[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-P02] Petrographic and geochemical study of apatite in carbonatites of the Peshawar Plain Alkaline Igneous Province, NW Himalayas, Pakistan

*Mehboob ur Rashid^1,2, Hafiz Rehman¹, Muhammad Jawad Zeb², Nasir Ahmad¹, Muhammad Hussain¹ (1.Graduate School of Science and Engineering, Kagoshima University , Japan, 2.Geoscience Advance research Labs, Geological Survey of Pakistan)

Keywords:PPAIP, Carbonatite, Apatite, Trace elements, Geochemistry, Geochronology

This study presents petrographic, textural, and geochemical investigations carried out on apatite crystals hosted in carbonatite rocks of the Peshawar Plain Alkaline Igneous Province (PPAIP), Northwestern Himalayas, Pakistan. Four carbonatite bodies (1. Sillai Patti, 2. Loe Shilman, 3. Jambil, and 4. Warsak) are exposed in the PPAIP that are named after the villages in which these bodies were first reported. Petrographic analyses indicate that all the four bodies exhibit phaneritic texture and are predominantly composed of carbonates (calcite with minor dolomite), pyroxene, biotite, amphibole, and apatite. Whole-rock geochemistry, characterized by relatively high CaO contents (> 40 wt.%), classify the four bodies as calcio-carbonatites. Based on textural observations, apatite in the studied carbonates can be categorized into two forms: (1) randomly distributed euhedral and well-developed crystals with size ranging from 50µm to up to 1 mm, (2) well-developed crystals, mainly elongated, and oriented along a particular direction. Backscattered electron and cathodoluminescence (CL) imaging reveal apatite crystals from Sillai Patti, Loe Shilman, and Warsak carbonatites are oscillatory zoned with dark inner domains and bright outer overgrowth/rim domains. In contrast, apatite crystals from the Jambil carbonatite show dirty and murky textures with weak or no zoning pattern. EPMA analyses confirm the studied apatites are fluorapatites (fluorine content: 1.44 to 2.76 wt.%) with limited variability in CaO and P2O5. Trace element analyses show apatite crystals from Sillai Patti and Loe Shilman carbonatites are enriched in Sr, Ba, and Ce and rare earth elements (REE). Apatite crystals from the Warsak carbonatite, although, sharing textural features with apatite crystals from the afore-mentioned two localities, are geochemically distinct and exhibit relatively lower or intermediate REE contents. In contrast, apatite crystals from Jambil carbonatite show decreased trace element concentrations, lower REE contents, and may suggest a different origin of their formation. Based on petrographic and textural features, and geochemistry, the studied apatite crystals from the four carbonatite bodies suggest their geodynamic evolution under a broader geological framework, possibly with the magmatic crystallization during Cretaceous, evidenced by the U-Pb isotope lower intercept values of 73 to 60 Ma from the analysis of apatite crystals from Loe Shilman and Warsak while the lower intercept age value of 29 Ma from apatite crystals from the Jambil carbonatite may indicate rehomogenization or geochemical resetting during the late-stage metamorphic event or hydrothermal activities in the post-India-Asia collision-related tectonic event that likely affected the magmatic growth-related geochronological records.