华北克拉通大青山地区古元古代晚期钾长伟晶岩脉SHRIMP锆石U-Pb定年和Hf同位素组成

报道了华北克拉通大青山地区哈德门沟钾长伟晶岩脉的锆石SHRIMP U-Pb年龄和Hf同位素组成。钾长伟晶岩脉广泛分布,不定向切割早期地质体。部分钾长石具出溶结构,利用三元长石温度计获得其形成温度为750～800℃。锆石粒度粗大,大多呈等轴状,锆石的阴极发图像中部具板状环带或扇形结构,边部出现振荡环带。SHRIMP锆石U-Pb定年,数据点位于谐和线上,207Pb/206Pb年龄加权平均值为1841Ma±9Ma(MSWD=1.5)。锆石的εHf(t)和tDM1(Hf)分别为-10.4～-6.6和2498～2648Ma。结合其它资料,可得出如下结论:①大青山地区至少在1.84Ga已进入伸展体制;②钾长伟晶岩脉形成于高温条件;③钾长伟晶岩脉的物质来自该区早前寒武纪变质基底。

Late Paleoproterozoic K-feldspar pegmatite veins in Daqingshan area, North China Craton: SHRIMP age and Hf composition of zircons

This paper reports SHRIMP age and Hf isotope composition of zircons from a K-feldspar pegmatite vein in the Wulanbulang-Hademengou area of southern Daqing Mountain,North China Craton.Pegmatite veins widespread in this area cut early Precambrian geological bodies.Some K-feldspar grains show exsolution structures with formation temperatures from 750 to 800℃ in terms of ternary-feldspar geothermometry.The zircons are large and mostly equant.In cathodoluminescence images,they show banded or sector zoning in the center of the crystal and oscillatory zoning in the outer domain.Data are distributed on concordia with a weighted mean 207Pb /206Pb age of 1841±9 Ma(MSWD=1.5).They have εHf(t) and tDM1(Hf) values ranging from-10.4 to-6.6 and 2498 Ma to 2648 Ma,respectively.Combined with data available,some conclusions can be drawn as follows: ① the tectonic environment changed from compression to extension at least before 1.84 Ga in the Daqing Mountain area;② the K-feldspar pegmatite veins were formed under the condition of high temperature;③ the K-feldspar pegmatite veins were derived from the early Precambrian basement.