Reconstructing mantle–crust boundary magmatism through Cimmerian orogenic events: evidence from deep crustal cumulates in northeastern Pamir

Abstract

Deep crustal cumulates directly represent the geochemical composition of the lower crust and can provide insights into magmatism at the mantle–crust boundary. However, the scarcity of exposed deep crustal cumulates, which is due to their high density causing such rocks to sink into the mantle, limits our access to deep crustal samples. This study investigated hydrous late Mesozoic mafic–ultramafic cumulate rocks from northeastern Pamir. These rocks are the first of their kind identified in this region and exhibit features typical of deep sub-arc hydrous cumulates worldwide. Petrography, zircon U–Pb ages and zircon Lu–Hf isotopes, whole-rock geochemistry and Sr–Nd isotopes, and mineral major and trace element chemistry were used to constrain the magmatic evolution from source to surface and the crystallization conditions of the primary magma at depth. In situ zircon U–Pb dating yielded a concordant age of 199 ± 1.3 Ma. The mafic cumulates are hornblende gabbros, which had a crystallization sequence of amphibole/magnetite → plagioclase → biotite → apatite. Hornblende geobarometry yielded an equilibrium pressure of 0.65–0.80 ± 0.14 GPa, corresponding to depths of 20–26 km. The ultramafic cumulates, are lherzolites and olivine clinopyroxenites that have a crystallization sequence of olivine/spinel → clinopyroxene → ± orthopyroxene. The estimated pressure, based on published experimental constrains, suggests high-pressure crystallization occurred at ~ 1 GPa. The elevated magmatic oxygen fugacity (ƒO₂) is consistent with values expected for sub-arc conditions, where FMQ is 1–4 log units more oxidized than mid-ocean ridge basalts. The trace element composition of melts calculated to be in equilibrium with clinopyroxene is comparable to the global average composition of continental calc-alkaline basalts. Based on the petrography, mineral chemistry, and uniform whole-rock Sr–Nd isotopic data, the mafic–ultramafic cumulate rocks are inferred to have formed by fractional crystallization of a common hydrous (~ 2 wt% H₂O) parental melt derived from a depleted mantle source (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7132 ε_Nd(t) = 1.5–3.3, ε_Hf(t) = 1.1–11). These results support the notion that the polybaric differentiation in the lower crust can significantly influence the diversity of geochemical composition in the upper crust and highlight that the final closure of the Paleo-Tethys in the northeastern Pamir may not have occurred before the early Jurassic.