Petrology, geochemistry and Re-Os isotopes of peridotite xenoliths from Maguan, Yunnan Province: Implications for the Cenozoic mantle replacement in southwestern China

Petrology, geochemistry and Re - Os isotopes of peridotite xenoliths from Maguan (Yunnan Province) are reported in this paper with the aims of constraining the age and evolution of the lithospheric mantle beneath the western margin of the Cathyasia block. The Maguan mantle xenoliths contain predominantly fertile lherzolites with whole-rock Al2O3 contents of 2.42-4.99 wt.%, and subordinate clinopyroxene-poor lherzolites with Al2O3 contents of 1.19-1.98 wt.%. Their whole-rock CaO, Al2O3 and Na2O decrease along with the increase of MgO, following melt depletion trends. This suggests that the Maguan Iherzolites represent mantle residues after variable degrees of partial melting. Clinopyroxenes in the fertile Iherzolites display flat to depleted REE patterns, whereas those in the clinopyroxene-poor Iherzolites are variably enriched in LREE. Modeling results of Y and Yb contents in clinopyroxenes suggest that the fertile lherzolites have experienced similar to 1-5% degrees of partial melting, in contrast with similar to 10-15% for the clinopyroxene-poor lherzolites. Both fertile and clinopyroxene-poor lherzolites have similarly high equilibrium temperatures, i.e., 911-1120 degrees C versus 919-941 degrees C, respectively. The whole-rock Os-187/Os-188 ratios of clinopyroxene-poor lherzolites vary from 0.11764 to 0.12506, which are slightly lower than most fertile lherzolites (0.12272-0.12854). Their Os-187/Os-188 ratios show no correlation with Re-187/Os-188 ratios or bulk-rock Al2O3 contents. The rhenium depletion ages (T-RD) of the lherzolites range from 0.15 to 1.08 Ga, whereas the clinopyroxene-poor lherzolites have T-RD ages of 0.64-1.67 Ga. This suggests the co-existence of Phanerozoic and Proterozoic mantle beneath the western Cathyasia block. Alternatively, the whole lithospheric mantle beneath Maguan was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter explanation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of olivine modes versus Fo contents. We suggest that the enriched mantle that was existed beneath the western Cathyasia block during the Late Cretaceous or Eocene-Oligocene has been replaced by juvenile and depleted mantle, which probably occurred during the Cenozoic. (C) 2013 Elsevier B.V. All rights reserved.