Abstract:

The Lishui Basin is located in the eastern part of the middle and lower reaches of Yangtze River metallogenic belt. It is
similar to the Luzong and Ningwu basins, where Mesozoic volcanic rocks are widespread. In order to investigate the formation time,
magma source and tectonic background of volcanic rocks in different cycles and their relationships with the Fe-Cu mineralization, this
study presents the new zircon dating results, Hf isotopic compositions, and major and trace element analysis of widely exposed
Longwangshan and Dawangshan volcanic rocks which are mostly related to mineralization of Lishui Basin. It is suggested that the
magmatism began at ~135 Ma, and peaked at 135~124 Ma in the Lishui Basin. Zircons from the Longwangshan and Dawangshan
volcanic rocks have similar Hf isotopic characteristics with εHf(t) values of -6~-11, suggesting similar magma sources. The
geochemistry characteristics suggest that the compositions of magma range from intermediate-basic to intermediate-felsic and rich

alkali, and the distribution of outcrops from NW to SE from Longwangshan Formation to Dawangshan Formation, which is related to the
iron, copper, gold, and strontium deposits. The positive anomaly of Sr of the Longwangshan volcanic rocks suggests that they could be
the source of the Sr deposit. Combined with the geochemical and isotopic characteristics, it is suggested that these magmatic rocks were
probably derived from an enriched mantle, which was contaminated by minor crustal materials during eruption. The formation time and
tectonic setting of the Lishui Basin is similar to those of the other volcanic basins along the middle and lower reaches of the Yangtze
River, suggesting that they could be controlled by the same tectonic background. Based on analysis of previous studies, this study
confirmed that the change of the subduction of the Pacific Plate occurred at ~135 Ma, when subduction weakened, and subsequently
the subducted slab rollback occurred, and finally formed regional extensional environment, resulting in the formation of a series of
volcanic basins.

Key words: the middle and lower reaches of Yangtze River, Lishui basin, Zircon U-Pb dating, Hf isotope, geochemistry, plate rollback