Identification of UHT Granulites in the Pan-African Dahomeyide Suture Zone in SE Ghana: Implications for Evolution of Collisional Orogens

This study presents the petrology, geochemistry, U-Pb ages, Lu-Hf and oxygen isotope compositions of Adaklu mafic granulites (ADMGs), from the Pan-African Dahomeyide suture zone in southeastern Ghana. The ADMGs show mafic precursor with low-K tholeiitic affinity. They display convex rare earth and trace elements characteristics without any obvious anomalies of Eu, Ti, Nb and Ta. The geochemical characteristics of ADMGs mimic those of N-MORB. Zircon U-Pb dating on the ADMGs reveals granulite facies metamorphic ages of ca. 595-602 Ma. However, a few zircons yield relatively older apparent Pb-206/U-238 ages of ca. 620 Ma, representing prograde metamorphic age. ADMGs preserve mean zircon epsilon(Hf) (t) values of +7.0 to +9.7 and delta O-18 values of 6.1-8.0 parts per thousand. Based on petrographic observations, geothermobarometric calculations using conventional thermobarometry, mineral equilibria modeling, Ti-in-zircon and Zr-in-rutile thermometers reveal peak granulite facies P-T conditions of 0.95-1.2 GPa/940-1000 degrees C, and retrograde amphibolite facies conditions of 0.83-0.93 GPa/575-710 degrees C. The prograde metamorphic stage is inferred to be amphibolite or eclogite facies metamorphism. Hence, a clockwise a P-T-t path is proposed for the ADMGs. The overall results indicate that ADMGs are ultra-high temperature (UHT) granulites, and the protolith is altered oceanic crust consumed during the Pan-African collisional events. Asthenospheric upwelling induced by lithospheric delamination in the earliest extension of the thickened orogen, or by slab break-off in the background of collision may have provided the additional heat for UHT granulite facies metamorphism.