华北克拉通西部陆块北缘新生代岩石圈地幔特征的初步研究

A preliminary study on the Cenozoic lithospheric mantle beneath the northern part of the western North China Craton

凉城、四子王旗、三义堂和大同的地幔包体的岩石学和矿物的主、微量元素成分显示华北克拉通中、西部北缘总体为过渡型岩石圈地幔,为原始地幔经过不同程度的熔体抽取和后期交代富集作用的残留。凉城岩石圈地幔经历的熔体抽取程度最低,后期交代富集作用比较强烈,这与其橄榄岩中尖晶石的Cr#较低,并且共存的单斜辉石的Mg#较低一致。三义堂岩石圈地幔经历的熔体抽取程度最高,后期交代富集作用最强烈,不同样品甚至同一样品中不同单斜辉石颗粒的La/Yb和LREE配分模式变化范围非常大,且矿物边部比核部更富集微量元素。大同、四子王旗岩石圈地幔经历的熔体抽取程度介于凉城和三义堂之间,但后期的交代富集作用明显不如凉城和三义堂强烈。研究区的岩石圈地幔主要受硅酸盐熔体交代作用的影响,只有三义堂岩石圈地幔还可能也受到了碳酸盐交代作用的影响。

Petrology and major-and trace-element compositions of minerals of the peridotite xenoliths from the Liangcheng,Siziwangqi,Sanyitang and Datong suggest that the lithospheric mantle beneath the northern parts of the Western Block and Central Zone of the North China Craton mainly consists of transitional peridotites.It is the relict of the primitive mantle having experienced initial melt extraction and subsequent metasomatism.The lithospheric mantle beneath the Liangcheng has been subjected to the lowest degree of melt extraction and intermediate metasomatism,which is in accordance with the lower Cr# of spinels and lower Mg# of coexisting clinopyroxenes in the Liangcheng peridotites.The lithospheric mantle beneath the Sanyitang has experienced the highest degree of melt extraction and the strongest metasomatism.The Sanyitang clinopyroxenes,even within one single sample,show large variations in La/Yb and chondrite-normalized REE patterns（from LREE-depleted to LREE-enriched）,and the Sanyitang peridotitic minerals show marked enrichment in some trace elements on the rim than in the core.The Datong and Siziwangqi lithospheric mantles have experienced intermediate melt extraction,but weaker metasomatism than the Liangcheng and Sanyitang lithospheric mantles.The lithospheric mantles were mainly affected by silicate melt metasomatism,and only the lithospheric mantle beneath the Sanyitang was probably affected by carbonatite melt metasomatism as well.