A tectonized ultramafic-mafic-pelitic package in Stockbridge, Vermont: Metamorphism resulting from subduction and exhumation

Abstract

A tectonized ultramafic-mafic-pelitic package southwest of the Burgess Branch Fault Zone in the Rowe/Prospect Rock slice in Stockbridge, Vermont preserves Taconian subduction zone metamorphism. The polydeformed package consists of carbonatized and chloritized serpentinite lozenges that are faulted between graphitic phyllite and an intercalated zone of mafic greenstones and pelitic schists that retain primarily Taconian structures. Amphiboles zoned chemically in greenstones display decreases in PL (Na^M4) and TK (Al^VI + Fe³⁺ + 2Ti + Cr) from cores to rims. Pseudosection and amphibole isopleth calculations show that different greenstone bodies experienced peak subduction zone metamorphism at different depths between ∼19 and 32 Km (0.55–0.95 GPa) and/or at different temperatures between ∼425 and 500 °C. Barroisite and winchite cores preserve the highest pressure and lowest temperature peak metamorphism, which is interpreted as lower blueschist facies. All greenstones followed decompression cooling paths to lower greenschist facies where actinolite rims grew. Assembly of the ultramafic-mafic-pelitic package probably occurred in a Taconian exhumation channel containing mantle serpentinite, oceanic crust, and clastic rocks of the Laurentian margin and Iapetan Ocean basin.