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

Abstract

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 187 Os/ 188 Os 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 187 Os/ 188 Os 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 ( 187 Os/ 188 Os: 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.

Modification of Lithospheric Mantle by Melts/Fluids With Different Sulfur Fugacities During the Wilson Cycle: Insights From Lesvos and Global Ophiolitic Peridotites

“…Tick marks indicate 5% melting increments. Also plotted are the ophiolitic peridotites from the settings of ocean‐continent‐transition (OCT; data from Becker et al., 2006; Fischer‐Gödde et al., 2011; Lorand et al., 1999, 2000, 2008, 2010; Luguet et al., 2007; Rampone et al., 1995; Reisberg & Lorand, 1995; Riches & Rogers, 2011; Saal et al., 2001; Snow et al., 2000; van Acken, Becker, Hammerschmidt, et al., 2010; van Acken, Becker, Walker, et al., 2010; van Acken et al., 2008; Wang et al., 2013), mid‐ocean‐ridge (MOR; data from Aldanmaz et al., 2012; Batanova et al., 2008; Dijkstra et al., 2010; Gong et al., 2020; Hanghøj et al., 2010; Hu et al., 2020; Schulte et al., 2009; Sergeev et al., 2014; Uysal et al., 2012) and supra‐subduction‐zone (SSZ; data from Agranier et al., 2007; Aldanmaz et al., 2012, 2020; Büchl et al., 2002, 2004; Chen et al., 2020; Haller et al., 2021; C. Z. Liu et al., 2018; O'Driscoll et al., 2012, 2015, 2018; Prelević et al., 2015; Secchiari, Gleissner, et al., 2020; Snortum & Day, 2020; Uysal et al., 2012; Xu & Liu, 2019). Arrows show the possible additional processes to account for the data.…”

Deformation, reactions, and phase mixing in upper mantle shear zones

“…Orthopyroxene + low-Si melt ↔ Olivine + high-Si melt (← high stress, → low stress) (reaction 4.I, Assuming a formation by percolating melt, the questions arise of its extent and composition. Melt percolation was reported to form lherzolite, plagioclase-bearing peridotite and grt-peridotite replacing harzburgite and dunite in a process called "refertilization" (Beyer et al, 2006;Hu et al, 2020;Le Roux et al, 2007). This process was also ascribed to form cpx-enriched tectonites in the refertilization front a few hundred meters ahead of the melting front between Ronda's coarse grained peridotites and the tectonite unit (Lenoir et al, 2001).…”