Zinc Isotopic Compositions Of Cenozoic Potassic Volcanic Rocks In Northeast China

Potassic and ultrapotassic volcanic rocks have attracted wide attention because of their high potassium contents and peculiar tectonic setting in which they formed.Cenozoic potassic volcanic rocks occur in Wudalianchi(WDLC),Erkeshan(EKS),Keluo(KL)and Nuominhe(NMH)in northeastern China.The petrogenesis of potassic volcanic rocks from northeastern China has been widely studied in recent years but remains highly debated.Proposed mechanisms include:potassic basalts originated from phrogopite-bearing garnet-peridotite in the sub-continental lithospheric mantle,which has been metasomatized by delaminated old,lower continental crust;a carbonated asthenosphere mantle and EM-1 lithospheric mantle interaction model;carbonated EM-1 melts derived from mantle transition zone and interacted with lithospheric mantle.In order to provide new constraints on the origin of these K-rich lavas,it is necessary to use other geochemical indexes to further constrain the mantle source composition of potassic volcanic rocks in northeastern China.Zinc(Zn)is a large lithophile element and widely distributed in mantle and sedimentary carbonates.Limited Zn isotope fractionation was resulted by subduction-related dehydration,magmatic differentiation and partial melting.There is a large Zn isotope difference between marine carbonates and the terrestrial mantle.Therefore,Zn isotope is a novel tracer for deep carbon cycles.In the master's thesis,we reported the major,trace elements contents and Sr,Nd,Zn isotopic compositions for Cenozoic potassic volcanic rocks in NE China.These rocks are characterized by high CaO/Al₂O₃ and LREEs that reflect the presence of garnet in the mantle source.The Sr-Nd isotopic composition constrains the mantle source in the EM1-type lithospheric mantle,and enrichment of Ba,Rb,K indicates that the mantle sources have been modified by melts/fluids from subducted plates.We first report the heavy Zn isotopic compositions of potassic rocks from NE China.There are no correlations among ?⁶⁶Zn and ⁸⁷Sr/⁸⁶Sr,¹⁴⁴Nd/¹⁴³Nd,TiO₂,Ni,Cr,Zn,which indicate that the heavy Zn isotopic signatures were not results of crustal contamination,fractional crystallization,diffusion effect and recycled carbonated eclogites in the sources.The lacking of correlation between ?⁶⁶Zn and ⁸⁷Sr/⁸⁶Sr and the absence of stagnant Paleo-Pacific slab in the research area further suggest that reaction of deeply-derived melts with the lithospheric mantle is not required.The negative correlation of ?⁶⁶Zn with CaO/Al₂O₃ and ?²⁶Mg indicates that the mantle sources were modified by recycle carbonates.In addition,the relationships among Zn isotopic composition,Sr isotopic composition and the content of Zn are not support the interaction model of asthenosphere mantle and lithospheric mantle.Therefore,together with the EM-1 like Sr-Nd isotopic compositions,we propose that the mantle source of potassic rocks from NE China was located in the lithospheric mantle,and the anomalies of heavy ?⁶⁶Zn and light ?²⁶Mg of these samples were the results of metasomatism by recycled carbonate-bearing ancient sediments.