拉萨地块西部狮泉河地区典中组火山岩年代学、地球化学特征及其构造意义

Geochronology,Geochemistry and Geological Significance of Volcanic Rocks of the Dianzhong Formation,Shiquanhe Area,Western Lhasa Block

广泛分布于拉萨地块南部的林子宗群火山岩被认为是新特提斯洋北向俯冲消减到陆陆碰撞构造背景转化过程中的岩浆作用产物,记录了从新特提斯洋俯冲到印度-欧亚板块碰撞、后碰撞的丰富信息.对狮泉河地区的林子宗群典中组火山岩样品进行了详细的年代学、元素及同位素地球化学研究.结果显示典中组火山岩样品LA-ICP-MS锆石U-Pb年龄集中在67.1±1.7 Ma至70.8±1.1 Ma,表明其形成于白垩纪晚期.火山岩岩性主要包括玄武安山岩、安山岩、流纹岩等,其中,中基性火山岩为一套低钾拉斑-钙碱性系列岩石,具有低TiO2(平均值<1%),高Al2O3(平均值>15%),均富集K、Rb、Th、U等大离子亲石元素(LILE),亏损Nb、Ta、Zr、Ti等高场强元素(HFSE),具有弧岩浆岩的地球化学特征.两件中基性岩样品(87Sr/86Sr)i比值分别为0.707437、0.707672,εNd(t)值分别为-5.06和-4.31.酸性火山岩为一套高钾钙碱性岩石,具有高的SiO2(76.01%~76.77%)、高K2O(4.93%~4.98%)、高K2O/Na2O(1.41%~1.45%)以及相对高的Mg#(平均值为45.38),富集K、Rb、Th、U等大离子亲石元素(LILE),亏损Nb、Ta、Zr、Ti等高场强元素(HFSE),具有明显的Eu和Sr负异常.以上特征暗示典中组中基性火山岩可能是新特提斯洋北向俯冲消减过程中板片流体引起上覆地幔楔部分熔融的产物,且在上升过程中受到地壳混染的影响,酸性岩则可能来源于上覆地壳的部分熔融,且受到少量幔源物质的影响.此外,年代学及地球化学特征均表明本研究样品不属于前人认为的则弄群火山岩,而应重新厘定为林子宗群典中组火山岩.

Volcanic rocks of the Linzizong Group are widely distributed in the southern part of the Lhasa Block,and are considered to related with tectonic settings during the transition from the northward subduction of Neo-Tethys to continent-continent collision.In this paper,we present detailed chronology,elemental geochemistry and isotope geochemistry of volcanic rock samples from the Linzizong Group,Shiquanhe area.The results show that LA-ICP-MS U-Pb zircon ages from volcanic rock samples of the Dianzhong Formation are mostly between 67.1±1.7 Ma and 70.8±1.1 Ma,establishing a Late Cretaceous age.The Dianzhong Formation in the study area comprises basaltic andesite,andesite and rhyolite.Medium-basic volcanic rocks belong to the potassium calcium-alkaline series,with low TiO2(average<1%)and high Al2O3(average>15%)contents,and are enriched in large ion lithophile elements(LILE),such as K,Rb,Th,and U,and depleted in high field strength elements(HFSE)such as Nb,Ta,Zr and Ti,thereby showing obvious island-arc volcanic affinity.The isotope study yields(87Sr/86Sr)iratios of 0.707437,0.707672,andεNd(t)of-5.06 and-4.30.The acid volcanic rocks have high SiO2(76.01%-76.77%),high K2O(4.93%-4.98%)and high K2O/Na2O(1.41%-1.45%),and belong to the high potassium calc-alkaline series,with a relatively high Mg#(average 45.38),enrichment of large ion lithophile elements(LILE)such as K,Rb,Th,U,depletion of high field strength elements(HFSE)such as Nb,Ta,Zr,Ti,and have significant negative Eu and Sr anomalies.The above characteristics suggest that the medium-basic volcanic rocks of the Dianzhong Formation were generated from partial melting of the overlying mantle wedge during northward subduction of Neo-Tethys,and were crustally contaminated during emplacement,whereas the acidic rocks may have been derived from partial melting of the overlying crust and the addition of a small amount of mantle material.In addition,both chronology and geochemical characteristics indicate that rock units sampled during this study are not part of the Zenong Group,as considered by the previous studies,but should be redefined as volcanic rocks of the Linzizong Group.