Metamorphic evolution of the Yuka eclogite in the North Qaidam,NW China:evidences from the compositional zonation of garnet and reaction texture in the rock.

Abstract

The typical mineral assemblage of the Yuka eclogite in North Qaidam is composed of garnet (Grt),omphacite (Omp), phengite(Phe) and rutile(Ru).Coarse-grained garnet porphyroblast with obvious zoning in both composition and mineral inclusions, can be divided,from core to rim,into four rings,i.e.,core (GrtⅠ),mantle (GrtⅡA),rim (GrtⅡB) and outermost rim (GrtⅡC). Petrographic and mineral chemistry studies indicate that the Yuka eclogite has undergone three successive stages of pre-eclogite, eclogite and post-eclogite,recording a process evolved from subduction to exhumation of the rock.The pre-eclogite stage (Ⅰ) is represented by the assemblage in the core part of garnet porphyblast (GrtⅠ+AmpⅠ+Zoi+PⅡ+Q),P-T metamorphic conditions are estimated to be 450 to 500℃ and 0.6 to 0.7 GPa.Eclogite facies stage can be further divided into three sub-stages of early-,peak-and retrograde-eclogites.Early-eclogite stage (ⅡA) is characterized by the assemblage of GrtⅡA+OmpⅡA+PheⅡA+Ru preserved in the mantle part of garnet porphyblast,which recorded the metamorphic conditions of 580 to 640℃ and 2.4 to 2.5 GPa.Peak-eclogite stage (ⅡB) is represented by the rim part of garnet porphyblast,and the core part of omphacites and phengites in the matrix (GrtⅡB+ OmpⅡB+PheⅡB+Ru),P-T estimates give peak metamorphic conditions of 620 to 680℃ and 3.0 to 3.4 GPa.Retrograde-eclogite stage (ⅡC) is characterized by the outermost rim part of garnet porphyblast,Mg - kataphaprite,the rim of omphaeite and phengite in the matrix (GrtⅡC+OmpⅡC+PheⅡC+AmⅡ),P-T estimates give the first retrograde conditions of 700 to 720℃ and 2.3 to 2.4GPa. Post-eclogite stage is mainly of granulite-amphibolite facies,which is inferred from the breakdown of omphaeite to produce diopside+ plagioelase amphibole symplectites.This was overprinted by abundant amphibole and plagioclase of amphibolite facies formed from garnet,clinopyroxene and Mg-kataphaprite.P-T estimates give the metamorphic conditions of 550～600℃ and 0.6～1GPa.The metamorphic conditions of different stages of the Yuka eclogite construct a clockwise metamorphic evolution path from prograde to retrograde,and the peak stage was formed at 630～680℃ and 3.0～3.4GPa,indicate that the Yuka eclogite underwent an ultrahigh pressure metamorphism.The preservation of prograde mineral assemblages and compositional zoning in garnet porphyblast suggest that the Yuka eclogite underwent an evolution history of rapid burial and uplift.