Major, trace element, and Nd, Sr and Pb isotope studies of Cenozoic basalts in SE China: mantle sources, regional variations, and tectonic significance

摘要

Major, trace element, and Nd-Sr-Pb isotopic compositions of mantle xenolith-bearing Cenozoic basalts in southeastern China are measured to provide an insight into the nature of their mantle sources and processes. Application of a modified dynamic melting inversion (DMI) method presented here to SE China basalts suggests that Nushan and Fangshan basalts are formed by 4-11% partial melting of a light-rare-earth-element-enriched mantle source. The negative correlation between Nd-143/Nd-144 and Pb-206/Pb-204 and the positive relationship between Sr-87/Sr-86 and Pb-206/Pb-204 strongly suggest a mixing of an intermediate-deplete asthenospheric mantle source and an EM2 component in the study area. The occurrence of the EM2 signature and the diminishing of this signature from south to north in the study area are consistent with the hypotheses that SE China was a part of Gondwanaland. In addition, isotopic constraints from SE China basalts are consistent with the crustal detachment model of Li [Li, Z.X., 1994. Collision between the North and South China blocks: a crustal-detachment model for suturing in the region east of the Tanlu fault. Geology, 22, 739-742] that a subsurface suture between South China and North China Blocks runs eastward through Nanjing. When published isotopic data for Cenozoic basalts from NE China are included, basalts in the whole of eastern China have high Pb-208/Pb-204 and Pb-207/Pb-204 at a given Pb-206/Pb-204, a feature that is commonly observed in the Southern Hemisphere Dupal oceanic island basalts. In addition, Pb-206/Pb-204 decreases from south to north. Such a regional variation of Pb isotopic compositions in the whole of eastern China cannot simply be attributed to mixing of two mantle endmembers because Nd and Sr isotopic compositions show opposite regional variations in SE China and NE China. The SE China basalts suggest mixing between an asthenospheric mantle and EM2, while the NE China basalts reflect mixing between an asthenospheric mantle and EM1. The central-eastern China basalts from Nushan, Fangshan, and Tashan have the most depleted Nd and Sr isotopic compositions that may represent the isotopic composition of the asthenospheric mantle. The occurrence of Pb-Dupal signatures in these central-eastern China basalts may imply that the asthenospheric mantle already had a Pb-Dupal signature before its mixing with EM1 or EM2. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 42]