Geochemical and Pb-Sr-Nd isotopic compositions of granitoids from western Qinling belt: Constraints on basement nature and tectonic affinity  

作　　者：Nigel HARRIS 

机构地区：[1]Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK

出　　处：《Science China Earth Sciences》2007年第2期184-196,共13页中国科学（地球科学英文版）

基　　金：Supported by the National Natural Science Foundation of China (Grant Nos. 40234052 and 40521001);the State Key Laboratory of Geological Process and Mineral Resources,China University of Geosciences and the Key Laboratory of Continental Dynamics, Northwest University

摘　　要：Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04-1.86 and are dominantly metaluminous with A/CNK=0.90-1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compo- sitional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with Isr=0.70682-0.70845, εNd(t)=?4.85 to ?9.17 and TDM=1.26-1.66 Ga. They are characterized by high ra- diogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996-18.468, 207Pb/204Pb=15.565-15.677 and 208Pb/204Pb=38.082-38.587. Geochemical and Sr-Nd isotopic composi- tions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900-1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the east- ern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04–1.86 and are dominantly metaluminous with A/CNK=0.90–1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compositional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with I Sr=0.70682–0.70845, ? Nd(t)=?4.85 to ?9.17 and T DM=1.26–1.66 Ga. They are characterized by high radiogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996–18.468, 207Pb/204Pb=15.565–15.677 and 208Pb/204Pb=38.082–38.587. Geochemical and Sr-Nd isotopic compositions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900–1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the eastern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.

关 键 词：GRANITOID geochemistry BASEMENT NATURE tectonic AFFINITY West QINLING 

分 类 号：P588.1[天文地球—岩石学]