The North Qaidam Mountains on the northeastern edge of the Tibetan Plateau show evidence of ultrahigh-pressure (UHP) metamorphism, indicating subduction and exhumation of continental materials to > 90 km deep, within the mantle. The area most likely represents a suture zone, as an ophiolite is present, indicating the closing of an ocean. This ocean was located between the North China and South China cratons about 514 million years ago, as dated from zircons in the ophiolite. The UHP metamorphism occurred during the closing of the ocean ~495 million years ago. The North Qaidam Mountains were uplifted and the ultrahigh-pressure rocks were exhumed later, in the Cenozoic, by the India-Asia collision and the resulting formation of the Himalayas. This late tectonic event has effectively masked the older rock histories, so little is known about the early history of the North Qaidam area.
By understanding the pressure-temperature history of the eclogites included in the UHP terrane, we can reconstruct the formation of the NE Tibetan Plateau. Toward this purpose, I analyzed an eclogite under the petrographic microscope as well as with an electron microprobe. The primary minerals included garnet, quartz and clinopyroxene, with accessory minerals of rutile, titanite, amphibole and white mica. There are no reaction textures around the garnets, indicating a well preserved sample of (ultra)high-pressure conditions. The garnets are small, generally 1/2 mm or less in diameter, and euhedral with few inclusions. The white mica grains are well-formed flakes with no sign of retrogression. A titanite rim around the rutile does indicate that the rock experienced a change to lower pressure and temperature after peak pressure. These data correlate well with reconnaissance studies of eclogites from across the area; mineral zoning and pressure–temperature estimates will be collected to better constrain the tectonic history of the (U)HP rocks.
Robin Drucker, ’12
Sponsor: Emily Walsh