Geochemical Characteristics of the Late Mesozoic Basalts in Southeastern Zhejiang Province and Constraints on Magma Source Materials

Abstract

The late Mesozoic basalts occur in the southeastern Zhejiang Province together with acidic volcanic rocks,which can be divided into upper and lower rock series(URS and LRS) . Major,trace elements and Sr-Nd-Pb isotopic features of thesebasalts are studied in this paper to provide insights into the nature of their magma sources and evolution processes. Both the URS and LRS basalts are enriched in alkaline and Al elements. However,their characteristics of trace elements are different. The samples from Tiantai and Qingtian of LRS are enriched in LREEs,Ba,Pb and Sr,and depleted in Nb,Ta,Zr and Hf with Eu negative anomalies. The basalts from Yongjiahuatan,Ningboxuantandi,Xinchangjingling and Yongjiajingjiashan basalts of URS show similar trace elements and REEs natures to those of the samples from LRS,but basalts from Wuyi of URS show no negative Ta,Nb anomalies,while the trace element features of the basalts from Jinqu Basin are on the average. Correspondingly,isotopic compositions of these basalts are also variable apparently. The initial isotopic ratios of basalts from LRS range are as follows:I Sr = 0.70850～ 0.70897,e Nd(t) =-5.6～-4.1,(206 Pb/204 Pb) i =18.21～18.38,(207 Pb/204 Pb) i =15.55～15.58,(208 Pb/204 Pb) i =38.26～38.49,which are approximate to those of the intermediate-acid magmatic rocks in LRS,indicating that their magmatic sources were contaminated significantly by the lower crustal materials. Basalts from URS demonstrate a marked difference from those of the LRS in isotopic features by showing similar groups as the characteristics of trace elements. The samples from Yongjiahuatan,Ningboxuantandi,Xinchangjingling and Yongjiajingjiashan of URS have enriched Sr-Nd isotopic compositions with I Sr =0.70734～0.70936,e Nd(t) =-7.1～-2.1,(206 Pb/204 Pb) i =18.01～18.40,(207 Pb/204 Pb) i =15.54～15.62,(208 Pb/204 Pb) i = 37.99～38.62,suggesting that these basalts are most likely generated from active continental margin. The samples from Wuyi and Jinqu Basins have the characteristics similar to OIB with I Sr = 0.70533～0.70589,e Nd(t) = 0.4～3.3,(206 Pb/204 Pb) i = 17.23～18.11,(207 Pb/204 Pb) i =15.46～15.53,(208 Pb/204 Pb) i = 36.91～38.43,towards the end member of depleted mantle. Based on elements and isotopic compositions of the Late Mesozoic basalts from the southeastern Zhejiang Province,the sources of the basalt are significantly different and varied obviously with time. It could be concluded that the initial magma of basalts of LRS may be derived from the ancient lithospheric mantle,subducted altered oceanic crust and contaminated by lower crustal materials. The source of the basalts of URS may contain the materials of the lithospheric mantle which is similar to basalts from LRS,depleted OIB-like mantle and lower crust materials. It is also concluded the formation of the late Mesozoic basalts in the southeastern Zhejiang Province may be related to the Pacific plate subduction,but the influence of delamination of the lithospheric mantle could not be ruled out. Since that,much further information of petrology and/or mantle xenoliths is needed for the specific discussion about the dynamic process.