International Geologiical Congress - Oslo 2008

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MPC-01 General contributions to geochronology and isotope geology

 

Rb-Sr Isotopic age and Isotopic Geochemistry of Pb, Sr and Nd from the Rhyolites at Eastern Tianshan in Xinjiang

 

Wang Yinxi, Center of Modern Analysis (China)
Li Himing, Center of Modern Analysis (China)
Gu Lianxing, State Key Laboratory of Mineral Deposit Research (China)
Zhang Zhunzhong, State Key Laboratory of Mineral Deposit Research (China)
WU ChangZhi, State Key Laboratory of Mineral Deposit Research (China)
 

 


The Bogda Mountain is a part of the eastern Tianshan mountain range with the Junggar basin to the north and the Turpan-Hami basin to the south. The authors have determined Rb-Sr isochron age and Sr initial ratios of ten rhyolites from the Dashitou-Sepigou belt. The age and initial ratio are 307 Ma and 0.703296, respectively. This age indicates that Bogda region volcanic activity is Late Carboniferous. This age also indicates the time when the Bogda rift closed. Sr isotopes of the rhyolites indicated that the volcanic rocks were chiefly derived from a depleted mantle source. The Dashitou-Shepikou area belong to the northern part of the eastern section of the Bogda orogen. Rhyolite of the Dashitou group gave a Rb-Sr age of 306.7±2.3Ma, and thus is suggested to have been produced at the uplifting stage following the closure of the Bogda rift.
With εNd (t) of +5.30∼+6.40, (87Sr/86Sr)i of 0.703289∼0.703496, (206Pb/204Pb)i of 18.037∼18.425, (207Pb/204Pb)i of 15.524∼15.567 and (208Pb/204Pb)i of 37.198∼37.810, the rhyolite is comparable in Nd, Sr and Pb isotopes to the basalt and rhyolite of the Lower and Middle Carboniferous Qijiaojing formation that were formed during extension and subsidence of the Bogda intra-continental rift. Minor rhyolite accompanying a large amount of basalt in the Qijiaojing formation is though to have been produced by fractional crystallisation of the basaltic magma, but a large quantity of rhyolite accompanied by minor basaltic rocks precludes this possibility of their formation.
The rhyolite of the Dashitou group is likely to have been derived from an underplated basaltic body by remelting of this body in response to mantle heating. Highly positive εNd (t), and negative εSr (t) (low 87Sr/86Sr initial ratio), and low Pb isotope ratios suggested that, like the pre-collision basalt of the Qijiaojing formation, the magma came from the depleted mantle.
Reference
Gu Lianxing, Hu Shouxi, Yu Chunshui, Li Hongyu et al.. 2000a. Carboniferous volcanites in the Bogda orogenic belt of eastern Tianshan: Their tectonic implications. Acta Petrologica Sinica, 16(3): 305-316
Wang YX, Gu LX, Zhang ZZ, Wu CZ et al. 2006. Geochronology and Nd-Sr-Pb isotopes of the bimodal volcanic rocks of the Bogda rift. Acta Petrologica Sinica, 22(5): 1215-1224 SrSr

 

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