Temporal variations in the composition of Cretaceous Cordilleran arc volcanism through a high-frequency record of bentonites
Temporal variations in the composition of Cretaceous Cordilleran arc volcanism through a high-frequency record of bentonites
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DOI：
10.1016/j.epsl.2022.117470
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发表时间：
2022-04
影响因子：
5.3
通讯作者：
S. Allen;Cin-Ty A. Lee;D. Minisini
中科院分区：
文献类型：
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作者：
S. Allen;Cin-Ty A. Lee;D. Minisini
大陆弧火山作用产生了各种各样的岩浆成分，但成分变化的节奏尚不清楚。在这里，我们调查了在北美西部西部内海沉积的白垩纪鹰滩群中700万年来的火山灰层记录。低氧条件阻止了显著的生物扰动，使得数百个薄薄的火山灰层以膨润土的形式得以保存。具有已建立的高分辨率年龄模型的钻芯提供了在100KYR时间尺度上调查火山灰成分的时间变化的机会。由于成岩过程中强烈的蚀变作用，由钛/锆值重建了火山灰原岩成分。我们首先证明了在成岩作用中，钛和锆的活动性是有限的。通过对6个大陆弧未蚀变火山岩资料的整理，发现不同时间、不同地理位置的大陆弧中，Ti/Zr与SiO_2的关系相似，证实了Ti/Zr是SiO_2的可靠分异指标和间接量度。应用钛/锆-二氧化硅经验关系对52个鹰福特膨润土进行了钛/锆测量，重建了火山灰原岩二氧化硅。灰分组成从玄武岩/玄武岩安山岩到英安岩/流纹岩，但变化不是随机的。在∼100KYR时间尺度上，火山灰成分在高二氧化硅(>；60wt.%二氧化硅)和低二氧化硅(<；60wt.%)火山活动期间波动。如果这些火山灰的时间变异性代表了科迪勒大陆弧的广泛快照，这些结果表明大陆弧系统可能在快速(<；100Kyr)时间尺度上经历岩浆分异程度或喷发性质的幕式变化。
Continental arc volcanism generates a wide diversity of magma compositions, but the tempos of compositional variation are unclear. Here, we investigate a 7-million year record of volcanic ash layers in the Cretaceous Eagle Ford Group, which was deposited in the Western Interior Seaway in western North America. Low oxygen conditions prevented significant bioturbation, allowing for the preservation of hundreds of thin volcanic ash layers in the form of bentonites. A drill core with an established high-resolution age model provides an opportunity to investigate temporal changes in ash composition on <100 kyr timescales. Because of intense alteration during diagenesis, ash protolith compositions were reconstructed from Ti/Zr. We first show that Ti and Zr had limited mobility during diagenesis. We then show, based on a compilation of data from unaltered volcanic rocks from six continental arcs, that the relationship between Ti/Zr and SiO2is similar across continental arcs from different times and geographic locations, confirming that Ti/Zr is a robust differentiation index and indirect measure of SiO2. Applying an empirical Ti/Zr-SiO2relationship to Ti/Zr measurements of 52 Eagle Ford bentonites allowed for reconstruction of ash protolith SiO2. Ash compositions vary from basalt/basaltic andesite to dacite/rhyolite, but the variations are not random. Ash compositions fluctuate between periods of high silica (>60 wt. % SiO2) and low silica (<60 wt. %) volcanism over ∼100 kyr timescales. If the temporal variability of these ashes represents broad snapshots of the Cordilleran continental arc, these results suggest that continental arc systems may undergo episodic changes in the extent of magmatic differentiation or the nature of eruption on rapid (<100 kyr) timescales.