华北太古代上地幔钕同位素组成、演化及对该区岩石圈地幔不均一性的制约

Nd ISOTOPIC COMPOSITION AND EVOLUTION OF THE UPPER MANTLE IN THE ARCHAEAN OF NORTH CHINA: CONSTRAINTS ON THE MANTLE HETEROGENEITY OF THE UNMETASOMATIC CONTINENTAL LITHOSPHERE

近年所获得的华北各地区太古代变质火山岩系9个较准确的Sm-Nd同位素年龄及初始Nd同位素比值表明,华北太古代(3500—2500Ma)上地幔都是亏损的,但在1000Ma范围內其ε_(Nd)值一直恒定在+3左右,与全球太古代亏损地幔Nd同位素演化规律一致。这表明华北太古代的构造体制有可能使大量地壳物质重新进入地幔从而保持ε_(Nd)值的恒定。如果华北大陆岩石圈地幔的主体是在太古代时从对流上地幔中分离出来的,利用该区太古代上地幔Nd同位素组成及其可能的sm/Nd值进行计算,可获得该区岩石圈非交代地幔应是与N型大洋中脊玄武岩(以下简称MORB)源类似或更亏损的高亏损地幔,其现代ε-(Nd)值最大变化范围在+7—+23之间。至今在华北我们尚未发现由这种高亏损非交代大陆岩石圈地幔产生的大陆玄武岩。如果某些大陆玄武岩可能产生于大陆岩石圈地幔,则其源区必定是经过地幔交代作用再富集了的地幔。

The available 9 precise Sm-Nd isotopic ages and their initial Nd isotopic ratios of the Archaean terranes in North China show that the upper mantle of the Archaean (3.5-2.5 Ga) in North China was depleted and the initial εNd values (=+2.2-+3.3) were fairly constant during that time period. This is consistent with the depleted mantle evolution curve of εNd for other parts of the world. These observations suggest that the tectonic regime in the Archaean would cause large quantities of continental material to enter into the mantle, thus maintaining a fairly constant value of εNd. If the major part of the mantle in the continental lithosphere of North China was separated from the convecting mantle during the Archaean time, calculations from the Nd isotopic data and ages of the Archaean terranes in North China and possible Sm/ Nd ratios show that the unmetasomatic mantle in the continental lithosphere of North China is similar to or more depleted than the source of N-type MORB The ENA values are variable at most between+ 7 and+ 23. So far, we have not yet found any continental basalt derived from this kind of unmetasomatic continental lithosphere mantle in North China. If certain continental basalts might be derived from the continental lithosphere, their sources should be enriched as a result of mantle metasomatism and their εNd values should be less than+ 10.