Ancient Refertilization Process Preserved in the Plagioclase Peridotites: An Example From the Shuanggou Ophiolite, Southwest China

Petrology, geochemistry, platinum group elements (PGEs: Os, Ir, Ru, Rh, Pt, and Pd), and Re-Os isotope data of harzburgites and plagioclase lherzolites from the Shuanggou ophiolite (southwest China) are presented in this study, in order to determine whether ancient refertilization process can be preserved in ophiolitic plagioclase peridotites. The harzburgites in the Shuanggou ophiolite are divided into two groups: (1) the Group-1 harzburgites have Os-187/Os-188 ratios (0.12297-0.12727) and PGE abundances similar to those of oceanic peridotites, thereby representing the convecting upper mantle; (2) the Group-2 harzburgites have extremely low Os-187/Os-188 ratios (0.11307-0.11651) with considerable fractionation in iridium group PGEs (Os, Ir, and Ru), indicative of an ancient refractory mantle in the oceanic lithosphere. The plagioclase lherzolites were formed by melt impregnation of the Group-2 harzburgites, according to the PGE compositions and the petrographic features. However, the large Os isotope difference between the lherzolites (Os-187/Os-188: 0.12470-0.12666) and the Group-2 harzburgites indicates that the refertilization process took place much earlier than the exhumation of the mantle at 0.4 Ga. In addition, clinopyroxene composition suggests that the percolating melt is different from the Shuanggou diabases and typical mid-ocean ridge basalts. This study therefore demonstrates that some plagioclase lherzolites possibly were not related to mantle exhumation beneath the spreading center but formed by much older melt impregnation processes in the mantle instead.