Geochemistry, chronology and zircon Lu-Hf isotopic characteristics of the volcanic rocks of Yeba Formation in Riduo area on the southern margin of Lhasa massif and their geological significance

The volcanic rocks of Yeba Formation in Riduo area on the southern margin of Lhasa massif are characterized by the absolute superiority of intermediate acid lava and pyroclastic rock.In this paper, the authors selected geochemistry, LA-ICP-MS zircon U-Pb geochronology and zircon Lu-Hf isotopes of representative rock combination of Yeba Formation in Riduo area of eastern Mozhugongka County as the study objects.Geochemical analysis shows that the volcanic rocks of the Yeba Formation are characterized by LREE enriched type pattern, enrichment of LILES such as Rb, Th and K, and depletion of HFSE such as Nb, Ta and Ti (P, Hf).The basic volcanic rocks are characterized by low potassium and titanium as well as rich sodium and aluminum.The values of Nb, Zr and ratios of Th/Y, Th/Yb and Ta/Yb are relatively high, but the ratio of La/Nb is relatively low, showing a trend of increasing continental crustal components.The intermediate acid volcanic rocks belong to the middle-high potassium calc alkaline series.They are characterized by low titanium and magnesium as well as high aluminum.The values and ratios of trace elements are close to those of andesite from the continental island arc.The overall geochemical characteristics of the volcanic rocks of the Yeba Formation are similar to features of the continental margin arc volcanic rocks.LA-ICP-MS Zircon U-Pb dating of dacite and rhyolitic crystal tuff yielded ²⁰⁶Pb/²³⁸U weighted average ages of 176.9±2.3Ma and 162.2±3.3Ma, showing that acidic volcanic rocks of Yeba Formation in the study area were formed in Middle Jurassic.Zircon Hf isotope analytical results show that the values of ε_Hf(t) range from 2.43 to 11.42, and the values of tDM^C range from 482Ma to 1065Ma, suggesting that the source area of the acid volcanic rocks of Yeba Formation was affected not only by the partial melting of the new crustal material but also by the ancient crystallization basement.Combined with previous research results, the authors hold that the volcanic rocks of Yeba Formation was formed in a tectonic setting of continental arc, which was the subduction of the Yarlung Zangbo Ocean to the southern margin of Lassa massif during Early-Middle Jurassic period.