ended at essentially the same time.

The SFMS exhibits certain relations suggestive of an inverted metamorphic gradient generated at the initiation of subduction, but other relations do not match this model very well. Instead, it is more likely that accretion, metamorphism, and cooling of the SFMS and the near-simultaneous rejuvenation of the Sierran arc mark some major change in subduction (acceleration?, shift to more orthogonal convergence?) at about 122 Ma. Speculatively, the Pacific-Farallon-Phoenix triple junction jumped about 1200 km to the SE at virtually this time and reorganization of oceanic plates may have altered relative plate velocities at the Franciscan trench.