Paper No. 6
Presentation Time: 2:30 PM


KOHN, Matthew J., Dept. of Geosciences, Boise State University, 1910 University Dr, Boise, ID 83725,

The ages of undeformed vs. deformed leucogranites and migmatites are commonly used to constrain Himalayan tectonic processes. Leucogranite and migmatite zircon (Zrc) ages and temperatures (T), however, generally violate petrogenetic models – creating a “leucogranite-migmatite problem.” Key observations include: (1) Many leucogranites are texturally and geochemically linked to migmatization via the muscovite dehydration-melting reaction at T≥~700 °C. (2) From eastern Nepal to western Bhutan, leucogranites intrude low-P granulite-facies migmatites (cordierite- and spinel-bearing) with thermobarometrically- and petrogenetically-constrained T~800 °C. Isothermal exhumation at T~800 °C is invoked to catalyze partial melting, leucogranite intrusion, and granulite-facies metamorphism.

This scenario implies: 1) Leucogranite Ti-in-Zrc and Zrc-saturation T’s should be ≥700 °C (melts are Zrc-saturated and should be solid below 700 °C). 2) In migmatites, Zrc solubility increases during isothermal exhumation, so no new Zrc should crystallize during this interval and older Zrc should dissolve, leaving an age gap. 3) Minimum Ti-in-Zrc T’s should track the T-t history (e.g. T≥~800 °C at the peak, systematically decreasing afterwards).

Instead, published data show: 1) Zrc ages span 20 Myr for migmatites and leucogranites, including all proposed times of isothermal exhumation. Why would Zrc crystallize during melting, and why wouldn’t older Zrc dissolve in increasingly Zrc-undersaturated melts? 2) Half of Ti-in-Zrc T’s and 90% of Zrc-saturation T’s are ≤700 °C. Only 5/150 Ti-in-Zrc T’s exceed published T-t curves, most are ~100 °C lower. How can a 700 °C melt or Zrc crystallize when the host rock is 800 °C?

Possible questions and counterarguments include: 1) Are Zrc and melt T’s underestimated? Experimental calibrations are extremely robust; 800 °C Ti-in-Zrc T’s would require unrealistically low a(TiO2) [≤0.25] and contrast with independent estimates [≥0.95]. 2) Are metamorphic T’s overestimated? The consistency of phase equilibria and thermobarometry is persuasive. 3) Are U-Pb ages reset or mixed? All ages are concordant. 4) Does diffusion matter? Pb and Ti diffusivities are far too slow. A community effort is needed to resolve this issue, which otherwise casts doubt on past tectonic interpretations.