Jadeitite in Metalherzolite of the El’denyr Massif, Chukotka: Mechanism and Setting of Its Formation

The paper presents the first data on the petrography, mineralogy, and geochemistry of jadeitites from the El’denyr massif, Chukotka, Russia, as well as host metalherzolites and amphibolite inclusions in the jadeitites. The jadeitite is composed of an association of jadeite, omphacite, analcime, and pectolite with a Ba−Ti−Si accessory mineral. The host metalherzolite is made of an association of olivine, antigorite, diopside, chlorite, ferrite-chromite, chromium magnetite, and accessory awaruite, heazlewoodite, and pentlandite. The jadeitite contains inclusions with a relict coarse-grained hypidiomorphic-granular texture, which are considered to be relics of the metasomatized protolith of the jadeitite. This protolith was probably high-temperature hydrothermal diopsidite. The inclusions show local recrystallization of primary diopside to aegirine-augite and pseudomorphic development of a fine-grained aggregate of amphiboles (several generations of richterite, actinolite, magnesiokatophorite, K-richterite, and eckermannite), omphacite, pectolite, analcime, phlogopite, accessory maucherite and heazlewoodite after diopside/aegirine-augite and an associated unidentified mineral. The protolith was transformed in several stages before the onset of jadeite crystallization, and these transformations included metasomatic recrystallization and a complete change in its texture. During the last stage, crystallization of the euhedral concentrically zoned jadeite with analcime and pectolite from fluid was accompanied by the recrystallization and dissolution of the last reworked relics of the protolith represented by high-calcium omphacite in microgranular omphacite-jadeite aggregates of jadeitite. The formation of jadeitites and the accompanying metamorphism of the host lherzolites occurred at 500°C and 8.5 kbar, which corresponds to P–T conditions typical of the metamorphism of mantle wedge peridotites in the “warm” subduction regime. The presence of jadeitites in the El’denyr massif and high-pressure metamorphic rocks in the Ust’-Belaya massif, which were studied previously, allows us to consider the Ust’-Belaya terrane as a mélange of a subduction zone active in the Early–Middle Triassic that was deformed and disintegrated during its subsequent exhumation in the Cretaceous.