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Crustal melting and granite genesis during the Himalayan collision orogenesis

Published online by Cambridge University Press:  03 November 2011

Christian France-Lanord  and
Patrick Le Fort
Christian France-Lanord
Affiliation:
Centre de Recherches Pétographiques et Géochimiques, B.P. 20, 54501 Vandoeuvre-lès-Nancy, France.
Patrick Le Fort
Affiliation:
Centre de Recherches Pétographiques et Géochimiques, B.P. 20, 54501 Vandoeuvre-lès-Nancy, France.
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Abstract

This paper reviews the petrogenesis of Himalayan leucogranites (HHγ) on the basis of field, petrological and geochemical data collected over the last fifteen years. HHγ are intruded at the top of the 2 to 8km-thick High Himalayan Crystallines (HHC). These are metamorphosed (Ky to Sill) and present much evidence of partial melting. During the MCT thrusting, the already metamorphosed HHC were thrust on top of the weakly metamorphosed Midland Formations, inducing the main phase of Himalayan metamorphism. The genesis of HHγ and North Himalaya leucogranites (NHγ) associates thrusting along the MCT, propagation of inverted metamorphism, liberation of large quantities of fluid in the Midlands, and partial melting of the HHC.

The restricted compositions of the granites are close to minimum melt compositions; variations in the alkali ratio probably relate to the variable amount of B, F and H2O. The HHγ were issued from the migmatitic zone around 700°C and 800 MPa., and still emplaced some 10 to 15 km below the surface. This syn- to late-tectonic emplacement of the leucogranites lasted for more than 10 Ma according to isotopic ages (25 to 14 Ma).

O, Rb–Sr, Nd–Sm and Pb isotope studies corroborate the unambiguous filiation between the HHC and the leucogranites in central Nepal. They also imply that the plutons are generated as numerous poorly mixed batches of magma produced preferentially in specific zones of the source rock. δD values may be explained by infiltration of water from the Midlands in the melting zone, and/or by water degassing during crystallisation. The positive covariations between Sr-, Nd- and O-isotope ratios relate to the variations in the original sediment composition of the source gneisses. Whereas trace element characteristics often date back to the anatectic process, limited magmatic differentiation is recorded by the biotite. These granites are typical crustal products, keeping track of some of the pre-Himalayan evolution together with that of their own origin.

Keywords

leucogranitemagma batchesmigmatitesOSrNd isotope heterogeneitypartial meltingthrustingwater infiltration
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 79 , Issue 2-3: The Origin of Granites , 1988 , pp. 183 - 195
Copyright
Copyright © Royal Society of Edinburgh 1988

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