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Origin of the deep fluids in the paleosubduction zones in western Tianshan:Evidence from pb-and Sr-isotope compositions of high-pressure veins and host rocks

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Science China Earth Sciences 2005年第10期48卷 1627-1636页

作者： Reiner Klemd Mineralogisches Institut Universit?t Würzburg Am Hubland 97074 Würzburg Germany

Fluids in the deep subduction zones play an important role in crust-mantle ex-change related to the subduction process. Identification of fluids origin, internal or external, can help us to evaluate the contribution of crust materials to mantle during this process. High-pressure veins, which developed in the western Tianshan HP-metamorphic belt extensively, are the direct products of the fluids in subduction zones. In western Tianshan, high-pressure veins and host rocks have overall pb- and Sr-isotope compositions. At t = 340 Ma, the high-pressure metamorphic time, the ratios of 206pb/204pb, 207pb/204pb, 208pb/204pb and 87Sr/86Sr of high pressure veins are 17.122―18.431, 15.477―15.611, 37.432―38.689 and 0.70529―0.70705, and are 17.605―17.834, 15.508―15.564, 37.080―38.145 and 0.70522―0.70685 for host rocks. However, high-pressure veins show a much larger variation in pb-isotope composi-tions than host rocks. Plots of samples are distributed roughly along a line parallel to and far away from the Northern Hemisphere reference line (NHRL) on the plot of 206pb/204pb vs 207pb/204pb. On the plot of 206pb/204pb vs 208pb/204pb, plots of data are distributed nearby mid-ocean-ridge basalts (MORB) area or along a line parallel to and not far away from NHRL, which shows that the data plots are farther away from MORB and ocean island basalts (OIB) areas on the plot of 206pb/204pb vs 207pb/204pb than on the plot of 206pb/204pb vs 208pb/204pb. The compositions of pb- and Sr-isotopes of these high-pressure metamorphic rocks are between that of enriched mantle 1(EMI) and of sediments or upper crust. A notable characteristic can be seen from plots of pb-isotope ratio vs pb-isotope ratio and 206pb/204pb vs 87Sr/86Sr that array of plots shows an evident tendency of mixture of two end members. Ratios of Rb/Ba, Ce/pb, Nb/U and Ta/U of high-pressure veins and host rocks are between that of OIB or MORB and that of conti-nental crust, which indicates that continental crust materials probably

关键词： deep fluids in the subduction zones, pb- and Sr-isotope, high pressure vein, host rock, western Tianshan.

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