Effective radium concentration across the Main Central Thrust in the Nepal Himalayas

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 metamorphic 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-1, 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-1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69 ± 0.11 Bq kg-1, 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 ECRa value, separating LHS from GHS, of 0.8 Bq kg-1, 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-1), 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. (authors)