A review of thermal history and timescales of tectonometamorphic processes in Sikkim Himalaya (NEIndia) and implications for rates of metamorphic processes

Abstract

The lesser and greater Himalayan sequences (LH and GH) in Sikkim have been studied in detail recently. The LH consists mainly of pelitic lithologies with detrital zircon ages >1800 Ma and e Nd (0) of À27.7 to À23.4 (with intercalated calcsilicates, metabasites and a sheared granite gneiss -the Lingtse gneiss). The metamorphic sequence is characterized by (i) a remarkably continuous and systematic metamorphic field gradient (~60-70°C kbar À1 ), (ii) peak pressures as well as temperatures of metamorphism increasing at higher metamorphic grades (~5 kbar, 500°C at the garnet zone tõ 8 kbar, 700°C at the sillimanite-muscovite-zone), (iii) clockwise P-T paths of a tight 'hairpin' shape with a prograde slope of 20-30°C kbar À1 , (iv) initiation of garnet growth (Lu-Hf chronometry) at systematically older ages at higher grades (11-10.5 Ma at garnet zone to 16.8 Ma at sillimanite-K-feldspar zone), (v) peak of metamorphism simultaneously at all grades (13-11 Ma), (vi) deformed and folded metapelitic rocks but with little evidence of pervasive shearing and (vii) duration of melt formation and segregation on the order of a few 100,000 years. The degree of melting was low, as indicated by the presence of muscovite all through the sequence. This evidence indicates that the LH behaved as a coherent block, and thermomechanical models quantitatively reproduce many of these features as well as explain the formation of inverted metamorphic sequence in such a coherent block. The greater Himalayan sequence (GH), also largely metapelitic, has different protoliths (with components as young as 800 Ma) and a different metamorphic history (extensive migmatization at higher grades~8-12 kbar, 750-850°C at an older age of largely >20 Ma along a different clockwise P-T path characterized by sharp, isothermal decompression followed by near isobaric cooling). Geochronological data (monazite, zircon, garnet) as well as cooling rate calculations (geospeedometry) reveal that the GH is made up of at least two blocks with distinct, complex and multistage P-T-t histories. Different stages of the high-T cooling were rapid (100's°C Ma À1 to 10's°C Ma À1 at different stages) and at least some of it was unrelated to exhumation; synthesis of these results with low-T thermochronometry allows the detailed multistage cooling and exhumation history of this block to be reconstructed. The reconstructed cooling and exhumation histories show that a number of interlinked processes operating on a hierarchy of timescales leads to the overall evolution. The P-T history with rapid cooling can be reproduced by thermomechanical models that consider conductive as well as advective heat transport and there is no need to invoke additional processes (such as the role of fluids). Metabasic inclusions within both the LH and the GH appear to record a different (? older in part) history. The role of these in the overall evolution, as well as the difference in metamorphic ages of the LH and GH remain open questions that need to be addressed in future research.

Garnet: A Rock-Forming Mineral Petrochronometer

“…This entire remarkable dataset is shown to be consistent with models of a coherent Lesser Himalayan block being buried during collision (Anczkiewicz et al 2014;Chakraborty et al 2016). All rocks reached their respective peak metamorphic conditions simultaneously but with those samples that eventually reach highest grade passing through garnet-in reactions first and thus recording earliest garnet growth (Anczkiewicz et al 2014).…”

Modeling High‐Resolution Pressure‐Temperature Paths Across the Himalayan Main Central Thrust (Central Nepal): Implications for the Dynamics of Collision

“…The MCT places the high‐grade (>600 °C) Greater Himalayan Crystallines (GHCs) over lower grade Lesser Himalayan Formations (LHFs) along a broad scale 8–12‐km thick shear zone (see review in Mukhopadhyay et al, ). The MCT footwall is characterized and defined by inverted metamorphism, where metamorphic grade increases toward structurally shallower levels (e.g., Chakraborty et al, ; Larson et al, ; Pêcher, ; Searle et al, ). Pressure‐temperature (P‐T) conditions and paths obtained from garnet‐bearing assemblages from the shear zone have long been applied to decipher the origin of this apparent inverted metamorphism (Anczkiewicz et al, ; Imayama et al, ; Kaneko, ; Larson et al, ; Metcalfe, ; Manickavasagam et al, ; Mottram et al, ; Staeubli, ; Vannay & Hodges, ; Vannay & Grasemann, ).…”

Rapid high‐T decompression recorded by Archean granulites in the northern Wyoming Province: Insights from petrological modelling

“…“geospeedometry”) have been used extensively to understand metamorphic processes on modern Earth (e.g. Brown, ; Chakraborty et al., ; Chambers et al., ; Spear, Pattison, & Cheney, ; and many more), with fewer examples of their application to Archean settings (notable exceptions include e.g. Bhadra & Nasipuri, ; Bhowmik, Wilde, Bhandari, & Basu Sarbadhikari, ; Cutts et al., ; Dumond, Goncalves, Williams, & Jercinovic, ; François, Philippot, Rey, & Rubatto, ; Nicoli, Stevens, Moyen, & Frei, ; White, Palin, & Green, ) to reveal the P−T– t paths that Archean HT rocks took through the crust.…”