Whole-rock rare-earth element (REE) abundances in the compositionally zoned Tuolumne Intrusive Series vary systematically from marginal quartz diorites with light REE at 60 times chondritic abundances and heavy REE at 8 times chondritic abundances to core granites with light REE at 100 times chondritic abundances and heavy REE at less than 3 times chondritic abundances. Similar highly fractionated REE distributions have been previously interpreted as resulting from (1) partial melting with residual garnet or hornblende or (2) fractional crystallization of hornblende. In the Tuolumne rocks, hornblende separation from the magma has caused systematic, relative depletion in heavy REE. We suggest that such hornblende fractionation may have been important in generating other granitoid rocks with highly fractionated REE abundances. Therefore, unusually low heavy REE abundances in granitoid rocks do not unequivocally indicate residual hornblende or garnet during melting.

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