Geochemical Study On The Paleoproterozoic Basement And Neoproterozoic Granulites From The Dahomeyide Belt Of Southeastern Ghana

The Dahomeyide belt of West Africa marks the southeastern margin of the West African craton(WAC)along which the WAC was incorporated into the Western Gondwana orogen during the Neoproterozoic Pan-African tectono-thermal event.It represents a subduction-collisional zone between the WAC and the Benin-Nigerian shield.The Dahomeyide belt is composed of three main lithotectonic zones,namely the external nappe zone made up of the deformed eastern margin of the WAC with its cover rocks and the Dahomeyide basement complexes,the suture zone and the eastern internal nappes zone belonging to the Benin-Nigerian shield.The suture zone is dominated by UHP-HP/UHT mafic metamorphic assemblages,which can be traced continuously for ca.1000 km along the Dahomeyide belt.This study seeks to explore the origin and the tectonic setting of the Dahomeyide basement complexes with the purpose of establishing the evolutional connection between the Dahomeyide basement terrane and the Birimian-Eburnean of the WAC.The study also aims to constrain the origin and the P-T-t path of the Neoproterozoic Pan-African granulites in the Dahomeyide suture zone of Ghana.This information is crucial for a better understanding of how the continental crust was formed and evolved during the Paleoproterozoic Birimian-Eburnean and the Neoproterozoic Pan-African orogenies.In order to achieve these objectives,a combined study of petrology,mineral chemistry,geochemistry,Lu-Hf and O isotopes compositions and zircon U-Pb dating is employed.The Dahomeyide basement samples were collected between Tsito and Nyive.They are mainly amphibole-biotite orthogneisses,biotite-muscovite orthogneisses and high-K,siliceous metagranites.All the basement rock suites are calc-alkaline and metaluminous to weak peraluminous with aluminum saturation index(A/CNK)of 0.86-1.03 and 1.0-1.07 for the orthogneisses and the metagranites,respectively.They all display fractionated trace element patterns characterized by high LREE and LILE,and low HREE and HFSE.The orthogneisses show negative to weak positive Eu anomalies(Eu/Eu*=0.68-1.44),high and variable Sr/Y(11-115),(La/Yb)N(10-104),Nb/Ta(11-50),and Zr/Sm(23-90).The metagranites exhibit positive Eu anomalies with higher and wider Eu/Eu*values(1.25-7.20),Sr/Y(73-369)and(La/Yb)N(42-288)and Zr/Sm(56-226),and lower Nb/Ta(19-42).The overall geochemistry of the metagranites is similar to the transitional TTGs or potassic TTGs.Zircon U-Pb dating of six orthogneiss and three metagranite samples yield ages of 2.15-2.11 Ga and 2.12-2.10 Ga,respectively.They all display similar subchondritic zircon Lu-Hf isotopes compositions with sHf(t)values of-10.6 to-1.7 and-11.1 to-1.6 for the orthogneisses and metagranites,respectively.However,a few inherited zircons from the metagranites yield older ages of ca.2.6 Ga and have 7?Hf(t)values of+1.5-+1.9.In addition,all the samples preserve similar oxygen isotope compositions with?18O values of 9.0-9.9 ‰ for quartz,7.0-7.7 ‰ for K-feldspar,7.2-7.8 ‰ for plagioclase,5.8-7.6 ‰ for zircon,3.2-4.4 ‰ for biotite and 7.3-8.5 ‰ for whole-rock.These imply that both the orthogneisses and metagranites are Paleoproterozoic rocks,derived from melting of a similar Archaean source.Theoretical modelling of partial melting process using trace element compositions revealed that the orthogneisses were derived by partial melting of metabasalt in the stability field of garnet amphibolite,whereas the metagranites were formed in the stability field of rutile-eClogites.Thus,the orthogneisses were formed at the normal lower crust depth(<35 km),and the metagranites were formed at a thickened lower crust depth(>50 km).Crustal thickening as a result of collision between the Birimian arc and an unknown Archaean terrane to the east of the WAC is inferred as a plausible setting for the Dahomeyide basement rocks.This marks gradual crustal thickening during the Eburnean orogeny in the background of the Columbia supercontinent assembly.Collectively,the results suggest that tectonic process analogous to the present day Himalayan orogen began at least 2.1 Gyr ago.The Neoproterozoic granulites were collected from the Adaklu area of the Dahomeyide suture zone.They show mafic precursor with low-K tholeiitic affinity,and display convex rare earth and trace elements characteristics without obvious anomalies of Eu,Ti,Nb and Ta.They exhibit low(La/Sm)N of 0.68-1.22 and(La/Yb)N of 0.96-2.63.The geochemical characteristics of the granulites mimic that of N-MORB.Zircon U-Pb dating of two selected samples yield similar ages of 595 ± 3.6-602 ± 4.7 Ma.However,relatively older metamorphic age of ca.620 Ma is obtained from a few zircons.The granulites show suprachondritic zircon Lu-Hf isotope compositions with?Hf(t)values of+7.0 to +9.7,and ?18O values of 5.92-8.05 ‰.Geothermobarometric calculation using traditional method,pseudosection modelling,Zr-in-rutile and Ti-in-zircon thermometers reveal peak granulite P-T conditions of 940-1000?/0.95-1.29 GPa,followed by retrograde under amphibolite facies conditions of 576-707?/0.83-0.93 GPa.The results demonstrate that the Dahomeyide granulites are UHT metamorphic rocks and represent remnants of oceanic crust consumed during subduction-collision between the West African craton and the Benin-Nigerian shield during the Pan-African orogeny at ca.600 Ma.Metamorphism of subducted oceanic crust to form eclogite facies rocks at ca.620 Ma,followed by decompression and heating in which part of the eclogites were transformed to UHT granulites at ca.595-602 Ma,and subsequent cooling under amphibolite facies conditions is proposed based on the data of this study and literature.Although the UHT granulite in this study did not preserve clear evidences of eclogite prograde metamorphism,the older metamorphic age of ca.620 Ma is similar to the ages obtained for eclogites in the Hoggar and Gourma regions within the northern extension of Pharuside-Dahomeyide belt.Also,in the nearby terrane(Hodzo Hills located at about 35 km away from this study area),eclogites with similar N-MORB geochemical characteristics,and P-T conditions 600?/1.8 GPa have been documented.Hence,it is plausible to infer similar eclogites as the prograde stage condition for the granulites.This reveals clockwise P-T-t path for the Dahomeyide granulites.Heating associated with post-collisional extension of the thickened orogen and asthenosphere upwelling accounted for the UHT metamorphism.The occurrence of the UHT granulites following UHP/HP metamorphism in the Dahomeyide collisional orogen indicates a change from gravity instability to a state of isostatic gravity.This implies that UHT metamorphism after UHP and HP metamorphism found in many collisional orogens record a change from collision to extension.Collectively,the results obtained for the Paleoproterozoic basement complexes and the Neoproterozoic granulites suggest that the Dahomeyide belt experienced at least two collisional events.And the Birimian-Eburnean orogen was reactivated in Dahomeyide belt during the Neoproterozoic Pan-African orogeny.