Petrogenesis of Cenozoic Basalts in Bohai Bay Basin:Geochemical and Sr-Nd-Hf-Pb Isotopic Evidences
The Cenozoic basalts in the Bohai Bay basin are an important part of the Cenozoic basalts in the northeast of the North China craton.However,the limited exposure of basalt on the surface in the region hinders research of this place.In this study,we selected drill core samples from Liaohe oil field and analyzed their major and trace elements,and Sr-Nd-Hf-Pb isotopic compositions.The Cenozoic basalts from the Bohai Bay basin in this study are mainly tholeiite and trachybasalt.Their SiO2 and MgO contents are ranging from 49.08％to 50.70％,and 2.63％to 5.80％,respectively.Moreover,they are enriched in LREEs with highly fractionated LREEs and HREEs((La/Yb)N=7.96-11.61 and(Dy/Yb)N=1.71-1.84).These samples are enriched in high field strength elements(HFSE)and large ion lithophile elements(LILE),with evidently positive Nb,Ta and Sr and without Eu and Ce anomalies.The Sr,Nd and Hf isotope ratios of the whole rock are(87Sr/86Sr)i=0.704622-0.706581,εNd(t)=1.1-1.9,εHf(t)=1.6-4.6,respectively.The ratios of(206 Pb/204 Pb)i,(207 Pb/204 Pb)i and(208 Pb/204 Pb)i are 17.257 7-17.409 9,15.201 5-15.335 4 and 37.185 8-37.912 9,respectively.All these characteristics are similar to those of the oceanic island basalts(OIBs).The geochemical features,combined with petrographic observations and Sr-Nd-Hf-Pb isotopic compositions,suggest weak crustal contamination and fractional crystallization in their origin.In conclusion,the Bohai Bay Cenozoic basalts were derived from slight partial melting of asthenospheric mantle.
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