Monazite-xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U-Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677-535 degrees C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the similar to 25.4-24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded Pb-207/U-235 dates that are statistically indistinguishable from the Pb-207/U-235 dates of coexisting xenotime crystals, permitting the application of both quantitative Y- partitioning and semi-quantitative Nd-partitioning thermometers as a cro ss-check for internal consistency. One of these sub-populations of accessory minerals, with a mean 207pb/235U date of 22.364 +/- 0.097 Ma, provides inconsistent Y-partitioning (641 +/- 39 degrees C) and Nd-partitioning (515-560 degrees C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO2) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO4 binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean Pb-207/U-235 date = 22.11 +/- 0.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535 +/- 49 and 525-550 degrees C, respectively. This consistency strongly suggests that the sample experienced a temperature of similar to 535 degrees C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by similar to 100 degrees C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U-Pb discordia with indistinguishable upper intercept ages (465.5 +/- 8.7 and 470 +/- 11 Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 degrees C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite-xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.
