浙闽沿海早白垩世玄武岩锶、钕、铅同位素特征——古老富集型地幔的证据

Sr,Nd AND Pb ISOTOPIC CHARACTERISTICS OF EARLY CRETACEOUS BASALTIC ROCKS FROM THE COAST OF ZHEJIANG AND FUJIAN: EVIDENCES FOR ANCIENT ENRICHED MANTLE SOURCE

浙闽沿海大面积出露的中生代酸性火山岩区有少量早白垩世玄武岩分布，它们具典型钾富集和铌等元素亏损特征，其同位素组成表现为较高ＩＳｒ（０．７０５５—０．７１０６）、低的εＮｄ（１．２—－１０．６，大多介于－３．２—－１０．６之间）及富放射性成因铅（２０６Ｐｂ／２０４Ｐｂ＝１８．３５５—１８．７２６，２０７Ｐｂ／２０４Ｐｂ＝１５．４５５—１５．７９９，２０８Ｐｂ／２０４Ｐｂ＝３８．５３０—３９．３１９）。这些特征表明玄武岩源区为一富集型的陆下岩石圈地幔，由古老的俯冲地壳物质再循环进入并交代地幔而形成。没有证据表明本区早白垩世基性和酸性岩浆之间发生过大规模的化学混合，但不排除同位素之间的交换以及局部的化学和机械混合。

There are two stage effusions of Early Cretaceous (early and late) basaltic rocks partly distributed in the extensive Mesozoic acidic volcanic province generally as interlayers along the coast of Zhejiang and Fujian, which provide important samples for exploring the nature of the upper mantle source, the crust mantle interaction and the origin of those contemporaneous voluminous acidic volcanic rocks. These rocks are mainly basalts, trachybasalts and basaltic trachyandesites without any mantle xenoliths or xenocrystals found till now. These basaltic rocks are chemically characteristic of high Al 2O 3 (15.24%—20.45%), low TiO 2 and MgO (Mg number less than 60), especially enriched K and Sr and depleted Nb (primordial mantle normalized), and have slightly higher LREE/HREE ratios with slightly positive Eu anomaly. Isotopically, they show distinctive high initial Sr isotopic ratios (0.7055—0.7106), low initial Nd isotopic ratios ( ε Nd =1.2—-10.6, mostly within -3.2—-10.6) and enriched radiogenic Pb( 206 Pb/ 204 Pb=18.355—18.726, 207 Pb/ 204 Pb=15.455—15.799 and 208 Pb/ 204 Pb=38.530—39.319) which are quite different from those of the Cenozoic basalts in the same area. It shows that the source of the basalts is an enriched subcontinental lithospheric mantle having been modified by input and metasomatism of the recycled ancient subducted crustal materials. No evidences show that there had been large scale chemical mixing between Cretaceous basic and acidic magmas in the area, but isotopic exchange as well as part mechanical mingling and chemical mixing cannot be excluded. The extensive acidic melts were generated by intrusion of basaltic magmas into the continental crust, which in turn prevented the penetration of the basaltic magma. The basic magma had not been obviously contaminated by crustal materials during its ascent to the surface when tectonics permitted. The early and late basalts of Early Cretaceous show slight difference in isotopic compositions which might be due to the volume difference of the involved enriched components in the mantle source.