ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract In closed magma systems SiO2 approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO2. The main role of δ 18O is as a sensitive indicator of contamination, either at the start of differentiation (δ 18Oinit) or as a proportion of fractionation in AFC. Plots of δ 18O vs SiO2-allow to determine initial δ 18O values for different sequences for source comparison. For NBS-28=9.60, the δ 18O at 48% SiO2-varies between a high 6.4‰ for Kiglapait (Kalamarides 1984), 5.9‰ for Transhimalaya, 5.8‰ for Hachijo-Jima (Matsuhisa 1979), 5.6‰ for Koloula (Chivas et al. 1982) and a low 5.3‰ for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the ‘Ladakh-Lhasa terrane’, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous (δ 18O vs SiO2-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in 18O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low-δ 18O (5.0‰−0‰) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya δ 18O trend line, but the distribution of low total rock or feldspar δ 18O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any 18O-contamination by crustal material.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00372217
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