DEHYDRATION-MELTING EXPERIMENT OF THE BIOTITE-GNEISS,EASTERN HEBEI,AT 1 GPa PRESSURE
Wu Zongxu1, Deng Jinfu2, P. J. Wyllie3, S. Newman3
1. Institute of Geology, State seismological Burean, Beijing 100029; 2. China University of Geosciences. Beijing 100083; 3. Division of Geological and Planetary Sciences, California Institute of Technology, USA
This paper presents phase relationships of dehydration-melting experiment of the biotite——gneiss, Eastern Hebei Province, China. The temperatures of the solidus, biotite-out, pyroxen-in and amphibole-out, are 812℃, 837℃, 862℃ and 887℃, respectively. The experimental results in tills paper show that the resulting granitic magma at temperature of 812-950℃ is water-undersaturated. The fact of the waterundersaturated granitic magma dominated in the nature suggests that the formation of the granitic magma from lower crust is controlled mainly by the mechanism ofdehydration-melting. The dehydration-melting of the biotite gneiss of Eastern Hebeiat IGPa pressure shows that after the generation of 20%-30% of granitic magma, the refractory residue is Opx+Cpx+Gax+Ga+Qz+Am. The latter is consistent with the assemblage of themineral assemblage of the granulite metamorphic factes, which suggests that the refractory residue after removing the granitic melts by the melting event of the lower continental crust will be the granulite-factes rock. It also showsthat the granulite is lesulted at least partly from the dehydration-melting which is satisfied with the water——undersaturated condition for the formation of the granulite. The upper amphibolite factes with the development of migmatization at the depth of 10-20km is formed at a temperature of about 650-700℃ where both the biedte and amphibole are stable and muscovite is absent. However, the lower amphibolite factes without migmatization is characterized by the developing of muscovite. It seems that the migmatization widespread by occurred in the upper amphibolite factes regions is dliven mainly from the muscovite-dehydration-melting.