1. 中国科学院广州地球化学研究所, 广州, 510640; 2. 中国地质大学, 武汉, 430074; 3. Tectonic Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907, Australia
PETROGENESIS OF NEOPROTEROZOIC BIMODAL VOLCANICS IN WESTERN SICHUAN AND ITS TECTONIC IMPLICATIONS:GEOCHEMICAL AND Sm-Nd ISOTOPIC CONSTRAINTS
Li Xianhua1, Zhou Hanwen2, Li Zhengxiang3, Liu Ying1
1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640; 2. China University of Geosciences, Wuhan 430074; 3. Tectonic Special Research Centre, Department of Western Australia, Nedlands, WA 6907, Australia
The geochemical and Nd isotopic data were reported for the Neoproterozoic Suxiong bimodal volcanic rocks in the Kangdian Rift,western Sichuan.Most samples of basaltic rocks were characterized by high positive εNd(T) values(+5 to +6),pronounced enrichment in the incompatible trace elements,smooth LREE-enriched and “humped” trace element distribution patterns.They closely resembled the alkali basalts in the OIB and CFB provinces,such as the Hawaii OIB and the Ethiopian CFB.These features suggested that these basaltic rocks were most probably derived from an OIB-like mantle source.The differentiated basalt and trachyandesite samples showed relatively low εNd(T) values(+1.7 to +2.4) and Nb-Ta depletion due to contamination by the mafic lithosphere and/or crustal materials.The rhyolite and dacite samples had small positive εNd(T) values(+1.1 to +2.6),general enrichment in most incompatible trace elements but significant depletion of Nb,Ta,Ba,Sr,P,Eu and Ti.They shared geochemical characters of A2-type granites,and were interpreted to be formed through assimilation of OIB-type basaltic magma followed by fractional crystallization in a.double diffusive’magma chamber at the middle crust level.The geochemical and Nd-isotopic characters of the Suxiong bimodal volcanic successions formed within a typical continental rift environment,similar to those volcanics erupted in the Ethiopian Rift.The Kangdian Rift was likely part of a wider continental rift system triggered by a Neoproterozoic mantle plume beneath South China during the breakup of Rodinia.
Li Xianhua,Zhou Hanwen,Li Zhengxiang et al. PETROGENESIS OF NEOPROTEROZOIC BIMODAL VOLCANICS IN WESTERN SICHUAN AND ITS TECTONIC IMPLICATIONS:GEOCHEMICAL AND Sm-Nd ISOTOPIC CONSTRAINTS[J]. Chinese Journal of Geology, 2002, (3): 264-276,287.