HIGH-TEMPERATURE PARTIAL-MELT EVENT RECORDED IN THE FIRST CONFIRMED ECLOGITE FROM THE SIERRA NEVADA METAMORPHIC BELTS, CALIFORNIA

The Triassic-Jurassic Central Belt of California’s Sierra Nevada is a collage of island arc and ophiolitic material. The presence of rutile-bearing amphibolite-facies metabasalt in the belt indicates that some of these rocks have been subducted, but there has been no confirmed report of eclogite or blueschist. One of the most structurally intact exposures of Central Belt ophiolite is at the Tuolumne Ophiolite, near Lake Don Pedro. There, an intact ultramafic sheet is structurally underlain by a thin unit of highly strained amphibolite-facies metaclastite, mafic garnet amphibolite, serpentinite, and metachert which has been previously interpreted as a metamorphic sole. These rocks are unconformably overlain by rocks of the ~200 Ma Peñon Blanco arc.

The mafic garnet amphibolite preserves eclogite relics overprinted by a high-temperature (HT) melt event. Garnets are embayed, in contact with high-Al Ca-amphibole, and contain inclusions of omphacite—indicating early eclogite—along with Ca-amphibole, epidote, rutile, and polymineralic aggregates. The polymineralic inclusions consist of K-feldspar, Ca-amphibole, phengite, and barite and are contiguous with thin cracks filled with K-feldspar that cross-cut omphacite. We interpret these aggregates and veins as recrystallized melt derived from phengite. Garnet-pyroxene, garnet-amphibole, and Zr-in-rutile thermometry on inclusions indicate temperatures of about 550 to 650 °C for the eclogite, too low to melt phengite. These textural relationships and temperatures indicate that the HT event post-dated eclogite-facies metamorphism.

The ages of the eclogite and HT metamorphic events are not clear. Two zircon separates from these rocks yielded U-Pb ages of 216 and 220 Ma, however zircon Th/U and REE values are most similar to those of igneous zircon. Poorly documented K-Ar and Ar-Ar hornblende dates from the amphibolite range from 178 to 200 Ma and may record cooling at the tail end of the HT event.

The observations indicate that in Central Belt, the lack of eclogite in the structurally highest part of the subduction complex can be at least partially explained by a HT event that overprinted the rocks, destroying earlier evidence of HP metamorphism. This suggests that other rutile-bearing amphibolite found in the Central Belt may record an earlier HP history.