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摘要
Charoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 +/- 2.4Ma, K-Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by Ar-40/Ar-39 incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation Ar-40/Ar-39 for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 +/- 2.9Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 +/- 3.4Ma for charoitite K-feldspar overlaps with the youngest of published K-Ar ages (112 +/- 5Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 +/- 3Ma to 115.7 +/- 4.3Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.
- 出版日期2014-6
- 单位中国科学院地质与地球物理研究所; 中国科学院地质与地球物理研究所兰州油气资源研究中心
