白云鄂博矿床基底岩石的年代学研究及其地质意义

Chronology and geological significance of the basement rock of the giant Bayan Obo REE-Nb-Fe ore deposit

白云鄂博裂谷带发育于古老基底之上，并在裂谷带中形成了白云鄂博超大型REE—Nb—Fe矿床。但是由于古老基底在白云鄂博矿区出露较少，研究工作开展不多。本研究对象是华北克拉通北缘白云鄂博地区宽沟断裂东段被第四纪沉积层覆盖的冲沟中的基底片麻岩。主元素特征是，SiO2含量介于4707％～7910％之间，低钛，低钙，Al2O孙Fe2O3和MgO含量变化较大，全碱含量为335％～751％，样品的里特曼指数为0.31-13.9，A／CNK比值为124-265；微量元素特征是，岩石富集大离子亲石元素，亏损高场强元素，轻稀土元素富集，重稀土元素亏损和Eu负异常等。原岩恢复图解表明基底岩石中既有正片麻岩，又有副片麻岩。同时，基底的地质与地球化学性质说明基底与成矿的关系不大。本次研究两个岩石样品给出的锆石SIMSu—Pb年龄介于（2914±20）Ma和（19460±44）Ma之间。所有年龄大致分布在谐和线上的三个区域，大部分年龄集中在20Ga附近。20Ga附近的年龄又可以大致分为三段，1950-2000Ma、2000-2050Ma和2050-2100Ma。第一段的年龄测点分布于锆石最边部，ThAJ比值较低，可能代表了岩石的变质年龄，第二段与第三段的年龄测点分布于锆石的幔部与核部，Th／U比值较高，可能代表了岩石的形成年龄。对后两阶段的年龄进行加权平均，得出的平均年龄为（2029±54）Ma，即为岩石的形成年龄，高的Th／U比值显示该年龄反映了该区的一次重要的岩浆作用事件，该年龄还限定了白云鄂博群沉积地层的最老年龄。其他两个区域（约24Ga和约29Ga）的年龄也可能分别反映了当时的岩浆作用，同时也为白云鄂博裂谷带的发育过程和华北克拉通的演化历史提供了新的年龄制约。

Abstract： The Bayan Obo rift valley zone, the host of the giant Bayan Obo REE-Nb-Fe ore deposit, developed on an old basement. The studied basement rocks were sampled from a newly exposed gully along the Kuangou fault, where the basement rock is covered by Quaternary sediments. These samples have low TiO2 and CaO with varied major elements, e.g., SiO2 ranges between 47.07%-79.10%, total alkali between 3.35%-7.51%, R-index （o-= （K20＋Na20）2/（SiO2-43）） between 0.31-13.9, and the A/CNK ratio, 1.24-2.65. They are enriched in LILE and LREE, and depleted in HFSE and HREE, with negative Eu anomaly, which implies the basement rocks originated from crustal sources. Protolith recovery diagrams indicate that the samples contain both orthogneiss and paragneiss.Also, the basement is not related to minemlization, based on the geologic and geochemical characteristics. The SIMS zircon U/Pb ages range from 2914±20 to 1946.0±4.4 Ma. Most of the ages distribute on the concordant line. and can be grouped into 3. Most ages are close to 2.0 Ga, which can be divided into 3 subgroups： 1950-2000 Ma, 2000-2050 Ma, 2050-2100 Ma. The first one most likely represents metamorphic age, as they distribute on the edge of zircons, with low Th/U. The others likely represent formation age of rocks （with a weighted mean of 2029±54 Ma）, as they distribute on the mantles or cores of zircon grains, with high Th/U, representing the formation age of the basement. It also gives the upper limit of Bayan Obo group stratum. The other ages also reflects two magmatism events as suggested by high Th/U. These ages provide new constraints on the geologic evolution of the Bayan Obo rift valley zone and the history of North China craton.