高温变泥质岩石中石榴石-黑云母地质温度计的应用——以胶北荆山群富铝岩石为例

APPLICATION OF THE GARNET-BIOTITE GEOTHERMOMETER IN HIGH GRADE METAPELITE:Al-RICH ROCK FROM THE JINGSHAN GROUP IN NORTH JIAODONG, CHINA

在对胶北荆山群麻粒岩相富铝岩石中石榴石、黑云母的成分环带进行深入研究基础上,选取不同粒径、与不同矿物相邻的石榴石、黑云母各微区点成分,利用石榴石-黑云母温度计分别进行了温度估算。确定在黑云母含量较高的岩石(V_(Grt)/V_(Bt)≤1)中,利用大颗粒石榴石(d≥1500μm)晶体核部(或靠近长英质矿物一侧的晶体幔部)成分与基质中远离石榴石等镁铁矿物处于长英质矿物之间的黑云母核部成分配合。通过石榴石-黑云母温度计可以获得相当可信的变质峰期温度。但是对于黑云母含量极低的岩石(V_(Grt)/V_(Bt)≥6),由于黑云母的成分普遍遭到了强烈改造。使得温度估算结果异常偏低,因此不适合采用石榴石-黑云母温度计估算峰期温度。同一岩石中,采用不同的相邻石榴石-黑云母矿物对晶体边缘成分获得的温度值差异较大,反映它们在峰期后发生Fe-Mg交换反应并达到封闭温度平衡状态的程度不同,因此利用石榴石-黑云母温度计难以获得准确的封闭温度。通过热力学计算,建立了一个新的石榴石-黑云母温度计公式。确定胶北荆山群所经历的变质峰期温度为720～770℃,峰期后最低相对封闭温度为480～500℃。

On the basis of detailed studies on the compositional zoning of garnet and biotite in Al-rich rocks from the Jingshan Group which had experienced granulite facies metamorphism in north Jiaodong, microdomain-compositions of garnets and biotites of different size and with different neighboring minerals are chosen to calculate metamorphic temperature by garnet-biotite geothermometer. The calculation indicates that, for Al-rich rocks with high biotite and low garnet contents (VGrt/VBt≤1), the core (or the mantle for the garnets next to felsic minerals) compositions of coarse-grained garnets and the core compositions of matrix biotites far away from garnet but scattered among felsic minerals can yield relia- ble peak metamorphic temperatures by the garnet-biotite thermometer. However, it is impossible for the rocks with extremely low content of biotite (VGrt/VBt>6) to be used to calculate the peak conditions because the peak biotites were strongly overprinted by cooling events. In a same sample, temperatures calculated from different rim compositions of garnets with the coupling biotite crystals are different from each other, suggesting that the Fe-Mg exchange between garnet and biotite did not reached equilibrium in the temperature decreasing post metamorphic peak. In this case, the garnet-biotite thermometer could not give a real closure temperature. According to thermodynamic calculations for the Fe-Mg exchange between garnet and biotite, a new formulation of garnet-biotite geothermometer has been obtained. The peak temperatures of the Jingshan Group ranging from 720 ℃to 770℃ and the lowest relative closure temperatures in post-peak stage ranging from 480℃to 500℃.