扬子块体南缘四堡群Sm－Nd同位素体系及其他壳演化意义

详细的主元素和Sm-Nd同位素体系研究表明,扬子块体南缘元宝山地区四堡群中镁铁质-超镁铁质岩可能来源于Al亏损地幔;而宝坛地区镁铁质-超镁铁质岩的源区则可能包括了A1未亏损和A1亏损两种地幔端元组分,部分样品可能受到围岩混染。镁铁质-超镁铁质岩的Sm-Nd数据构成了一条无地质意义的假等时线,由其斜率获得的年龄约2.2Ga明显偏老。四堡群浅变质沉积岩的Nd模式年龄限定了镁铁质-超镁铁质岩和四堡群的地层年龄应小于1.8Ga.扬子南缘最老的基底四堡群(及相应地层)主要是由地壳存留年龄为1.8-1.9Ga的未成熟陆壳再循环物质组成,明显不同于华南块体(华夏古陆)的早-中元古代变质基底。迄今为止获得的沉积岩和花岗岩的Sm-Nd同位素资料都不支持扬子南缘存在早元古代-晚太古代基底。

Sm-Nd LSOTOPIC SYSTEMATLCS OF SIBAO GROUP FORM THE SOUTHERN MARGIN OF YANGTZE BIOCK:IMPLICATIONS FOR THE CRUSTAL EVOLUTION

Major element and Sm-Nd isotopic studies suggest that the mafic-ultra mafic rocks in the Yuanbacshan and Baotan, southern margin of the Yangtze Block were probably derived from a Al-depleted mantle source.and from the mixture of Al-depleted and Al-undepleted mantle sources, respectmely. A few of Baotan samples might have been contaminated by the crustal wall rocks. The Sm-Nd isotopic data of these samples yield a geologically nieaivngless pseudoisaehron, whose slope gives an age of about 2.2Ga sighficautly older than the Eormatoon age of the samples.The consistent Nd model ages(T_DM) of the low-grade metamorphic sedimentary rock samples from the Sibao Group constrian a maximum age of about 1.8Ga for the Sibao sediments and the interlayered mafic-ultra mafic rocks. The oldest basement rocks, i. e. the Sibao Group and its equivalents, in the southern margin of the Yangtze Slock are dominantly composed of the unmature crustal recycled materials with crustal residence ages of f.8-1.9Ga. The Sm-Nd stotopic data obtained so far indicate that there is probably no early Proterozoic-late Archean basement rocks along, the Southern margin of the Yangtze Block.