Oxygen isotope trends and anomalies in granitoids of the Tibetan plateau Peter Blattner a, * , Rainer Abart b,1 , Chris J. Adams a , Kevin Faure a , Lan Hui c a Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand b Mineralogisches Institut der Universitaet, Universitaetsplatz 2, A-8010 Graz, Austria c Chengdu Institute of Geology and Mineral Resources, 82/3, No.1 Ring Road (N), Chengdu 610082, People’s Republic of China Received 5 September 2001; revised 7 November 2001; accepted 10 April 2002 Abstract Extreme Transhimalayan (oceanic, mantle, I-type) and High Himalayan (anatectic, collision, S-type) granitoids in the Himalaya, Kashmir, and Tibet range in their original oxygen isotope composition from 6 to 7.5 and from 10 to 12.5‰ d 18 O SMOW , respectively. In the sutured Paleozoic and Mesozoic orogen of the eastern Tibetan plateau, purely oceanic batholiths seem absent (Shalouli Shan 8 – 10‰), but High Himalayan values are matched in Dongda Shan and at Luxi. Towards the east, oceanic values are found again only on the margin of the Yangtse platform. The regional pattern of normal oxygen isotope data provides a background for anomalous feldspars (to 2 1‰) in Lhasa Granite and in the active geothermal system at Yangbachen, recharged by , 6000 m a.s.l. meteoric water. For Lhasa Granite, oxygen isotope exchange paths and preliminary K – Ar dating of feldspars suggest exchange with meteoric water from over 3000 m altitude before 20 Ma ago. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Oxygen isotopes; Tibetan plateau; Fluid-rock interaction; Paleoclimate; S-type granites; I-type granites 1. Introduction With the passing of half a century of oxygen isotope research (Urey et al., 1951), the normal range of crustal d 18 O values may be summarised in terms of a controlling influence by the Earth’s mantle near about 5.5‰, and by actively coexisting ocean water near the defined 0‰ SMOW mark (Muehlenbachs, 1998). The unequal mantle/ ocean system and the strong 18 O-enrichment required at low, near-surface, temperatures, determines the high d 18 O of marine precipitates. The mantle ‘buffer’ also sets the initial level of d 18 O of fractionating magma, which rises gradually up to some 7.5‰, as the 18 O-depleted mafic minerals crystallise early (see review of oxygen isotopes in seven oceanic sourced magmas in Blattner et al. (1989)). The d 18 O of magma may increase more substantially when sediments or their metamorphic and anatectic products are assimilated (AFC, DePaolo, 1981). 1.1. The twin uses of d 18 O in granite minerals Next to fluid-dominated dissolution and precipitation in the ocean, a similarly rapid and effective way of separating oxygen isotopes is evaporation and precipitation in the atmospheric water cycle. The resulting d 18 O- and d D vs temperature functions, and d D vs d 18 O relationship for atmospheric precipitation (Meteoric Water Line) have turned out to be a particularly useful ‘side show’ of the oxygen isotope cycle. This process can leave sensitive climatic imprints in solid rock. Whereas quartz often retains an original d 18 O value, feldspars are known to exchange oxygen relatively easily and often without mineralogical, chemical or textural effects (O’Neil and Taylor, 1967; Taylor, 1974). Late magmatic and secondary hydrothermal oxygen isotope exchange with meteoric water may therefore lead granite feldspars to constrain latitude and/or altitude. Fig. 1 shows the ranges of d 18 O of meteoric H 2 O, and of normal rocks, as well as of some altered rocks. Zircons are found to be still more retentive of their original oxygen isotope composition than quartz, and Monani and Valley (2001) have given comparative data for d 18 O of zircon, quartz, and total rock (in lieu of feldspar) for some 25 granites of the Scottish Tertiary Igneous Province. On account of hydrothermal alteration, and using 1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(02)00046-9 Journal of Asian Earth Sciences 21 (2002) 241–250 www.elsevier.com/locate/jseaes 1 Present address: Mineralogisch-Petrographisches Institut der Universitaet, Bernoullianum, CH-4056 Basel, Switzerland. * Corresponding author. Tel.: þ 64-4-562-7550; fax: þ64-4-570-4600. E-mail address: p.blattner@gns.cri.nz (P. Blattner).