Mg-Ca-Sr-Nd isotopes of orogenic peridotites from the North Qaidam orogen tracing deep carbon transfer during continental collision

Subduction serves as the primary mechanism for recycling surface materials into the mantle. While the carbon cycle in oceanic subduction zones has been extensively studied, the migration, storage and speciation of carbon in continental subduction zones remain poorly understood. Here, we present an integrated study of whole-rock major and trace element compositions, Sr-Nd-Ca-Mg isotopes and in situ clinopyroxene major-trace elements and Sr isotopes in orogenic peridotites from the North Qaidam orogen, Tibetan Plateau. The results show that the garnet peridotites originated from highly depleted subcontinental lithospheric mantle and experienced variable degrees of metasomatism. Garnet dunites and garnet lherzolites exhibit higher δ 44/40 Ca values (0.96 ± 0.08‰ to 1.22 ± 0.13‰) than the normal mantle (0.94 ± 0.05‰), indicating modification by partial melting. From garnet dunites through garnet lherzolites to garnet pyroxenites, Ca content increases while δ 44/40 Ca decreases progressively, indicating metasomatism by a light-Ca-enriched agent. These ultramafic rocks have nearly uniform δ 26 Mg values (−0.37 ± 0.03‰ to −0.17 ± 0.05‰), slightly lower than or consistent with the normal mantle, suggesting metasomatism by a light-Mg-enriched agent. The combined Ca-Mg isotopic systematics point to metasomatism by carbonate-bearing fluids. High 87 Sr/ 86 Sr and negative ε Nd (t) values further indicate metasomatism by a silicate melt derived from subducted granitic gneiss. Clinopyroxenes show high Ca/Al, (La/Yb) N , and Nb/Yb ratios, consistent with carbonatitic melt metasomatism, however their covariation of Zr/Hf and Ti/Eu ratios deviate from typical carbonate metasomatism. Together with the high 87 Sr/ 86 Sr ratios, these features suggest that the metasomatic agent was not a pure carbonatitic melt but a carbonated silicate melt, generated by reaction between carbonate and silicate melt derived from subducted continental crust. However, the low MgO content of carbonated silicate melts is insufficient to explain the Mg isotopic fractionation observed in the Lüliangshan peridotites. Instead, the P-T conditions and zircon characteristics indicate incorporation of carbonate into refractory peridotites via supercritical fluids, which effectively induced the observed Mg isotopic fractionation. Geochronological constraints suggest that Mg-rich supercritical fluid metasomatism likely occurred during ultrahigh-pressure metamorphism, whereas carbonated silicate melt metasomatism took place during slab exhumation. Thus, continental subduction can transfer substantial surface carbon into the subcontinental lithospheric mantle, with potential later recycling to the surface via post-collisional magmatism