Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

Isotopic and geochemical data indicate that intrusions in the eastern Creig nish Hills of central Cape Breton Island, Canada represent the roots of arc s active at similar to 540-585 Ma and similar to 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U-Pb zircon age of 553 +/- 2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U-Pb zi rcon age of 540 +/- 3 Ma that is within analytical error of Ar-40/Ar-39 hor nblende plateau isotope-correlation ages of 545 and 550 +/- 7 Ma in the Riv er Denys diorite; and (3) an upper intercept U-Pb zircon age of 586 +/- 2 M a in the Melford granitic stock. On the other hand, similar to 441-455 Ma A r-40/Ar-39 muscovite plateau ages in the host rock adjacent to the Skye Mou ntain granite provide the best estimate of the time of intrusion, and are c onsistent with the presence of granitic dykes cutting the Skye Mountain gab bro-diorite previously dated at 438 +/- 2 Ma. All the intrusions are calc-a lkaline; the Skye Mountain granite is peraluminous. Trace element abundance s and Nb and Ti depletions of the intrusive rocks are characteristic of sub duction-related rocks. The similar to 540-585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent th e youngest Neoproterozoic are magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic roc ks, which bracket the transition from convergence to extension between simi lar to 540 and 505/520 Ma. This transition varies along the Avalon are: 590 Ma in southern New England, 560-538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transiti on is attributed to northwestward subduction of two mid-ocean ridges border ing an oceanic plate, and the migration of two ridge-trench-transform tripl e points. Following complete subduction of the ridges, remnant mantle upwel ling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the are in both southern New Brunswick and central Cape Breton Island. The similar to 440 Ma are ma gmatism in the Creignish Hills extends through the Cape Breton Highlands an d into southern Newfoundland, and has recently been attributed to northwest erly subduction along the northern margin of the Rheic Ocean.