Paleoproterozoic tectonic evolution of the Yangtze Block: New evidence from ca. 2.36 to 2.22 Ga magmatism and 1.96 Ga metamorphism in the Cuoke complex, SW China

Abstract

The infrequent early Paleoproterozoic rocks are essential for investigating the early evolution of the Yangtze Block and its reconstruction in the global supercontinent. Here, we carried out an integrated study of petrology, whole-rock geochemistry, as well as zircon U-Pb-Hf isotope compositions for granitoids from the recently discovered Cuoke complex in the southwestern Yangtze Block. SHRIMP zircon U-Pb dating yields magmatic crystallization ages of ca. 2.22 and 2.35 Ga for two monzogranites, ca. 2.36 Ga for a granodiorite, and ca. 2.34 Ga for a granitic gneiss. The granitic gneiss also contains metamorphic zircons that give an age of ca. 1.96 Ga. These results, compiled with published geochronological data, suggest that the western part of the Yangtze Block (WYB) features extensive magmatism with ages of ca. 2.5–2.2 Ga and multiple regional metamorphism with peaks ca. 2.50, 2.36, 1.96 and 1.85 Ga. This characteristic is significantly different from the eastern part of the Yangtze Block (EYB), which is dominated by the ca. 2.15–1.95 Ga subduction- and collision-related metamorphism and magmatism with a peak ~2.0 Ga. Our studied granitoids show negative zircon εHf(t) values from −14.0 to −1.9, with corresponding two-stage model ages of ca. 3.50–2.93 Ga, indicative of their derivation by reworking of ancient crustal materials. Geochemically, the ca. 2.22 Ga monzogranites have high HFSEs (Zr + Nb + Ce + Y = 777–1136 ppm) and 10,000*Ga/Al ratios (3.73–4.56) with dramatically negative Eu, Ba, Sr and Ti anomalies. Together with high Y/Nb (2.75–3.15) and Yb/Ta (2.11–2.94) ratios, they are classified as A2-type granites. The ca. 2.36–2.35 Ga granitoids have high CIPW-normative corundum contents of 4.92–6.06 vol% and are strongly peraluminous with A/CNK values of 1.46–1.64, similar to S-type granites. They are enriched in LREEs ((La/Yb)N = 14.55–25.33) with moderately negative Eu anomalies (Eu/Eu* = 0.56–0.66), but depleted in Nb, Sr and Ti. The ca. 2.34 Ga granitic gneisses show intermediate geochemical signatures between S-type and I-type granites. They are featured by no discernible negative Eu and Sr anomalies and relatively steep HREE patterns with (Gd/Yb)N ratios between 1.94 and 2.50. In combination with available observations, our new geochronological and geochemical dataset indicates that these ca. 2.36–2.34 Ga granitoids represent the post-collisional magmatism immediately following a possible collision event in the WYB, while the 2.22 Ga A2-type monzogranites were likely formed in a back-arc extensional setting. In addition, we further suggest that the Yangtze Block and NW Laurentia share a common succession of major Paleoproterozoic tectonothermal events, favoring their tight spatial linkage in the global Columbia supercontinent.