Origin of a Miocene alkaline–carbonatite complex in the Dunkeldik area of Pamir, Tajikistan: Petrology, geochemistry, LA–ICP–MS zircon U–Pb dating, and Hf isotope analysis

Abstract

Intrusive carbonatite complexes host the world’s largest rare-earth element (REE) deposits. However, the genesis of carbonatite complexes remains unclear. A recently discovered carbonatite–syenite complex in the Dunkeldik area of Pamir, Tajikistan, is interpreted as a carbonatite-related REE deposit. The deposit is hosted in an alkaline–carbonatite belt comprising alkali basalt, alkali trachyte, syenogranite, syenitic porphyry, and carbonatite. The present study focuses on a 1–3-m-wide, 25-m-long, NE-trending carbonatite dike and associated syenitic porphyries. The carbonatite comprises mainly calcite with minor apatite, bastnäsite, allanite, and zircon. Relatively high proportions of anhydrite, gypsum, fluorite, quartz, and barite post-date the formation of the carbonatite–syenite complex. The syenitic porphyries contain phenocrysts of microcline, microperthite, and fluorite ± nepheline or quartz in a feldspathic groundmass.Carbonatite samples are characterized by low SiO2 (<5 wt%), FeO (<1.84 wt%), and MgO (<0.25 wt%), and relatively high CaO (29.39–35.04 wt%) concentrations, distinguishing them from primary magnesiocarbonatites. The associated syenitic porphyries yield high SiO2 (54.26–69.59 wt%), Al2O3 (12.73–16.08 wt%), and alkali (K2O + Na2O = 10.84–13.48 wt%; K2O > Na2O) concentrations, and low MgO (0.14–0.97 wt%) and TiO2 (0.21–0.50 wt%) concentrations. The carbonatite and syenitic porphyries show similar mantle-normalized incompatible-trace-element trends, characterized by negative gradients, extreme large-ion lithophile element (e.g., Sr and Ba) and light REE enrichment, and depletion in high-field-strength elements (e.g., Nb, Ta, P, Zr, Hf, and Ti). Laser-ablation–inductively coupled plasma–mass spectrometry U–Pb dating of zircon from carbonatite and syenite porphyry samples yields weighted mean ages of 10.66 ± 0.23 Ma (2σ) and 10.76 ± 0.09 Ma (2σ), respectively. Zircon grains in the carbonatite have moderately radiogenic Hf isotope compositions (εHf = −7.9 to +0.9). Zircons from carbonatite and syenitic porphyry samples show similar REE compositional trends, characterized by enrichment in heavy REEs relative to light REEs. Zircon grains from carbonatite and syenitic porphyry yield Th/U ratios of 0.77–1.18 and 0.85–1.62, respectively. We therefore interpret zircon grains in the carbonatite as xenocrysts inherited from the associated syenites.The similar emplacement ages and trace-element compositions of the spatially associated syenites and carbonatite suggest an origin involving liquid immiscibility. The geochemistry and Hf isotope data indicate that the magmas were derived from thickened lower crust that received an input of enriched mantle-sourced material hybridized by fluids derived from an ancient subducted oceanic slab. We infer that the carbonatite and syenitic porphyries formed in a post-collisional setting during the Miocene. The upwelling of an asthenospheric mantle diapir during the Cenozoic induced the partial melting of metasomatized lithospheric mantle and lower crust beneath the western margin of the Indian–Asian collision zone. The large-scale Karakorum strike-slip fault may have acted as a conduit for the emplacement of the syenite–carbonatite magma.