新路盆地火山岩Sr-Nd同位素地球化学特征及其地质意义

在浙西新路盆地中生代酸性火山岩的Sr-Nd同位素组成和Nd同位素模式年龄基础上,探讨了其源岩性质及其大地构造意义。结果显示,构成新路盆地主体的黄尖组晶屑熔结凝灰岩和花岗斑岩的平均锶初始值分别为0.714 10、0.710 39,ε_(Nd)(t)值分别为-5.5、-5.7;黄尖组晶屑熔结凝灰岩和花岗斑岩的两阶段模式年龄均值分别为1 366Ma、1 387Ma,与扬子地块基底双溪坞群变质岩年龄(900~1 000 Ma)差异明显,而与华夏地块基底陈蔡群变质岩年龄(1 400 Ma)接近;2个单元火山岩Nd同位素均位于陈蔡群Nd同位素演化域。研究认为,新路盆地黄尖组熔结凝灰岩和花岗斑岩具有相似的物质来源,均表现出壳源和幔源物质混合成因特点,壳源物质可能源自南部华夏地块的陈蔡群变质岩,与扬子地块变质基底双溪坞群无关;推测发生于1 000~900 Ma B.P.时期或其后的浙西地区扬子地块与华夏地块之间的陆陆碰撞,具有华夏地块向扬子地块俯冲的动力学特点。上述大地构造运动方式以及岩石成因特点,或许是导致赣杭火山岩带分属华夏地块和扬子地块的火山岩均呈现出与铀成矿关系密切的重要原因。

Characteristics of Sr-Nd isotope and geological significance of the volcanic rocks in Xinlu basin,Zhejiang, China

The volcanic faulted Xinlu basin in the southern margin of the Yangtze block is formed in Yanshanian epoch.Sr-Nd isotopic compositions and Nd model age of the Mesozoic acid volcanic rocks in the Xinlu basin are studied and the properties of source rocks and dynamics of regional tectonic evolution are discussed.It reveals that the average value of Isr for crystal debris tuff from Huangjian Formation and for granite-porphyry is 0.714 10,0.710 39,and the average value of (εNd(t) is-5.5,-5.7,respectively.Two stage model ages for crystal debris tuff from Huangjian Formation and for granite-porphyry is 1 366 Ma,1 387 Ma,respectively,close to the age (1 400 Ma) of the Chencai Group in the basement of Cathaysia block,and different from the age (900 ～ 1 000 Ma) of the Shuangxiwu Group from the basement of Yangtze block.The εNd (t) values both from Huangjian Formation volcanic tuff and granite-porphyry are located in Nd isotopic evolutionary domain of Chencai Group.It realizes that the crystal debris tuff of Huangjian Formation and granite-porphyry have similar source rocks originated from mixing material of crust and lithospheric mantle.The crust source part is probably derived from the metamorphic rocks of Chencai Group in Cathaysia block,and has no connection with Shuangxiwu Group in Yangtze block.It is considered that continent-continentcollision occurred in the period of 1 000～900 Ma B.P.or later is characterized by the subduction from the Cathaysia block to the Yangtze block in western Zhejiang area.