CRUSTAL MINERALS IN OPHIOLITIC MANTLE ROCKS

A variety of crustal minerals, including in varying proportions, zircon, corundum, feldspar, almandine garnet, kyanite, sillimanite, quartz and rutile have been recovered from podiform chromitites of the Luobusa and Dongqiao ophiolites of Tibet and the Semail ophiolite of Oman. In the Luobusa ophiolite, these minerals are associated with in-situ diamonds and moissanite. In-situ grains of corundum have been found in chromitite and some of the other crustal minerals are attached to chromite grains, metallic alloys or rutile. Zircon is common in all three ophiolites and occurs as rounded to subangular grains, about 50-300 microns across, typically with very complex internal textures. A few euhedral prisms have regular oscillatory zoning indicative of a magmatic origin. ²⁰⁶Pb/²⁰⁸U SIMS dates for the Luobusa zircons range from 549+/-19 to 1657+/-48 Ma, whereas those from Dongqiao have ages of 484+/-49 to 2515+/-276 Ma, all much older than the ophiolites. Sixteen dates on zircons from the Semail ophiolite ramge from 84+/-4 to 1386+/-46 Ma. Most zircons from Oman are older than the ophiolite but 3 grains are essentially the same age (92+/-4 to 99+/-5). These are euhedral prisms with oscillatory zoning. Most of the zircons contain a variety of low-pressure inclusions, including quartz, rutile orthoclase, mica, ilmentie and apatite. In addition all of zircons from the three ophiolites have REE and trace element compositions compatible with a crustal origin. The assemblage of crustal minerals, combined with the morphology and age of the zircon, strongly suggest derivation from crustal sedimentary rocks subducted into the mantle, where they were mixed with UHP and highly reduced phases. The preservation of these minerals can be explained by their occurrence as inclusions in chromite grains.