The Changed Sr-Nd-Pb Isotopic Compositions With Time Of The Late Mesozoic Basalts In Eastern Zhejiang Province

Detailed Sr-Nd-Pb isotopic compositions and zircon U-Pb dating were carried out on Late Mesozoic basalts from the eastern area of Zhejiang province, to explore geochemical evolution histories of the lithosphere with time. The volcanic rocks cropped out in this area were normally separted into two groups, the older one is Lower rock series (LRS), and the younger one is Upper rock series (URS), the two sreies both contain basaltic rocks. The average weighted ²⁰⁶Pb/²³⁸U age of the basalts from LRS is 120±1 Ma dated by SHRIMP, and the Th/U ratios are ranged from 0.65 to 2.44, which suggest that these zircons have a magmatic origin and the age obtained represents the formation time of the basalts. The average weighted ²⁰⁶Pb/²³⁸U age of the basalts from URS is 104±2 Ma, with Th/U ratios ranging from 0.57 to 3.03, which suggest that these zircons were magma in origin and the age representing the formation time of the basalts of URS. Thus, it can be concluded that the formation time of the URS and LRS are significantly different, and the ages are consistent with the former researchers. It can also be found that most of the zircons from the LRS basalts have a relatively homogeneous ages with only a few ancient zircon relicts, which indicates that they have little crustal contaminated during rock forming. The yield of the magmatic zircon from URS basalts are very low, many detrital grains were dated and gave the ages covering almost all of the magmatic events in the South China block. It is suggested that the URS basaltic rocks were contaminated by clastic rocks which contain the detrital zircon grains with different ages.Meanwhile, major, trace element and Sr-Nd-Pb isotopic compositions of Late Mesozoic basalts are studied to provide an insight into the nature of their mantle sources and processes. Major elements of Late Mesozoic basalts in eastern Zhejiang show an enrichment in alkaline and Al and other features. Tiantai and Qingtian samples of LRS show significant positive Ba, Pb and Sr anomalies and negative Nb, Ta, Zr and Hf anomalies and are highly enriched in LREEs with negative Eu anomalies to different degrees. Trace elements and REEs of Huatan, XuanTandi, Xinchang and Yongjia Jingjiashan samples from URS show the similar nature of samples from LRS, but Wuyi samples from URS show no positive or negative Ta, Nb, Hf, Zr anomalies.The initial magma of most of the Late Mesozoic basalts from LRS and URS in east Zhejiang province has been little contaminated by curstal materials during the magmas ascending based on geochemical and isotopic evidences. Therefore, the isotopic characteristics of the basalts from LRS and URS can be used to approximate those of their mantle sources. But Xuantandi samples from URS have been contaminated siginificantly by curstal materials during the magmas ascending, so it can not show information of their mantle sources accurately. We discuss the highest value ofεNd (T) for Xuantandi samples from URS as subject. Basalts from LRS show the range of the initial isotopic ratios as follows: ISr=0.70850⁰.70897,εNd(T)= -5.6 -4.1 , (²⁰⁶Pb/²⁰⁴Pb)i=18.21¹8.38 , (207Pb/²⁰⁴Pb)i=15.55¹5.58 ,(²⁰⁸Pb/²⁰⁴Pb)i=38.26³8.49. Basalts from URS show the range of the initial isotopic ratios as follows: ISr=0.70533⁰.70936 ,εNd(T)=-7.1³.3 , (²⁰⁶Pb/²⁰⁴Pb)i=17.23¹8.40,(207Pb/²⁰⁴Pb)i=15.46¹5.62,(²⁰⁸Pb/²⁰⁴Pb)i=36.91³8.62, which are similar to the findings of other workers. Besides, the initial isotopic ratios of Huatan, Xuantandi, Xinchang and Yongjia Jingjiashan samples from URS are: ISr=0.70734⁰.70936 ,εNd(T)= -7.1^-2.1 , (²⁰⁶Pb/²⁰⁴Pb)i=18.01¹8.40 ,(207Pb/²⁰⁴Pb)i=15.54¹5.62,(²⁰⁸Pb/²⁰⁴Pb)i=37.99³8.62. Sr, Nd and Pb initial isotopic ratios of Wuyi and Jinqu samples are: ISr=0.70533⁰.70589,εNd(T)=0.4³.3,(²⁰⁶Pb/²⁰⁴Pb)i=17.23¹8.11 ,(207Pb/²⁰⁴Pb)i=15.46¹5.53, (²⁰⁸Pb/²⁰⁴Pb)i=36.91³8.43. Initial Sr and Nd isotopic compositions of Cenozoic basalts are 0.70382 and 7.6 respectively. There are significantly higher than that of the Mesozoic basalts and show depleted features.Based on major element, trace and rare earth element and isotopic compositions from Late Mesozoic basalts of the eastern Zhejiang province, we can conclude that the initial magma of basalts from LRS may be derived from the enriched lithospheric mantle, crustal material and subducted altered oceanic crust. Basalts from URS may be derived from the enriched lithospheric mantle which is similar to basalts from LRS, depleted asthenospheric mantle and crust material. Cenozoic basalts show depleted mantle character clearly. The dynamic process of the late Mesozoic lithospheric evolution in the southeastern Zhejiang province may be related to the Pacific plate subduction or delamination of the lower crust, but the specific discussion about the dynamic process will require additional information of petrology and / or mantle xenoliths.