拉萨地块南缘日多地区叶巴组火山岩地球化学、年代学、锆石Lu-Hf同位素特征及其地质意义

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

拉萨地块南缘日多地区叶巴组火山岩以中酸性熔岩及火山碎屑岩占绝对优势为特征。以墨竹工卡县以东日多地区叶巴组火山岩代表性岩石组合为对象进行了地球化学、LA-ICP-MS锆石U-Pb年龄及锆石Lu-Hf同位素研究。研究结果表明,叶巴组火山岩具有轻稀土元素富集,富集大离子亲石元素Rb、Th、K,亏损高场强元素Nb、Ta、Ti(P、Hf)的地球化学特征。其中,基性火山岩具低钾、低钛和富钠、富铝的特征,Nb、Zr含量和Th/Y、Th/Yb、Ta/Yb值较高,而La/Nb值较低,呈现出大陆地壳组分增加的趋势。中酸性火山岩属中钾-高钾钙碱性系列,具有低钛、低镁和高铝的特征,微量元素含量及比值与大陆岛弧安山岩接近。叶巴组火山岩总体地球化学特征与陆缘弧火山岩相似。LA-ICP-MS锆石U-Pb测年获得英安岩和流纹质晶屑凝灰岩^(206)Pb/^(238)U年龄加权平均值分别为176.9±2.3Ma和162.2±3.3Ma,表明研究区叶巴组酸性火山岩形成于中侏罗世。锆石Hf同位素测试结果显示,εHf(t)值为2.43~11.42,二阶段模式年龄(t_(DM)~C)为482~1065Ma,暗示叶巴组酸性火山岩源区除新生地壳物质的部分熔融外,还明显受到古老结晶基底的影响。结合前人研究成果,认为叶巴组形成于早中侏罗世雅鲁藏布江洋北向俯冲于拉萨地块南缘之下的陆缘弧环境。

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 ofεHf(t)range from 2.43 to 11.42,and the values of tDMC 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.