EXOTIC(?) DETRITAL ZIRCONS IN THE NEOPROTEROZOIC PLACERITA FORMATION OF SAN GABRIEL MOUNTAINS PROVIDE CLUES TO IDENTITY OF CONJUGATE RODINIA RIFT FRAGMENT(S)

We present a compilation of 2112 detrital zircon ages from 25 samples of quartzite and meta-arkose in the Neoproterozoic Placerita Formation that record syn-rift deposition during ca. 750 Ma breakup of Rodinia. The Placerita Formation comprises a 90 km-long belt of metasedimentary pendants in Jurassic and Late Cretaceous plutons between Placerita Canyon and Penstock Ridge. These rocks delineate the westernmost Rodinia rift facies at the southwest margin of Laurentia. Due to severe Late Mesozoic lead loss, we employed the method of Stewart et al. (2001) to individually calculate upper intercept ages anchored at the time of peak metamorphism (80 -76 Ma). Despite the thermal disturbance, we were able use our new projected ages to assess provenance with probability density and kernel density estimation diagrams.

The detrital zircon record is dominated by Proterozoic detritus best explained by erosion from currently exposed southwest Laurentia bedrock sources; e.g., maxima at 1.78-1.64, 1.50-1.35, and 1.15-1.04 Ga. Neoproterozoic grains are rare, but sufficient concordant analyses from Placerita Canyon and Limerock Canyon yield a composite maximum depositional age of 787±35 Ma (N = 14). Of particular interest are age populations suggesting extra-regional (central or eastern Laurentia) or exotic (non-Laurentia) sources. A few Late Archean grains may be derived from the Wyoming Province, but such ages are non-unique to Laurentia. Significant Late Grenville age populations (1.03-0.93 Ga) potentially record transcontinental river transport from eastern Laurentia, but erosion from Grenville orogenic belts on other continents is not precluded. Well-developed 1.28-1.2 Ga maxima may be derived in part from Texas, but peak strengths suggest a proximal source that has been rifted away. Likely exotic sources required for Rodinia fit are recorded by distinct maxima at 2.0-1.8 Ga and less-defined populations between 1.63 and 1.51 Ga that occur throughout the Placerita belt. These grains have potential ties to orogenic belts in Australia or Siberia, but apparent polar wander paths are problematic. The Yangtze and South China blocks may offer the best paleomagnetic fit, but 2.0-1.8 Ga bedrock is missing. Antarctica lacks a match to both exotic detrital populations. The source of 1.28-1.2 Ga grains remains cryptic and elusive.