Re-Os systematics of the Raobazhai peridotite massifs from the Dabie orogenic zone, eastern China

The Raobazhai ultramafic massif of the ultrahigh pressure Sulu-Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re-Os isotopic study of periclotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and Os-187/Os-188 ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. (OS)-O-187/(OS)-O-188 is roughly correlated with Re-187/(OS)-O-188, and strongly correlated with Al2O3. These correlations can be explained by radiogenic ingrowth of (OS)-O-187 since an ancient partial melting event. T-MA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the (OS)-O-187/(OS)-O-188 vs. Al2O3 correlation at similar to 1% Al2O3. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re-Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the (OS)-O-187/(OS)-O-188 vs. Re-187/Os-188 correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids. Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between (OS)-O-187/(OS)-O-188 and Al2O3, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones. (C) 2009 Elsevier B.V. All rights reserved.