Other
Scientific paper
Nov 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982gecoa..46.2279v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 46, Issue 11, pp.2279-2292
Other
29
Scientific paper
The Himalayan orogeny is characterized by an intraplate subduction zone. The thrusting of the Tibetan Slab over the foreland is probably responsible for the origin of the Manaslu-type leucogranites. This geochemical study tries to examine the crustal origin of the granite, to verify whether the paragneisses of the Tibetan Slab are the most probable parent material and to determine the subsequent petrogenetic processes. The Rb-Sr systematics suggest that 1. (1) the scatter of data points in the 87 Sr / 86 Sr vs . 87 Rb / 86 Sr diagram may be due to the heterogeneity of the initial 87 Sr / 86 Sr ratios. 2. (2) the initial isotopic ratios are very high (0.730 0.760) and are in agreement with those of the gneisses of the Tibetan Slab at the time of the melting. Pb isotopic ratios are homogeneous. They are also consistent with a crustal prehistory. The REE and Th contents are unexpectedly low in comparison with the much higher contents of the parent material. Monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed REE and Th abundances. Alternatively, field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity. In consequence, the low REE and Th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the Sr and Pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying Tibetan Slab.
Cocherie Alain
Le Fort Patrick
Vidal Ph.
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