Melting a fine-scale heterogeneous mantle source and the formation of the Hongliuxia shoshonitic volcanic rocks, NW China: Constraints from Sr and Nd isotope and trace element studies

The ~100 Ma Hongliuxia shoshonitic volcanic rocks occurred near the junction of the Altyn Tagh fault and the Qilian Shan frontal thrust fault. Detailed Sr and Nd isotope and trace element studies on this suite of rocks show that they can be subdivided into two groups. Group A rocks have intitial 87 Sr/86Sr and ɛNd values ranging from 0.70616 to 0,70620, and -4.48 to -5.34, respectively, which are lower than 0.70649 to 0.70666, and -4.11 to -4.43 for Group B rocks. As compared to those in the Group B rocks, Group A rocks have lower contents of LREE, Nb, Ta, Sr, Rb, SiO2, Al2O3, K2O, and Na2O and ratios of (La/Gd)N and Rb/Sr, but higher MgO, Th/U and Sm/Nd. Group B rocks have Sr concentrations greater than 1066 ppm which are correlated with their high contents of K2O (>3.0 wt%) and Ba (>1250 ppm), and Rb/Sr and initial 87 Sr/86Sr ratios. Several lines of evidence suggest that the elemental geochemistry and Sr-Nd isotope systematics for the Hongliuxia volcanic rocks were derived from their mantle source regime, not by crustal contaminations. Such evidence includes: (1) seemingly positive correlation between initial Sr and Nd isotopic compositions; (2) high Sr (>1066 ppm) but low MgO (<6.0 wt%) contents in Group B rocks; (3) high MgO contents (7.49-7.92 wt%) in Group A rocks; (4) relatively low Rb (<58 ppm) concentrations. These observations lead us to propose that Group A and B rocks have sampled a lithospheric mantle with fine-scale geochemical heterogeneities. Group A rocks were derived from partial melting of metasomatised ancient lithospheric mantle without the involvement of phlogopite and apatite, in contrast, enhanced involvement of phlogopite and apatite might have contributed to elevated Rb/Sr, and Sr and Nd isotopic compositions in Group B rocks. This study suggests that such a fine-scale geochemical heterogeneity may be a common feature for the continental lithospheric mantle.