TIMING OF METAMORPHISM AND DEFORMATION IN THE FOOTWALL OF THE MAIN MANTLE THRUST, SWAT VALLEY, PAKISTAN

The geology of the Swat region in northern Pakistan was the subject of investigation in the 1980s and early 1990s before political instability made fieldwork there difficult. That early work outlined a multiphase overprinting deformation history at least partially coeval with metamorphism that reached sillimanite grade locally. Published pressure-temperature estimates and geochronologic constraints are limited to the techniques available ~ 30 years ago. Zircon U-Pb rim dates and ⁴⁰Ar/³⁹Ar dates on hornblende from the region bracket metamorphism between ~47 and 31 Ma. The hornblende dates have also been combined with the observation of hornblende overgrowth across a late crenulation cleavage in some rocks to interpret that the majority of the deformation in the region is pre-31 Ma. Alternative interpretations regarding the timing of the youngest phase of deformation suggest that it may be pre-36 Ma, post-24 Ma or even ~5 Ma.

New geologic mapping, metamorphic petrology and monazite petrochronology data from the Loe Sar dome in the Swat region offer refined constraints on the timing of metamorphism and deformation. Phase equilibria modelling and textural observations indicate that metamorphism in the area followed a prograde burial path from ~510 ˚C and 2.8 kbar to ~630 ˚C and 6.9 kbar before a tight return path. Monazite petrochronology indicates metamorphism occurred between ca. 39 Ma and 28 Ma. Furthermore, partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite indicate that they grew in equilibrium. This relationship is confirmed through the monazite-garnet thermometer, which indicates a temperature of ~ 611 ± 35 ˚C that overlaps with the ~630 ˚C garnet rim temperature estimate.

Microtextural relationships indicate that garnet growth occurred before the development of the main foliation in the area. That foliation is folded around the regional Loe Sar dome and is crenulated at the meso and micro-scale. These relationships indicate that such deformation must be younger than 30-28 Ma monazite that grew with garnet rims at peak pressure and temperature conditions. This is consistent with interpretations of past studies that suggest the regional crenulation cleavage developed post-24 Ma, but inconsistent with those that suggest it developed either pre-31 Ma or pre-36 Ma.