QUANTIFYING PATHS IN UHP TERRANES: A 3-D APPROACH USING THERMOBAROMETRY AND FIELD RELATIONSHIPS

Ultra high-pressure eclogites from the Tso Morari gneiss dome in Ladakh, India are the focus of this study. The Tso Morari dome is located in the Himalayas and was formed during the continental collision of India and Asia. The UHP unit consists of numerous discrete blocks of eclogite ranging in size from 1 m to 100s of m within a leucocratic quartzo-feldspathic orthogneiss. Eclogite blocks within the dome represent the leading edge of the subducted Indian continental margin. The occurrence of eclogite boudins within leucogneisses is a common feature of many other UHP localities including Dabie Shan, China and the Western Gneiss Region, Norway. Mafic eclogite blocks preserve the best mineral indicators that the rock has experienced ultra high -pressure conditions. Garnet, omphacite, and phengite are compositionally zoned suggesting changing pressure and temperature conditions during growth. Electron microprobe analyses of the mineral composition in the eclogites including garnet traverses and spot analysis of omphacite and phengite were collected. This data has been converted to atoms per formula unit, which was then used to calculate pressure and pressure conditions using published thermometers and barometers. Preliminary thermobarometric calculations on the eclogite facies assemblage yield UHP conditions and subsequent mineral retrograde growth records exhumation conditions. Reconstruction of the growth history using these P-T plots provides a proxy for position in crust during collision, depth of subduction, and exhumation paths. Donaldson et al age calculations on metamorphic zircon from the same sample (DD71710-2b) used in this study suggest that the UHP stage occurred at 44.1 ± .9 Ma. P-T path reconstruction will help to clarify the timing of UHP metamorphism, and subsequent exhumation, helping to better understand the India-Asia collision and Himalayan orogeny.