PROGRADE METAMORPHIC HISTORY OF THE NANGA PARBAT MASSIF: A RECORD OF PLIO-PLEISTOCENE GEODYNAMICS, PALEOGENE COLLISION, OR BOTH?

The Nanga Parbat Massif (NPM) in the western Himalayan Syntaxis exposes some of Earth’s youngest high-grade metamorphic rocks and leucogranites (melt crystallization as young as 0.7 Ma), which have been exhumed at rates of ~10 km/Ma. The geodynamic mechanisms for such rapid exhumation rates in the Himalayan Syntaxes remain debated. One outstanding problem centers on the prograde metamorphic history of the NPM prior to the initiation of rapid exhumation: did rocks in the core of the NPM reside in the mid-crust since the early (Paleogene) stages of Himalayan orogenesis, or was their metamorphism at mid-crustal depths solely a result of Plio-Pleistocene geodynamic processes? Answering these questions are required to assess permissible geodynamic scenarios to explain rapid exhumation of the NPM, including the role of tectonic and climatic factors that have been previously proposed.

We present a petrochronologic study of rocks from the core of the NPM in an attempt to reconstruct its prograde metamorphic evolution. In-situ monazite (mnz) U-Th-Pb + trace element (TE) analyses and garnet TE mapping from one sample show that mnz grew during prograde garnet growth and apatite/zircon breakdown from ~6-4 Ma. Thermodynamic modelling predicts that garnet growth in this sample requires heating ± burial at pressures < 0.7 GPa; thus, our data suggest the NPM was undergoing mid-crustal heating ± burial at ~6-4 Ma. Mnz growth continued from ~3-1 Ma during near-isothermal decompression, consistent with our previously published results. Zircon U-Pb + TE depth profiling from multiple samples suggests metamorphic zircon recrystallization only occurred in the Plio-Pleistocene, in the presence of apatite and garnet. Diffusion modelling of preserved major element zonation in peritectic garnet rims of a restitic pelite suggests brief residence (<1 Ma) at peak temperatures, requiring that garnet rims are young - in agreement with published mnz data from the same sample.

At face value, our data suggest both prograde and retrograde metamorphism were the result of Plio-Pleistocene geodynamic processes. Sm-Nd garnet geochronology will be presented to determine the absolute timing and rate of prograde garnet growth. We will discuss the implications of our data on permissible geodynamic scenarios for extreme exhumation rates in the NPM.