Cenozoic Lamprophyres From NE Türkiye: New Constraints on Post-Collisional Metasomatism and Alkaline Magma Origin in the Juvenile Subcontinental Lithospheric Mantle

The Middle Eocene (~43 Ma) and Late Miocene (~6 Ma) lamprophyres cropping out as sills from NE Türkiye can be classified as alkaline lamprophyres. These lamprophyres exhibit moderate contents of K2O (2.22–2.59 and 2.58–2.98 wt%), K2O/Na2O ratios (0.78–1.14 and 0.62–0.94) and Mg# (40.55–46.32 and 56.93–61.52), respectively, and low concentrations of mantle compatible elements. The lamprophyres also have a moderate to relatively high abundance of rare earth elements (ΣREEs = 140–159 and 547–604 ppm, respectively) with significantly fractionated REE patterns (LaN/YbN = 6.82–9.10 and 65.17–71.31, respectively), and are enriched in large-ion lithophile and light REEs but depleted in high-field-strength and heavy REEs. The lamprophyres have homogeneous and moderately low (87Sr/86Sr)i of 0.705048–0.705474 and 0.705252–0.705401, and nearly uniform εNd(i) values varying from −0.47 to 0.20 and 0.54 to 0.79 and εHf(i) values varying from 4.51–6.71 and 5.83–6.18, respectively. They also have a narrow and moderately high Δ7/4Pb range of 6.19–15.41 and 10.62–10.88 and Δ8/4Pb range of 49.56–56.14 and 46.86–47.99, respectively. Petrological modelling suggests that the parental magmas of both the Eocene and Miocene lamprophyres were generated by the low-degree partial melting of phlogopite-bearing spinel-garnet and amphibole-bearing garnet lherzolite sources, respectively. By integrating whole-rock trace elements and isotope compositions with regional geological observations, we propose that the post-collisional Middle Eocene and Late Miocene lamprophyres in the Eastern Pontides Orogenic Belt originated from the juvenile subcontinental lithospheric mantle sources metasomatized initially by Neotethyan subduction-related fluids during the Eocene, and subsequently overprinted by delaminated lithosphere-derived carbonate components in the Miocene. The lamprophyres likely record a temporal magmatic response to lithospheric delamination and regional thermal upwelling associated with Eocene extensional tectonics, followed by lithospheric thinning during the Miocene in a post-collisional tectonic setting.