Cenozoic volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, China: Petrochemical evidence for partial melting of the mantle-crust transition zone  

作　　者：LAI Shaocong QIN Jiangfeng LI Yongfeng LIU Xin 

机构地区：[1]State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi＇an 710069, China

出　　处：《Chinese Journal Of Geochemistry》2007年第3期305-311,共7页中国地球化学学报

基　　金：This project was financially supported by the National Natural Science Foundation of China （Grant Nos. 40572050 and 40272042） ;the Teaching and Research Award Program for 0utstanding Young Teachers at Higher Education Institutions, Ministry of Education （M0E）, China.

摘　　要：Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association, with latite-trachyte as the main rock type. The results of chemical analysis are: SiO2=52%–62%, Al2O3>15%, Na2O/K2O>1 and MgO<3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10–13, (La/Yb)N=15–19, and show a weak negative Eu anomaly with δEu=0.71–0.89. The close relationship between Mg# and SiO2 and the co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values (Sm/Yb=3.23–3.97) imply that this association of volcanic rocks should have originated from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the result of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet Plateau.Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association, with latite-trachyte as the main rock type. The results of chemical analysis are： SIO2=52%-62%, A1203〉15%, Na20/KzO〉l and MGO〈3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10-13, （La/Yb）N=15-19, and show a weak negative Eu anomaly with 8Eu=0.71-0.89. The close relationship between Mg# and SiO2 and the co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values （Sm/Yb=3.23-3.97） imply that this association of volcanic rocks should have originated from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the result of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet Plateau.

关 键 词：新生火山岩石 地幔 变迁地带 青藏高原 

分 类 号：P3[天文地球—地球物理学]