Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians
Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians
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DOI：
10.1016/j.lithos.2016.03.003
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发表时间：
2016-06
期刊：
影响因子：
3.5
通讯作者：
D. Kellett;C. Warren;K. Larson;H. Zwingmann;C. V. Staal;N. Rogers
中科院分区：
文献类型：
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作者：
D. Kellett;C. Warren;K. Larson;H. Zwingmann;C. V. Staal;N. Rogers
白云母40ar /39Ar分析可能对构造过程的时间提供有用的约束，但复杂的地质和热历史可能以各种方式扰乱Ar系统。韧性剪切带为探索白云母动态重/新结晶与同期增强流体流动之间的联系及其对40ar /39Ar年代的影响提供了极好的研究案例。在加拿大纽芬兰阿巴拉契亚造山带多佛断层（Dover Fault）附近，采集了成分相似的花岗岩的白色云母40ar /39Ar数据，这些花岗岩记录了不同的变形时期，40ar /39Ar数据采用激光烧蚀和逐级加热单晶法收集。结果相互比较，并针对互补的U-Pb锆石和独居石，以及K-Ar断层泥分析。虽然step-heat40Ar/39Ar是一种广泛应用于造山带背景的方法，但该数据表明，相对平坦的step-heat40Ar/39Ar光谱与来自同一样品的广泛分布的原位40ar /39Ar日期相矛盾，并且在某些情况下，高原日期产生的混合日期具有模棱两可的地质意义。这一结果表明，白云母中Ar的逐步释放可能使剪切岩石中白云母中空间控制的Ar同位素储层均匀化和模糊化。相比之下，在激光烧蚀中，40ar /39Ar分析保留了40ar储层的空间分辨率，这些储层因变形和流体相互作用而发生了变化。因此，这项研究表明，激光烧蚀是确定白云母记录的变形时间的最佳方法。对结果的最终解释应以显微结构分析、变形温度估计、白色云母的化学特性和互补的计时器为指导。总体而言，花岗岩原岩位于430 ~ 422 Ma （U-Pb锆石）之间。沿Wing Pond剪切带的高应变变形发生在约422 ~ 405 Ma （U-Pb单氮石和40ar /39Ar）之间。在低T切变（< 400℃）和变形后流体相互作用期间，白色云母中局部出现了斑片状Ar损失。在约385 Ma （40Ar/39Ar）时，多佛断层（Wing Pond剪切带的一个窄段）发生了低温再激活。宽共线性脆性Hermitage湾断层断层泥中自生伊利石的K-Ar定年表明，沿地界的滑动至少持续到密西世。
White mica40Ar/39Ar analyses may provide useful constraints on the timing of tectonic processes, but complex geological and thermal histories can perturb Ar systematics in a variety of ways. Ductile shear zones represent excellent case studies for exploring the link(s) between dynamic re-/neo-crystallization of white mica and coeval enhanced fluid flow, and their effect on40Ar/39Ar dates. White mica40Ar/39Ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the Dover Fault, a terrane-bounding strike-slip shear zone in the Appalachian orogen, Newfoundland, Canada.40Ar/39Ar data were collectedin situby laser ablation and by step heating single crystals. Results were compared to each other and against complementary U–Pb zircon and monazite, and K-Ar fault gouge analysis.Although step-heat40Ar/39Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat40Ar/39Ar spectra are in contradiction with wide spreads inin-situ40Ar/39Ar dates from the same samples, and that plateau dates in some cases yielded mixed dates of equivocal geological significance. This result indicates that the step-wise release of Ar from white mica likely homogenizes and obscures spatially-controlled Ar isotope reservoirs in white mica from sheared rocks. In contrast,in situlaser ablation40Ar/39Ar analysis preserves the spatial resolution of40Ar reservoirs that have been variably reset by deformation and fluid interaction. This study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica. Final interpretation of results should be guided by microstructural analysis, estimation of deformation temperature, chemical characterization of white mica, and complementary chronometers.Overall the dataset shows that granitic protoliths were emplaced between 430 and 422 Ma (U–Pb zircon). High strain deformation along the Wing Pond Shear Zone occurred between ca. 422–405 Ma (U–Pb monazite and40Ar/39Ar). Subsequent patchy Ar loss in white mica occurred locally during low T shear (< 400 C), and via post-deformation fluid interactions. Low-temperature reactivation of the Dover Fault, a narrow segment of the Wing Pond shear zone, occurred at ca. 385 Ma (40Ar/39Ar). K-Ar dating of authigenic illite in fault gouge from the broadly co-linear brittle Hermitage Bay Fault indicates that slip along the terrane boundary persisted until at least the Mississippian.