Grain-size effects on the closure temperature of white mica in a crustal-scale extensional shear zone — Implications of in-situ 40Ar/39Ar laser-ablation of white mica for dating shearing and cooling (Tauern Window, Eastern Alps)
Grain-size effects on the closure temperature of white mica in a crustal-scale extensional shear zone — Implications of in-situ 40Ar/39Ar laser-ablation of white mica for dating shearing and cooling (Tauern Window, Eastern Alps)
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DOI：
10.1016/j.tecto.2016.02.014
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发表时间：
2016-04
期刊：
影响因子：
2.9
通讯作者：
A. Scharf;M. Handy;S. Schmid;S. Favaro;M. Sudo;R. Schuster;K. Hammerschmidt
中科院分区：
文献类型：
--
作者：
A. Scharf;M. Handy;S. Schmid;S. Favaro;M. Sudo;R. Schuster;K. Hammerschmidt
原位40 Ar/39 Ar激光烧蚀测年的白云母颗粒的样品从下盘的地壳规模的伸展断层（Katschberg正断层; KNF），容纳向东造山平行位移的阿尔卑斯造山地壳在东部的陶恩窗。这一测年主要产生冷却年龄范围从31到13百万年，与大多数年龄之间的聚类21和17百万年。早于主要Katschberg叶理的折叠白色云母，在误差范围内，与长在叶理上的白云母颗粒的年龄相同。然而，两个世代的绝对年龄在底部（2000万年），其粒度较大（300-500 μm），而在顶部和邻近该剪切带的上盘（1700万年），其粒度较小（< 100-300 μm）。KNF中白云母粒度的这种向上细化趋势与从底部（~ 445 °C）到顶部（< 400 °C）的闭合温度降低有关，并解释了下盘冷却年龄向下增加的反直觉趋势。新的数据表明，根据40 Ar/39 Ar白云母冷却年龄，东陶恩窗KNF内的快速冷却开始于2100万年前的某个时候，根据新的Rb/Sr白云母年龄，在25- 2100万年之间，即，大约2500万年前，陶恩窗达到热峰后不久。这些新的数据，结合文献数据，支持早期冷却在东部的Tauern窗口比西部的一些3-5万年。
In-situ40Ar/39Ar laser ablation dating of white-mica grains was performed on samples from the footwall of a crustal-scale extensional fault (Katschberg Normal Fault; KNF) that accommodated eastward orogen-parallel displacement of Alpine orogenic crust in the eastern part of the Tauern Window. This dating yields predominantly cooling ages ranging from 31 to 13 Myr, with most ages clustering between 21 and 17 Myr. Folded white micas that predate the main Katschberg foliation yield, within error, the same ages as white-mica grains that overgrow this foliation. However, the absolute ages of both generations are older at the base (20 Myr) where their grain size is larger (300–500 μm), than at the top and adjacent to the hangingwall (17 Myr) of this shear zone where grain size is smaller (< 100–300 μm). This fining-upward trend of white-mica grain size within the KNF is associated with a reduction of the closure temperature from the base (~ 445 °C) to the top (< 400 °C) and explains the counter-intuitive trend of downward-increasing age of cooling in the footwall. The new data show that rapid cooling within the KNF of the eastern Tauern Window started sometime before 21 Myr according to the40Ar/39Ar white-mica cooling ages and between 25–21 Myr according to the new Rb/Sr white-mica ages, i.e., shortly after the attainment of the thermal peak in the Tauern Window at ~ 25 Myr ago. These new data, combined with literature data, support earlier cooling in the eastern part of then Tauern Window than in the western part by some 3–5 Myr.