Blueschist Facies Metamorphism In Northeastern China

The studies of the blueschists in northeastern China(NE China)have provided insights into understanding the nature as well as temporal information of the suture between the microcontinents and the deep dynamic process of the microcontinent amalgamation in NE China.In this paper,blueschist samples from the Toudaoqiao,Yilan and Mudanjiang area are investigated from the view of systematically detailed petrology and mineralogical chemistry.Combined with the geochronology,our phase equilibrium modeling gives the P-T path of the metamorphism,which provides constraints on the tectonic evolution of these areas.Blueschists in the Toudaoqiao area could be classified into two groups,epidotechlorite glaucophane schist(chl-ep-gl-phn-ab-hem-sph-q),the pressure and temperature peak metamorphic conditions are estimated to be 1.3~1.5 GPa,360~400?;0.75~0.96 GPa,505~532?,repectively;phengite-albite glaucophane schist(chl-ep-wnc-ab-phn-gl-sph-q).the pressure and temperature peak metamorphic conditions are estimated to be 1.3~1.4 GPa,440~450?;0.9~1.1 GPa,560~600?,respectively.The formation and evolution of the representative blueschist in the Toudaoqiao area could be divided into three stages.(1)The early prograde metamorphic stage with representative minerals of glaucophane and phengite inclusions in epidote porphyroblast and albite porphyroblast.The mineral assemblage is gl+phn+sph.(2)The near-peak blueschist facies stage with representative minerals of epidote porphyroblast and the glaucophane with composition zoning.The mineral assemblage is ep+amph+phn+gl+sph.(3)The retrograde cooling stage with representative minerals of the albite porphyroblast and matrix mineral.The mineral assemblage is chl+ep+gl+ab.These stages constitute a typical clockwise(western Alps type)P-T path in the Toudaoqiao area,and show the characteristic of the peak temperature lagging behind the peak pressure.Geochemical features show that the protoliths of the blueschists are alkaline basalts which are derived from partial melting of an enriched lithospheric mantle and contaminated by crustal components.The protoliths also exhibit the affinities of oceanic island basalt(OIB).Combined with the previously reported geochronological data,we proposed that the Toudaoqiao blueschist had been formed during the continental collision between the Erguna Block and the Xing‘an Block in the early Paleozoic.Blueschists in the Yilan area could be classified into three groups,garnet-barroisite schist(g-brs-wnc-act-phn-chl-ep-ab-stlp),chlorite-epidote glaucophane schist(gl-brswnc-act-chl-ep-stlp)and phengite-glaucophane schist(gl-wnc-phn-ab-ep-chl-sph).Peak metamorphic conditions are estimated to be 1.14~1.2 GPa,500~525?;1.4 GPa,480?;0.95~1.2 GPa,450~500?,respectively.The formation and evolution of the representative blueschist in the Yilan area could be divided into three stages.(1)The early prograde metamorphic stage with representative minerals of glaucophane,phengite and epidote inclusions in garnet porphyroblast and the core component of garnet.The mineral assemblage is ep+phn+gl+g+amph.(2)The near-peak epidote-blueschist facies stage with representative minerals of inclusions in albite porphyroblast and the mantle composition of garnet.The mineral assemblage is ep+phn+g+amph+chl.(3)The retrograde cooling stage with representative minerals of the rim composition of garnet and albite porphyroblast and matrix mineral.The mineral assemblage is chl+amph+act+ep+stlp+ab.Blueschists in the Mudanjiang area could be classified into two groups,phengite-chlorite glaucophane schist(chl-ep-phn-gl-amph-act-sph)and epidote-chlorite glaucophane schist(chl-ep-gl-amph-act-phn-pa-bi-sph).Peak metamorphic conditions are estimated to be 1.2~1.3 GPa,490~500?;1.2~1.3 GPa,520~530?,respectively.The formation and evolution of the representative blueschist in the Mudanjiang area could be divided into three stages.(1)The early prograde metamorphic stage with representative minerals in epidote porphyroblast and the strong orientational glaucophane.The mineral assemblage is gl+phn+chl+ep+stlp+sph.(2)The near-peak epidote-blueschist facies stage with representative minerals of epidote porphyroblast and the rim composition of glaucophane.The mineral assemblage is chl+ep+phn+gl+amph+sph+pa+sph.(3)The retrograde cooling stage with representative minerals of albite porphyroblast and matrix mineral.The mineral assemblage is chl+ep+gl+amph+act+phn+bi+sph.These stages constitute typical clockwise(Franciscan type)P-T paths in the Yilan and Mudanjiang area,which have recorded the P-T increasing process before the metamorphic peak.The maximum temperature was coincident with the maximum pressure in the process,which indicates that the epidote-blueschist facies mineral assemblage was formed in the process of subduction instead of exhumation.The retrograde trajectory nearly coincidently retraced the prograde trajectory,which represents the similar geothermal gradient in the subduction and the relatively slow exhumation process.The formation of blueschists in the study area marks the young oceanic crust subduction.Geochemical features show that the protolith of blueschist are mainly alkaline basalts and some of them are subalkaline basalts,which are derived from partial melting of enriched lithospheric mantle and mixed by ocean sediments.The blueschist has the property of oceanic island basalt(OIB),and a minority of samples present mid-ocean ridge basalt(MORB)characteristics.Zircon U-Pb geochronology shows that the protolith age of the blueschist is~261 Ma.Combined with previous studies and regional geological observations,we suggest that there was an ocean basin between the Jiamusi Block and SongnenZhangguagncai Range Block during the late Paleozoic-early Mesozoic.The blueschists in the Yilan and Mudanjiang area were formed in the oceanic crust subduction during the early-middle Jurassic,and experienced the metamorphism and deformation during the late Jurassic.