Geochemistry of amphibolites in Hualong Group of eastern Middle Qilian massif and its tectonic significance

Petrologic characteristics and analyses of major and trace elements of the amphibolites in Hualong Group within the eastern segment of Middle Qilian massif are reported in this paper.The amphibolites consist mainly of amphibole (52%) and plagioclase (40%),with the accessory minerals comprising ilmenite (6%),aprtite (2%),quartz and sphene.In the rocks,SiO2=43.78%～48.09 %,TiO2=2.26%～2.5%,Al2O3=14.99%～16.64%,P2O5=0.38%～0.43%,CaO=8.11%～10.93%,FeOt=11.83%～12.54%,and Mg#=0.53～0.54.The amphibolites in Hualong Group crop out in layers,and their protoliths are alkalic basalts characterized by significant LREE[(La/Yb)cn=4.3～8.3)] and HFSE (e.g.,Nb,Ta) enrichment (Nb/La=1.2～1.5,Th/Ta=1.2～1.4) and insignificant Eu anomalies (Eu/Eu=0.90～1.11,with an average of 1.04 ),as well as trace element patterns similar to those of ocean island basalts.Various geochemical diagrams indicate that these amphibolites mainly formed in an intraplate environment.Compared with OIB,amphibolites have slightly lower LILE (e.g.,Th) concentrations,implying an effect of asthenosphere-lithosphere interactions.The formation of the amphibolites was probably related to the partial breakup of the Neoproterozoic Rodinia Supercontinent in the Qilianshan region or the breakup of Neoproterozoic Rodinia Supercontinent in part of the Qilianshan region.