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 ...

The metapelitic rocks of the Sikkim Himalayas show an inverted metamorphic sequence (IMS) of the complete Barrovian zones from chlorite to sillimanite + K-feldspar, with the higher grade rocks appearing at progressively higher structural levels. Within the IMS, four groups of major planar structures, S 1 , S 2 and S 3 were recognised. The S 2 structures are pervasive throughout the Barrovian sequence, and are sub-parallel to the metamorphic isograds. The mineral growth in all zones is dominantly syn-S 2 . The disposition of the metamorphic zones and structural features show that the zones were folded as a northerly plunging antiform. Significant bulk compositional variation, with consequent changes of mineralogy, occurs even at the scale of a thin section in some garnet zone rocks. The results of detailed petrographic and thermobarometric studies of the metapelites along a roughly E-W transect show progressive increase of both pressure and temperature with increasing structural levels in the entire IMS. This is contrary to all models that call for thermal inversion as a possible reason for the origin of the IMS. Also, the observation of the temporal relation between crystallization and S 2 structures is problematic for models of post-/late-metamorphic tectonic inversion by recumbent folding or thrusting. A successful model of the IMS should explain the petrological coherence of the Barrovian zones and the close relationship of crystallization in each zone with S 2 planar structures along with the observed trend(s) of P-T variation in Sikkim and in other sections. A discussion is presented of some of the available models that, with some modifications, seem to be capable of explaining these observations.

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