TECTONIC EXHUMATION OF MID-CRUSTAL ROCKS RECORDED BY TEXTURAL UNCONFORMITIES IN GARNET FROM THE NEW ENGLAND APPALACHIANS

Neoproterozoic (Z) to Ordovician (O) rocks in the Appalachians of western New England record deformation from both the O Taconic and Devonian (D) Acadian orogenies. In contrast, Silurian (S) and D rocks in the region were only affected by the Acadian. Rosenfeld (1968) documented garnets in the older rocks that preserve a two-stage growth history, and he argued that the older cores grew during Taconic metamorphism and that the younger rims were Acadian. Thompson et al. (1977) suggested that the garnets could have formed during a single prograde event in which an intermediate garnet-consuming reaction partially resorbed garnet.

Garnets with textural unconformities are widespread in the Z Gassetts Schist in VT, but are not found in S to D rocks. Inclusions of rutile are ubiquitous in garnet cores but absent in the second stage rims and matrix, indicating that the first stage of garnet growth occurred under higher P conditions than the second. A critical observation is that the hiatus in garnet growth is not correlated with metamorphic grade but it is texturally linked to the formation of a new cleavage. Thus, the textural unconformity cannot be explained by an intermediate garnet-consuming reaction during a single prograde event.

Isograd maps for each stage of garnet growth, created using inclusion and matrix assemblages, reveal a strong correlation between the high-grade areas during each stage. Thus, it seems more likely that both growth stages occurred during a single orogeny rather than during two events separated by 80 m.y. Mineral assemblages and chemical zoning suggest that garnet growth in the older rocks occurred during decompression of ca. 3 kbars, whereas metamorphism of S and D rocks occurred during increasing P conditions.

The two-stage garnet growth history records a cleavage-forming event that tectonically exhumed the older sequence of rocks, together with underlying Grenvillian basement rocks at 380 Ma, the time of peak metamorphic conditions. The boundary between rocks with different metamorphic histories coincides with a high strain zone with normal displacement that separates structurally lower rocks with complex garnets from structurally higher rocks with simple garnets. The lower rocks were extruded northward and upward, parallel to the orogen, into west-directed nappes during the Acadian orogeny.