OVERSTEPPED REACTIONS, ANATEXIS AND MAGMA INJECTION: ZONAL DIFFERENCES IN THE CONTACT-AUREOLE OF THE MARCY META-ANORTHOSITE MASSIF, NEW RUSSIA GNEISS COMPLEX, NORTHEASTERN ADIRONDACK HIGHLANDS, NY

The close spatial association of massif anorthosites, jotunites, mangerites, and charnockites occur as: 1) approximately concentric envelopes or partial envelopes surrounding the massif with silica and alkali content increasing with distance from anorthosite or 2) dikes and relatively small igneous bodies that crosscut anorthosite and other igneous or meta-igneous rock associated with the massif. Recognizing the distinct natures of these two associations is important to understanding the underlying cause of these relationships.

The Marcy meta-anorthosite massif of the Adirondack Highlands is bounded on its east by the New Russia gneiss complex with textural and chemical zonation subparallel to the massif border, an association of the first kind. These zones arise from different responses of a mangeritic protolith to contact heating in the aureole of the Marcy. Rocks closest to the massif were heated rapidly and so overstepped melt-producing reactions. Metastable reactions produced a refractory assemblage (Grt-Opx-Cpx-Pl-Or-Qtz-Oxide) as vapor was driven from the rock, outward into more slowly heating gneiss. In this second zone, significant amounts of melt formed, aggregated and moved outward leaving a restitic assemblage (Pl-Cpx-Opx-Hbl-Ilm ±Qtz) and minor leucosomes (antiperthite-Qtz-Opx-Cpx-Grt). Migrating melt injected a third zone to produce charnockite with ≈ 50% alaskitic leucosome (mesoperthite + Qtz). Mineral assemblages and whole rock chemistry support this hypothesis. The data imply that at least some mangerites and charnockites associated with massif anorthosite are produced by in situ differentiation during contact metamorphism. If the anatectic reactions that affected the mid- to lower-crustal rocks in the aureole of Marcy meta-anorthosite are representative, then alaskitic intrusions should be found with large mangerite and charnockite bodies found near massif anorthosites if they originated as lower crustal melts in the early stages of anorthosite magmatism.