Petrogenesis of the Paleoproterozoic granitoids in Kapatu, northern Zambia: constrains from geochemistry, zircon U-Pb chronology and Hf isotopes

Paleoproterozoic granite is widely exposed in northern Zambia, which is a natural laboratory for studying the magmatism and tectonic evolution in the Banweulu block. In this paper, systematic studies of petrology, LA-MC-ICP-MS zircon U-Pb geochronology, petrogeochemistry and Lu-Hf isotopes were conducted on the exposed granites in Kapatu area, northern Zambia. The results show that the main granites in the Kapatu area were formed between 2012±11~1970±20Ma, and the emlocation age of the late shallow-flesh-red porphyritic syenogranite was 1959±16Ma, both of which were paleoproterozoic magmatic activities. A large number of Archean captured zircons show that there may be remnants of Archean crustal components in the Bangweulu block. The Kapatu granites have high SiO2 (67.93%~72.80%), high aluminum (13.85%~14.91%), high Na2O/K2O (0.29~0.89), low MgO (0.38%~1.93%), and low P2O5 (0.10%~0.23%). A/CNK values range from 1.04~1.19, and the CIPW corundum standard molecular index is greater than 1% (1.01%~2.51%). The chondrite-normalized REE patterns of the granites show right oblique. The main granites have relatively high total rare earth elements (ΣREE=191.79×10-6~294.25×10-6) with evidently negative Euanomalies (δEu=0.26~0.53), where as the late shallow-flesh-red porphyritic syenogranites have relatively low total rare earth elements (ΣREE=48.58×10-6~54.00×10-6) with positive Eu anomalies (δEu=1.04~1.37). The Kapatu granites were enriched in large-ion lithophile elements (LILEs) (e.g.Rb, K), but depleted in Ba, Sr and high-field-strength elements (HFSEs) (e.g. Nb, Ta), and have low Cr (3.56×10-6~65.6×10-6), Co (1.99×10-6~12.5×10-6), and Ni (2.16×10-6~33.7×10-6). The above results show that the Paleoproterozoic granites belong to S-type granites. Hf isotopic results show that the εHf(t) values and two-stage Hf model age (tDM2) values were -15.9~+4.7 and 2289 ~ 3596Ma, respectively, far older than the crystallization age of zircon, indicating strong heterogeneity in the composition of the magma source, which was derived from the ancient Archean-Paleoproterozoic crust source mixed with different contribution of mantle materials. In addition, the higher CaO/Na2O and the lower Rb/Sr ratios of the main granites indicate that they were derived from the partial melting of arenaceous rocks. Meanwhile, the low CaO/Na2O and high Rb/Sr ratios of the shallow-flesh-red porphyritic syenogranite indicate partial melting of argillaceous rocks. Combined with the regional and global tectonic evolution history, it is suggested that the Kapatu granites were formed in syn-collision tectonic setting of Bangweulu Block and Tanzania Craton and the paleoproterozoic magmatic events in the Bangweulu block may be the response of the Columbia supercontinent. © 2021, Science Press. All right reserved.