Mg-Zn Isotopic Evidence for the Involvement of Recycled Carbonates in the Petrogenesis of Cenozoic Basalts in Central Mongolia
The light δ26Mg and heavy δ66Zn of basaltic lavas are generally thought to be derived from recycled carbonates that are subducted into the mantle along with the slab,making Mg-Zn isotopes a powerful tool for tracing recycled carbonates.To identify the possible recycled carbonates carried by the subducted Mongolic-Okhotsk oceanic plate and its control on the isotopic and geochemical compositions of Cenozoic basalts in Central Mongolia,this paper reports the Mg-Zn isotopic compositions of Cenozoic alkaline basalts from Central Mongolia(Jargalant,Tariat,Togo and Orhon)in the north of the Mongolian-Okhotsk suture belt.These basalts show low δ26Mg values(-0.43‰ to-0.28‰)and high δ66Zn values(0.33‰ to 0.52‰),which are significantly lower than terrestrial mantle δ26Mg values(average δ26Mg=-0.25‰±0.04‰)and higher than terrestrial mantle δ66Zn values(average δ66Zn=0.18‰±0.05‰).The Mg-Zn isotopic anomalies cannot be explained by surface processes,magmatic differentiation and evolution,or isotopic diffusion effects,but are consistent with the isotopic fractionation effect caused by the partial melting of carbonated pyroxenite.Quantitative modelling of Mg-Zn-Sr isotope suggests that the addition of 5%-10%recycled Mg-rich carbonate into the pyroxenite source could well match the low δ26Mg and high δ66Zn of Cenozoic basalts in central Mongolia.Given that the studied volcanic fields are located near the Mongolia-Okhotsk suture belt,the carbonate recycling was most likely derived from the subducted Mongolia-Okhotsk oceanic crust during the Paleozoic-Mesozoic.Both the isotopic and geochemical compositions of the basalts exhibit symmetrical variations from South to North,that is,δ26Mg value,87Sr/86Sr,Ti/Ti*,and Hf/Hf* ratios decrease,while δ66Zn value and Sm/Yb ratios increase.We suggest that more magnesites are released from the subducted slab,with an increase in the depth of subduction.The geochemical and isotopic variations of Cenozoic basalts in Central Mongolia are controlled by the melting depth and dolomite/magnesite ratios.
万方数据
维普
