Geological History of the Daling Formation of Himalaya, Bhutan

Abstract

The Daling Formation of Bhutan is a significant Precambrian lithostratigraphic unit of the Lesser Himalayan Sequence of the eastern Himalaya, which holds important geological records concerning the tectono-sedimentary history of the northern continental margin of India. The Daling Formation is made up primarily of low to moderate grade metamorphosed rocks such as phyllite, quartzite, quartz-mica schist, slate, and occasional carbonaceous beds. Geologic and stratigraphic studies have established that the Daling Formation is a part of the Daling-Shumar Group, formed in the Paleoproterozoic in the shallow marine shelf settings on the passive northern margin of the Indian continent. Evidence of sedimentary features such as ripple marks and cross bedding present in the quartzitic beds suggests deposition in a stable tectonic setting prior to Himalayan deformation. The detrital zircon geochronology of sediment deposition ranges from 1.8 to 1.9 Ga, indicating the continental crust origin related to the Indian shield. Geological history of the Daling Formation was greatly altered due to the Himalayan orogenesis resulting from India–Asia continental collision event. Regional tectonic compression has led to folding, thrusting, development of schistosity and regional metamorphism. The formation is tectonically connected with major Himalayan tectonic zones, especially the Main Central Thrust (MCT) that had a major role in crustal shortening and tectonic stacking. Petrographic study of thin sections shows extremely deformed and recrystallized quartz–mica schist that is highly foliated and schistic under crossed polars. The quartz has undulose extinction and dynamic recrystallization, whereas the micas have lepidoblastic to granolepidoblastic texture. Feldspar, amphibole and opaque minerals are present in small amounts, which implies medium to high-grade regional metamorphism under directed pressure. The formation is correlated with comparable Lesser Himalayan sequences in India, Nepal, and Sikkim.