Origin of migmatite granite and its relations to iron deposits—An experimental study

Experiments have been conducted over a wide range of temperatures and pressures (300°–950°C, 500–2000 bars) in an attempt to gain some better understanding of the geological features of Precambrian migmatite granites and banded iron formation (BIF) widespread in Northeast China. Results indicate that the BIF of Anshan type is unstable under the action of sufficient alkaline solutions with some iron or aluminum silicates formed at the expense of quartz. Rock melting experiments show that the initial melting temperatures of phyllite, biotite plagioclase gneiss and migmatite granite range from 630° to 640°C, but those of BIF and plagioclase amphibolite are 150°–200° C higher. The authors suggest that migmatites in this region have resulted from partial melting and metasomatism. With respect to the relationship between migmatization and iron deposits, it has been experimentally revealed that the migmatites will not “digest” the BIF if they are separated by other wall rocks. But melting to various extents will take place in the BIF where migmatites are in direct contact with it. Additionally, the BIF will even become richer in iron under favorable conditions. This information may probably help guide our efforts to prospect for Precambrian iron deposits in regions with extensive occurrence of migmatites.