Geochronology, mineral chemistry and genesis of REE mineralization in alkaline rocks from the Kohistan Island Arc, Pakistan

Alkaline igneous-related rare earth element (REE) deposits constitute a major global economic resource of REE. However, our understanding of their geodynamic settings and magmatic-hydrothermal processes remains uncertain. Here, we present an investigation of petrography, geochronology and mineral chemistry of newly discovered REE mineralization in the alkaline syenite and pegmatite from the Kohistan Batholith in the Kohistan Island Arc, Pakistan. The pegmatite contains abundant chevkinite and monazite (up to 10–35% in volume), which formed from crystallization of a REE-oversaturated evolved melt and are the main REE-bearing phases. U-Th-Pb isotopic dating of zircon from the syenite yields the emplacement age of 31.2 ± 0.3 Ma, which closely matches the ages of zircon and monazite from the pegmatite (30.1 ± 0.3 Ma and 30.9 ± 0.3 Ma, respectively). Zircons from the syenite and the pegmatite have similar ɛHf(t) values, ranging from + 0.2 to + 2.5, indicating that the syenite and the pegmatite are co-magmatic in origin. The alkaline magma likely formed by partial melting of the enriched lithospheric mantle induced by the asthenospheric upwelling, which was related to slab break-off at the post-collisional stage following the collision of the Kohistan Island Arc with the Indian Plate. Zircons show the evolutional trend in trace element compositions from the syenite to the pegmatite, marked by increases in REE, Hf, Y, U, volatiles and fo2. These changes reflect prolonged fractional crystallization processes of the alkaline magma, which were responsible for the REE enrichment in the residual pegmatitic melt. Post-collisional alkaline magmatic rocks are widely distributed in many places along the Tethyan orogenic belt. These observations suggest that these post-collisional alkaline rocks may be good targets for REE exploration.