Geochemical comparison between Himalayan and Hercynian leucogranites

Details Geochemical comparison between Himalayan and Hercynian leucogranites Geochemical comparison between Himalayan and Hercynian leucogranites

Oct 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984pepi...35..179v&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 35, Issue 1-3, p. 179-190.

Physics

25

Scientific paper

Isotopic (Sr, Pb, Nd, O), REE and trace element data from three Himalayan (Nepal) and six Hercynian (Brittany, France) leucogranites are compared. For the Himalayan granites-Makulu, Mustang and especially Manaslu-the 87Sr/86Sr versus 87Rb/86Sr scatter diagrams, and hence variability of the initial 87Sr/86Sr ratios, reflect heterogeneity of the source materials. These ratios are very high (0.730- > 0.770). For Manaslu, the very high 207Pb/204Pb ratios (~ 15.8) at the corresponding 206Pb/204Pb ratio of ~ 18.8, the very low 143Nd/144Nd ratios (ɛi Nd ~ -13 to -17), and the very high 18O/16O ratios (~ 12‰) confirm the crustal origin with the paragneisses of the Tibetan Slab Formation I as the probable parent source. For the six Hercynian granites with an individual Rb-Sr isochron established for many of them, the initial 87Sr/86Sr ratios range from 0.706 to 0.717. The lead, neodymium and oxygen isotope compositions confirm their crustal source, of presumably Palaeozoic age but not specifically identified. The rare earth patterns and thorium contents are comparable to those for Himalayan granites.
The similarities of the geochemical signatures (isotopes and trace elements) of the Himalayan and Hercynian leucogranites suggest that the petrogenetic processes were comparable and that the Himalayan continental collision model can be applied to the Hercynian. The depletion in REE and Th implies either interaction of the granitic liquids with fluid or, more probably, early precipitation of monazite. The increase in Sm/Nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages.
In contrast to the Hercynian leucogranites, the extremely radiogenic Sr of the Himalayan leucogranites reflects the very long crustal residence time before the melting event. The apparent uniformity of the initial 87Sr/86Sr ratios of the individual Hercynian granites may be related to an ``aging effect'' where the straight line fitting improves with increase in age of the pluton.

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