The Pocologan metamorphic suite of southern New Brunswick, Canada: New constraints on age and conditions of medium- to high-pressure metamorphism on the Ganderian margin of the Rheic Ocean

Geologically diverse fault-bounded crustal slices form the present-day southern margin of Ganderia in southern New Brunswick. One such slice, the Pocologan metamorphic suite (PMS), was previously shown to contain high-pressure metamorphic rocks and interpreted as part of the forearc of the adjacent Silurian Kingston arc terrane. In this study, the protolith age of metasedimentary rocks in the PMS has been constrained by U-Pb dating of detrital zircon to less than about 470 Ma. The age spectrum is dominated by 500 to 600 Ma ages and sparse older ages, typical of other Lower Paleozoic sedimentary components of Ganderia. An orthogneiss sample yielded a U-Pb concordia age of 433.5 ± 0.7 (2σ) Ma, interpreted as the igneous crystallization age and further supporting a link to the Kingston arc. Electron-microprobe dating of monazite from mica-schist samples yielded a mean age of 388.3 ± 2.8 (2σ) Ma, an older population at about 417 Ma, and older detrital grains at 450 Ma. Pseudosection modeling and quartz-in-garnet barometry showed that garnet cores in these mica-schist samples formed at 7 kbar and 550 °C; similar modeling combined with phengite geobarometry showed a second event at similar pressure but higher temperature. This event was followed by pressure decrease to 5 kbar and slight cooling, when biotite and staurolite porphyroblasts formed. Mylonitization started at the peak of the second metamorphic event and ended during early exhumation. The P-T and monazite age data suggest that the PMS was metamorphosed in the Early Devonian by burial to middle-lower crustal levels during the collision of Avalonia with Ganderia and metamorphosed again about 30 million years later, also at middle-lower crustal levels, during dominantly transcurrent tectonic activity. During this time different crustal slices were juxtaposed to form the PMS and probably the entire fault-bounded Ganderian and Avalonian terrane assembly in southern New Brunswick.