Helium Isotope Geochemistry of Ultrahigh-Pressure Metamorphic Eclogites From the Dabie-Sulu Terrane in East China

Abstract: The ³He/⁴He ratios of most eclogites from the Dabie‐Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the ³He‐⁴He diagram. The ³He/⁴He ratios of eclogites are obviously correlated with the types of their surrounding rocks. The helium isotope composition of the eclogites from the Bixiling complex possesses characters of mantle‐derived rocks with the ³He/⁴He ratio being 5.6 Ra. The ⁴He concentration of the eclogites exhibits visible inverse correlation with the δ¹⁸O value of the quartz in the eclogites from the Sulu area. The δ¹⁸O values of the eclogites change synchronously with those of the country rocks. Those results suggest that protoliths of the eclogites were basic‐ultrabasic rock bodies or veins intruding into the continental crust in the early stage; strong exchange and hybridization between the basic‐ultrabasic rocks and continental rocks and the atmospheric water during the intrusion led to abrupt increase of the ³He/⁴He ratios, δ¹⁸O values and _Nd(0) values of the intrusive bodies or veins, which show characters of continental rocks. This indicates that the eclogites are autochthonous.