Copper isotope constraints on the origins of basaltic and andesitic magmas in the Tengchong volcanic field, SE Tibet

The Tengchong volcano field (TVF), situated at the southeastern margin of the Tibetan Plateau, holds crucial information regarding Cenozoic volcanic activities and geotectonic evolution of the SE Tibet. To provide new constraints on petrogenesis and evolution of the Tengchong volcanism, here we conducted copper (Cu) elemental and isotopic analyses on a suite of samples that document the evolution from basalts to andesites in the TVF. The basalts are Cu-depleted (29.7–36.9 ppm) and have higher δ⁶⁵Cu values (0.19‰–0.40‰, mean = 0.31‰ ± 0.05‰; n = 11) than those of mid-ocean ridge basalts (MORBs, ∼0.09‰) and the mantle (∼0.06‰) as well as the majority of island arc lavas. Along with the low Cu/Zr ratios, these characteristics are interpreted to reflect the fractionation of isotopically light sulfides in the S-saturated systems during magma ascent, rather than source heterogeneity induced by recycled materials and redox reactions. Compared with the basalts, the andesites have slightly lower Cu contents (14.4–29.4 ppm) and lighter Cu isotopic compositions (mean = –0.14‰ ± 0.06‰; n = 13). These differences cannot be attributed to progressive sulfide fractionation of basaltic magmas but require the assimilation of lower crustal materials with low δ⁶⁵Cu values during evolution of the andesitic magmas. Our results collectively suggest that Cu isotopes can provide valuable insights into magma origin and evolution.