Hydrogen and oxygen isotope variations in the high himalaya peraluminous Manaslu leucogranite: Evidence for heterogeneous sedimentary source

Abstract

The Manaslu granite belongs to the High Himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting. δD and δ 18 O values have been determined for whole rock and coexisting minerals from the ~8 km thick Manaslu massif and its 50 km long dyke sheet, its country rocks and the Formation 1 (F1) paragneisses which are the source of the granite. For the granite, δD musc range from −70 to −85%. and δ 18 O W.R. from 10.9 to 12.8%. H and O-isotope fractionations among minerals are consistent with high temperature equilibrium and, for oxygen, closed system evolution. A few samples, coming mainly from the dyke swarm, have very low δD values (down to −188%.) and biotite-muscovite H-isotope fractionations indicative of disequilibrium; the D H ratios of associated magmatic tourmaline are essentially unmodified. From the distribution of δD values in the granite and its country rocks, circulation of very low deuterium meteoric hydrothermal waters was extremely localized. Because these waters are depleted in deuterium by up to 50%. relative to modern meteoric waters, the Manaslu area was either at an altitude substantially higher than that of today (2500–6000 m for the analyzed samples) or a mountain chain once existed to the south. The F1 gneisses have δ 18 O quartz between 12 and 14.3%. which confirms that the granite was generated from F1, but δD values are ≈20%. higher than in the granite. Such a difference can be a result of degassing of the magma and/or introduction of fluid in the melting zone. Infiltration of low δD fluid (≈−90%.) into the hot but dry F1 probably triggered partial melting; these fluids could have come from the dehydration of the Midlands sediments which are separated from the overlying F1 by the Main Central Thrust. The correlations among δ 18 O, ( 87Sr 86Sr) 20 Ma and ϵNd values in both F1 and the granite indicate that the variations of these isotopic ratios in the Manaslu are inherited from those in F1 at the time of melting. In turn, these ratios in F1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of Nd and O, and to the quantity of radiogenic Sr generated within the sediment, which is a function of age and Rb content (amount of phyllosilicate). Some other Himalayan leucogranites require either other crustal source rocks or the δ 18 O and 87Sr 86Sr ratios of F1 vary along the Himalaya.