Crustal anatexis and its relation to the exhumation of collisional orogenic belts, with particular reference to the Himalaya

Abstract

We review the causes, mechanisms and consequences of crustal anatexis during the exhumation of metamorphic terranes, from a petrological perspective. During both prograde and retrograde metamorphism, limited influx of free hydrous fluids may result in small volumes of very hydrous melts, which cannot ascend far (if at all) before reaching their solidus. If thermal conditions for dehydration melting are attained in fertile micaceous crustal layers, much larger volumes of waterundersaturated granitic magmas may result, especially where limited external fluid influx raises water activities above those that may be buffered by dehydrating hydrous phases. Magmas have specific trace element characteristics depending on the reaction which formed them which, combined with accessory phase thermometry, may enable the (P-T) conditions of melting to be ascertained. Small volumefraction magmas will typically remain as in situ migmatites unless their extraction is assisted by deformation. In turn, deformation will be focused in weaker partially molten zones, so that waterundersaturated magmas may often be mobilized. Once segregated, their ascent is limited by the rate of dyke propagation, and they may reach shallow levels (<2 kbar) before crystallizing. The complex interplay between deformation and melting is exemplified by the Miocene evolution of the central Himalaya, where thrust and normal faulting, melting and exhumation were all simultaneously active processes which were linked by feedback relations. In the Nanga Parbat Massif of the western Himalaya, rapid post-Miocene denudation and vigorous fluid flux enabled rocks to experience more than one episode of melting simultaneously, at different levels of the same exhuming crustal section.

Petrologic and thermal constraints on the origin of leucogranites in collisional orogens

“…The links can also be made in the Black Hills and the Himalayas, where granite intrusion was, in some cases, accompanied by rapid denudation (Winslow et al 1995;Davidson et al 1997;Whittington & Treloar 2002). In the three terranes, regional metamorphism and deformation of the host metapelitic sequences began several tens of millions of years prior to leucogranite magmatism during early stages of continental collisions, as shown by dated monazite and garnet growth (Dahl & Frei 1998;Harris et al 2000).…”

The exhumation of gneiss domes in bivergent wedges: Geometrical concepts and examples from the Himalayan syntaxes

“…Fertile schists carried in the hanging walls of reverse shear zones will undergo rapid decompression (assuming that erosion is vigorous) and may experience shear heating, both of which will facilitate the crossing of dehydration melting reactions. The "chicken and egg" argument over the relative timing of shear zone development and partial melting remains unresolved in the central Himalaya, where parallel normal and reverse shear zones facilitated exhumation (e.g., the review by Whittington and Treloar, 2002). In either geometrical arrangement (Fig.…”

“…Dehydration melting of fertile crustal protoliths may be triggered by shear heating, decompression, or a combination of the two (e.g., Whittington and Treloar, 2002). Decompression would only trigger melting of material that had already entered the wedge, resulting in internal differentiation of the wedge but no A F C B D E Figure 3.…”

Dehydration melting studies in a ‘Kyanite terrain’, Manali, NW Himalayas

“…Another point where there is much disagreement is the generation of granite melts during the Himalayan orogeny and its relationship to the phases of metamorphism and deformation (e.g. Le Fort, 1986;Searle and Fryer, 1986;Pognante and Benna, 1993;Whittington and Treloar, 2002).…”