Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications
Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications
复制标题
DOI：
10.1007/bf00344081
复制
发表时间：
1992-05
影响因子：
3.5
通讯作者：
R. Pidgeon
中科院分区：
文献类型：
--
作者：
R. Pidgeon
振荡环带是酸性火成岩中锆石的一个共同特征，据信是在岩浆中锆石结晶过程中通过一种尚未了解的机制形成的。许多具有振荡环带的锆石还表现出环带被非环带锆石的拼凑替换。未分区的锆石呈圆形，海侵斑块分布在整个分区的锆石和过渡置换区，其中分区的锆石逐渐被未分区的锆石取代，只有微弱的痕迹，原来的区域仍然存在。这种结构被解释为一个渐进的再结晶的振荡带锆石不稳定的岩浆结晶过程中掺入高浓度的污染元素。重结晶覆盖了振荡区，似乎是在初级结晶完成后发生的。它伴随着U，Th和Pb的损失和振荡带的去除。重结晶的未分区锆石相对于后来的Pb损失是非常稳定的，并倾向于保留一致或稍微不一致的U-Pb年龄。重结晶提供了一种机制，重新设置锆石U-Pb年龄，这是独立的锆石晶格的辐射损伤程度。这不同于其他模型的不一致性，其中涉及的放射性铅的浸出作为一个结果的锆石结构的逐步分解，通过时间积分的辐射损伤进一步增强高浓度的微量元素污染物。未分带锆石的U-Pb年龄可以确定重结晶事件的年代，这可能接近于初次结晶的年龄或反映了后期的变质作用。
Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U−Pb age. Recrystallisation provides a mechanism for resetting zircon U−Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U−Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.