DIFFUSION IN ECLOGITE GARNETS FROM THE NORTH QAIDAM (U) HP METAMORPHIC BELT, CHINA, AND ITS IMPLICATIONS FOR THERMOBAROMETRY

The northeastern region of the Tibetan plateau is composed of multiple terranes and tectonically altered zones, including the North Qaidam (ultra)high-pressure metamorphic belt (NQMB). The NQMB is roughly 15 km wide, extends 350 km NW-SE, and contains well-exposed continental collision-type (U) HP metamorphic rocks. Studies in the NQMB actively question the timing and nature of metamorphic processes that occurred before the Himalayan uplift. Thermobarometric studies of eclogites from the NQMB depict a clockwise pressure-temperature (PT) path from subduction to exhumation, starting with prograde metamorphism to amphibole-eclogite facies with retrogression to epidote-amphibolite facies.

NQMB eclogites are spread throughout the host orthogneiss and record HP conditions within their mineral assemblage due to their fluid-poor nature. The host gneiss in the NQMB also contains HP mineral assemblages, implying that the eclogites and their host rocks were subducted as a single unit to (U) HP depths. Sample E9814B1 is from an eclogite located in the NE corner of the NQMB, in the Luliangshan; this region records metamorphic dates of 495-485 Ma. We analyzed sample E9814B1 by electron microprobe and laser-ablation ICP-MS to reconstruct its PT path using diffusion rates of major elements in garnets.

This sample contains grt+cpx+bt+qtz and exhibits no symplectite. The garnets are 3-5 mm across and have well-preserved euhedral rims and minimal inclusions. The grossular and spessartine components exhibit bell-shaped profiles, implying prograde growth with the core recording maximum pressures. The Mg# is zoned in an inverse bell-shaped curve indicating growth under increasing temperature. Major element patterns reveal two zones of retrograde equilibration “rims”, implying two stages of garnet growth. The calcium concentration generally increases towards the core with slight variations within the mantle. Trace element zoning reveals additional complexities with variations in the LREE and transition metal concentrations. Most trace elements show bell-shaped curves; although, some transition metals show an off-center peak. By using a Jacobian matrix, with changing pressure and temperature, composition and distance across the grain as variables, we were able to further refine the clockwise PT path of the NQMB.