西藏拉萨地块南部啊扎侵入体锆石U-Pb年龄、地球化学特征及其对新特提斯洋演化历史的指示

Zircon U-Pb age and geochemical characteristics of the Azha intrusion in the southern Lhasa Block,Tibet and their indications for the evolutionary history of the Neo-Tethys

拉萨地块南缘广泛出露的岩浆岩是研究新特提斯洋沿该区域北向俯冲演化历史的良好对象。为研究新特提斯洋俯冲及演化机制,对西藏山南地区啊扎侵入体进行了系统的岩石学、年代学和地球化学研究。结果表明,其为一套形成于晚白垩世(95.0±1.4 Ma、100.2±1.4 Ma)的石英二长闪长岩;地球化学组成显示其高硅、富钠(Na_(2) O>K_(2) O),属于高钾钙碱性岩;微量元素具有高Sr(588.47×10^(-6)、649.65×10^(-6)),低Y(8.66×10^(-6)、9.03×10^(-6))和Yb(0.87×10^(-6)、0.92×10^(-6))及高的Sr/Y值(65.17、75.02),显示典型的埃达克岩地球化学特征。同时富集Rb、Sr等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,稀土元素配分曲线呈右倾模式,具正Eu异常;显示低的(^(87)Sr/^(86)Sr)_(i)(0.703825、0.703836),高的^(143)Nd/^(144)Nd(0.512790、0.512798)及正的εNd(t)值(4.08和4.25)。结合前人的研究成果,认为啊扎岩体是新特提斯洋洋脊俯冲消减背景下洋壳部分熔融的产物。

A large number of magmatic rocks are exposed in the southern margin of the Lhasa Terrane,which provides great objects for studying the evolution history of the northward subduction of the Neo-Tethys along the southern margin of the Lhasa Terrane.In order to study mechanism of subduction evolution of Neo-Tethys,this paper conducts systematic petrological,geochronological and geochemical analyses of the Azha intrusions in the Shannan area of Tibet.Results show that the Azha intrusive rocks are mainly composed of quartz monzodiorite,formed during Late Cretaceous(95.0±1.4 Ma,100.2±1.4 Ma).The samples are characterized by high silicon and sodium(Na 2 O>K 2 O),belonging to high-K calc-alkaline rocks.They have high Sr(588.47×10^(-6),649.65×10^(-6)),low Y(8.66×10^(-6),9.03×10^(-6))and Yb(0.87×10^(-6),0.92×10^(-6))and high Sr/Y ratio(65.17,75.02),showing the geochemical characteristics of typical adakitic rocks.Their large-ion lithophile elements such as Rb and Sr are enriched,and their high field strength elements such as Nb,Ta and Ti are depleted,along with the enrichment of LREE and the positive anomaly of Eu.The samples show low(87 Sr/86 Sr)i(0.703825,0.703836),high 143 Nd/144 Nd(0.512790,0.512798)and positive values ofεNd(t)(4.08 and 4.25).Combined with previous results,it is suggested that the Azha instrusion is the product of partial melting of oceanic crust due to ridge subduction of the Neo-Tethys.