EVIDENCE FOR COMPLEX MID-CRUSTAL FLOW IN THE NORTHERN APPALACHIANS FROM REGIONAL GARNET INCLUSION AND ZONING PATTERNS

Garnet-bearing pelitic and mafic schists are common in VT. Rosenfeld (1968) described garnets with two-stage growth histories from the Neoproterozoic Gassetts Schist in the Chester dome, and argued that the first-stage cores were Taconic and that the second-stage rims were Acadian. Thompson et al. (1977) suggested that both stages could have grown during a single prograde event that included an intermediate garnet-consuming reaction.

Garnets that grew in two stages are widespread in VT. Many contain zoning reversals indicating that garnet was partially resorbed during the growth hiatus. Rutile inclusions are common in garnet cores but absent in the rims and matrix indicating that the first growth stage occurred under higher pressure conditions than the second.

The hiatus in garnet growth is texturally linked to the formation of a new cleavage, and does not correlate with metamorphic grade. The wide variation in grade of rocks containing two-stage garnets indicates that the hiatus cannot be attributed to an intermediate garnet consuming reaction during a single event. Two isograd maps, one for each stage of garnet growth, show that the grade during the second growth stage is consistently higher than the first, but there is a striking spatial correlation between the highest-grade areas during each stage. This suggests that both growth episodes occurred during a single orogeny, rather than separated by 80 m.y. The age of peak metamorphism is well constrained to be 380 Ma. Further, inclusions patterns and mineral chemistry indicate ca. 3 kbars of decompression during garnet growth. Thus, the hiatus in garnet growth was triggered by a tectonic event that altered the P-T conditions at a particular mid-crustal level.

In contrast, Sil. and Dev. pelitic and mafic schist samples contain garnets with simple zoning patterns and no evidence for two stages of growth. Metamorphism in these rocks occurred during increasing P.

The spatial distribution of distinctive garnet zoning and inclusion patterns is consistent with structural evidence for a high-strain zone with normal displacement that separates structurally lower rocks with complex garnets from higher rocks with simple garnets. Structurally lower rocks were extruded northward and upward into Sil. and Dev. nappes that were transported westward during the Acadian orogeny.