TRACE ELEMENT VARIABILITY IN MIGMATITES

The connection between rhyolites and plutons is fundamental to our understanding of the differentiation of the continental crust. Whereas in major element geochemistry granites and rhyolites are very similar, there are distinct differences in trace element characteristics between them. Leucosomes in lower crustal migmatites have similar major element geochemistry to rhyolites and granites. However, there is scant data on the trace element geochemistry of leucosomes. We are measuring the trace element geochemistry of leucosomes in order to test the hypothesis that rhyolites may be sourced directly from the lower crust with little to no upper crustal evolution. We analyzed amphibolite facies migmatites in order to better understand magma generation processes. Metaigneous migmatite samples were collected in Spruce Pine, North Carolina, and in Taos Range, New Mexico. In addition pelitic migmatites were collected at Winding Stair Gap, North Carolina.

Metaigneous leucosomes have compositions ranging from intermediate to high silica rocks. These samples have generally flat REE trends but are slightly enriched in LREEs relative to HREEs. Leucosomes are depleted in REE relative to the melanosomes in the migmatites. Pelitic leucosomes are granitic in composition and have REE trends with higher La/Lu ratios compared to metaigneous leucosomes. The metaigneous leucosomes have slightly negative Eu anomalies with an average Eu/Eu* value of 0.7. This differs greatly from pelitic leucosomes that have positive Eu anomalies with an average Eu/Eu* value of 1.5. Rhyolites generally have a Eu/Eu* value of ~0.5, similar to the metaigneous leucosome values. The data suggest that rhyolites may be derived from metaigneous migmatites. If so, they are only partially extracted, leaving some leucocratic material in the migmatite. The data also suggest that if rhyolites are derived by melting of metasedimentary sources significant post-melt modification is required.