Crustal evolution of the Southern Benin-Nigerian shield : insights from late post-orogenic lithium-cesium-tantalum (LCT) pegmatite mineralization

Lithium-cesium-tantalum (LCT) pegmatites are a key source of economic lithium. The timing of LCT pegmatite crystallization within the context of an orogenic system (i.e., pre-, syn- and post-orogenic) appears to vary globally and exerts a primary influence on the distribution of lithium mineralization in orogenic belts. The recent advances in in-situ Rb–Sr mica geochronology have enabled the direct dating of lepidolite, a major phase in LCT pegmatites. When combined with in-situ Rb–Sr, Lu-Hf, and U–Pb geochronology on other related major and accessory phases, the timing of LCT pegmatite crystallization can be accurately determined and placed in the context of a region’s magmatic and thermometamorphic history. Here, we present new in-situ Rb–Sr, Lu–Hf, and U–Pb geochronology data and results of petrological modeling from an east to west transect of the Benin-Nigeria shield to constrain the timing of magmatism, metamorphism, and LCT pegmatite crystallization related to the Pan-African orogeny. Crystallization ages of Pan-African granites across our transect range from ca. 625–600 Ma. Magmatism is followed by a thermal metamorphic peak at ca. 550–530 Ma, as recorded by Lu-Hf systematics of post-kinematic garnet and monazite U–Pb. The southwesteern Benin-Nigeria shield cooled through biotite Rb–Sr closure temperatures at ca. 500 Ma. Lepidolite in an LCT pegmatite swarm found in the western part of our transect crystallized at ca. 489 Ma, consistent with post-orogenic pegmatite genesis. In the absence of the wide-scale detachment structures associated with orogenic collapse, we propose that the LCT pegmatites were generated by remelting of highly fractionated Pan-African granite due to post-orogenic thermal maturation of the Benin-Nigeria shield. Garnet-staurolite schists record a static increase in geothermal gradients from ~1040 °C/GPa to ~1160 °C/GPa. The cause of this late thermal pulse is not yet determined but may be associated with lithosphere delamination, the break-off of the subducting slab, or post-magmatic radiogenic heating.