Asynchronizing paleo-Pacific slab rollback beneath SE China: Insights from the episodic Late Mesozoic volcanism

Extensive Late Mesozoic igneous rocks in SE China have been widely considered to be generated under the paleo-Pacific tectonic regime. Previous studies suggested a fundamental oceanward younging trend for the igneous rocks, further implying a NW-ward paleo-Pacific subduction model with gradual slab rollback. New LA–ICP–MS zircon U–Pb dating results show that early stage of the Late Mesozoic volcanic sequences (also known as the lower volcanic series) in Fujian Province was formed in three episodes: 160–148 Ma for the Changlin Formation, 145–130 Ma for the Nanyuan Formation, and 130–127 Ma for the Xiaoxi Formation, among which the second episode made the volcanism climax. Thus the entire lower volcanic series in Fujian were formed earlier than those in Zhejiang (140–118 Ma), displaying a NE-ward younging trend parallel to the potential subduction belt. Besides, in situ Hf isotope analyses on dated zircons yield an ε_Hf(t) range of − 19.2 to + 1.7 for the lower volcanic series in Fujian. The majority of the studied volcanic rocks have more radiogenic Hf than that of the metamorphic basement, requiring the involvement of juvenile components in their origin. Moreover, the zircon ε_Hf(t) value increases with time in each single area, as well as the igneous rocks elsewhere in SE China, except for the Changlin Formation which shows very opposite isotopic varying trend. Increasing zircon ε_Hf(t) values imply an increasing material contribution of contemporaneous underplated mantle-derived magmas, which was plausibly induced by gradual crustal decompression. As well as the NE-ward younging trend of the lower volcanic series, it is also identified that the juvenile material contribution in the igneous petrogenesis gradually took place in the same direction along the coastal area. Thus here we propose an asynchronizing paleo-Pacific slab rollback model during 150–120 Ma to account for the episodic magmatism and crustal extension in SE China. On the contrary, decreasing zircon ε_Hf(t) values of the Changlin Formation volcanics indicate that they were formed under enhanced crustal compression probably induced by slab advance. On the other hand, the upper volcanic series in Fujian were formed during 110–88 Ma coevally to those in Zhejiang, with more depleted zircon Hf isotopic compositions than the lower volcanic series as well, indicating that the entire coastal SE China was under the back-arc tectonic setting during that time.