花岗岩体高温热年代学研究的新思路、方法及计算实例

摘要

对国内外花岗岩体723对锆石U-Pb年龄（tZr）和全岩Rb-Sr等时线年龄（tRb）进行的相关分析,拟合出相关系数很高（R=0.997）,回归系数接近l的线性回归方程（tZr=1.0005×tRb+0.493041）。ΔtZr-Rb（tZr-tRb）频数统计分析表明：ΔtZr-Rb呈对称正态分布（偏度系数CSK=0.193；峰度系数CKU=6.722）,其均值为0.624 Ma,众数值为1.0 Ma。这表明花岗岩体锆石U-Pb定年的测定结果与全岩Rb-Sr等时线定年测定结果在允许的误差范围内是一致的。不存在花岗岩体锆石U-Pb年龄必定大于全岩Rb-Sr等时线年龄的规律表明，同位素热年代学方法只适用于研究花岗岩结晶固结后的低温热演化史。前人根据锆石U-Pb年龄和全岩Rb-Sr等时线年龄差值及相应同位素体系封闭温度研究的10个花岗岩体的冷却速率（CRZr-Rb）表明，它们与岩体体积尺度不相关，这有悖于“热物体的体积（质量）愈大，则在相同热物理条件下其冷却速率愈小”的热物理学基本定律。根据热传导理论及本文作者（2010）提出的侵位结晶时差概念我们得出“在相同热物理学条件下，体积尺度是决定花岗岩体冷却速率最主要因素”的结论。以上述10个花岗岩体为例，本文计算得出它们在结晶固结前高温阶段的冷却速率（CRECTD）并拟合出冷却速率与岩体体积尺度呈幂函数关系：CRECTD=7544.7×D-2.1686,计算结果符合热物理学基本定律。%Using least squares regression procedure, a best regression equation (tZr=1.0005×tRb+0.493041) with high correlation coefficient (R=0.997) is fitted for 723 pairs of zircon U-Pb ages (tZr) and whole rock Rb-Sr ages (tRb) of granite plutons. The frequency analysis of 723 individual values of differences between tZr and tRb (ΔtZr-Rb) for granites shows symmetrical normal distribution (skewness CSK=0.193;kutrocess CKU=6.722) with the Mean of 0.624 Ma and the Mode of 1.0 Ma. These statistical characteristics indicate that for the granites as a whole, both the zircon U-Pb dating ages and the whole-rock Rb-Sr isochron ages are consistent within permissible errors. However, because of a time difference between emplacement age and crystallization-solidification age, this isotope thermochronological method can be applicable only for the stage after their crystallization-solidification. Based on the differences between zircon U-Pb ages and whole-rock Rb-Sr isochron ages and the closure temperatures of corresponding isotopic systems, previous researchers studied the cooling rates (CRZr-Rb) and thermal evolution history of 10 granite plutons, but these cooling rates are incorrelate to the volume change of granite plutons and contrary to the basic law of thermophysics, i.e., the cooling rate of a larger hot body should be slower compared with a smaller one. In this paper, based on the theory of thermal conductivity and using the concept of time difference between emplacement age and crystallization-consolidation age, developed by present authors (2010), we come to a conclusion that, at the same thermo-physical conditions, the volume scale is the most important parameter that controls cooling rate of the granite plutons. On the example of existing 10 granites, and for the high temperature thermal stage from magma emplacement to magma crystallization-consolidation, we derived a power function regression equation to describe the relationship of cooling rate with volume parameter D:CRECTD=7544.7×D-2.1686. The calculated results are obviously more reasonable and evidently conformable to the basic law of thermophysics..