Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges
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 δ18O is as a sensitive indicator of contamination, either at the start of differentiation (δ18Oinit) or as a proportion of fractionation in AFC. Plots of δ18O vs SiO2-allow to determine initial δ18O values for different sequences for source comparison. For NBS-28=9.60, the δ18O 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 (δ18O 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-δ18O (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 δ18O trend line, but the distribution of low total rock or feldspar δ18O 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.
- Publication:
-
Contributions to Mineralogy and Petrology
- Pub Date:
- April 1989
- DOI:
- 10.1007/BF00372217
- Bibcode:
- 1989CoMP..101..438B
- Keywords:
-
- Fractionation;
- Oxygen Isotope;
- Eurasia;
- Trend Line;
- Assimilation Rate