ZIRCON-CLINGING INFERRED ANATECTIC MELT INCLUSIONS IN ADIRONDACK GARNET

Micron-scale, multiple-solid inclusions occur inside garnet from metapelitic gneiss extracted from the hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The garnet host grains are small (2 mm diameter), euhedral neoblasts that were interpreted by Florence et al. (1995, CMP, v. 121, p. 424-436) as forming peritectically during anatexis accompanying the Ottawan Orogeny (ca. 1040 Ma). Florence et al. (1995) determined partial melting conditions of 4 to 6.4 kbar and 735^oC. The inclusions are typically 10-20 micrometers in diameter and occur in garnet as isolated clusters with no preferred orientation. Many, if not all, of the inclusions comprise a large zircon grain surrounded by roughly equal amounts of finer-grained (cryptocrystalline) biotite, quartz, and K-feldspar (determined by electron microprobe analysis and backscatter electron imaging). The zircon grains frequently occupy greater than 50 vol. % of the inclusion.

Because of the inferred peritectic origin of the host garnet, the occurrence of inclusions as random, isolated clusters, and the roughly consistent phases ratios of cryptocrystalline biotite, quartz and K-feldspar, the multiple solid inclusions are interpreted as crystallized melt inclusions. Similar melt inclusions in peritectic garnet were recently described by Cesare et al. (2009, Geology, v. 37, p. 627-630) and provide evidence of anatectic melt compositions in granulate terranes, although the inferred melt compositions are controversial (Clemens, 2009, Geology, v. 37 p. 671-672).

Experimental work on Zr solubility in silicate melts (Watson and Harrison, 1983, EPSL, v. 64, p. 295-304) suggests the large zircon grain in each inclusion is a non-igneous phase. Consequently, we infer that micron-scale droplets of anatectic melt clung to refractory zircon grains while garnet grew around them. This inferred mineral-melt textural relationship suggests that anatectic melt has a chemical affinity for zircon grains.