Abstract

New chemical data from a widespread Precambrian stratigraphic unit, the Popple Hill Gneiss in the northwest Adirondacks, have meaning for depositional setting and correlation of Grenville metasediments. Average chemistry of the predominant rock type is appropriate for dacite volcanics rather than the previously proposed elastic sediments. Particularly unsuitable for shale are the high total alkali, Na2O, and SiO2 content; slightly peraluminous composition; and low index of chemical alteration.

This gneissic unit and leucogneisses of probable ash-flow-tuff origin lower in the section (Hyde School Gneiss) are interpreted as massive volcanic outpourings within a platformal-type sedimentary sequence. A recently proposed caldera complex in anorogenic or rift-related environments is an appropriate setting, but vulcanism interrupted as well as preceded carbonate deposition. Popple Hill and Hyde School Gneisses belong to a common depositional sequence with associated metasedimentary units and should be placed in the same stratigraphic group.

Popple Hill leucosomes are variable in shape and texture and, most likely, in age and mode of origin. Most are foliated and show grain size reduction; others cut each other and are isoclinally folded. Physical appearance seems unrelated to chemical composition. Confmement of leucosomes to Popple Hill Gneiss and chemical correlations with host mesosome suggest insitu origin and equilibration with mesosome. Many are enriched over mesosomes in K2O, Sr, Ba, and Rb; have low total REE and Zr; and have positive Eu anomalies. Restite is not readily observed. Such leucosomes could have formed throughout various phases of folding. Given the dry state of metamorphism as proposed elsewhere for the gneiss, metamorphic differentiation is a likely mode of origin. Other leucosomes with negative Eu anomalies and REE content similar to those of mesosome may be partial melts that formed during early Grenvillian metamorphism.

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