Late Paleozoic plutons in the Sylvester Allochton, northern British Columbia

Abstract

The Sylvester allochthon of the Slide Mountain terrane, which in north-central British Columbia lies structurally above shelf to platformal miogeoclinal strata, consists of a wide variety of lithologies suggestive of oceanic, island-arc, and cratonal environments. Included among these are distinctive suites of plutonic rocks which either intrude, or are structurally juxtaposed with, the layered volcanic and sedimentary rocks of the allochthon. Although these plutonic rocks occur in a range of bulk compositions from potassic granite, to intermediate diorite and tonalite, to mafic gabbro, they appear to represent only two distinct periods of intrusive activity. Foliated and mylonitic hornblende diorite from three different bodies near Dease River have been dated by U-Pb zircon and titanite methods at 353 +/- 4 Ma, 362 +/- 5 Ma, and 350 +/- 3 Ma, demonstrating consistent latest Devonian and Early Mississippian ages. Zircons from each of these rocks have an important inherited Precambrian component. Muscovite from the siliceous tectonite that these plutons intrude also gives a Mississippian 40Ar/39Ar cooling age, indicating a regionally developed, mid-Paleozoic metamorphic event that affected some of the rock units within the allochthon. A zoned hornblende gabbro-tonalite-granodiorite body near the northwest end of the allochthon has been dated at 265 +/- 3 Ma (zircon); a gabbro to tonalite body near Zus Mountain in the centre of the allochthon has a U-Pb zircon age of 268.6 +6.8/-3.4 Ma; farther to the south, granodiorite, monzodiorite, and potassic granite near Four Mile River have zircon dates of 262 +/- 0.5 and 270 +/- 4 Ma; a small body of hornblende diorite nearby has an age of 266.5 +/- 1.5 Ma (zircon); and a tonalite sill near Rapid River has zircon and titanite ages of 267-269 Ma. These ages suggest a significant pulse of plutonism in the mid-Permian.