中文摘要：

南美洲南部的Pali Aike火山岩区第四纪碱性玄武岩中普遍发育含石榴石的斜方辉石岩包体。这种斜方辉石岩既作为独立的捕掳体存在又以细脉的形式穿插于橄榄岩捕掳体中。斜方辉石岩普遍含富Ti矿物,并且次生斜方辉石含橄榄石和单斜辉石残晶。与含石榴石橄榄岩中的斜方辉石相比,这种次生的斜方辉石以高TiO2、中等含量的Al2O3以及低Mg#为特征,表明它是在一种高度分异演化的富Ti熔体交代作用下通过消耗橄榄石和单斜辉石方式形成的。斜方辉石岩全岩的Co、Ni略低,Cr和铂族元素（PGE）含量与地幔橄榄岩相当,表明这些元素在交代作用过程中相对稳定,而交代介质带入的组分以碱质（K2O＋Na2O）、Ti、Si、Al和S为主。交代的斜方辉石在现代活动岛弧和古克拉通的地幔橄榄岩捕掳体中多有报道。与这些环境中地幔样品的斜方辉石相比,PaliAike地区的次生斜方辉石含有相对高的Ti和Al,以及相对低的Mg。高Ti低Mg属性反映了交代介质可能来源于下伏的软流圈地幔并且经历了高度的分异和演化过程。Pali Aike地区所见到的这种交代斜方辉石和斜方辉石岩在其他被上涌软流圈影响的陆下岩石圈地幔中可能普遍存在。这些研究对了解中国华北-东北中生代以来的岩石圈地幔减薄机制有着重要的借鉴意义。

英文摘要：

Garnet-bearing orthopyroxenites occurred as both discrete xenoliths and veinlets in peridotite xenoliths that were brought to the surface by Quaternary Pali Aike alkali basalts in southern South America. Orthopyroxenites commonly contain Ti-rich minerals and also relict grains of olivine and clinopyroxene occurring as inclusions in secondary orthopyroxene. The secondary orthopyroxene contains high TiO2,moderate Al2O3 and low Mg# compared with orthopyroxene in garnet-bearing peridotites. This suggests that the secondary orthopyroxene formed at the expense of olivine and clinopyroxene during the metasomatism by an evolved Ti-rich melt. Compared with peridotites,the orthopyroxenites contain slightly low Co and Ni,and similar Cr and PGE,suggesting that these metals were essentially immobile during metasomatism. The components introduced by the metasomatizing melt are mainly alkalis （K＋Na）,Ti,Si,Al and S. Secondary orthopyroxene has also been documented in mantle peridotite xenoliths derived from beneath both active arcs and ancient cratons. In comparison with such orthopyroxene in these mantle samples,the secondary orthopyroxene in the Pali Aike xenoliths contains relatively high Ti and Al and low Mg. High Ti and low Mg,which reflects the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. The type of secondary orthopyroxene,typified by these samples,may be common elsewhere in subcontinental mantle lithosphere affected by asthenospheric upwelling. The study may provide a new clue for understanding the thinning mechanism of lithosphere beneath North and Northeast China since the Mesozoic Era.