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The crystalline units of the High Himalayas in the Lahul–Zanskar region (northwest India): metamorphic–tectonic history and geochronology of the collided and imbricated Indian plate

Published online by Cambridge University Press:  01 May 2009

U. Pognante ,
D. Castelli ,
P. Benna ,
G. Genovese ,
F. Oberli ,
M. Meier  and
S. Tonarini
U. Pognante
Affiliation:
Dipartimento di Scienze della Terra, Via Valperga Caluso 37, 10125 Torino, Italy
D. Castelli
Affiliation:
Dipartimento di Scienze della Terra, Via Valperga Caluso 37, 10125 Torino, Italy
P. Benna
Affiliation:
Dipartimento di Scienze della Terra, Via Valperga Caluso 37, 10125 Torino, Italy
G. Genovese
Affiliation:
Dipartimento di Scienze della Terra, Via Valperga Caluso 37, 10125 Torino, Italy
F. Oberli
Affiliation:
Isotope Geochemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
M. Meier
Affiliation:
Isotope Geochemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
S. Tonarini
Affiliation:
Laboratorio di Geocronologia e Geochimica Isotopica – C.N.R., Via Cardinale Maffi 36, 56100 Pisa, Italy
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Abstract

In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb–Sr and U–Th–Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.

Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler’ Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Fault.

Type
Articles
Information
Geological Magazine , Volume 127 , Issue 2 , March 1990 , pp. 101 - 116
Copyright
Copyright © Cambridge University Press 1990

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