Cretaceous extension of the Ganhang Tectonic Belt, southeast China.pdf

Cretaceous Research 27 (2006) 663e672 /locate/CretResCretaceous extension of the Ganhang Tectonic Belt, southeastern China: constraints from geochemistry of volcanic rocks Xinqi Yu a,b,*, Ganguo Wu a,b, Da Zhang a,b, Tiezeng Yan c, Yongjun Di a,b, Longwu Wang c a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China b Key Laboratory of Lithosphere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences, Beijing 100083, China c Geological Survey Institute of Zhejiang Province, Hangzhou 311203, Zhejiang, China Received 14 February 2005; accepted in revised form 21 November 2005 Available online 30 June 2006 Abstract New geochemical and isotopic data are reported from Cretaceous volcanic rocks of the Ganhang Tectonic Belt, southeast China. Two periods of basaltic eruptions (143
1.1e139
0.7 Ma and 105e98 Ma), each representing results of previous tectonic events, are recognized in the belt. Rock types associated with the eruptions are significantly different in mineral and chemical composition. For the 143
1.1e139
0.7 Ma basalts, 87Sr/86Sr ratio (0.7041e0.7048) and SmeNd isotopic compositions (3Nd ?t3.78 to t4.38) suggest that the magma came from the depleted mantle and had not mixed with crustal material. During the second episode of magmatic and tectonic activities (105e 98 Ma), volcanism was accompanied by intense regional extension and the development of a series of ‘‘red basins’’. The geochemical charac- teristics of these basalts exhibit both tholeiite and alkaline olivine-types, indicating inhomogeneous mixing of magma with crustal material. The tholeiites have a mediumehigh 87Sr/86Sr ratio (0.7054e0.7070) and a low 3Nd value (1.69e0.70), indicating that the magma of these basalts came from an unchanged mantle. The alkaline olivine basalts, on the other hand, have a relatively higher 87Sr/86Sr ratio (0.7076e0.7080) and a relatively lower value of 3