Ion Microprobe Depth Profile Analysis of Micron-Scale Metamorphic Rims on Detrital Zircons from the Pelona Orocopia Rand Schist

Flat-slab subduction of the Farallon Plate beneath western North America in the Late Cretaceous is believed to be the principal driver of extinction of the Sierra Nevada arc and the eastward migration of the locus of magmatism and deformation. Prior work has established that the sedimentary protoliths of the Pelona Orocopia Rand Schist of southern California experienced subduction, erosion and accretion synchronous with this flat-slab subduction event. Recent work has attributed underthrusting of the forearc system beneath north America more specifically to subduction of the Shatsky Rise conjugate in the Late Cretaceous. Thus, knowledge of time scale and thermal conditions of PORS emplacement is critical to understanding the tectonic evolution of western North America and, more generally, flat-slab subduction margins.

Recently recognized metamorphic zircon overgrowths from Cemetery Ridge and the Plomosa Mountains raise the possibility of refining the chronology of subduction and accretion of the PORS. Application of depth profile laser ablation split stream ICP-MS methodologies to detrital zircons from exhumed Orocopia Schist exposed within the Chocolate Mountain Anticlinorium of southeastern California has revealed that many grains exhibit very thin rims (< 1 μm) of metamorphic zircon. Accordingly, we are applying secondary ionization mass spectrometry to acquire U-Pb and trace element depth profiles with SHRIMP-RG. The much lower sputtering rates achievable with the ion probe (5-10 nm/sec) provides the ability resolve U-Pb ages from submicron-scale metamorphic rims and provides complementary trace element data to assess their petrologic significance.