GEOCHEMISTRY, PETROLOGY, AND STRUCTURE OF THE TILLOTSON PEAK AND BELVIDERE MOUNTAIN MAFIC COMPLEXES, NORTHERN VERMONT

The Tillotson Peak Complex (TPC) and Belvidere Complex (BC) are situated between the same two lithotectonic packages. Both lie above albitic schists and gneisses of the Hazens Notch and Fayston Formations, and both are overlain by phyllite and schist of the Ottauquechee, Stowe, and correlative units. However, geochemically and petrologically the mafic rocks from the TPC and BC are distinct.

For TPC mafic rocks, chondrite-normalized REE patterns are LREE depleted with flat HREE. Overall, LREE abundances range from 7-10X chondrite whereas HREE are 15X chondrite. MORB-normalized spider diagrams show irregular LILE abundances, variable Th, flat signatures near unity from Nb to Sc, and variably depleted Cr abundances relative to MORB. Trace element tectonic discrimination diagrams consistently show MORB affinity. These data are consistent with an Iapetan MORB origin for TPC mafic rocks with subsequent subduction to high pressures (garnet-omphacite pairs range from 12 Kb, 520 oC to 14 Kb, 620oC).

Based on reaction space analysis, metamorphism preserved in the BC mafic rocks is lower pressure than in TPC. BC mafic rocks do not show the same geochemical coherence. REE patterns range from LREE-depleted to slightly LREE-enriched. MORB-normalized spider diagrams show irregular LILE for both BC amphibolites and greenstones; however, greenstones have flat patterns from Zr-Cr whereas amphibolites have more irregular patterns that slope gently upward from Zr-Cr. Tectonic discrimination diagrams (V vs. Ti and Zr/Y vs. Zr) show that most amphibolite and some greenstone samples fall in the island arc field.

The oldest structures in each of the complexes and the surrounding packages may not be coeval. Early Taconian deformation in both TPC and BC is dominated by faults and isoclinal folds, later deformed by open to tight plunging, reclined folds that control the map pattern, but the orientation differs from about N-S (BC) to E-W (TPC). The latter difference may be related to rotation of the TPC during emplacement. All rocks in the area are overprinted by broad Acadian folds with associated NE-trending spaced cleavage and minor shears.