UC Santa Cruz

The Pelona-Orocopia-Rand Schist (PORS) of southern California and southwest Arizona are interpreted as Late Cretaceous correlatives of the Jurassic-Cretaceous Franciscan accretionary complex that were subducted and accreted beneath cratonal and Mesozoic magmatic rocks during flat-slab subduction of the Farallon oceanic lithosphere. The Orocopia Schist (OS) of southeast California and southwest Arizona occurs east of the San Andreas fault and represents the furthest inboard member of the 800 km PORS belt. While the bulk of the Orocopia Schist exposures occur within the core of the 230-km-long, NW-SE– to E-W–trending Chocolate Mountains Anticlinorium (CMA), recent discoveries at Cemetery Ridge and the Plomosa Mountains significantly increase the known areal distribution of the PORS in southwestern Arizona. These exposures occur 300 km inland of the present-day continental margin and up to 450 km inboard of the reconstructed position of the trench.

The Orocopia Schist provides a unique opportunity to investigate the timing of the underplated schist and the metamorphic conditions the schist underwent during low-angle subduction. Previous work determined the evolution of the PORS by constraining the time between the youngest detrital U/Pb zircon ages and oldest 40Ar/39Ar metamorphic mineral cooling ages, thus determining the PORS “cycling interval”. However, the recent discovery of metamorphic zircon growth in samples from Cemetery Ridge and the Plomosa Mountains has permitted more direct estimates of the time of metamorphism that attended accretion of subducted sediment beneath cratonal southwestern Arizona.

Here, we present U/Pb age data and chemical proxies for recrystallization (e.g., Th/U) from overgrowths upon detrital zircon grains from several different major exposures of Orocopia Schist. This study utilized cycle-by-cycle depth-profile laser-ablation split-stream ICP-MS methods to target <5µm-thick zircon metamorphic overgrowths in an attempt to refine the timing of schist subduction and emplacement.

Preliminary results identify metamorphic rims with young 206Pb/238U ages and low (<0.1) Th/U that support an interpretation of metamorphic recrystallization of ~57-86 Ma. Collectively, these data yield a weighted-mean 206Pb/238U age of 64.14 ± 2.80 Ma which is consistent with previous assertions that metamorphism began during the late Cretaceous. Youngest rim ages (~57 Ma) observed in this analysis are comparable to other recent studies that record ages as low as 40 Ma in the easternmost outcrops of the Orocopia Schist. The ages recorded here represent a west-to-east younging trend in schist exposed along the CMA, which may indicate that metamorphism was time-transgressive.