Disequilibrium Melting of Granite at the Contact with a Basic Plug: A Geochemical and Petrographic Study

Abstract

Mantle-derived intrusions are potentially an important source of heat for melting in the crust, yet there are few studies of anatectic melts occurring at the contacts of such intrusions to provide natural analogs. This is a study of anatectic melts developed in a Cretaceous granite at the contact with a 12 Ma trachyan-desite plug in the Sierra Nevada batholith, California, where a detailed textural and geochemical study was possible because the melt is now represented as a glass that has undergone limited devitrification. A Rb-Sr isochron age of Ma for glasses and granite whole rocks is in agreement with the K-Ar age of Ma for a similar trachyandesite plug nearby, confirming that the melting was associated with the trachyandesite intrusion. Melting occurred at about 900°C or higher and at less than 1 kb. The restite minerals consist dominantly of zoned plagioclase and sanidine (both have rims of anorthoclase of similar composition), quartz, and magnetite. The whole-rock major and trace element abundances are essentially the same for rocks with 0-56% modal glass. This suggests that the melt did not separate from its restite, even though the melted granite clearly shows flow texture. The glasses have REE patterns subparallel to those of the whole rock, with abundances similar to or lower than those of the whole rock. The high REE abundances of residual magnetite prevented an increase in the REE abundance of the glasses relative to that of the rock. Most samples have both clear and brown glass grading into each other. The clear glass appears to have developed earlier, at lower extents of melting. Only the cations with high diffusion rates have the same abundances in the two glasses, which suggests that melting and quenching occurred over a period of weeks to months. On longer timescales, corresponding to more deep-seated crustal anatexis, a greater degree of cation homogenization between co-existing melts is expected.