Petrology of an inverted Barrovian sequence of metapelites in Sikkim Himalaya, India: Constraints on the tectonics of inversion

Abstract

The lesser Himalayan sequence in Sikkim exposes an unusually complete inverted Barrovian sequence of metapelitic rocks. A number of different bulk compositions are interlayered along the prograde sequence, providing an excellent natural laboratory for studying the controls of pressure, temperature and composition on stability of mineral assemblages. Using three different approaches for determining pressures and temperatures, and making use of the bulk chemical constraints, we show that all three methods yield consistent estimates of pressures and temperatures once the strengths and weaknesses inherent to each approach are critically evaluated. The metamorphic field gradient along two separate traverses are found to be positive (60 °C/kbar in east Sikkim and 70 °C/kbar in north Sikkim) -pressure as well as temperature increases continually up to the highest grade. The various bulk compositions allow different assemblages to be used to establish the coherence of the sequence. In combination these place tight constraints on the permissible tectonic mechanisms for the production of this inverted metamorphic sequence. Neither older hot iron type models, nor more recent models that invoke thrusting (post-or synmetamorphic) as a mechanism of inversion can produce such a coherent package inverted in pressure as well as temperature. On the other hand, the observations are more consistent with the predictions of channel flow type tectonics. In particular, a recent geodynamic model of subduction-collision that takes into account high heat generation (as found in these Himalayan metapelites) and incorporates the effects of the resulting melting (reduction of viscosity and enhancement of buoyancy) on the tectonic evolution, predicts such inversion of coherent blocks as an unavoidable consequence. Such melting triggered inversion and exhumation as a coherent, fault bounded block explains many features of the inverted sequence in Sikkim and it is not necessary to invoke inversion by multiple thrusting events. introduction One of the best developed Barrovian sequences in metapelitic rocks occurs over the entire length of the Himalayan mountain chain. The sequence is inverted so that progressively higher grade rocks occur at higher structural levels. This inverted metamorphic sequence (IMS) in turn is spatially closely related to the Main Central Thrust Zone (MCTZ) that juxtaposes rocks of the Lesser Himalaya with those of the Higher Himalaya. Much of the literature on metamorphism and origin of the IMS has focussed on thermobarometry because of the obvious significance of these data for tectonic models of development of the MCTZ and the evolution of the Himalayan range as a whole (see Pecher, 1989;Guillot, 1999; as well as reviews in Macfarlane and others, 1999;Hodges, 2000; Jain and others, 2002 for an overview; more recent contributions include Fraser and others, 2000; Stephenson and others, 2000; Beyssac and others, 2004). Quite aside from this tectonic significance, the wide extent and continuous nature of the Barrovian ...

Higher Himalayan Shear Zone, Sutlej section: structural geology and extrusion mechanism by various combinations of simple shear, pure shear and channel flow in shifting modes

“…Thus, the southwestward shearing seemingly migrated southward, but not necessarily continuously, within a span of *24 Ma. It is important to mention here that not only at its boundaries, but the ductile shearing is found to be distributed throughout the HHSZ in contrary to Grasemann et al's (1999) and Dasgupta et al's (2009) previous findings from this section and from the Sikkim Himalaya, respectively.…”

“…In the model, flow lines and the inactive marker are comparable with the primary shear planes and shear fabrics, respectively. Ductile slip along individual shear planes throughout the HHSZ has been utilized in developing the model, which is, therefore, distinctly different from the channel flow mode of extrusion by tectonic inversion of the HHSZ as a 'coherent block' as recently proposed by Dasgupta et al (2009), possibly a result of extrusion of rocks with non-Newtonian rheology through a channel with parallelsided walls, from the Sikkim Himalaya.…”

“…These are (1) thermal-, (2) coupled thermomechanical-, (3) post-metamorphic deformation-, and (4) syn-metamorphic deformation models (Yin 2006 and his review; also see Hodges 2006 andDasgupta et al 2009 for other grouping). However, given a wide variation in tectonic framework of the HHSZ in different sections, none of these could embrace all the tectonic and metamorphic constraints (see Hodges 2000 for review).…”

Developing an inverted Barrovian sequence; insights from monazite petrochronology

“…1 and supplementary material S2.1-2.3; Mohan et al, 1989). Peak metamorphic conditions increase structurally upwards from ∼480 to 530 • C and 5 kbar in the structurally low garnet zone, through ∼540-565 • C and 6 kbar in the staurolite zone, ∼565-625 • C and 6-7 kbar in the kyanite zone, ∼675 • C and 7.5 kbar in the sillimanite zone and ∼625-700 • C and 6-9 kbar in the uppermost sillimanite-Kfeldspar zone (Dasgupta et al, 2009(Dasgupta et al, , 2004Dubey et al, 2005).…”

Early Oligocene partial melting in the Main Central Thrust Zone (Arun valley, eastern Nepal Himalaya)

“…It has however to be noted that in other areas of the eastern Himalaya, MCT and MCTZ have been defined differently (e.g. Catlos et al, 2004, Dasgupta et al, 2009) and thus the data presented here are compared to rocks occupying a similar structural level independently of their proposed structural unit.…”