CR-SPINEL GEOCHEMISTRY OF ULTRAMAFIC ROCKS FROM THE BELVIDERE MOUNTAIN COMPLEX, NORTHERN VERMONT: EVIDENCE FOR AN EARLY PALEOZOIC FOREARC SETTING

Ultramafic rocks, in the form of serpentinite, occur throughout the Appalachian Mountains. These bodies provide critical information about the tectonic development of the Laurentian margin of northeastern North America. However, in many cases, deformation has obscured such information. We have been able to identify relic Cr-spinel in samples from a metamorphosed ultramafic body within the Belvidere Mountain Complex, northern Vermont; which had a protolith of dunite, olivine pyroxenite, and possibly harzburgite. Four samples from this body were analyzed on a JEOL 8900R Superprobe Electron Probe MicroAnalyzer at the Florida Center for Analytical Electron Microscopy. The spinels from these samples display hydrothermal reaction rims of magnetite. Because of this, 32 of the least altered spinels were analyzed, and rims were avoided. To account for possible compositional zoning, each spinel analysis reported here is the average of 3 different spots. These spinels have: Cr# that range from 82.62 to 88.26; Mg# that range from 32.89 to 37.97; TiO₂ that range from 0.01 to 0.08 wt. %; and Al₂O₃ that range from 10.44 to 15.17 wt. %. Hydrothermally altered spinels commonly have high Fe³⁺ and an intergrown silicate phase resulting in anomalously high SiO₂. Our spinels do not display these traits suggesting they have primary compositions. They plot along the Cr-Al trend on the Cr-Al-Fe³⁺ triangular diagram. They also plot within the arc, supra-subduction zone, or forearc fields on tectonic discrimination diagrams. The low Al and Ti in these spinels suggest a high degree of partial melting of a mantle peridotite in an arc setting. The high Cr# is indicative of a mantle source that interacted with a boninitic melt. Alternatively, the high Cr# could have resulted from a high degree of melting within a previously partially-melted, hydrous mantle peridotite. Collectively, the data from these spinels suggest that the ultramafic rocks associated with the Belvidere Mountain Complex formed from a high degree of partial melting of a depleted mantle source within a forearc setting. This setting would be analogous to the modern Izu–Bonin–Mariana forearc. Our conclusions are in agreement with published data for spinels from the East Dover and Ludlow ultramafic bodies in southern Vermont.