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In this paper, we report an occurrence of ultramafic pseudotachylytes, providing fault-rock evidence of paleo-earthquakes, from the Luobusha ophiolite complex in the Yarlung Zangbo suture zone. The pseudotachylytes form hairline-thin aphanitic veinlets and vein networks bounded by micro-damage zones cutting through the host harzburgite, forming flow banding in some places. The pseudotachylyte veins are dominated by close-knit ultrafine-grained minerals consisting of olivine, orthopyroxene, serpentine, spinel, and magnetite, cemented by an extremely fine matrix. As the primary component of the pseudotachylyte veins, olivine occurs as microphenocryst showing zoning from core to rim and as irregularly shaped microlite immersed in the interstitial material. Zoned crystals of olivine developed with Mg-rich cores and more Fe-rich rims. Microlite diagnosis of crystallization in a quenched melt includes dendritic, skeletal, and poikilitic olivine crystals, which are typical of ultramafic pseudotachylyte. The olivine microlites contain higher amounts of Ca, Al, and Cr but a lower Ni content compared with the host harzburgite olivine. Irregularly shaped chrome-spinel crystals are chemically zoned as well, indicating an Fe-rich rim overgrowth. Ni sulfide droplets interspersing among the matrix imply melt occurrence. The presence of a micro-fibrous and micro-vesicular interstitial matrix also indicates a melting-related origin. Ultracataclastite veins associated with pseudotachylyte transecting serpentine are observed, which convey that heat was generated during rapid comminution and injection. The characteristic petrography, microtextures, and chemical inhomogeneities meet the criteria of ultramafic pseudotachylyte and reveal a mixed genesis via a combination of crushing and melting. The development of extremely tiny globular prograde serpentine inclusions (∼100 nm) in the olivine microlites is ascribed to the dehydration reaction of serpentine to olivine within the pseudotachylyte. The Luobusha metamorphogenic peridotite was subjected to serpentinization after having emplaced in the crust and subsequently to high-pressure metamorphism. The pseudotachylytes were generated in the crust after the high-pressure metamorphism and did not descend to a greater depth. Flash ultra-comminution associated with frictional heating may release fluids via localized heat-driven prograde reactions in the crust.

Podiform chromitite with a dunite envelope occurs in harzburgite in the Luobusha ophiolite in the Yarlung-Zangbo suture zone, Tibet, SW China. The chromite and olivine grains come in two main forms in the chromitite: larger spherically-shaped (nodular), and dispersed (disseminated) smaller-grains. The nodular chromite and olivine grains, and their "melt inclusions," indicate that their formation was via a magmatic process, most likely in the upper mantle. The disseminated chromite and olivine are fragments of the nodular grains that were recrystallized in a metamorphic process. In contrast to the magmatic grains, these metamorphic (that is non-magmatic) chromite and olivine grains are characterized by the presence of inclusions of lizardite, brucite, chlorite, talc, actinolite, uvarovitic garnet, and magnetite. This occurrence suggests that the high-pressure metamorphism was not coeval with the chromitite formation but occurred later as an independent event, with chromitite formation and high-pressure metamorphism separated by passage to near-surface conditions. There, lizardite grew, now occurring as inclusions in the preserved high-pressure metamorphic minerals. Serpentinization took place in the shallow crust, prior to the high-pressure metamorphism. [ABSTRACT FROM AUTHOR]

Podiform chromitites crop out in ophiolitic harzburgites as pod‐like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt‐rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes (e.g., Arai, 2013; Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent (e.g. Chen et al., 2014; Weyer and Ionov, 2007; Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, δ56Fe (relative to the standard, IRMM‐014) values range from −0.02‰ to 0.11‰ in olivines and from 0.03‰ to 0.08‰ in chromites. Chromites in Dazhuqu chromitites show δ56Fe values varying from −0.03‰ to 0.02‰. In nodular and densely disseminated chromitites from Luobusha, olivines have δ56Fe values of olivines and chromites are 0.09–0.35‰ and −0.15–0.08 ‰, respectively. Chromites from Luobusha massive chromitites have δ56Fe values of 0.07–0.12 ‰. Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions Δ56FeOl‐Chr ≈–0.08‰ at 1300 °C according to the ionic model (e.g., Macris et al., 2015; Sossi and O'Neill, 2017). However, most of our samples, except for two samples, have Δ56FeOl‐Chr values that are greater than zero, indicating a disequilibrium inter‐mineral Fe isotopic fractionation. There is a positive correlation between Fo and δ56Fe (or Δ56FeOl‐Chr) of olivines but no positive correlation between Mg# and δ56Fe (or Δ56FeOl‐Chr) of chromites. This phenomenon suggests that the Fe isotopic dis‐equilibration may be caused by migrating melts in dunitic channels rather than by the sub‐solidus Fe‐Mg exchange (Xiao et al., 2016; Zhang et al., 2019). Additionally, the wide δ56Fe range of chromites is similar to those of the subduction‐related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone. [ABSTRACT FROM AUTHOR]