PROGRESSIVE, LONG-LIVED, VERSUS PUNCTUATED HP METAMORPHIC EVENTS: A REGIONAL PERSPECTIVE FROM THE FRANCISCAN COMPLEX, CALIFORNIA

For many orogenic belts with HP or UHP metamorphism, a single age is commonly reported or sought for a metamorphic event, as if a subduction event were expected to result in but a single age of peak burial and metamorphism. Implicit in such an analysis/interpretation is that (1) metamorphism resulted from an exceedingly brief subduction episode and/or (2) a combination of subduction erosion or exhumation processes resulted in preservation of deeply subducted rocks representing a small time fraction of a subduction episode. Whereas many of the HP or UHP rocks may have been generated or preserved by short-lived subduction and exhumation episodes, the Franciscan Complex of California preserves a record of HP metamorphism and exhumation that spans at least 80 million years. This span of ages results from subduction-accretion of different packets of rocks at different times during an extended episode of subduction instead a single peak event with later fluid infiltration events or thermal events, as shown by protolith age constraints of many of the blueschist-facies rocks. The youngest coherent Franciscan HP rocks have protolith ages no older than about 80 Ma, based on the younger detrital zircon populations in them, whereas the oldest Franciscan HP rocks, including both blocks-in-mélange and coherent units, have metamorphic ages (hbl Ar-Ar, grt Lu-Hf) of about 150-170 Ma. Accordingly, the oldest rocks were subducted and metamorphosed 70-90 m.y. before the protoliths of the youngest HP rocks were formed. Many of the HP clastic rocks and sedimentary mélanges contain clasts that record an earlier episode of HP (subduction) metamorphism followed by surface exposure, redeposition, and subduction. At one locality two cycles of subduction-exposure-subduction-exposure may have taken place in less than 50 to 60 m.y. Other localities suggest that the earliest surface exposure of the oldest and highest grade rocks may have taken place in less than 10 to 35 m.y. after their initial subduction and metamorphism, based on high-grade metamorphic ages and depositional ages of the unit the blocks occur in.