Structural framework, protolith affinity, and <i>P-T-t-D</i> evolution of the metasediments exposed in the Lalong dome, eastern Himalaya: Evidence of the upper South Tibetan detachment system in the northern Himalaya

The North Himalayan gneiss domes serve as a type example of domal culminations that record the complex crustal thickening, metamorphic, and magmatic histories of continental collisional processes. While the basic geological information about the North Himalayan gneiss domes has been established, there have been multiple different mechanisms put forth to explain their development. This study investigates the Lalong dome in southern Tibet using detailed field observations, structural kinematics, petrology, phase equilibrium modeling, and new monazite, titanite, and detrital zircon U-Th-Pb geochronology. These data are used to construct pressure-temperature-time-deformation (P-T-t-D) paths, which confirm the overprinting of ca. 27−26 Ma Barrovian-type metamorphism (565−620 °C, 7.4−8.5 kbar, ∼23 °C/km) by younger 23−20 Ma Buchan-type metamorphism (530−660 °C, 3.5−5.5 kbar, ∼40 °C/km). The Buchan-type metamorphism is directly related to granite emplacement in the dome core, which is also associated with contemporaneous skarnization (590−700 °C) and enrichment of Li, Be, and Sn. Biotite in situ Rb-Sr dates of ca. 13 Ma are interpreted to reflect cooling within the dome. Detrital zircon dates and metamorphic conditions are consistent with Tethyan Himalayan sequence (THS) affinity for mantle schists and skarns within the Lalong area. Finally, a top-to-the-north extensional structure, interpreted to be equivalent to the upper South Tibetan detachment system (upper STDS), was identified within the THS rocks in the Lalong dome. The data and observations collected as part of this study are interpreted to reflect a magmatic diapirism origin for the Lalong dome with horizontal stretching and vertical thinning facilitated by the upper STDS. In the Early Miocene Himalaya, shallower extensional structures were active simultaneously with crustal shortening at depth and crosscut the stratigraphic boundary into the THS, providing chances for high-silica magma emplacement into much shallower levels.