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首页> 外文期刊>Contributions to Mineralogy and Petrology >Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera
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Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

机译：伊利石/白云母 40 Ar / 39 Ar年龄谱在渐进性低等变质过程中的变化：以美国山脉为例

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40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.
机译：收集了来自美国西部的富含伊利石和白云母的寒武纪粉质岩的 40 Ar / 39 Ar阶跃加热数据从浅成岩带（沸石相）到表生带（绿片岩相）。随着变质等级的增加， 40 Ar / 39 Ar释放光谱的最大年龄降低，总气体年龄和保留年龄也降低。先前的研究已经解释了类似的结果，这些结果主要或全部来自碎屑白云母的溶解和新形成的伊利石的沉淀/重结晶。在认识到这些过程对评估结果的重要性的同时，我们建议，表观年龄与变质等级之间的反相关关系主要是通过热活化体积扩散来控制的，这与在许多钻孔中的测温仪观察到的深度随深度的减少类似实验。我们的结果表明，伊利石/白云母Ar温度计时计的完全复位发生在高棘足动物区和附生区之间，或大约在300°C处。该经验结果与先前基于白云母扩散参数的计算结果一致，后者表明半径为0.05–2μm的白云母晶粒的封闭温度应在250至350°C之间。在较高的裂殖区条件下，我们观察到年龄/晶粒尺寸关系的逆转（最细的晶粒尺寸产生了最古老的表观年龄），这可能标志着新形成的伊利石/白云母微晶通常超过碎屑白云母晶粒的尺寸。这也是新形成的伊利石/白云母微晶发展出足够的Ar保持力以产生具有地质意义的 40 Ar / 39 Ar年龄的阶段。寒武纪地层取样样面的结果建立了一个解释伊利石/白云母 40 Ar / 39 Ar年龄光谱在低等变质不同阶段的框架，并阐明了这种转变伊利石到白云母。在内华达州弗朗西斯山（Frenchman Mtn。），寒武纪明亮天使组处于沸石相条件下，伊利石/白云母 40 Ar / 39 Ar数据表明碎屑白云母组分具有明显的表观特征。年龄≥967Ma。相关的卡拉拉Fm。处于东部加利福尼亚州的Panamint和Resting Spring Ranges的裂殖带条件下，在这些位置，伊利石/白云母 40 Ar / 39 Ar数据表明亚二叠纪早二叠纪发生相变质作用。内华达州南部Bullfrog / Fluorspar峡谷脱离活动下盘的落岩带条件（绿片岩相）等价地层的相同类型的数据显示，白垩纪晚期冷却较为缓慢。

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