FURTHER STUDY OF GARNET XENOCRYSTS IN NEW YORK CITY MIGMATITES

The authors have previously published an article at the 2004 Geological Society of America Annual Meeting, Petrogenetic Significance of Garnets in the Bedrock of New York City, in which they suggested that the garnet xenocrysts observed in the quartzofeldspathic zones of the migmatites, so frequently seen both at the surface, and below the surface pursuant to the construction of the Manhattan portion of New york City Water Tunnel #3, were a refractory residue of the anatectic melting of the immediately surrounding schistose bedrock. Although the relatively high melting point temperature of garnet compared to other minerals in the surrounding schists and schistose gneiss supports this suggestion, subsequent analysis of the composition of the garnets in the pegmatitic zone of the migmatites shows that they are quite different from the garnets in the immediately surrounding bedrock. This implies that the garnets in the quartzofeldspathic parts of the migmatites were not derived from the bedrock immediately adjacent to them. Further study of garnetiferous bedrock in the area, specifically for the composition of the garnets, could reveal the source rocks of the garnets. This is a project that students could be involved in and which we intend to pursue.