THE P-T-T-D EVOLUTION OF THE MAHABHARAT KLIPPE, EAST-CENTRAL NEPAL: IMPLICATIONS FOR THE OUT-OF-SEQUENCE EVOLUTION OF THE HIMALAYA

The Mahabharat klippe in east-central Nepal is one of a series of such structures that occurs along frontal portion of the Himalaya. The rocks hosted therein have received little attention relative to those exposed farther north in the High Himalaya. In order to better understand the development of the klippe this work presents new metamorphic, geochronological and deformation data from a garnet + biotite + muscovite + chlorite + quartz schist. Thermodynamic modelling indicates a prograde path from garnet nucleation (c. 510 ˚C, 4.6 kbar) to peak conditions (c. 570 ˚C, 5.8 kbar). Quartz c-axis fabrics from the same specimen indicate post-peak deformation at 520 ± 50 ˚C, coincident with Ti-in-biotite thermometry from grains replacing garnet in strain shadows (c. 520 ˚C). Garnet Lu-Hf geochronology yielded 32.4 ± 0.3 Ma. Considering that most of the Lu is sequestered in grain cores, this age is interpreted to represent early prograde growth.

Previous chronological research elsewhere in the Mahabharat klippe indicates that prograde metamorphism occurred between 35 and 28 Ma, while the retrograde history passed through Ar closure at c. 17 Ma. The data presented here confirm those results and establish the start of prograde metamorphism robustly in the earliest Oligocene. The data further substantiate the contrast in metamorphic history between these frontal-Himalayan rocks and amphibolite-facies rocks in the High Himalaya to the North; the latter record younger prograde metamorphism, peak conditions at ~ 22 Ma, and a retrograde monazite history that lasted at least until 14 Ma with cooling through Ar closure at c. 9 Ma. A similar diachronous evolution is observed in west Nepal. There, high-grade rocks in a frontal klippe record Eocene prograde metamorphism followed by Oligocene to early Miocene retroression, whereas metamorphism in the nearby High Himalaya to the North is mostly of late Miocene age. These resolvable and laterally recurring contrasts characterize an out-of-sequence development of the Himalayan system that reflects large-scale tectonic adjustments.