Trace element and Sr isotope records of multi‐episode carbonatite metasomatism on the eastern margin of the North China Craton

Abstract

AbstractLherzolite xenoliths entrained in the Changle Cenozoic basalts were analyzed to infer mantle process beneath the eastern block of the North China Craton. These xenoliths were classified into two types. Clinopyroxenes (Cpx) in the type‐1 xenoliths are strongly enriched in large ion lithophile elements and light rare earth elements (LREE) but depleted in high field‐strength elements and heavy rare earth elements, and show high Ca/Al, Zr/Hf, and (La/Yb)N ratios but low Ti/Eu ratios. These features indicate that they were crystallized from a carbonatitic melt. Cpx in the type‐2 xenoliths are mostly characterized by a chemical zonation, that is, LREE and Sr contents and (La/Yb)N and Eu/Ti ratios gradually increase from the cores to the rims. Some fresh cores preserve the original signatures of the depleted mantle. These observations indicate partial modification of pre‐existing Cpx by carbonatite metasomatism. Two episodes of metasomatism were identified based on Sr isotopic compositions of Cpx and carbonate inclusions within olivines. Both the carbonate inclusions and Cpx cores in the type‐2 xenoliths have relatively high 87Sr/86Sr ratios (>0.7033), suggesting metasomatism due to CO2‐rich silicate melt derived from the recycled oceanic crust. However, low 87Sr/86Sr ratios of Cpx rims in the type‐2 xenoliths suggest a late stage of metasomatism by a low‐87Sr/86Sr carbonatitic melt (<0.7024), which might have been caused by upwelling of the asthenosphere mantle. Such multiple metasomatism could have played an important role in changing the chemical composition of mantle from refractory and depleted to fertile and enriched.