Serrated quartz grain boundaries, temperature and strain rate: testing fractal techniques in a syntectonic granite
Serrated quartz grain boundaries, temperature and strain rate: testing fractal techniques in a syntectonic granite
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锯齿状石英晶界、温度和应变率：在同构造花岗岩中测试分形技术
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发表时间：
2010
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通讯作者：
R. O. Greiling
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作者：
M. A. Mamtani;R. O. Greiling
摘要石英晶界缝合线的分维（Dr）和石英颗粒的面积周长分维（Da）分别与温度（T）和应变率有关。然而，这些方法的应用，以测量温度和应变率在自然变形侵入岩尚未进行过测试。在本研究中，博士和Da计算12个薄切片从同构造花岗岩（戈得拉花岗岩，印度）的不同部分。其中，六个属于花岗岩的北方部分，两个属于花岗岩的中部，四个属于花岗岩的南部。早期对戈得拉花岗岩的研究显示出应变和温度梯度，北部温度高，南部应变高。显微结构的研究表明，石英晶界缝合线是锯齿状的北方样品相比，从花岗岩的其余部分。北方样品含有丰富的高温固态变形组构，形成于675 ~ 725 °C之间（石英棋盘图案温压法）。使用早期工作者给出的Dr与T的关系图，预计上述T范围的Dr值为1.05-1.14。对北方样品的石英缝合线进行Dr计算，得出中值为1.11，大多数缝合线的Dr <1.14。这些数据与石英棋盘图案在戈得拉花岗岩中形成的预期温度范围吻合得很好。花岗岩的中部和南部主要是myrmekites（500-670 °C），重结晶的长石（450-600 °C），长石中的变形孪晶（400-500 °C）和扭结黑云母（<300 °C）。石英缝线在上述中低温范围内的预期Dr分别为1.07-1.23、1.11-1.25、1.16-1.28和<1.27。Dr计算结果表明，中部和南部石英缝合线的Dr值大多大于1.14，中间值分别为1.18（中部）和1.17（南部）。使用Dr v. T图，这些Dr值表明，戈得拉花岗岩中部和南部的大部分结构形成于450-600 °C的温度范围内，这与控制花岗岩这一部分的中低温组构发展所需的温度范围非常吻合。因此，石英缝合线的Dr值可以作为同构造花岗岩的地质温度计。北方和南方样品的Da分别为1.10和1.14。使用面积-周长分形维数（Da）值，分别获得高温（675 °C）和低温（300 °C）下约10−7和10−11 s−1的应变率。虽然这些数值高于自然界已知的地质应变率（10−12-10−15 s−1），但低温范围的计算值与侵入体的估计应变率（10−10-10−12 s−1）相似。计算结果表明，用石英颗粒的Da值计算应变率的方法不能给出同构造花岗岩在高温下的地质合理的应变率。然而，该方法可能有助于获得合理的应变率估计较低的温度。
Abstract In the past fractal (ruler) dimension (Dr) of quartz grain-boundary sutures and area–perimeter fractal dimension (Da) of quartz grains, respectively, have been shown to depend on temperature (T) and strain rate. However, the application of these methods to gauge temperature and strain rate in naturally deformed intrusive rocks has not yet been tested. In the present study Dr and Da are calculated in 12 thin sections from different parts of a syntectonic granite (Godhra Granite, India). Of these, six belong to the northern part, two to the central part and four to the southern part of the granite. Earlier work on the Godhra Granite showed both a strain and a temperature gradient, with high temperature in the north and high strain in the south. Microstructural studies reveal that the quartz grain-boundary sutures are less serrated in the northern samples compared to those from the remaining part of the granite. The northern samples contain abundant high-temperature solid-state deformation fabrics that formed between 675 and 725 °C (quartz chessboard pattern thermobarometry). Using a Dr v. T plot given by earlier workers, a Dr value of 1.05–1.14 is expected for the above T range. Dr calculations of quartz sutures from the northern samples give a median of 1.11 and most of the sutures have Dr <1.14. These data fit well with the expected temperature range in which the quartz chessboard pattern formed in the Godhra Granite. The central and southern parts of the granite are dominated by myrmekites (500–670 °C), recrystallized feldspars (450–600 °C), deformation twins in feldspar (400–500 °C) and kinked biotite (<300 °C). The expected Dr of quartz sutures under the above medium–low temperature ranges are 1.07–1.23, 1.11–1.25, 1.16–1.28 and <1.27, respectively. Dr calculations reveal that most of the quartz sutures from the central+southern part have Dr >1.14, and the median values are 1.18 (centre) and 1.17 (south). Using the Dr v. T plot, these Dr values indicate that most of the textures in the central+southern part of the Godhra Granite formed in the temperature range of 450–600 °C, which fits well with the temperature range required for the development of medium–low temperature fabrics that dominate this part of the granite. Thus, it is concluded that Dr of quartz sutures can be used as a geothermometer in syntectonic granites. Da for northern and southern samples is 1.10 and 1.14, respectively. Strain rates of the order of approximately 10−7 and 10−11 s−1, respectively, are obtained for high (675 °C) and low temperature (300 °C) using area-perimeter fractal dimension (Da) values. Although these are higher than geological strain rates that are known in nature (10−12–10−15 s−1), the calculated values for the lower-temperature range are similar to strain rates estimated for intrusions (10−10–10−12 s−1). The calculations indicate that the method to calculate strain rate using Da of quartz grains fails to give geologically reasonable strain rates for high temperature in a syntectonic granite. However, the method maybe useful in obtaining reasonable strain rate estimates for lower temperatures.