Effective radium concentration (EC Ra) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured in the laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to Eastern Nepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade meta-morphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence (GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. EC Ra values range from 0.03 ± 0.03 to 251.6 ± 4.0 Bq kg À1 , and appear to be representative of the formation and clearly related to the local lithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350 available EC Ra values, LHS rocks are characterized by a mean value of 5.3 ± 1.3 Bq kg À1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69 ± 0.11 Bq kg À1 , thus leading to a LHS/GHS EC Ra ratio of 7.8 ± 2.2. This behavior was systematically confirmed by other transects (ratio of 7.9 ± 2.2 in all other sites), with a threshold EC Ra value, separating LHS from GHS, of 0.8 Bq kg À1 , thus bringing forward a novel method to characterize, within the MCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks, occurred in several transects and were characterized by high EC Ra values (17.9 ± 4.3 Bq kg À1), easy to distinguish from the GHS gneisses, characterized by low EC Ra values at the bottom of the GHS, thus providing a further argument to locate the MCT. The measurement of EC Ra data, thus, provides a cost-effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurements of EC Ra provides additional information to discriminate different geological formations, and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial.
Effective radium concentration across the Main Central Thrust in the Nepal Himalayas
Résumé
en
Effective radium concentration (EC Ra) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured in the laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to Eastern Nepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade meta-morphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence (GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. EC Ra values range from 0.03 ± 0.03 to 251.6 ± 4.0 Bq kg À1 , and appear to be representative of the formation and clearly related to the local lithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350 available EC Ra values, LHS rocks are characterized by a mean value of 5.3 ± 1.3 Bq kg À1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69 ± 0.11 Bq kg À1 , thus leading to a LHS/GHS EC Ra ratio of 7.8 ± 2.2. This behavior was systematically confirmed by other transects (ratio of 7.9 ± 2.2 in all other sites), with a threshold EC Ra value, separating LHS from GHS, of 0.8 Bq kg À1 , thus bringing forward a novel method to characterize, within the MCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks, occurred in several transects and were characterized by high EC Ra values (17.9 ± 4.3 Bq kg À1), easy to distinguish from the GHS gneisses, characterized by low EC Ra values at the bottom of the GHS, thus providing a further argument to locate the MCT. The measurement of EC Ra data, thus, provides a cost-effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurements of EC Ra provides additional information to discriminate different geological formations, and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial.
1
IPGP -
Institut de Physique du Globe de Paris
( 1852 )
- IPGP, 1 rue Jussieu, 75238 Paris cedex 05 ; Université Paris Diderot, Bât. Lamarck A case postale 7011, 75205 Paris CEDEX 13
- France
Institut national des sciences de l'Univers ( 300045 )
;
Université Paris Diderot - Paris 7 ( 300301 )
;
Université de La Réunion ( 300316 )
;
Institut de Physique du Globe de Paris ( 300354 )
;
Centre National de la Recherche Scientifique UMR7154 ( 441569 )
2
ISTerre -
Institut des Sciences de la Terre
( 222134 )
- BP 53 - 38041 Grenoble cedex 9
- France
Université Joseph Fourier - Grenoble 1 ( 51016 )
;
Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux ( 222110 )
;
Institut national des sciences de l'Univers ( 300045 )
;
Institut de recherche pour le développement [IRD] : UR219 ( 302663 )
;
PRES Université de Grenoble ( 303008 )
;
Université Savoie Mont Blanc ( 432896 )
;
Centre National de la Recherche Scientifique UMR5275 ( 441569 )
3
National Seismic Centre, Nepal
( 106502 )
- Lainchaur, Kathmandu, Nepal
- Népal
Department of Mines and Geology ( 325540 )
4
CEA -
Commissariat Energie Atomique
( 141852 )
- Bruyeres-le-Chatel
- France
Commissariat à l'énergie atomique et aux énergies alternatives ( 300016 )
5
PRODIG -
Pôle de recherche pour l'organisation et la diffusion de l'information géographique
( 1339 )
- PRODIG - 2 rue Valette - 75005 Paris
- France
Université Paris 1 Panthéon-Sorbonne ( 7550 )
;
Institut de Recherche pour le Développement ( 67872 )
;
École Pratique des Hautes Études ( 110691 )
;
Université Paris Sciences et Lettres ( 564132 )
;
Université Paris-Sorbonne ( 123821 )
;
Université Paris Diderot - Paris 7 ( 300301 )
;
Centre National de la Recherche Scientifique UMR8586 ( 441569 )
6
CRPG -
Centre de Recherches Pétrographiques et Géochimiques
( 262474 )
- CNRS / Université de Lorraine, 15 rue Notre-Dame des Pauvres, 54500 Vandoeuvre-les-Nancy
- France
Institut national des sciences de l'Univers UMR7358 ( 300045 )
;
Université de Lorraine ( 413289 )
;
Centre National de la Recherche Scientifique UMR7358 / UPR2300 / EP2031 / UPR9046 ( 441569 )
Langue du document
Anglais
Nom de la revue
Geochimica et Cosmochimica Acta
(ISSN : 0016-7037, ISSN électronique : 0016-7037)
Publié par Elsevier
Revue non référencée dans Sherpa-Romeo
Frédéric Girault, Frédéric Perrier, Ananta Prasad Gajurel, Mukunda Bhattarai, Bharat Prasad Koirala, et al.. Effective radium concentration across the Main Central Thrust in the Nepal Himalayas. Geochimica et Cosmochimica Acta, 2012, 98 (100), p.203-227. ⟨10.1016/j.gca.2012.09.001⟩. ⟨hal-01258425⟩