HR: 10:50h
AN: T22C-03    [Abstracts]
TI: Proterozoic mantle lithosphere beneath the Tariat Depression and Dariganga Plateau, Mongolia: in situ Re-Os evidence
AU: * Wang, K
EM: kwang@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, 11529, Taiwan
AU: * Wang, K
EM: kwang@earth.sinica.edu.tw
AF: GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Balaclava Road, North Ryde, Sydney, NSW 2109, Australia
AU: O'Reilly, S
EM: soreilly@els.mq.edu.au
AF: GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Balaclava Road, North Ryde, Sydney, NSW 2109, Australia
AU: Griffin, W
EM: wgriffin@els.mq.edu.au
AF: GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Balaclava Road, North Ryde, Sydney, NSW 2109, Australia
AU: Pearson, N
EM: npearson@els.mq.edu.au
AF: GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Balaclava Road, North Ryde, Sydney, NSW 2109, Australia
AU: Zhang, M
EM: mzhang@els.mq.edu.au
AF: GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Balaclava Road, North Ryde, Sydney, NSW 2109, Australia
AB: The Os isotope compositions of sulfides in mantle peridotites from the Tariat Depression (TD) in central Mongolia and the Dariganga Plateau (DP) in southeast Mongolia reveal the presence of Proterozoic lithospheric mantle beneath these regions. The least-disturbed sulfides, with ¹⁸⁷Re/¹⁸⁸Os<0.07, yield T_MA model ages of 1.5±0.3, 1.1±0.1, 0.7±0.1 and 0.48±0.08 Ga (2σ) for TD and 2.0±0.2 and 1.4±0.4 Ga (2σ) for DP. Beside these low Re/Os sulfides, T_RD model ages of other sulfides without later introduction/loss of Os, can still provide minimum estimates for the age of lithospheric mantle and record later metasomatic events. Ten TD Sulfides have T_RD model ages ranging from 2.6 to 0.5 Ga, with peaks around 1.6 and 0.8-0.6 Ga; two DP sulfides have T_RD model ages of 1.8 and 1.2 Ga. These old depletion/melting events are consistent with those from Nd model ages of peridotites from the same region (~2 Ga for TD [1]; >1 Ga [2] and 1.6 Ga [3] for DP). In the Tariat region, these mantle events are consistent with those known in the overlying crust as recorded by Mesoproterozoic Nd model ages (T_DM = 1.4-1.1 Ga) of Paleozoic and Mesozoic granitoids from the Khangai microcontinental block, where the Tariat Depression is located, and similar Nd model ages of 1.5-1.0 Ga for crustal granulite xenoliths from the Shavaryn-Tsaram volcano in the Tariat Depression [4]. Younger sulfide Os ages (1.1, 0.7 and ~0.5 Ga) may date mantle thermal events that also affected the overlying crust, marking the beginning of the Central Asia Orogeny in Neoproterozoic time. Although the South Mongolia domain, where the Dariganga Plateau is located, is believed to consist of late Paleozoic accretionary complexes and arc terrains [5], the presence of Precambrian zircon xenocrysts in magmatic rocks and ancient detrital zircons in arc-derived sediments [6], and Proterozoic Nd model ages of basement rocks in the Xilinhot region (B. Chen, pers. comm.) suggest substantial reworking of old crust. The presence of a Precambrian crustal terrain in the Dariganga region, corresponding to the underlying Mesoproterozoic lithospheric mantle indicated by the DP sulfide Os data, appears probable. References [1] Stosch, H.-G. et al. (1986) GCA 50, 2601-2614; [2] Wiechert, U. et al. (1997) CMP 126, 345-364; [3] Deng, F.-L. et al. (1992) Nature 360, 333-336; [4] Kovalenko V.I. et al. (2004) JAES 23, 605-627; [5] Windley, B.F. et al. (2007) JGS 164, 31-47; [6] Kroner, A. et al. (2007) GASM, in press.
DE: 1040 Radiogenic isotope geochemistry
SC: Tectonophysics [T]
MN: 2007 Fall Meeting