Anomalously old biotite 40Ar/39Ar ages in the NW Himalaya
Anomalously old biotite 40Ar/39Ar ages in the NW Himalaya
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DOI：
10.1130/l586.1
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发表时间：
2017-06
期刊：
影响因子：
2.4
通讯作者：
K. Stübner;C. Warren;L. Ratschbacher;B. Sperner;R. Kleeberg;J. Pfänder;D. Grujic
中科院分区：
文献类型：
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作者：
K. Stübner;C. Warren;L. Ratschbacher;B. Sperner;R. Kleeberg;J. Pfänder;D. Grujic
黑云母40 Ar/ 39 Ar年龄大于相应的白云母40 Ar/ 39 Ar年龄，与这些矿物的扩散特性相反，在喜马拉雅和其他变质带中很常见。在这些情况下，黑云母40 Ar/ 39 Ar年龄通常被认为“太老”，因为“过量的Ar”。我们从印度喜马偕尔邦喜马拉雅地区中部的17个样品中获得了32个阶梯加热40 Ar/ 39 Ar年龄。在几乎所有的情况下，黑云母的年龄都比根据冷却历史预测的要早。我们记录了寄主岩石的岩性和化学组成，云母的微观结构，黑云母的化学组成，以及黑云母分离物的绿泥石和白云母成分，以证明这些因素不能解释40 Ar/ 39 Ar年龄异常古老的黑云母。我们讨论了可能的机制，可以解释这些样品中外来的Ar（遗传或过量的Ar）。造成“太老”黑云母的最可能原因是过量的Ar，即与母岩k分离的40 Ar。我们认为这种污染是由以下一种或几种机制造成的：(1)40 Ar是在新生代前进变质作用中释放的；(2)晶间介质暂时干燥，限制了40 - Ar输运；(3)熔体在结晶过程中析出40 Ar进入含水流体相。来自主中央逆冲剪切带的样品可能受到不同的过量氩富集机制的影响，可能与剪切带后期流体循环和绿泥化作用有关。黑云母和白云母中不同的Ar扩散率和/或溶解度可以解释为什么黑云母比白云母更容易受到过量Ar的影响。
Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as “too old” on account of “excess Ar.” We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for “too-old” biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.