MÉLANGES WITH HP METAMORPHIC ROCKS IN SUBDUCTION COMPLEXES: DEFORMED OLISTOSTROMES RATHER THAN EXHUMED SUBDUCTION CHANNELS?

Many believe that the incorporation of high-pressure (HP) metamorphic rocks into mélanges results from large-scale return flow in a subduction channel, and that such mélanges represent exhumed subduction channels or plate interfaces. In the Franciscan Complex of California, the most striking examples of such mélanges are those with "high-grade", primarily metamafic, blocks, including HP amphibolite and eclogite. For examples with serpentinite-poor/free matrix, the blocks are clearly higher grade than the matrix, whereas this relationship is less clear for serpentinite matrix, due to the greater probability of retrogression of the latter. The high-grade metamorphic ages of the blocks are more than 50 million years older than the accretion age of the enclosing mélanges. The block-matrix metamorphic contrast, and the large difference between metamorphic and accretion ages, is difficult to explain with a simple subduction channel model. Franciscan mélanges with high-grade blocks display widespread evidence for a sedimentary (submarine sliding) origin rather than a subduction channel (tectonic) origin. The matrix comprises sedimentary breccia and conglomerate, including shale-rich, sandstone-rich, and serpentinite-rich examples. In several examples, the matrix metamorphism indicates subduction of the olistostromes to blueschist facies conditions. Many of the blocks in such units record at least two burial-exposure cycles to blueschist facies or greater depth. The first cycle involved exhumation of the blocks to the sea floor prior to olistostromal deposition, and the second involved subduction and exhumation of the olistostrome and the underlying block-free unit together. Metasomatic rinds around high-grade blocks apparently formed by reaction with surrounding ultramafic rocks. Ar-Ar ages of some rinds are less than 10 million years younger than high-grade metamorphic ages, indicating incorporation of the blocks in serpentinite mélanges 40 million years before the introduction of blocks into the oldest olistostromes. The first exhumation of the high-grade blocks may have taken place in the earlier serpentinite mélange(s). These older mélanges have yet to be found, so their origin (diapiric, shear zone, or sedimentary) remains unknown.