Different embryos of TTGs in garnet at Hooper mine, Adirondacks (New York State, US)

Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called  nanogranitoids once crystallized, whose characterization allows us to clarify deep crustal melting processes. the Adirondacks (US), is an ideal location to investigate mafic melting and crustal growth . As a portion of the Grenville Province, this massif mainly consists of intrusive bodies metamorphosed during the Ottawan Orogeny (1090-1050 Ma). Nanotrondhjemites were previously reported in in the giant garnets of Barton mine, Gore Mountain area (Ferrero et al., 2021) in the central-southern part of the Adirondacks. Melting is, however, not limited to such location: MI-bearing garnets are also found in the mafic granulites at Hooper mine, approximately 5 km NW of the Barton Mine. Such garnets have been divided in two types based on size, chemical zoning, habitus as well as the composition of the trapped melt.

Type 1 garnets are large, euhedral porphyroblasts of diameter >5 cm, with a rather homogeneous composition similar to the Barton mine garnets. The nanogranitoids here are scattered randomly and contain a constant assemblage consisting of quartz, kumdykolite/albite, amphibole(s) and minor amounts of phlogopite. Re-melting experiments conducted via piston cylinder led to the complete re-homogenization of the inclusions at 940°C / 1.0 GPa with the generation of a hydrous trondhjemitic glass.

Type 2 garnets are instead significantly smaller, <1 cm in diameter, and xenoblastic in shape. Their composition resembles type 1 garnets with the exception of low Ca and Y in the MI-bearing domains. The nanogranitoids in type 2 garnets contain quartz, kokchetavite/K-feldspar, kumdykolite/albite and phlogopite. Such phase assemblage is remarkably different from the previous nanogranitoids, i.e., amphiboles are notably absent whereas kokchetavite is present. Such inclusions re-homogenize to a less hydrous granitic glass at lower T, 900°C, and same P conditions (1 GPa) with respect to the previous MI type.

LA-ICP-MS analyses show different signatures for the two melt types, hence suggesting different melt production mechanisms. The trondhjemitic melt trapped in type 1 garnets shows the same enrichment in Th, U, Zr and Hf observed in the Barton Mine, thus suggesting a similar genesis for this melt, i.e., a H₂O-fluxed melting of a gabbro protolith (Ferrero et al., 2021). The granitic melt in type 2 garnets does not have such features, and we propose amphibole dehydration melting as the most likely genetic mechanism for this melt.

Altogether, microstructures, microchemistry and experiments indicate that the Adirondacks experienced multiple partial melting events at T≥ 900°C at in the deep crust. Moreover, the compositions of the melts generated at both Hooper mine and Barton mine defines a trend characteristic of primitive TTG melts or TTG embryos.

Bibliography

Ferrero S. Wannhoff I., Laurent O., Yakymchuk C., Darling R., Wunder B., Borghini A. & O’Brien P.J., 2021. Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. Earth Planet. Sci. Lett., 569, 117058.