Quartz petrofabrics and RSCM thermometry constrain the P-T-t history of the UHP Tso Morari metamorphic complex, northern India: cool, shallow, and possibly slow

Abstract:

The Tso Morari metamorphic complex (TMMC) in northern India is one of only two UHP localities in the Himalaya. As such, its P-T-t history can provide important insight into Indo-Asian collisional tectonics. Numerous studies of UHP P-T conditions, prograde and retrograde P-T paths, and geochronology of the TMMC yield disparate results. These disparities challenge the use of these rocks to interpret either the initial stages of Himalayan orogenesis or the processes by which UHP metamorphic rocks form and are exhumed. Here, we use Raman spectroscopy of carbonaceous material (RSCM) and quartz petrofabrics to constrain peak and deformation temperatures for the TMMC, critically evaluate previous P-T estimates, and (together with published data) provide limits to likely durations at maximum T (Tmax).

Since the discovery of coesite in TMMC rocks in 2001, different studies have estimated diverse P-T-t conditions, including Pmax = 23 kbar (below coesite stability) to 46 kbar (well into the diamond stability field), Tmax = 600 to 800 °C, exhumation rates of 1.2 cm/yr to 12 cm/yr, and durations at Tmax of <100 kyr to >5 Myr. Higher T’s commonly correspond with higher P’s and with shorter durations at Tmax, although an amphibolite-facies overprint at <10 kbar represents a second high-T point. Our new RSCM data indicate Tmax of 550±50 °C, consistent with textures indicative of grain boundary migration recrystallization in quartz. Quartz c-axis opening angles preserve lower deformation temperatures of 525 to 350 °C, likely reflecting post-peak-metamorphic shearing.

From these data, we interpret a maximum T of 600 °C for the TMMC, at the cool end of most prior estimates, and well below some proposed UHP and amphibolite-facies overprint T’s. In combination with previous work, the most likely peak UHP condition is c. 600 °C, 27 kbar, at the shallow limit of UHP conditions. Available petrochronology still allows for widely-ranging durations near peak T’s (0 to >5 Myr), while reevaluation of published diffusion modeling of chemical gradients in garnet also permits durations up to 5 Myr, i.e., all data can be reconciled with relatively slow exhumation. While our data help limit possible P-T-t histories, many parameters –the timing of UHP metamorphism, shape of the exhumation P-T path, rate of exhumation, etc. – remain poorly constrained.