Precambrian ocean, life, and syn to post-collisional records in the Himalayas (Kumaun)

Abstract

The Ph.D. thesis focuses on the central sector of the Himalayas, i.e., Kumaun. Here, an attempt has been made to understand the poorly understood genesis of magnesium carbonates (sparry magnesite) of the Precambrian (Neoproterozoic) Deoban Formation in the Lesser Himalayas, and poorly constrained post-collisional metamorphism and anatexis in Kumaun and its impact on the Lesser Himalayas carbonates. Some of the key findings during the doctoral work are 1) First time identification of Neoproterozoic oceanwater droplets from the sparry magnesite crystals of Kumaun, 2) Identifying missing links between Snowball Earth glaciation, magnesite formation and oxygenation of Earth’s oceans and atmosphere, 3) Identification of structural and temporal relations between metamorphisms, anatexis, and tectonics, that shaped the overall architecture of the orogenic belt. The sparry magnesite-bearing horizons of the Deoban Formation preserve evidence of rampant growth of the photosynthetic stromatolites during the upward transition of dolomite to magnesite horizons (sparry magnesite), which is identified as an ecological response of the microbial mats (oligotrophs) towards oligotrophic conditions (nutrition deficient, Ca poor) during Snowball Earth glaciations potentially arrived due to low riverine input of dissolved product of weathering (freezing of rivers). The trapped fluid inclusions in the magnesite represent the Neoproterozoic seawater, glacial meltwater, and their interaction. Hence, long-lived global glacial events favour magnesite precipitation, population expansion of the cyanobacteria, and, through positive feedback, contribute to large oxygen production. The Deoban basin potentially contributed to increased oxygen levels in the Neoproterozoic—the Neoproterozoic Oxygenation Event and the sparry magnesites are potential time capsules of the paleo-oceans. The central sector also preserves an immediate (~ 49 Ma monazite) and peak (~ 21 Ma) metamorphic response towards the India-Asia collision (52- 50 Ma). The Greater Himalayan Sequence (GHS) of Kumaun show an inverted metamorphism, i.e., middle to upper (~ 610 to 680 oC at ~ 4.5 to 7 kbar) at the top and lower to middle amphibolite facies metamorphism (~ 550 to 590 oC at ~ 4.5 to 7.5) at the base. The klippes in the Lesser Himalayan Zone (LHZ) are texturally and metamorphically (~ 360 to 580 oC at ~ 1 to 6.5 kbar) related to the base of the Main Crystalline Zone (MCZ). The ~ 21 Ma peak metamorphism is potentially related to the activation (23-17 Ma) of the Main Central Thrust (MCT), and most probably, the klippes reached LHZ through southward thrusting along the Main Central Thrust. The MCT activation and peak metamorphism are synchronous to the post-collisional anatexis in the Kumaun. The field relations, feldspar exsolution, and mineral chemistry suggest that the Higher Himalayan Leucogranites (HHL) crystallized under hypersolvus conditions. The maximum solidus temperature reached ~ 650 oC (two mica granite) to ~ 750 oC (tourmaline granite). The MgO and TiO2 in mica suggest a different melting source, i.e., the tourmaline granite is possibly formed due to the melting of muscovite-rich and the two-mica granite from biotite-rich metapelites. The ~ 22 Ma HHL emplacement is synchronous to peak metamorphism and the Main Central Thrust activation. Suggesting the activation of MCT played an important role in the metamorphism, HHL generation and overall tectonic evolution in the Himalayas. Furthermore, the metamorphism and anatexis mostly impacted the Greater Himalayas. In contrast, the carbonates in the Lesser Himalayas have undergone very little or no change. As a result, the primary information of these carbonates remains intact, providing valuable insights into the sedimentation, life, and ecology of the Precambrian period.