FABRIC ANALYSIS AND METAMORPHIC HISTORY OF THE WESTOBORO FORMATION IN THE BREAKHEART RESERVATION, SAUGUS, MA

The Proterozoic Westboro Formation (ca. 800 Ma), composed of massive quartzites and interbedded schists, is interpreted as a passive margin assemblage deposited on Avalon before the microcontinent rifted from Gondwana. The Westboro experienced various pulses of deformation and metamorphism during the Proterozoic. Of notable interest are 1 to 3-meter wide, N-NE striking, shallow dipping mylonitic zones exposed in and around the Breakheart Reservation in Saugus, MA. At one location blocks of eroded mylonite are contained in a Westboro debrite. A possible interpretation is that mylonites exposed along a rift-basin border fault were eroded and incorporated upper Westboro through mass gravity transport.

This study was undertaken to determine shear sense and the relative timing of mylonite formation in order to shed light on possible Proterozoic extensional and/or transcurrent activity related to the rifting and movement of Avalon while along the margin of Gondwana. Thirteen samples were collected for thin section and anisotropic magnetic susceptibility (AMS) analyses, and local field relationships were mapped.

The most informative data were obtained from field observations and measurements, and thin section analyses. AMS analyses were useful in redefining the principal foliation, but did not aid in determining lineations or direction of shear sense. Nevertheless, these analyses in combination with local mapping tentatively point to the following sequence of events: 1) deposition of the Westboro; 2) greenschist facies metamorphism forming the principal foliation; 3) contact metamorphism, recorded by the formation of large cordierite porphyblasts; 4) mylonitization; 5) extrusion and deposition of overlying felsic flows and pyroclastic deposits; 6) intrusion of the Dedham granite (ca. 610 Ma) and possible retrograde metamorphism of porphryblasts; 7) additional contact metamorphism, recorded by unaltered cordierite aggregates rimming porphyroblasts; and 8) brittle deformation evidenced by thin bands of cataclasite in the Dedham. Rotated porphyroblasts and S-C fabrics in the mylonites record a dextral transpressive sense of motion possibly along a restraining bend of a transcurrent system. Therefore mylonite blocks in the debrite are not easily explained by simple resedimentation in a rift basin.