苏皖地区幔源橄榄岩捕瞄虏体的锇同位素组成、模式年龄及其意义

苏皖地区发育的新生代玄武岩中富含地幔橄榄岩捕虏体，测定了25个橄榄岩全岩的锇同位素组成。结果表明大部分样品呈亏损特征,187Os/188Os=0.119～0.129.Os同位素比值与主量成分之间有显著的相关性.采用187Os/188Os-Al2O3代用等时线方法,由截距得到初始值(187Os/188Os)0=0.109,在对流地幔的187Os/188Os比值演化线上获得模式年龄t=2.5(±0.1)Ga,为晚太古-早元古代。用样品组中最低的锇同位素组成,即187Os/188Os=0.119,计算Re亏损模式年龄,t=1.2Ga,为中元古代。表明苏皖地区新生代玄武岩中的地幔橄榄岩捕虏体具有古老的形成年龄,它们是经过显生宙减薄作用后的残余地幔的碎片。大陆岩石圈地幔的古老形成年龄与上覆地壳克拉通的古老稳定年龄相耦合。

The osmium isotopic compositions and model age of mantle peridotite xenoliths from Jiangsu-Anhui area and their geological significance.

Cenozoic alkaline basalt contains abundant mantle peridotite xenoliths along the boundary between Jiangsu and Anhui provinces. This paper reports the osmium isotopic compositions of whole rock for the peridotite xenoliths, 187Os/188Os =0.119～0.129, which is mainly characterized as depletion. There is a significant linear correlation between the osmium isotopic compositions and the major element compositions among the samples. The osmium initial ratio (187Os/188Os)0 = 0.109 comes from alumichron, then the relevant model age t=2.5(±0.1)Ga is found from the evolution model of osmium isotopic composition in the convective mantle. The Re depletion age t=1.2Ga, is calculated from the lowest osmium isotopic ratio in the sample set. The longevity of sub-continental lithospheric mantle from osmium isotope systematics in peridotite xenoliths from Jiangsu-Anhui area indicates that they are the fragments of residual mantle after Phanerozoic thinning. The ancient formation age of continental lithospheric mantle is coupling with the age of overlying craton crust.