Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.t33b2041x&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #T33B-2041
Other
1000 Geochemistry, 3640 Igneous Petrology, 8104 Continental Margins: Convergent, 8109 Continental Tectonics: Extensional (0905), 8415 Intra-Plate Processes (1033, 3615)
Scientific paper
It has been recognized that ultra-potassic and potassic igneous rocks were dominant from 13 to 25 Ma in south Tibet, which is generally interpreted that break-off of the northward subducted Indian lithosphere or delamination of the thickened lithosphere resulted in eruption (or intrusion) of these ultra-potassic and potassic magmas (Miller et al., 1999; Maheo et al., 2002; Williams et al., 2001; Chung et al., 2005), as well as occurrence of the N-S striking extension faults ( at least before 13.5-14 Ma) on the Tibetan plateau (Coleman and Hodges, 1995; Blisniuk et al., 2001) and main elevation of the plateau in this period (e.g., 13- 18 Ma). Here we firstly report post-collisional Na-rich peralkaline volcanic rocks erupted at 12.3 (0.5) Ma within a south-north striking graben in south Tibet. These Na-rich rocks are phonolites with actual nepheline and aegirine-pyroxene minerals and significantly high Na2O/K2O ratios (higher than 1). Os-Nd isotopic and elemental compositions coupled with mineral composition analyses reveal that the Na-rich volcanic rocks cannot be generated by differentiation of the associated potassic or ultra-potassic magma; instead they were derived from a mantle source distinct from that of the latter, hinting that the mantle-derived volcanism in south Tibet experienced a temporary composition variation from potassic to sodic at about 12 Ma. Moreover, some Na-rich volcanic rocks erupted in 10-12 Ma were also found in eastern part of the Lhasa block in south Tibet. Furthermore, it has not been observed for a mantle-derived igneous rock younger than 10 Ma in south Tibet (Chung et al., 2005). Therefore, the observed transition from dominantly potassic to Na-rich magmatism represents an important tectonic event in south Tibet. We suggest that transition at about 12 Ma most possibly implies end of the break-off or delamination event, as well as temporary cession of E-W striking extension and the related uplifting process on the plateau. Thus, the south Tibetan plateau had possibly risen to its highest level before 12 Ma rather than at 2-3 Ma as suggested by other studies (Power, 1986; Li, 1991; Zheng et al., 2000). References: Blisniuk, et al., Nature, v. 412, 628-632 (2001) Chung, S.L. et al. Earth Sci. Rev., v. 68, 173-196 (2005) Coleman, M. and Hodges, K , Nature, v. 374, 49-52 (1995) Li, J., Quat. Sci. Rev., v. 10, 479-483 (1991) Maheo, G. et al.. Earth Planet. Sci. Lett., v. 195, 45-58 (2002) Miller, C.et al., Journal of Petrology, v. 40, 1399-1424 (1999) Power, C. M, Earth Planet. Sci. Lett., v. 81, 79-94 (1986) Williams, H., et al., Geology, v. 29, 339-342 (2001) Zheng, H.B. et al., Geology, v. 28, 715-718 (2000).
Chen Jiahua
Kang Zhaofeng
Li Jiying
Wang Binghong
Xu Jiajun
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