An ultrahigh-pressure (UHP) metamorphic zone has been identified in the North Qaidam Mountains of China. Located on the northeastern edge of the Tibetan Plateau, the North Qaidam Mountains contain UHP eclogites, which indicate subduction of continental materials to > 90 km deep. UHP metamorphism occurred ~495 million years ago; the presence of an older ophiolite (~514 million years old) indicates that the area is part of a suture zone, where two continental blocks collided after the closing of an ocean. The uplift of the Tibetan Plateau in the Cenozoic has added a layer of complexity to the area and has masked the tectonic histories of these older rocks.
The purpose of this study is to constrain pressure and temperature (PT) histories of eclogites from the Luliangshan, the northwestern range of the North Qaidam Mountains. Eclogite samples CM-5-27-04-7A, CM-6-7-04-7A and CM 6-6-04-8A range from well-preserved to moderately preserved and include primarily garnet, clinopyroxene and quartz. Accessory minerals include rutile, titanite, amphibole and white mica. Some titanite has grown around the rutile grains, indicating that the samples recorded lower pressures and temperatures after higher pressures. In some of the samples, there are also intergrowths between mineral grains. We used an electron microprobe to collect elemental data from each of the three samples; this elemental data was processed through the multivariable program THERMOCALC to estimate pressures and temperatures for each point. Preliminary calculations indicate that our samples retain multiple stages of the PT path of the rock. Our highest pressures come from near the cores of the garnet grains and are ~22 ±1.5 kbar, though chemical analysis indicates that this cannot be the “peak” pressure, which was not preserved. Likewise, our highest temperatures are from both the center and the edge of the garnet grains and are ~650 ±50˚C, with a dip in the middle to ~565 ±50˚C. Using the PT data, we created a model showing how the rocks moved through the Earth. This model accounts for the depth and velocity of subduction and exhumation, thus aiding in the construction of an overall theory of formation for the North Qaidam Mountains.
Robin Drucker, ’12
Sponsor: Emily Walsh