Early Paleoproterozoic mafic to felsic magmatism from the Cuoke Complex, South China: Implications for the early tectonic evolution of the southwestern Yangtze Craton

Abstract

The early Paleoproterozoic architecture and tectonic evolution of the Yangtze Craton in present-day southern China remain poorly understood due to the limited outcrops of rocks from this period. The recently discovered Cuoke Complex in the southwestern Yangtze Craton preserves numerous early Paleoproterozoic igneous rocks, providing a valuable opportunity to investigate the early evolution of the Yangtze Craton. On the basis of lithological features, magmatic zircon U–Pb–Lu–Hf isotopes, and whole-rock geochemical characteristics, four main lithological assemblages have been identified, and their petrogeneses have been revealed. (1) The ca. 2.36 Ga monzogranites have low Zr + Nb + Ce + Y contents (155–267 ppm), high A/CNK ratios (1.14–1.42), and low 10,000*Ga/Al ratios (1.66–2.50), which are typical of S-type granites. These monzogranites display negative whole-rock ε_Nd(t) (−4.7 to −7.6) and zircon ε_Hf(t) values (−2.4 to −4.8) and high CaO/Al₂O₃ (0.01–0.08) and CaO/Na₂O ratios (0.04–0.32), suggesting that they formed through the remelting of ancient pelites. (2) The ca. 2.33 Ga diorites are characterized by high Sr contents (556–619 ppm), high Sr/Y (63.0–83.1) and (La/Yb)_N ratios (24.7–28.1), and low Y (7.45–9.17 ppm) and Yb (0.94–1.11 ppm) contents; thus, these rocks resemble adakites. The elevated Mg^# values (46–56), high Cr (106–125 ppm) and Ni (38.9–43.9 ppm) contents, and positive ε_Nd(t) (+2.4 to + 4.2) and ε_Hf(t) values (+1.4 to + 8.4) indicate that these rocks originated from the partial melting of a subducted oceanic slab. (3) The ca. 2.32 Ga mafic rocks exhibit typical Nb-enriched basaltic features, with high TiO₂ (2.08–3.94 wt%) and Nb (7.63–16.7 ppm) contents and elevated Nb/Th (4.85–11.7), Nb/La (0.33–0.87), and Nb/U (13.4–32.1) ratios. The rocks have decoupled Nd–Hf isotopes with relatively low whole-rock ε_Nd(t) (+2.1 to + 3.6) values and relatively high zircon ε_Hf(t) (+2.2 to + 9.0) values, suggesting that they were derived from a mantle wedge modified by slab-derived adakitic melts. (4) The ca. 2.32 Ga granites display high Zr + Nb + Ce + Y contents (451–522 ppm), high 10,000*Ga/Al (2.58–2.69), Y/Nb (1.32–1.94), and Yb/Ta ratios (1.09–1.89), and significantly negative Eu, Ba, Sr, and Ti anomalies, indicating an affinity with A₂-type granites. The peraluminous features (A/CNK = 1.05–1.24) and negative whole-rock ε_Nd(t) (−2.9 to −5.5) values and zircon ε_Hf(t) values (−2.1 to −5.1) suggest that these rocks formed through the partial melting of ancient metasedimentary rocks. Considering previous geochronological and geochemical studies of other ancient complexes within the Yangtze Craton, we propose that the ca. 2.36 Ga monzogranites were most likely generated in a syn-collisional setting, whereas the ca. 2.34–2.32 Ga adakites, Nb-enriched mafic rocks, and A₂-type granites likely formed in a post-orogenic extensional setting. Furthermore, the southwestern Yangtze Craton and northern Yangtze Craton had independent and spatially distinct evolutionary histories during the Archean to early Paleoproterozoic, with the unified Yangtze Craton most likely forming between 2.00 and 1.93 Ga.