New zircon U-Pb ages for erratic emplacement of 2213-2130 Ma Paleoproterozoic calc-alkaline I-type granitoid rocks in the Lawra Volcanic Belt of Northwestern Ghana, West Africa

摘要

Paleoproterozoic Birimian granitoid rocks from the Lawra volcanic belt of northwestern Ghana have been analyzed for their major, trace, REE concentrations and their zircon U-Pb age. Based on the mineral assemblages, the calc-alkaline, metaluminous to weakly peraluminous I-type granitoids were classified as: (1) gneissic biotite granite, (2) hornblende granodiorite, (3) biotite granite, (4) two-mica granite and (5) pyroxene hornblende gneiss. The high La_N/Yb_N (~4-146) values of the granitoids, characterized by enriched LREE pattern and flat to depleted HREE pattern ((Dy/Yb)_N = 0.95-3.47), is typical of rocks derived from an evolved magma or partial melting of the depleted mantle. Similarly, the slightly negative Sr- and Eu-anomalies indicate varying degrees of plagioclase fractionation. Their high but varying SiO_2 contents (58.1-76.1 wt.%), K_2O/Na_2O ratios (0.17-1.2) and (FeO + Fe_2O_3)/MgO ratios (0.98-4.02), and low Ce and Ti suggest that they are volcanic arc granites (VAG). The granitoids have zircon U-Pb ages between 2130.8 ± 9.5 Ma and 2213±76 Ma, and preclude the contamination of the juvenile Birimian crust by notable amounts of reworked Archaean crustal material. However, these ages spanning ~81 Ma were recorded 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 pulses of granitic magma during the period. Furthermore, the oldest ages of 2211 Ma and 2213 Ma recorded in this study suggest that the emplacement of Birimian granitoids in Ghana may have commenced much earlier than previously reported in the literature. The results demonstrate the juvenile character of the granitoid rocks. We therefore infer that the Birimian granitoids of the Lawra volcanic belt formed from large-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantle in an island arc environment.