Apatite texture, trace elements and Sr[sbnd]Nd isotope geochemistry of the Koga carbonatite-alkaline complex, NW Pakistan: Implications for petrogenesis and mantle source

Abstract

The Koga Carbonatite-Alkaline Complex (KCAC) in northwestern Pakistan is emplaced within the Early Paleozoic Greater Himalayan Sequence of the region, marking a significant magmatic event. The complex primarily comprises quartz syenite, nepheline syenite, and carbonatite lithologies. Apatite grains extracted from these lithologies were systematically analyzed using cathode-luminescence (CL) imaging, trace elements and SrNd isotopic compositions, and LA-ICP-MS UPb geochronology. The homogeneous cathode luminescence of apatite grains from quartz syenite (Q-Ap1), nepheline syenite (N-Ap1) and carbonatite (C-Ap1) and their high concentrations of REE (2400 ppm – 22,130 ppm; 3832 ppm – 14,532 ppm; and 2494–9050, respectively), particularly light REE (with LaN/YbN = 12 to 78; 22 to 113; and 30–166; respectively), indicate their magmatic origin. Texturally and chemically zoned apatite from nepheline syenite (N-Ap2) and carbonatite (C-Ap2) are also enriched in light REE (LaN/YbN = 38 to 97 and 24 to 108; respectively) relative to heavy REE, which is consistent with a magmatic origin. The Q-Ap1 apatite are showing distinct negative Eu anomalies (0.18–0.75) indicative of crystallization from an evolved melt that experienced significant plagioclase fractionation. The N-Ap2 apatite exhibit a rim-ward increase in REE + Y contents, which can be attributed to fractional crystallization of pyroxene, alkali feldspar and biotite. While in C-Ap2 the rim-ward increase in REE + Y is coupled with increasing Na and decreasing Mn, suggesting early crystallization of calcite. Moreover, patchy texture and depleted REE + Y contents of N-Ap3 apatite from coarse-grained nepheline syenite compared to N-Ap1, reflect their metasomatic nature by interacting with fluids. However, N-Ap3 show similar initial Sr and Nd composition with magmatic apatite from nepheline syenite (i.e., N-Ap1) suggesting that the fluids involved in metasomatic alteration were likely originated from the cooling host magma. The LA-ICP-MS UPb apatite geochronology data from KCAC align closely with previously published zircon UPb ages, indicating rapid cooling following the emplacement of the complex. The UPb dating of apatite yields an emplacement age of 287 ± 16 Ma, 279 ± 9 Ma and 274 ± 9 Ma for the quartz syenite, nepheline syenite and carbonatite, respectively; implying their emplacement during the Permian rifting and breakup of the northern margin of Gondwana. The Sr and Nd isotopic compositions of apatite from the KCAC imply a sub-lithospheric mantle source, with minimal crustal contamination or interaction with metasomatized sub-continental lithospheric mantle. The study demonstrates that apatite textural features combined with in-situ trace elements and SrNd isotopic compositions can be an effective tool to unravel complex magmatic and hydrothermal processes and nature of the magma source of carbonatite- alkaline complexes.