Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses
Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses
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DOI：
10.1016/j.epsl.2013.03.002
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发表时间：
2013-05
影响因子：
5.3
通讯作者：
S. Schneider;K. Hammerschmidt;C. Rosenberg
中科院分区：
文献类型：
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作者：
S. Schneider;K. Hammerschmidt;C. Rosenberg
我们尝试通过对来自西陶恩窗（东阿尔卑斯山）左旋剪切带的相同样本内的运动前白云母、黑云母和钾长石、同运动白云母和钾长石以及后运动白云母进行原位40Ar/39Ar织构控制分析，改善对延性剪切带寿命和终止的时间限制。另外两个样品通过 Rb/Sr 方法（微量取样）进行了年代测定。通过原位 40Ar/39Ar 分析证实了基于微观结构、横切关系的矿物形成的相对顺序，表明同运动矿物一般比同一样品的前运动矿物年轻，而比后运动矿物年龄更老或年龄相同。从西Tauern Window的边缘到核心，同运动白云英岩和钾长石揭示了一组变化的形成年龄，最北端和外围剪切带（Ahorn剪切带）为33至15Ma，中间剪切带网络（Tuxer剪切带）为24至12Ma，最南端和中部剪切带（Greiner剪切带）为20至7Ma。同步运动白白云岩和钾长石分析的年龄变化大于所获得的每个年龄的分析误差。此外，同运动矿物的等时线计算揭示了类似大气的 40Ar/36Ar 截距。因此，所获得的同运动矿物的年龄值被解释为形成年龄，该年龄是长期持续变形期增量的日期。每个剪切带系统的变形时间范围由同运动多白硅岩和钾长石的最古老和最年轻的形成年龄划分。后运动多硅白云石板条显示出最年轻的形成年龄，并与最年轻的同运动形成年龄重叠。这种关系表明，在延性左旋剪切结束时，同运动矿物形成后立即发生后运动生长。因此，变形的终止是根据这些后运动白云岩爆炸的年龄来确定的。前运动矿物的特征是分解和解溶反应，它们的年龄值是异质的，并且经常受到外来氩的存在的影响。这些年龄值通常比同运动矿物的年龄更老，但有时与之重叠。利用前运动矿物年龄、同运动矿物年龄和后运动矿物年龄获得的时间约束，我们可以评估Ahorn剪切带19Myr（33-15Ma）、Tuxer剪切带13Myr（24-12Ma）和Greiner剪切带14Myr（20-7Ma）同运动矿物形成的部分重叠时间间隔。这表明陶恩窗西部的韧性剪切带从边缘的较低变质点向中心的较高变质点连续定位和扩展。
We attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally-controlled in situ40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral shear zones from the western Tauern Window (Eastern Alps). Additionally two samples were dated by the Rb/Sr method (microsampling). Relative sequences of mineral formation based on microstructural, cross-cutting relationships were confirmed by in situ40Ar/39Ar analyses, showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample. From the rim to the core of the western Tauern Window syn-kinematic phengite and K-feldspar reveal a set of formation ages varying between 33 and 15Ma for the northernmost and peripheral shear zone (Ahorn Shear Zone), between 24 and 12Ma for the intermediate shear zone network (Tuxer Shear Zones), and between 20 and 7Ma for the southernmost and central shear zone (Greiner Shear Zone). The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar. Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the termination of deformation is dated by the ages of these post-kinematic phengite blasts. Pre-kinematic minerals are characterized by break down and exsolution reactions and their age values are heterogeneous and often affected by the presence of extraneous Ar. These age values are usually older than, but sometimes overlapping with, ages of the syn-kinematic minerals. Using the temporal constraints obtained by the ages of pre-, syn-, and post-kinematic minerals, we could assess partly overlapping time intervals of syn-kinematic mineral formation of 19Myr (33–15Ma) in the Ahorn Shear Zone, 13Myr (24–12Ma) in the Tuxer Shear Zones and 14Myr (20–7Ma) in the Greiner Shear Zone. This indicates successive localization and propagation of ductile shear zones in the western Tauern Window from lower metamorphic sites at the rim towards higher metamorphic sites in the center.