Petrogenesis And Tectonic Implications Of The Northern Luobei Amphibolite And Metagabbro From The Western Margin Of The Jiamusi Massif

Controversy has long surrounded the tectonic relationship and amalgamation history between the Jiamusi Massif and Songnen-Zhangguangcai Range Massif.But previous studies mainly focus on the Heilongjiang complex to address these issues,especially in the Yilan and Mudanjiang areas,and ignore the researches of the Heilongjiang complex in the Luobei area and many intrusions along the western margin of the Jiamusi Massif.Consequently,we present LA-ICP-MS zircon U-Pb ages,geochemical data and zircon Hf isotope compositions of the Taipinggou amphibolite and metagabbro exposed along the western margin of the Jiamusi Massif,in order to ascertain the emplacement ages and geochemistry characteristics of the amphibolite and metagabbro,as well as the tectonic evolution between Jiamusi Massif and Songnen-Zhangguangcai Range Massif.1.The emplacement ages of the Taipinggou amphibolite and metagabbroLA-ICP-MS zircon U-Pb isotope data yield yield206Pb/238 U ages of 267±2Ma and 264±2Ma for the Taipinggou amphibolite and metagabbro,respectively,indicating that the Taipinggou amphibolite and metagabbro formed in the Middle Permian.2.Geochemistry of the Taipinggou amphibolite and metagabbroTaipinggou amphibolite samples in Luobei area have low SiO₂?50.67–53.35 wt.%?and total alkali?Na₂O+K₂O=2.30–4.01 wt.%?contents,and Na₂O/K₂ O ratio0.402¹.169;and high total Fe₂O₃?6.55–7.40 wt.%?;TiO₂ concentrations?0.405⁰.603 wt.%?;Al₂O₃ concentrations?14.38–16.67 wt.%?;P₂O₅ concentrations?0.047⁰.077 wt.%?;MgO concentrations?9.44–10.48 wt.%?and Mg-number?73–75?.These samples are enriched in large ion lithophile elements?e.g.,Rb,Ba,K?and light rare earth elements,and depleted in high field strength elements?e.g.,Nb,Ta,Ti,P?and heavy rare earth elements,with ?Hf?t?values of –6.63 to –3.26.In addition,the Taipinggou metagabbro samples have SiO₂ contents of 52.34–53.28 wt.%,total Na₂O+ K₂O contents of 5.68–5.94 wt.%,Na₂O/K₂O ratio 5.68⁵.94,TiO₂ contents of1.09¹.19 wt.%,Al₂O₃ contents of 17.26–17.86 wt.%,total Fe₂O₃ contents of8.79–9.41 wt.%,P₂O₅ contents of 0.242⁰.272 wt.%,MgO contents of 4.40–4.74 wt.%,and Mg-number of 49–51.The Taipinggou metagabbro samples are classified as sub-alkaline series,and are characterized by enrichment in large ion lithophile elements?e.g.,Rb,Ba,K?and light rare earth elements and depletion in Nb-Ta-P-Ti,with ?Hf?t?values of –3.09 to +1.16.3.Petrogenesis and tectonic setting of the Taipinggou amphibolite and metagabbroThe Taipinggou metagabbro and amphibolite have similar geochemical and Hf isotopic compositions,implying their derivation from a common parental magma source but with different degrees of magmatic differentiation.Based on new geochronological and geochemical data presented in this study,we propose that the parental magma of the Taipinggou metagabbro and amphibolite probably originated from an enriched lithospheric mantle.And the negative Nb,Ta,Zr,Hf anomalies,and high Mg#?49-75?,low Lu/Yb ratio?0.13-0.16?,high Ba/Nb and La/Nb ratio implying derivation from the mantle had been metasomatized by fluids derived from the subducted oceanic slab instead of been influenced by significant crustal contamination.And in the evolution process,the magma of amphibolite and metagabbro experienced varying degrees of magmatic fractionation.The magma of amphibolite mainly experienced accumulation with a shallow-level evolutionary process involving fractional crystallization.In conclusion,we suggest that the Taipinggou amphibolite and metagabbro were both formed in a Middle Permian continental arc setting,closely related to the eastward subduction of Songnen-Zhangguangcai Range Massif beneath the Jiamusi Massif.And therefore,we suggest that the Mudanjiang Ocean between the Jiamusi Massif and Songnen-Zhangguangcai Range Massif were not closed before the Permian.