Geochemical characteristics of Cretaceous basaltic rocks in South China and constraints on Pacific plate subduction

Middle-Late Cretaceous Basaltic rocks (85-110 Ma) are distributed in the contemporaneous downfaulted red basins and volcanic-sedimentary basins in South China (including Hunan, Jiangxi, Guangdong, Fujian, Zhejiang provinces). They are geographically divided into two groups delimited by Wuyishan Range, namely the west area basaltic rocks (WABs) distributing over the west of Wuyishan Range and the east area basaltic rocks (EABs) distributing over the east of Wuyishan Range, and observed having distinct geochemical and isotopic characters. WABs are characterized by relatively low in K₂O (0.44%-3.17%), alkalis (K₂O+Na₂O= 3.27%-6.80%) and Al₂O₃ (13.08%-16.75%) contents, but high in MgO (5.13%-8.78%) and TiO₂ (1.12%-3.35%) contents. WABs are similar to ocean-island basalts (OIB) or intraplate basalts on trace elements and isotopes: no negative Nb and Ta anomalies, relatively low ∑REE (74.66 μg/g - 287.72 μg/g) and La _N/Yb_N (4.49-22.10), low (⁸⁷Sr/ ⁸⁶Sr)_i (average ratio is 0.704679), high ε_Nd(f) (-1.81-8.00). In contrast, EABs display relatively high in K₂O (0.55%-4.86%), alkalis (K₂O+Na₂O=2.95%- 7.55%) and Al₂O₃ (15.80%-21.10%) contents, but low in MgO (2.63%-6.28%) and TiO₂ (1.19%-1.86%) contents, strong negative Nb and Ta anomalies, enrichment in LILE (K, Rb, Ba and Th), relatively high ∑REE (79.68 μg/g-327.61 μg/g) and La_N/Yb_N (5.25-26.37), high (⁸⁷Sr/⁸⁶Sr)_i (average ratio is 0.707658) and negative ε_Nd(f) (-8.50-1.22), similar to that of IAB (island arc basalts) or CAB (continental arc basalts). Accordingly, WABs were probably generated under continental rifting tectonic setting related to asthenosphere upwelling, and hence their source was characterized by mixing between EM II with DM. Source of EABs may be enriched lithoshperic mantle which had ever undergone the metasomatism of fluid/melt derived from the subducted Pacific plate.This indicates that subduction of a paleo-Pacific plate (Kula) beneath the Eurasian continent had ever occurred during the late Mesozoic period, but the metasomatism resulted from the subduction was only limited to the east of Wuyishan Range. Copyright by Science in China Press 2005.