Isotopic and Geochemical Evidence for Differing Magmatic and Tectonic Origins of Mafic Subduction Zone Rocks, Vermont Appalachians

Whole-rock major and minor element data suggest that polymetamorphosed mafic bodies within the subduction zone terrane of VT are tholeiitic rift basalts. Whole-rock REE and isotopic data reveal that different mafic bodies within the terrane have different magmatic and tectonic origins. Bodies associated with ultramafics display flat REE patterns or slight depletion in LREE relative to HREE, suggesting origination from depleted melts and/or high degrees of partial melting. These rocks retain age-corrected ɛNd (t) values ranging from 4.39 to 7.11, with higher values consistent with melting of MORB-type mantle (ɛNd 6 to 13) and lower values consistent with melting of enriched mantle compared to MORB-type. Samples with ɛNd (t) < 6 were derived from deeper, more primitive mantle melts than MORB-type, or originally depleted melts were contaminated by enriched lithospheric mantle. The latter is preferred because LREE enrichment is not correlated with low ɛNd (t) values. Mafic bodies surrounded only by metamorphosed sediments are moderately enriched in LREE relative to HREE, moderately depleted in LREE relative to HREE, or display flat patterns. The range in ɛNd (t) values is 3.46 to 7.82 for these rocks. Samples with ɛNd (t) values > 6 display flat REE patterns or are moderately depleted in LREE, consistent with melting of MORB-type mantle. Samples that retain ɛNd (t) values < 4 exhibit moderate enrichment in LREE relative to HREE abundances that are 30 times greater than chondrite abundances, similar to arc rift basalt signatures. These samples were derived from deep, moderately enriched, primitive mantle melts that resulted from moderate to low degrees of partial melting. One sample preserves ɛNd (t) = 4.48 and is moderately enriched in LREE relative to HREE abundances that are 15 times greater than chondrite abundances, comparable to ocean island signatures. This sample was derived from a relatively deep, moderately enriched, primitive mantle melt that resulted from moderate degrees of partial melting. The geochemical and isotopic variations among different mafic subduction zone rocks in VT may be explained by mixing of a mantle plume with MORB-type mantle melts in an oceanic rift environment that was evolving into an arc rift setting as a result of subduction.