DIACHRONY OF ANATEXIS DURING OROGENESIS IN THE NEPALESE HIMALAYA

The metamorphic core of the Himalayan orogen shows evidence for polyphase granite generation and emplacement due to anatectic melting. These melts range from millimeter-scale leucosomes to leucogranitic plutons outcropping over hundreds of square kilometers. The generation of these melts directly reflects the evolving thermal structure of the orogen, and likely relates to a combination of shear heat generated along faults, elevated heat flow due to high concentrations of radioactive heat-producing elements, and decompression due to denudation.

Evidence for the polyphase generation and emplacement of leucogranites can be seen in single outcrops where cross-cutting relationships between multiple intrusive phases are observed, or where intrusions are interspersed with multiple deformational events. In one such outcrop near Gokyo in the Everest region of the Nepalese Himalaya, three phases of leucogranitic dikes intrude a structurally high region of the orogen’s metamorphic core. ⁴⁰Ar/³⁹Ar biotite ages for samples from this outcrop are 23.73 ± 0.37 Ma for the country rock, and 20.31 ± 0.22 Ma, 19.98 ± 0.37 Ma, and 17.91 ± 0.47 Ma for the intruding leucogranites. Isotope correlation analysis of these data indicates that the nonradiogenic Ar component in the samples had isotopic characteristics comparable to that of modern day atmosphere. The biotite ages are distinct and agree with the crosscutting relations observed. Minimum age estimates for the intrusions of the dikes are provided by the three youngest biotite dates. The biotite date of the youngest dike, which is undeformed and cross-cuts all other fabrics, is comparable to the 17.8 Ma crystallization age of a similar dike sampled structurally below the Gokyo outcrop, as determined by U-Pb analysis of monazite. This suggests that the final melt phase was emplaced during a distinct pulse of magmatism which intruded relatively cool crust, as the magnitude and duration of heating were insufficient to reset host rock biotite.