Paleo-Mesoproterozoic meta-basalts within the Caiziyuan-Tongan accretionary complex in the southwestern Yangtze Block, South China: Evidence for the breakup of the Nuna supercontinent

In order to better understand the role and position of the Yangtze Block in the assembly and breakup of the Nuna supercontinent, we present zircon U-Pb ages, Lu-Hf isotopes, and the bulk-rock geochemistry of meta-basalt samples from the recently identified Caiziyuan-Tongan accretionary complex (CTAC) in the southwestern Yangtze Block, South China. Three meta-volcanic samples yielded zircon U-Pb ages of 1722 & PLUSMN; 15 Ma, 1736 & PLUSMN; 17 Ma, and 1552 & PLUSMN; 33 Ma. These meta-basalt samples were subdivided into two groups according to their sampling positions, crystallization ages, and geochemical characteristics. Group 1 (-1.7 Ga) meta-basalts are alkaline in composition and have consistently high TiO2 content (3.42-5.17 wt%). They are enriched in LREEs with (La/ Yb)N ratios of 2.86-12.08 and show positive zircon & epsilon;Hf(t) values from + 1.4 to + 6.8. These features indicated that the Group 1 may form at depths in the asthenospheric mantle in an intra-continental rift setting. Group 2 meta-basalts (-1.5 Ga) are tholeiitic in composition, and contain relatively low & sigma;REE (60.3-89.3 ppm) and slightly enriched LREEs, which were chemically similar to the features of MORB. These features indicated that they were derived from a moderately depleted asthenospheric mantle and formed in a mid-ocean ridge environment. Combined with the regional magmatic-sedimentary records, we propose that the southwestern Yangtze Block evolved from an intra-continental rift to a passive continental margin during the Paleo-Mesoproterozoic. This is the result of long-term lithospheric extension under the background of the global breakup of the Nuna supercontinent.