Monazite-xenotime thermochronometry: methodology and an example from the Nepalese Himalaya

Monazite-xenotime thermochronometry involves the integration of petrographi c, geochronological, and geochemical techniques to explore the thermal evol ution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss s ample from the Everest region of the Nepalese Himalaya that contains leucog ranitic segregations produced by in-situ anatexis. Observations of phase re lationships and the internal structure of accessory minerals made using bot h transmitted light and electron microscopy revealed the existence of multi ple generations of monazite and xenotime and guided microsampling efforts t o isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Ne arly concordant U-Pb isotopic ratios for 13 single monazite and xenotime gr ains ranged in age from 28.37 to 17.598 Ma, making determination of the tim ing of anatexis difficult without additional information. Presuming that mo nazite and xenotime were in equilibrium over that entire interval, temperat ures estimated from the yttrium contents of dated monazites range from 677- 535 degreesC. Only the highest temperatures are consistent with experimenta l constraints on the conditions necessary to produce anatectic melts of app ropriate composition, implying that the similar to 25.4-24.8 Ma dates for t he grains with high apparent equilibration temperatures provide the best es timates 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 quanti tative Y- partitioning and semi-quantitative Nd-partitioning thermometers a s 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, pro vides inconsistent Y-partitioning (641 +/- 39 degreesC) and Nd-partitioning (515-560 degreesC) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO2) in this subpopulation's monazi te. The Y-partitioning thermometer was derived from experimental data for t he (Ce, Y)PO4 binary and may be inappropriate for application to high-Th mo nazites. 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 degreesC, respectively. This consistency strongly sugges ts that the sample experienced a temperature of similar to 535 degreesC at 22.11 Ma. This finding is tectonically important because temperatures at hi gher structural levels were much higher (by similar to 100 degreesC) at the same time, lending support to earlier suggestions of a major structural di scontinuity 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 applicat ion of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 degreesC. Whether or not these temperatures are geologically meaningful is unclear without indep endent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite-xenotime thermochron ometry may be useful for "seeing through" high-grade metamorphism to extrac t temperature-time information about inherited mineral suites.