New zircon U–Pb ages for erratic emplacement of 2213–2130 MaPaleoproterozoic calc-alkaline I-type granitoid rocks in the LawraVolcanic Belt of Northwestern Ghana, West Africa
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Date
2014
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Precambrian Research
Abstract
tPaleoproterozoic Birimian granitoid rocks from the Lawra volcanic belt of northwestern Ghana havebeen analyzed for their major, trace, REE concentrations and their zircon U–Pb age. Based on the mineralassemblages, the calc-alkaline, metaluminous to weakly peraluminous I-type granitoids were classifiedas: (1) gneissic biotite granite, (2) hornblende granodiorite, (3) biotite granite, (4) two-mica granite and(5) pyroxene hornblende gneiss. The high LaN/YbN(∼4–146) values of the granitoids, characterized byenriched LREE pattern and flat to depleted HREE pattern ((Dy/Yb)N= 0.95–3.47), is typical of rocks derivedfrom an evolved magma or partial melting of the depleted mantle. Similarly, the slightly negative Sr- andEu-anomalies indicate varying degrees of plagioclase fractionation. Their high but varying SiO2contents(58.1–76.1 wt.%), K2O/Na2O ratios (0.17–1.2) and (FeO + Fe2O3)/MgO ratios (0.98–4.02), and low Ce andTi suggest that they are volcanic arc granites (VAG). The granitoids have zircon U–Pb ages between2130.8 ± 9.5 Ma and 2213 ± 76 Ma, and preclude the contamination of the juvenile Birimian crust bynotable amounts of reworked Archaean crustal material. However, these ages spanning ∼81 Ma wererecorded by all the rock types in the study area, with no distinct age difference among them. Accordingly,the area could be described as an undifferentiated terrain that experienced erratic emplacement of pulsesof granitic magma during the period. Furthermore, the oldest ages of 2211 Ma and 2213 Ma recorded inthis study suggest that the emplacement of Birimian granitoids in Ghana may have commenced muchearlier than previously reported in the literature. The results demonstrate the juvenile character of thegranitoid rocks. We therefore infer that the Birimian granitoids of the Lawra volcanic belt formed fromlarge-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantlein an island arc environment.
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Keywords
Birimian, Granitoids, Geochronology, Subduction, Volcanic Arca, Lawra belt