Neoproterozoic continental margin-arc magmatic activity of the Nanping-Ninghua tectonic belt, South China:Implications for tectonic evolution of the Wuyi orogenic belt

Numerous Neoproterozoic volcanic-sedimentary rocks, e.g., Wanquan Group, Louqian and Xixi Formation, are outcropped along the Nanping-Ninghua tectonic belt, South China. This paper presents a systematic petrological, geochronological, and geochemical study of metamorphic rocks from the Louqian Formation in Mingxi County of Fujian and Ruijin County of Jiangxi Province. Zircons from meta-dacite in Mingxi County and meta-crystal tuff in Ruijin County yielded U-Pb weighted mean ages of (729±4)Ma and (735±6.7) Ma (by LA-ICP-MS), respectively. The SiO₂ values of these meta-volcanic rocks range from 65.22% to 74.54%, and they have high Al₂O₃ (11.05%-16.80%) and Na₂O + K₂O content (4.88%-10.19%) but low CaO, MgO, and FeO_T content. Their ANK values and A/CNK ratios are 1.23-1.78 and 0.98-1.57 with Nb/Ta being 12.44-17.28, Nd/Th being 2.07-3.51, Ti/Zr being 6.08-10.37 and Ti/Y being 68.51-154.71, implying the S-type igneous rocks. All samples are enriched in large-ion lithophile elements (LILE; e.g., Ba and Rb) and depleted in high-field-strength elements (HFSE; e.g., Nb, Ta, Ti, and P)with Zr/Nb being 16.65-24.07 and Th/Ta being 12.94-16.93. Marked negative Eu anomalies 0.33-0.62 are similar to those in arc igneous rocks. Based on regional geological data and previous studies, the authors hold that the Nanping-Ninghua tectonic belt had been in an active continental margin setting until 713 Ma. The magmatism was caused by the subduction of oceanic crust and formed an intermediate-acid volcanic belt along the Nanping-Ninghua-Ruijin continental margin, which indicates that the collision between the North and South Wuyi blocks did not form a unified Wuyi Block before 713 Ma, and the Cathaysia Block had not a unified pre-Nanhua crystalline basement.