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

Abstract

Fluids in the deep subduction zones play an important role in crust-mantle exchange 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 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb and 87 Sr/ 86 Sr 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 compositions 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 206 Pb/ 204 Pb vs 207 Pb/ 204 Pb. On the plot of 206 Pb/ 204 Pb vs 208 Pb/ 204 Pb, 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 206 Pb/ 204 Pb vs 207 Pb/ 204 Pb than on the plot of 206 Pb/ 204 Pb vs 208 Pb/ 204 Pb. 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 206 Pb/ 204 Pb vs 87 Sr/ 86 Sr 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 continental crust, which indicates that continental crust materials probably have been incorporated into the rocks. The fluids in the paleosubduction zones in western Tianshan are mixtures of two sources, one originating from the devolatilzation of the host rocks with protolithes similar to ocean basalts from enriched mantle 1 (EMI), the other from the dehydration of subducted sediments.