Three Phases of Eocene–Miocene Igneous Activities in the Pamir Plateau: Implications for Continental Uplifting and Geodynamic Process after the Closure of the Neo-Tethys Ocean

Abstract A wide variety of Eocene–Miocene igneous rocks are widespread in the Pamir Plateau. The geodynamic processes of these igneous rocks and their coupling with the continental uplifting of the Pamir are poorly understood. In this study we report the field geological features, zircon and monazite U–Pb ages, major and trace elemental as well as Sr-Nd-Pb-Hf isotopic compositions of the Eocene–Miocene gabbroic diorite–granodiorite–leucogranite–pegmatite complex along the southeastern side of the Hindu Kush Mountains between Afghanistan and Pakistan, to provide new insights into the geodynamic process after the amalgamation of India and Asia in the Pamir. Age dating results reveal the diverse types of intrusions were emplaced at 27–22 Ma. Based on elemental and Sr-Nd-Pb-Hf isotopic compositions, the gabbroic diorites were derived from recently metasomatized lithospheric mantle source coupled with crustal assimilation and crystal fractionation effects (AFC effects) whereas voluminous leucogranite and pegmatites were derived from partial melting of the Precambrian basement. The voluminous Early Miocene diverse intrusions along the Hindu Kush Mountains are genetically ascribed to the Neo-Tethys slab delamination at an extensional setting. Integrating this study with previous research, three main phases of Eocene–Miocene igneous activities can be identified in the Pamir, i.e. the c. 40, 27–20 and 12–9 Ma. In addition to the distinct petrogenesis of these three phases of igneous rocks, they revealed the variation of the crustal thickening as well as deep geodynamic process from c. 40 to c. 9 Ma in the Pamir. We suggest that the earliest locally limited crustal thickening at the western section of the Central Pamir began at c. 40 Ma, induced by corner compression between Northern Pamir and Central–Southern Pamir. During 27–20 Ma, the spatially–temporally associated lamprophyres and high-Ba-Sr Baltoro granitoids in eastern Karakorum suggest the second phase of crustal thickening of the Eastern Pamir whereas the crustal thickness of the Western Pamir remained at a normal level at this time. This scenario was likely ascribed to the different speeds of northward movement of the Indian Plate during 40–27 Ma. The rapid crustal thickening of the Pamir occurred at Late Miocene (12–9 Ma), which was coupled with the Asian lithosphere delamination at the Central Pamir.