Abstract:As a result of protracted granitic magmatism （～150 myr）in the extinct Mesozoic Sierran continental arc, California, the metasedimentary framework rocks had undergone up to amphibolite facies intermediate- to high-grade metamorphism and partial melting of labile wall rocks within the proximity of invading plutons. These partial melting processes resulted in the formation of a number of migmatite complexes in the southern Sierra Nevada batholith. Such well-preserved migmatite complexes provide a natural laboratory to investigate the geochemistry of metasediment-derived melts. Rare earth element geochemistry data on the Goat Ranch migmatite complex, one of the best preserved migmatite complexes, show that the leucosomes, migmatites, and metapelites have: (1) pronounced negative Ce anomalies; (2) subparallel chondrite-normalized REE distribution patterns; (3) leucosomes have lower LREE contents than those in migmatites and pelites; and (4) contrary to the migmatites and pelites, leucosomes have various Eu anomalies. The Isabella metapelites contain a fair amount of graphite as shown in whole rock dissolution experiments and petrographic observations, which suggest that they are strongly reduced metasediments. A similar conclusion was alsoreached from previous studies on mineral geochemistry of biotites from these rocks and adjacent granitoids. These data and previous Sr, Nd isotopic compositiondata on the leucosomes, migmatites and metapelites demonstrate that pronounced negative Ce anomalies in leucosomes were inherited from their source rocks, not due to differential dissolution or fractionation crystallization of accessory phases (e.g. apatite, monazite or zir- con) during partial melting of metapelites. The finds from this study also suggest that if sediments with Ce anomalies above a subduction zone are introduced into the island arc magma source regime via either subduction or subduction erosion, and participate in island arc magmatism, then sediment-derived melts can contribute to the formation of those arc magmas with negative Ce anomalies.
