基于石榴石的变质岩年代学

程昊, 徐乃潇

Garnet Geochronology of Metamorphic Rocks

Hao Cheng, Naixiao Xu

图5 单颗粒微区石榴石耦合Lu-Hf/Sm-Nd定年 （a）白点线所示，通过微锯对1/4的石榴石单颗粒取样，获得的10份样品合并为5份用作同位素测试；(b)该直径为2.1 cm的石榴石按简单的一维模型冷却模拟Sm-Nd和Lu-Hf年龄差。灰色方框代表5份石榴石的Lu-Hf和Sm-Nd年龄平均差异，指示Lu-Hf年龄代表的是前进到峰期变质的时间。模拟结果显示经过峰期变质温度约720 oC后，样品经历了一个速率为2.0~1.5 °C/Ma的冷却过程；据参考文献［6］修改

Fig.5 Coupled Lu-Hf and Sm-Nd geochronology on a single microsampling garnet (a) Micro-sawing of a quarter of the mega garnet. Ten rectangle sections were micro-sawed to produce five garnet aliquots for isotope analysis indicated by the dotted white lines; (b) Sm-Nd age resetting profiles as a function of initial cooling rate and peak temperature for a 2.1 cm diameter garnet crystal. Square symbols represent the mean age differences between the calculated Lu-Hf and Sm-Nd ages of the five mega garnet zones, and implies that Lu-Hf ages solely represent prograde to peak metamorphic garnet growth. Note that to reset the Sm-Nd age of the garnet rim, would require peak metamorphic temperature of 720 °C and initial cooling rates of 2.0~1.5 °C/Ma, modified after reference[6]