黑龙江三道湾子金矿区龙江组安山岩的年代学与地球化学

Geochronology and geochemistry of the andesites of Longjiang Formation in the Sandaowanzi gold deposit,Heilongjiang Province

三道湾子金矿是我国东北地区重要的浅成低温热液金矿之一,金矿赋存在龙江组安山岩和安山质火山角砾岩中。本文对三道湾子金矿赋矿围岩进行了锆石U-Pb年代学和Hf同位素、岩石元素和Sr-Nd同位素地球化学研究。LA-ICP-MS锆石U-Pb测年获得龙江组安山岩加权平均年龄为121.4±1.8Ma,表明其喷出时代为早白垩世。在地球化学组成上,龙江组火山岩具有明显的轻重稀土分馏和弱Eu负异常,微量元素富集Rb、Ba、Th、U等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),岩石与岛弧安山岩地球化学特征相似。龙江组安山岩具有低的Sr初始比值[(87Sr/86Sr)i=0.704798~0.704919]和正的εNd(t)(0.7~1.8),说明岩浆源区具有弱亏损地幔的特征。三道湾子金矿区的龙江组安山岩可能形成于早白垩世岩石圈大规模伸展减薄的动力学背景下,由于伊泽奈崎板块运动方向的改变致使整个东北地区处于伸展构造背景中,软流圈上涌的同时下地壳与岩石圈地幔发生部分熔融,导致整个东北地区广泛发育晚中生代岩浆作用,构成了三道湾子金矿的赋矿围岩。

The Sandaowanzi epithermal gold deposit,located in the Northeast China,is hosted by Longjiang Formation andesite and andesitic breccia. This study carried out the research of zircon U-Pb geochronology,Hf isotopes and whole rock elemental and Sr-Nd isotopic geochemistry on the samples collected from Sandaowanzi gold deposit. LA-ICP-MS zircon U-Pb dating of this study gives a weighted average age of 121. 4 ± 1. 8Ma for andesite,indicating that the Longjiang Formation volcanic rocks were erupted in the Early Cretaceous. Geochemically,Sandaowanzi Longjiang Formation volcanic rocks show fractionation of light rare earth elements（ REE）relative to heavy REE,and less Eu anomaly. The rocks are enriched in large ion lithosphile elements（ LILEs,such as Rb,Ba,Th,U,etc.）,and depleted in high field strength elements（ HFSEs,such as Nb,Ta,Ti,etc.）,which show similar characteristics of island arc andesite. The host rocks are characterized by low87Sr/86 Sr initial values [（87Sr/86Sr）i= 0. 704798 ~ 0. 704919] and positive εNd（t） values（0. 7 ~ 1. 8）,indicate that the magma source has the characteristics of a weakly depleted mantle. Sandaowanzi gold deposit host rocks were generated under the geodynamic setting of large scale lithospheric extension-thinning in Early Cretaceous,the change in the direction of the Izanagi plate caused the northeast of China region in the extension tectonic setting. In the mean time,the upwelling of asthenosphere resulted in the partial melting of lower crust and lithosphere mantle,which formed the Late Mesozoic volcanic rocks of the Northeast China.