Mesozoic Lithospheric Mantle Modified by Delaminated Lower Continental Crust in the North China Craton:Constraints from Compositions of Amphiboles from Peridotite Xenoliths
XU Wen-liang,YANG De-bin,PEI Fu-ping,WANG Feng,WANG WeiCollege of Earth Sciences,Jilin University,Changchun 130061,China
Major and trace element analyses of amphiboles from the peridotite xenolith entrained by early Cretaceous high-Mg# diorites in western Shandong and southern Taihang Mountains(Mts.) were carried out with the aim of constraining the nature and derivation of the melt modifying lithospheric mantle.The results indicate that these amphiboles within the peridotite xenoliths are of metasomatic origin.Chemically,they belong to calcic amphibole group.The amphiboles from the peridotite xenoliths in the Fushan intrusion of southern Taihang Mts.are magnesio-hornblende,edenite,and edenitic hornblende,whereas ones from the peridotite xenoliths in the Tietonggou intrusion of western Shandong are parga-sitic hornblende,edenitic hornblende,and magnesio-hastingsitic hornblende.Compared with amphiboles from the intraplate lithospheric mantle,ones from the peridotite xenoliths entrained by early Cretaceous high-Mg# diorites in the southern Taihang Mts.and western Shandong are characterized by high Mg# values(western Shandong: 86.0-90.8;southern Taihang Mts.: 90.7-94.2) and SiO2 contents(western Shandong: 44.04%-46.98%;southern Taihang Mts.:47.09%-49.79%),and relatively low Na2O(1.92%-2.51%) and TiO2 contents(0.01%-1.46%),as well as low Nb(0.29×10-6-1.98 ×10-6) and Zr abundances(1.61×10-6-5.34×10-6),similar to those from the suprasubduction lithospheric mantle.Taken together with geochemical characteristics of these peridotite xenoliths,it is suggested that the lithospheric mantle had been modified by the melt derived from the delaminated lower continental crust in the early Cretaceous.