REGIONAL GEOCHEMICAL COMPARISON OF METAMORPHOSED IAPETAN BASALTS IN VERMONT: INTERACTIONS BETWEEN MAGMATISM, RIFTING, AND SUBDUCTION

Whole-rock geochemistry of metamorphosed Paleozoic basalts from the Iapetan subduction zone terrane in Vermont is compared north to south along strike, from Tillotson Peak to Stockbridge, VT. Spatial analysis of rare earth element (REE) data characterizes the geochemical nature of Iapetan Ocean rift magmatism prior to subduction, and, as well, constrains the tectonic configuration of the ancient oceanic spreading center. In the north, eclogite, blueschist facies amphibolite, and greenschist are depleted in light rare earth elements (LREE) with respect to heavy rare earths (HREE), and signify MORB melt sources. Farther south, lower blueschist to epidote-amphibolite facies rocks and greenschist retain mixed LREE and HREE signatures ranging from depleted MORB and E-MORB to OIB; geochemical heterogeneity occurs both within and between individual units.

Varied REE signals in the south reflect mixing of fertile and depleted mantle melt sources beneath the Iapetan spreading center. Mixing of melt sources in the upper mantle could have resulted from the incorporation of convection-driven E-MORB plumes and/or from melting of embedded E-MORB fragments derived from earlier, deeper mantle melting. Kew Hill in Waitsfield, VT is a mafic body that retains anomalously high LREE, and potentially formed as an off-axis dike sourced from fertile mantle. Spatial distribution of REE signatures throughout the region may be related to Paleozoic geometrical configuration of oceanic transform faults and ridge axes as the Iapetan rift system was subducted obliquely. Ultimately, subduction and exhumation paths of Iapetan crustal fragments were influenced by oceanic plate geometry along the subduction interface. Ultramafic and mafic phase equilibria and mineral chemistries provide key information regarding conditions of metamorphism and associated subduction zone dynamics.