Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges |
| |
| Authors: | P Blattner Jin Cheng-wei Xu Yong |
| |
| Institution: | (1) DSIR Geological Survey, PO Box 30 368, Lower Hutt, New Zealand;(2) Institute of Geology, Academia Sinica, PO Box 634, Beijing, China;(3) Geothermal Institute, University of Auckland, Private Bag, Auckland, New Zealand;(4) Institute of Nuclear Sciences, PO Box 31 312, Lower Hutt, New Zealand;(5) Present address: Department of Geology, University of Beijing, Beijing, China |
| |
| 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 
18O is as a sensitive indicator of contamination, either at the start of differentiation (
18Oinit) or as a proportion of fractionation in AFC. Plots of
18O vs SiO2-allow to determine initial
18O values for different sequences for source comparison. For NBS-28=9.60, the
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 (
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-
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
18O trend line, but the distribution of low total rock or feldspar
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. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
