Contact metamorphism of the Tethyan Sedimentary Sequence, Upper Mustang region, west-central Nepal

Abstract

The Upper Mustang region of west-central Nepal contains exposures of metamorphosed Tethyan Sedimentary Sequence rocks that have been interpreted to reflect either contact metamorphism related to the nearby Mugu pluton or regional metamorphism associated with the North Himalayan domes. New monazite geochronology results show that the Mugu leucogranite crystallized at c. 21.3 Ma, while the dominant monazite age peaks from the surrounding garnet ± staurolite ± sillimanite schists range between c. 21.7 and 19.4 Ma, generally decreasing in age away from the pluton. Metamorphic temperature estimates based on Ti-in-biotite and garnet–biotite thermometry are highest in the specimens closest to the pluton (648 ± 24°C and 615 ± 25°C, respectively) and lowest in those furthest away (578 ± 24°C and 563 ± 25°C, respectively), while pressure estimates are all within uncertainty of one another, averaging 5.0 ± 0.5 kbar. These results are interpreted to be consistent with contact metamorphism of the rocks in proximity to the Mugu pluton, which was emplaced at c. 18 ± 2 km depth after local movement across the South Tibetan detachment system had ceased. While this new dataset helps to characterize the metamorphic rocks of the Tethyan Sedimentary Sequence and provides new constraints on the thickness of the upper crust, it also emphasizes the importance of careful integration of metamorphic conditions and inferred processes that may affect interpretation of currently proposed Himalayan models.

A thermal event in the Dolpo region (Nepal): a consequence of the shift from orogen perpendicular to orogen parallel extension in central Himalaya?

“…Although there is a partial overlap between the timing of D2 and the D3 phases in literature (Fig. 8c), the middle Miocene ages (14-10 Ma) obtained in this study fall in the range of the Himalayan D3 phase linked to the E-W extension, that is younger than the time of emplacement and cooling of the large granite intrusions (Copeland et al 1990;Guillot et al 1994;Larson et al 2019;Lihter et al 2020;Jessup et al 2019 for a review). This suggests that, although in the present work structures related to S1 and S2 foliations have been analysed, the dated micas may have undergone re-equilibration due to a late recrystallization event linked to the M3 phase.…”

“…1a and Fig. 8b), hosted within the THS in the Upper Mustang region of west-central Nepal, emplaced ~21-19 Ma ago (Th-Pb monazite data) at a depth of about 18 km (based on garnet -biotite -muscovite -plagioclase barometer), thereby constraining a minimum age for the STDS shearing followed by a rapid exhumation at ~15-16 Ma (Guillot et al 1999;Lihter et al 2020). These data agree with those from the Manaslu leucogranite, in the Manaslu area (Fig.…”

“…Although a lateral variability is present, the ages of the three main Himalayan events D1, D2 and D3 are well distinguishable, falling in fairly distinct temporal ranges, as: 48/44-25 Ma for the D1 phase (Hodges et al 1996;Godin et al 1999Godin et al , 2001Carosi et al 2010;Dunkl et al 2011); 27-15 Ma for the D2 phase (Guillot et al 1994;Godin et al 2006b;Crouzet et al 2007;Leloup et al 2010;Dunkl et al 2011;Carosi et al 2013;Iaccarino et al 2017b;Soucy La Roche et al 2018a, b;Lihter et al 2020), with final cooling ages recorded up to the late Miocene (e.g., McDermott et al 2015); and, 17-15 Ma for the initiation of the D3 phase in the northern Himalaya (Godin 2003;Dunkl et al 2011;Lee et al 2011;Nagy et al 2015;Larson et al 2019;Brubacher et al 2020; see Jessup et al 2019 for a review). Particularly, the D3 phase coincides with the elevation increase of the Tibetan Plateau (Royden et al 2008), and two drops in the convergence rate between India and Eurasia, at c. 20 Ma and 13-11 Ma (see also Larson et al 2019 with references).…”

Prolonged Slip on the South Tibetan Detachment Constrains Tectonic Models for Synorogenic Extension in the Central Himalaya

“…Unlike previous authors, Baltz et al (2021) mapped the Mustang and Mugu bodies as a single pluton. However, both thermal ionization (Hurtado, 2002) and LA-ICPMS (Brubacher et al, 2020;Lihter et al, 2020) MnzPb data indicate that the Mustang body is significantly older. As we did for the Everest region samples discussed in Section 5.2, we have reinterpreted emplacement ages of the Mugu and Mustang plutons from the larger MnzPb data sets reported by Lihter et al (2020) and Brubacher et al (2020) using the BEE approach described in Section 4.…”

“…However, both thermal ionization (Hurtado, 2002) and LA-ICPMS (Brubacher et al, 2020;Lihter et al, 2020) MnzPb data indicate that the Mustang body is significantly older. As we did for the Everest region samples discussed in Section 5.2, we have reinterpreted emplacement ages of the Mugu and Mustang plutons from the larger MnzPb data sets reported by Lihter et al (2020) and Brubacher et al (2020) using the BEE approach described in Section 4. The results suggest that the older Mustang leucogranitic gneiss (23.8 ± 1.8 Ma) is approximately the same age as the older leucogranites found in both the Annapurna and Everest regions, whereas the younger Mugu pluton (18.78 ± 0.45 Ma) is intermediate in age between the oldest and youngest leucogranites in the two other regions.…”