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Longevity of the Permian Emeishan mantle plume (SW China): 1 Ma, 8 Ma or 18 Ma?

J. GREGORY SHELLNUTT^*,, MEI-FU ZHOU^*, DAN-PING YAN and YANBIN WANG

^* Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
State Key Laboratory of Geo-Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing, China
Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Science, Beijing, China

Author for correspondence: jgshelln{at}earth.sinica.edu.tw

After the formation of the ~ 260 Ma Emeishan large igneous province, there were two volumetrically minor magmatic pulses at ~ 252 Ma and ~ 242 Ma, respectively. Alkaline mafic dykes intruding both 260 Ma and 252 Ma felsic plutons in the Panxi region, southwestern China, have compositions similar to the Emeishan flood basalts. One dyke is dated using the SHRIMP zircon U–Pb technique at 242 ± 2 Ma, ~ 18 Ma younger than the start of Emeishan magmatism. The dykes have enriched light rare earth element patterns (La/YbN = 4.4–18.8) and trace element patterns similar to the Emeishan flood basalts and average ocean-island basalts. Some trace element ratios of the dykes (Zr/Nb = 3.8–8.2, La/Nb = 0.4–1.7, Ba/La = 7.5–25.6) are somewhat similar to EM1 source material, however, there are differences. Their Nd values (Nd = +2.6 and +2.7) and _ISr (_ISr = 0.704542 and 0.704554) ratios are indicative of a mantle source. Thus Emeishan magmatism may have lasted for almost 20 Ma after the initial eruption. However, geological evidence precludes the possibility that the post-260 Ma magmatic events were directly related to Emeishan magmatism, which began at and ended shortly after 260 Ma. The 252 Ma plutons and 242 Ma dykes represent volumetrically minor melting of the fossil Emeishan plume-head beneath the Yangtze crust. The 252 Ma magmatic event was likely caused by post-flood basalt extension of the Yangtze crust, whereas the 242 Ma event was caused by decompressional melting associated with the collision between the South China and North China blocks during the Middle Triassic.

Key Words: Emeishan large igneous province • mafic dykes • Middle Triassic • Late Permian • Middle Permian