Genetic Mineralogy Of The Garnet Mafic-ultramafic Rocks In The Mica Hill,Polar Urals,Russia

The garnet mafic-ultramafic rocks are located in Mica hill,southern part of the Marunkeu high pressure metamorphic terrane,Polar Urals.The rock types are mainly composed of peridotites,pyroxenite,amphibolites and metagabbroics,where many residual primary minerals and pyrogenic textures remained.The residual primary minerals mainly include olivine,orthopyroxene and clinopyroxene,while the metamorphic minerals mainly include amphibole,garnet and some orthopyroxene.The textures include cumulatic,polkilitic,sideronitic,exsolution,relict-intersertal,granoblastic,coronal,etc.Primary olivine belongs to chrysolite with Fo values 82.9 to 85.9.Primary orthopyroxene belongs to bronzite,and the composition is En79.2-84.4Fs12.9-16.SWo1.3-5.2,and characterized by high Al,Ca and low Mg.Primary clinopyroxenes are diopside and augite with a composition of En42.4-46.1Fs7.7-13.OWo410-48.2,showing the characteristics of high Al.The garnets appearing in plagioclase pseudomorph have Ca-rich characteristics,while others show Mg-rich,low LREE,high MREE and HREE,high Rb,U,Zr,low Ba,Sr.Meta-orthopyroxene is bronzite with the compositions of En83.4-87.8Fs11.0-16.3Wo0.1-1.8,indicating the features of high Mg,low Al and Ca.The amphiboles belong to pargasite,magnesiohastingsite and minor endenite with characters of rich Fe3+ and the REE distribution parrerns of LREE enrichment,low Ho,Yb,and the trace elements of high Rb,Ba,Nd Sr,and low Th.Amphiboles are metamorphic in origin due to having pargentic relation with garnet.The major elements of garnet mafic-ultramafic complex show high MgO,CaO and Al2O3.The REE distribution patterns show LREE enrichment and Eu positive anomaly.Trace elements show obvious Th,Nb,Zr and Hf negative anomaly and Ba,Sr,Ta,Eu positive anomaly.The transition elements show high Sc,Ti,V,and low Co,Ni.The range of epsilon Nd(t)values is from 0.36 to 3.15,indicating a weak depleted mantle source.SHRIMP U-Pb ages of igneous zircons in the leuco-metatroctolite are 517±10 Ma and 518±11 Ma,while SHRIMP U-Pb ages of igneous zircons in the kyanite eclogite yield a concordant age of 502±7Ma,indicating the protolith age of the garnet mafic-ultramafic rocks formed in the Middle Cambrian.According to the mineral chemistry of primary orthopyroxenes and clinopyroxenes,the temperature-pressure condition of the protolith of garnet mafic-ultramafic complex is estimated to be P=5±2 kbar and T=1100±25?;According to the symbiosis of garnet and orthopyroxene in the metatroctolite,the peak metamorphic condition yielded P= 18-21 kbar,T=740-866? by the garnet-orthopyroxene geothermobarometry,which belongs to the medium termperature eclogite facies.Considering the petrography,mineral chemistry,geochemisty and isotopic data of the garnet mafic-ultramafic rocks in the Mica hill,the protolith was identified to be continental troctolitic cumulate and formed in the post-collision extension/continental rift environment in light of zircon chronology and regional geological data.