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

Science China Earth Sciences 2005年第12期48卷 2104-2117页

作者： chen weifeng chen peirong xu xisheng zhang min The State Key Laboratory of mineral Deposit Research Department of Earth Sciences Nanjing University Nanjing 210093 China The State Key Laboratory of Mineral Deposit Research Department of Earth Sciences Nanjing University Nanjing 210093 China The State Key Laboratory of Mineral Deposit Research Department of Earth Sciences Nanjing University Nanjing 210093 China The State Key Laboratory of Mineral Deposit Research Department of Earth Sciences Nanjing University Nanjing 210093 China

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 LaN/YbN (4.49―22.10), low (87Sr/86Sr)i (average ratio is 0.704679), high εNd(t) (?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 ΣREE (79.68 μg/g―327.61 μg/g) and LaN/YbN (5.25―26.37), high (87Sr/86Sr)i (average ratio is 0.707658) and negative εNd(t) (?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 EMⅡ with DM. Source of EABs may be enriched lithoshperic mantle which had ever undergone the metasomatism of fluid/melt derived from the subducted Pacific *** indicates that subduction of a paleo-Pacific plate (Kula) beneath the Eurasian continent had ever occurred during the late Mesozoic period,

关键词： basalt, imprint of subducted plate, rifting, Cretaceous basins, South China.

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