Crust-Mantle Recycling Recorded In Tibetan Ophiolites

Numerous crustal silicate minerals, including zircon, corundum, quartz, kyanite, feldspar and almandine garnet have been recovered from podiform chromitites of the Luobusa and Donqiao ophiolites of Tibet. In addition, the Tibetan ophiolites also contain several ultrahigh pressure minerals such as diamond, coesite and moissanite (SiC), some of which are hosted as inclusions in Os-Ir alloys. The zircon grains range from 20 to 300 µ across, and are mostly well rounded with very complex internal structures. A few grains are euhedral to subhedral and have concentric zoning suggesting an igneous origin. Zircon grains from the Luobusa ophiolite contain only low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. 206Pb/238U SIMS dates for the Luobusa zircons range from 1657-549 Ma, whereas those for the Donqiao zircons range from 2515-484 Ma. In both cases the zircons are much older than the ophiolites (~125 Ma for Luobusa and ~175 Ma for Donqiao). Several grains from both ophiolites contain distinct cores with overgrowth rims. The older zircons in both ophiolites are interpreted as xenocrysts, probably derived from metasedimentary rocks and introduced into the mantle by subduction. The grains of other silicate minerals range up to about 0.5 mm and are moderately to well rounded. Smaller, angular fragments of such grains are also present. The morphology of these grains strongly suggests derivation from reworked sedimentary material, probably from the same sources as the zircon. Similar collections of minerals have been recovered from podiform chromites in other ophiolites. The recovery of the same minerals from samples of widely separated ophiolites, processed in different laboratories, argues against natural or anthropogenic contamination. The preservation of such minerals, particularly quartz and coesite, in the mantle implies isolation from the enclosing rock, perhaps in xenoliths.