|
Xuexiang Qi, Institute of Geology, CAGS (China)
Zhiqing Xu, Institute of Geology, CAGS (China)
|
|
The ilmenite-magnetite exsolutions in eclogite, peridotite and paragneiss from the CCSD-MH has the typical exsolutional lamellae of ilmenite and magnetite. The end member mineral components in ilmenite lamellaes are FeTiO3(70.6∼93.45mol.percent) , MgTiO3 (0.55∼25.68 mol.percent), and little CoTiO3, MnTiO3, Cr2O3 and Al2O3, and that in magnetite lamellaes are Fe2TiO4?i1.17∼62.63 mol. percent?j, FeFe2O4?i20.81∼90.26 mol. percent?j, MgFe2O4?i0.08∼12.98 mol. percent?jand little CoFe2O4?ANiFe2O4?AMnFe2O4?AFeAl2O4?AFeCr2O4. The exsolved temperatures for ilmenite-magnetite exsolutions from eclogite, peridotite and paragneisses are 1342∼809°C, 578∼526°C and 980∼706°C, and the oxygen fugacity (-logfo2) for them are 14.33∼5.97,22.4∼19.94 and 17.5∼10.9, respectively. Of them, there is typical zoning decreasing from core to rim within ilmenite-magnetite exsolution for eclogite. In logfo2-T, it is observed that the data plots for eclogite lie in the area between FMQ and NNO curves, but that in core on FMQ curve; the data plots for paragneisses lie between the FMQ and NNO curves, and for peridotite on NNO curves, which show that the exsolution extent and Δlogfo2(logfo2-FMQ) in the core of ilmenite-magnetite exsolution for eclogite are lower than that in its rim; the exsolution degree and Δlogfo2 for paragneisses are the same with that in exsolution rim for eclogite, and lower than that for peridotite. To sum up, the discovery of moissanite inclusion and the former research indicate the homogeneous solid solution of ilmenite-magnetite formed in the peak stage metamorphism (T1342°C), and the exsolution begins at 1007∼1342°C. The exsolution of ilmenite-magnetite from eclogite and paragneisses continues until 706°C, but that from peridotite stops at 526°C. The uplift rate for Sulu ultrahigh pressure metamorphic belt in the first exhumation stage is minor according to the speculation based on the fact that the exsolution of ilmenite-magnetite is a slow process of temperature decreasing and pressure releasing. The temperature and pressure decreased more rapidly with increasing uplift rate, resulting in the end of exsolution for ilmenite-magnetite in eclogite and paragneisses, but the exsolution of ilmenite-magnetite in peridotite continued until 526°C for high PH2O and Δlogfo2 in system, showing that is a slow exhumation and temperature decreasing from 1342°C to 706°C, and a very rapid exhumation and temperature decreasing from 706°C to 526°C. In a word, the UHP metamorphic rocks had experienced 3 stages, such as peak metamorphism (1342°C), slow exhumation retrogressive metamorphism (1342∼706°C) and rapid exhumation retrogressive metamorphism (706∼526°C).
|
