Cambrian–Triassic mafic magmatism in the northeastern Pamir: Implications for the transition from the Proto- to Paleo- Tethys oceans

The Pamir Plateau records the evolution of the Tethys Ocean, assembly of the main terranes of the Gondwana land, and transition from the Proto- to the Paleo-Tethys tectonic domains. In this study, newly identified multi-stage mafic intrusions in the northeastern Pamir we investigated to better elucidate these processes. Zircon U–Pb dating identified four stages of mafic magmatism: 539–512 and 211 Ma in the Northern Pamir Terrane (NP), and 277 and 206 Ma in the Central Pamir Terrane (CP). The 539–512 Ma gabbros in the NP have arc-like geochemical compositions, characterized by enrichments in large-ion lithophile elements (Rb, Ba, and Sr) and depletions in high-field-strength elements (e.g., Nb, Ta, and P). They have relatively uniform zircon ε_Hf(t) values (−2.7 to +2.3), elevated zircon δ¹⁸O values (5.97 ‰–7.66 ‰), and negative ε_Nd(t) values (−6.88 to −2.33). These gabbros were likely derived by partial melting of metasomatized sub-continental lithospheric mantle in a continental arc setting. The magmas then experienced significant assimilation and fractional crystallization processes. The ca. 211 Ma cumulate gabbros have ocean island basalt (OIB)-like compositions, mantle-like zircon δ¹⁸O values (5.05 ‰–6.69 ‰), and positive ε_Nd(t) values (+0.37 to +4.21), indicative of derivation from a water-rich asthenospheric mantle source in a subduction setting. The 277 Ma gabbros in the CP exhibit enrichments in light rare earth elements, and variable whole-rock ε_Nd(t) values (+1.56 to +4.74) and zircon ε_Hf(t) (−7.3 to +1.6) values, indicating the partial melting of a lherzolitic source in a fore-arc setting. The 206 Ma gabbros in the CP, exhibiting OIB-like elemental signatures and positive ε_Nd(t) values (+1.87 to +1.96), and were likely derived from spinel-facies mantle that had been metasomatized by oceanic crust-derived materials, as evidenced by significant negative Pb anomalies. These results and those of previous studies indicate that southward subduction of Proto-Tethys Ocean in the northeastern Pamir region began at ca. 540 Ma. The Proto-Tethys oceanic slab was subducted beneath the Mazar–Tianshuihai Terrane until the Late Ordovician, but the ocean did not close completely, leaving a remnant basin that was infilled by the Upper Ordovician–Lower Carboniferous clastic carbonate sedimentary rocks. The initial subduction of Paleo-Tethys Ocean began at ca. 330 Ma in the Pamir, as recorded by the Qushiman ophiolite in the NP and back-arc volcanic rocks between the Tarim and NP. The 277 and 206 Ma gabbros in the CP suggest that bi-directional subduction of Paleo-Tethys Ocean in the Pamir began before ca. 277 Ma. Finally, ca. 180 Ma granulite-facies metamorphism occurred during final closure of the Paleo-Tethys Ocean in the Pamir, due to the collision between the NP and CP.