Genesis of the Ray-Iz chromitite, Polar Urals: Inferences to mantle conditions and recycling processes

Highlights

• New EPMA data of olivine, pyroxene and Cr-spinel in Ray-Iz ophiolites are presented.

• The Ray-Iz chromitites have noticeably IPGE > PPGE, consistent with the SSZ setting.

• UHP inclusions in Ray-Iz chromitite provide in-situ clues of deep mantle recycling.

Abstract

Ophiolitic chromitites in the eastern sector of the Alpine-Himalayan and Polar Ural orogenic belts preserve evidence of formation under ultra-high-pressure (UHP), deep mantle conditions. The textural and compositional characteristics of microscopic and submicroscopic inclusions in magnesiochromite from the Ray-Iz ophiolites are considered as clues of their complex evolutionary history. Bulk-rock platinum-group-element (PGE) compositions of refractory and metallurgical chromitite ores (102–309 ppb) with generally IPGE>PPGE are consistent with supra-subduction zone (SSZ) metasomatism. The abundant unusual UHP phases (e.g., micro-diamonds, moissanite, coesite…etc.), together with clinopyroxene lamellae, and globular silicate inclusions in the Cr-spinel are seen as indications of deep mantle recycling of the mantle section of the Ray-Iz ophiolites. The calculated fO₂ values for dunite and chromitite (+1.17 to +4.16, generally above the FMQ buffer) are in line with interaction of mid-ocean ridge (MOR) or back-arc ophiolites with oxidized, Mg-rich silicic (boninitic) melts in a SSZ environment.

Introduction

Podiform chromitites in the mantle part of ophiolites occur commonly as lenticular or irregular bodies of massive to disseminated chromite and are commonly enveloped by dunite in a harzburgite matrix (e.g., Su et al., 2018, Su et al., 2019, Su et al., 2020; Zhang et al., 2019; Zhou et al., 1996, Zhou et al., 2005). Their heterogeneous mineralogical and chemical compositions may reflect a prolonged history of melting and magma mixing in the upper mantle (e.g., Nicolas and Prinzhofer, 1983), or alternatively effective segregation of chromite in separate, mini-magma conduits in the upper mantle (Zhou et al., 2005).

Recent studies document the occurrence of micro-diamond inclusions in ophiolitic chromitites from Luobusa ophiolite in the southern Tibet (Chen et al., 2018; Lian et al., 2017; Xiong et al., 2016; Xu et al., 2015; Yang et al., 2014) and from ophiolites in the Paleozoic Polar Ural orogenic belt (Yang et al., 2015). Occurrence of micro-diamonds in these chromitites reflects the ultra-high-pressure (UHP) conditions of their formation in deep upper mantle levels (>150 km). This precludes the previously suggested depths of ~8 to 30 km for formation of ophiolitic chromitites (e.g., Zhou et al., 2005 and references therein). Moreover, acicular silicate inclusions (i.e., diopside, coesite, and enstatite) in Cr-spinel grains have been reported in several ophiolite complexes (e.g., Yamamoto et al., 2009). Exsolution of diopside, enstatite and coesite from a UHP Cr-spinel polymorph with Ca-ferrite (CF), which is stable above 12.5 GPa (deeper than 380 km) is suggested (cf. Satsukawa et al., 2015). It is therefore suggested that these chromitites bodies form, at least partly, in a much deeper setting, near the Mantle Transition Zone (410–660 km) and then travel upwards in the upper mantle during their evolution (e.g., Griffin et al., 2016; Satsukawa et al., 2015; Xiong et al., 2015; Yamamoto et al., 2009; Yang et al., 2007; Yang et al., 2015).

Evolution of the associated peridotites from deep to shallow mantle conditions is supported by the occurrence of two pyroxene-spinel symplectites in some of the diamond-bearing Tibetan ophiolites (e.g., Hébert et al., 2003; Xiong et al., 2016, Xiong et al., 2018). Griffin et al. (2016) interpreted these intergrowths as a result of the breakdown of high-pressure majoritic garnet. Recent studies showed numerous crustal minerals (e.g., zircon, monazite, rutile, apatite) in many of these ophiolites (Akbulut et al., 2016; Lian et al., 2017; Liu et al., 2019; Robinson et al., 2015; Zhou et al., 2014). The presence of both deep and shallow mantle minerals in many podiform chromitites has been considered as clues of deep mantle recycling of ophiolitic chromitites and associated peridotites (e.g., Arai and Miura, 2016; Griffin et al., 2016).

In this study, comprehensive studies of the textural relationships and micro-analytical data of mineral constituents of the Ray-Iz chromitites and associated peridotites are used to reveal details of their genesis and source regions. Moreover, the spatial relationships between different chromitite bodies and details of their PGE composition are assessed here to better understand their evolution and mantle conditions during formation. The new data presented in this study are discussed in view of the conceptual genetic models of podiform chromitites.