Geology, mineralogy, geochemistry and Sr-Nd-Pb isotopic composition of the ultrapotassic rocks and associated shoshonites of Kef Hahouner (Northeastern Algeria): New data and geodynamic implications

The association of ultrapotassic rocks and shoshonites is common in the Alpine-Mediterranean region, with the exception of the Maghrebide Chain of Northern Africa, where only two small outcrops of ultrapotassic rocks have been documented. This study reports detailed field observations and new data on the petrology, mineral chemistry, whole-rock geochemistry and Sr-Nd-Pb isotopic compositions of the volcanic ultrapotassic rocks and shoshonites of the Kef Hahouner region (Northeastern Algeria). The results indicate that these rocks are composed of an association of ultrapotassic rocks that outcrop at the base of a lava flow sequence, and shoshonites that constitute shallow volcanic intrusions and the upper part of the abovementioned lava flow pile. The ultrapotassic rocks are K₂O-rich, with K₂O/Na₂O > 2, high Mg# (Mg/(Mg + Fe²⁺) = 0.70–0.72), and high Ni (∼173 μg.g⁻¹ [ppm]) and Cr contents (∼392 μg.g⁻¹). They are strongly enriched in LREE and extremely enriched in Th and LILE (Rb, Ba, Sr), with respect to High Field Strenght Elements (Ta, Nb, and Ti), showing an orogenic-type pattern. Shoshonites display lower K₂O and higher Na₂O and Al₂O₃ contents compared to the ultrapotassic facies. However, these rocks have similar trace and rare earth element (REE) patterns to those observed in the ultrapotassic rocks, albeit with slightly lower LILE and MREE contents. The initial ⁸⁷Sr/⁸⁶Sr values range from 0.706101 to 0.709500, and ¹⁴³Nd/¹⁴⁴Nd from 0.512246 to 0.512438, while the lead isotope ratios vary between 18.538 and 18.571 for ²⁰⁶Pb/²⁰⁴Pb, 15.649 to 15.657 for ²⁰⁷Pb/²⁰⁴Pb, and 39.806 to 39.998 for ²⁰⁸Pb/²⁰⁴Pb. Ultrapotassic rocks exhibit higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb and lower ¹⁴³Nd/¹⁴⁴Nd than shoshonites. The two volcanic suites most likely result from the increasing partial melting of the metasomatised mantle source in which the ambient mantle component dilutes the alkaline-rich component derived from the partial melting of the sole vein, explaining the transition from ultrapotassic to shoshonite magma. Compared to the North-African Cenozoic magmatic belt, the Kef Hahouner ultrapotassic and shoshonitic lithologies are unique and have no equivalent among the entire belt. We, therefore, propose a geodynamic model in which the subduction of the African continental margin beneath the Kabylian domain and its break-off during the Tortonian times caused the strong metasomatic episode in the source region, resulting in the formation and emplacement of the ultrapotassic rocks and shoshonites in the region. The observed Kef Hahouner–Djebel Debar fault may represent the surface expression of the tear fault related to the southern edge of the African subducted lithospheric slab.