DOCUMENTATION OF NEOGENE REGIONAL METAMORPHISM IN THE HIMALAYAS OF PAKISTAN USING U-PB IN MONAZITE

In the Himalayas, investigators have constrained the timing of metamorphism associated with the Eocene orogeny through the use of cooling ages on a number of minerals. The majority of such studies give minimum metamorphic ages of approximately 30-40 Ma [1-6], largely confirming theoretical studies predicting peak metamorphic conditions occurring tens of millions of years after large-scale collision [e.g. 7]. In northwestern Pakistan, however, the Nanga Parbat-Haramosh Massif (NPHM) exhibits much more recent activity, with rapid and accelerating cooling over the past 10 Ma [1,2]. The NPHM also exhibits unexpectedly recent igneous activity: three leucogranite dikes give minimum zircon U-Pb ages of 7, 5 and 2.3 Ma [8]. The problem we address in this paper is whether the thermal structure suggested by recent leucogranite production at depth is discernible in the shallower metamorphic rocks of the NPHM. To date metamorphism, we have used the U-(Th)-Pb system in monazite, a common accessory mineral in high-grade metamorphic rocks which can be a viable metamorphic geochronometer, dating the formation of the mineral under moderate metamorphic conditions [9,10]. Monazite separated from schists and gneisses of the NPHM gives U-Pb ages of 4-11 Ma. We interpret these young ages as evidence for Neogene metamorphism affecting the NPHM, an event heretofore unsuspected in the greater Himalaya and only intimated in the NPHM. This recent metamorphism roughly coincided with leucogranite production at depth [8] and with the onset of high denudation rates [1,2]. As such, our conclusions offer important constraints on further modeling of similarly anomalous behavior within tectonically active regions, suggesting that any reasonable model should strive to explain the relationship between recent, rapid uplift and coincident recent metamorphism.