Mafic whole-rock geochemistry and neodymium isotopes, Green Mountain and Rowe/Prospect Rock slices, Vermont Appalachians

Abstract

Mafic rocks containing sodic-calcic amphibole in the Green Mountain slice (GMS) and Rowe/Prospect Rock slice (R/PRS), Vermont Appalachians were originally subalkaline basalts emplaced as melts during Neoproterozoic rifting of Rodinia that led to the formation of the Iapetus Ocean basin. Relatively high degrees of partial melting of asthenosphere that was highly depleted in incompatible elements produced R/PRS magma(s), which may have been contaminated locally by continental crust and/or fluids. Trace element chemistry suggests that mafic bodies from different locations in the R/PRS may be fractionated magmatic equivalents. Mafic rocks in the GMS formed from magmas produced by relatively low degrees of partial melting of mantle that was relatively enriched in incompatible elements compared to depleted mantle. These enriched melts may have been derived from a plume and/or enriched lithospheric components, potentially continental crust, embedded in depleted upper mantle. A depleted mantle signature preserved locally in the GMS probably reflects increasing asthenospheric input during crustal thinning, opposed to crustal or fluid contamination.

Whole-rock minor and trace element data from the GMS and R/PRS are distinct from analyses of glaucophane schist from the Tillotson Peak Complex in northern Vermont, which may be exhumed Iapetan ocean floor. Exhumed mafic rocks in the GMS and R/PRS were formed as lower plate melts prior to relatively high pressure subduction zone metamorphism, providing evidence for subduction of the Laurentian margin, not subduction erosion. Low pressure greenschist facies mafic rocks that occur structurally between the GMS and R/PRS were sourced from depleted or highly depleted mantle in a supra-subduction zone environment, potentially a forearc or backarc basin; an ophiolitic origin is equivocal. Geochemical and isotopic data and interpretations are compatible with rift-related tectonomagmatic models for the peri-Laurentian realm of the northern Appalachians.