安徽女山地幔橄榄岩捕虏体的同位素组成:中国东部新生代岩石圈地幔时代制约

位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。

Isotopic compositions of the peridotitic xenoliths from the Nushan area, Anhui Province: Constraints on the age of subcontinental lithospheric mantle beneath the East China

Nushan volcano, located in Anhui Province, contains abundant and various mantle-derived peridotitic xenoliths, including spinel lherzolite, garnet lherzolite and spinel-garnet lherzolite, with minor harzburgite. Some spinel lherzolites contain amphibole, phlogopite and apatite, indicating that they had experienced mantle metasomatism. Petrological, mineralogical and geochemical data of the spinel lherzolites and harzbuigites revealed that the former are fertile and the latter are refractory. Depleted Sr-Nd-Hf isotopic characteristics of the spinel lherzolites indicate that they are typical of oceanic peridotites, rather than of Archean lithospheric mantle. Furthermore, most spinel lherzolites have high ¹⁸⁷Os/¹⁸⁸Os ratios. Although the lithospheric mantle beneath the Nushan are has been strongly metasomatismed by fluids/melts, the Os isotopes and PGE (Platinum Group Element) data display that the Os isotopic composition of the Nushan peridotites are not significantly affected by later mantle metasomatism processes. All these geochemical data of most spinel lherzolites indicate that the lithospheric mantle beneath Nushan are young (Phanerozoic) additions to the subcontinental lithospheric mantle (SCLM) from the convective asthenospheric mantle. However, one harzburgite has relatively lower ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1184 and ancient Re-depletion age of 1.5Ga, suggesting existence of some ancient component in the asthenosphere.