Nchekwube Nweke, MSc

N. Nweke¹, B. Dyck¹, K. Larson^1
1Department of Earth, Environmental and Geographic Science, The University of British Columbia, Kelowna, Canada 

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. 615–600 Ma. Magmatism is followed by a thermal metamorphic peak at ca. 545–530 Ma, as recorded by Lu-Hf systematics of post-kinematic garnet and monazite U–Pb. The western Benin-Nigeria shield cooled through biotite Rb–Sr closure temperatures at ca. 510 Ma whereas the more deeply exhumed eastern part of the shield cooled through biotite Rb–Sr closure temperature at ca. 480 Ma. Lepidolite in a LCT pegmatite swarm found in the western part of our transect crystallized at ca. 505 Ma, consistent with post-orogenic pegmatite genesis. In the absence of the wide-scale detachment structures associated with orogenic-collapse, we propose 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 ~900 °C/GPa to 1150 °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.