Geochemistry and petrogenesis of Early Cretaceous adakitic volcanic rocks from Chuzhou basin, eastern Tan-Lu Fault Belt

Late Mesozoic volcanic rocks from Chuzhou basin to the east of the Tan-Lu Fault Belt, are composed mainly of andesites. LA-ICP-MS zircon U-Pb dating for three samples yields mean Pb-206/U-788 ages of 128 +/- 3Ma, 128 +/- 2Ma, and 128 +/- 2Ma, respectively. These rocks have high alkali contents (sigma = 2. 34 similar to 4. 97) and Na2O/K2O ratios with variable SiO2 ranging from 60. 48% to 66. 20%, and are characterized by enrichment of LREEs and LILEs (e. g., Ba, U and K) and depletion of HREEs and HFSEs (e. g., Nb, Ta and Ti). All samples exhibit typical features of adakitic rocks, such as low Yb (0. 70 x 10(-6) similar to 0. 92 x 10(-6)) and Y (8. 84 x 10(-6) similar to 11. 8 x 10(-6)) contents, high Sr (944 x 10(-6) similar to 1213 x 10(-6) Sr/Y (80.0 similar to 113) and (La/Yb)(N) (23. 8 similar to 29. 7) ratios. Additionally, moderately high MgO (2. 39% similar to 3. 48%) and Mg-# (43 similar to 61) indicate that Chuzhou andesites belong geochemically to high-Mg adakitic rocks. They have (Sr-87/Sr-86)(i) = 0. 70564 similar to 0. 70579 and epsilon(Nd) ( t) = - 15. 7 similar to - 14. 0, similar to the thickened Yangtze LCC-derived low-Mg adakitic rocks. Zircons from these rocks show elevated Ce4+/Ce3+ ratios of 38 similar to 1260, and Eu/Eu* values ranging from 0. 68 to 1. 20. Calculated Ti-in-zircon temperatures range from 613 to 728 degrees C. These features are similar to adakites from the LYRB belt, but different from adakites from the STLF, indicating relatively high oxygen fugacity and low temperatures for their magmas. These data suggest that the high-Mg adakitic rocks from Chuzhou were formed by melts produced by partial melting of the thickened Yangtze LCC in the garnet and rutile stability field, subsequently mixing with high oxygen fugacity and hydrous magmas resulting from partial melting of an enriched mantle, which had been modified by fluids/melts released from subducted of the Paleo-Pacific plate.