Tale of the Kulet eclogite from the Kokchetav Massive, Kazakhstan: Initial tectonic setting and transition from amphibolite to eclogite

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt+Omp+Qz+Rt±Ph and fine-grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse-grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four-stage metamorphic evolution: (I) pre-eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre-eclogite stage or coronal eclogite, and so did not identify the four-stage evolution recognized here. P-T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P-T path and peak conditions of 27-33kbar and 610-720°C, and 27-35kbar and 560-720°C, respectively. P-T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE-enriched patterns [(La/Lu) _N=1.1-9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO ₂ (1.35-2.65wt%) and FeO (12.11-16.72wt%) and REE contents than those of coronal eclogite (TiO ₂<0.9wt% and FeO<12.11wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb-Zr/4-Y and TiO ₂-FeO/MgO diagrams, although displacement from the MORB-OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180-200km at 537-527Ma, followed by slab breakoff at 526-507Ma, exhumation and recrystallization at crustal depths is applied to explain the four-stage evolution of the Kulet eclogite.