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

Abstract

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ∼ 540–585 Ma and ∼ 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 zircon age of 540±3 Ma that is within analytical error of 40Ar/39 Ar hornblende plateau isotope-correlation ages of 545 and 550±7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586±2 Ma in the Melford granitic stock. On the other hand, ∼ 441–455 Ma 40Ar/39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438±2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ∼ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ∼ 540 and 505/520 Ma. This transition varies along the Avalon arc: 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 transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ∼ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Field relations, age, and tectonic setting of metamorphic and plutonic rocks in the Creignish Hills – North Mountain area, southwestern Cape Breton Island, Nova Scotia, Canada

“…Ar cooling age of 455 ± 1 Ma was reported for muscovite from the same area (Keppie et al 2000). Campbell (1990) obtained older muscovite ages of ca.…”

“…Those differences have been supported by a number of subsequent studies (e.g., Barr et al 1995Barr et al , 1998Potter et al 2008a, b), and led to the inclusion of the Bras d'Or terrane, along with similar areas in central Newfoundland and southern New Brunswick, in Ganderia by Hibbard et al (2006). In contrast, a number of other studies have focused on age similarities between rock units in the Bras d'Or and Mira terranes, and have produced alternative models based on the interpretation that these areas were not separate in the Neoproterozoic (e.g., Keppie and Dostal 1998;Keppie et al 1990Keppie et al , 1998aKeppie et al , 2000.…”

“…They also proposed local names for components of both of these metamorphic suites in different areas of the terrane because correlations among them could not be verified. Keppie (2000) termed the higher grade units . .…”

The Cape Porcupine Complex, northern mainland Nova Scotia – no longer a geological orphan

“…Their ages of ca. 560-550 Ma (e.g., Keppie et al 2000;White et al 2003) are not similar to the ca. 610 Ma and 478 Ma ages obtained from the Cape Porcupine Complex granitic and syenitic units, respectively.…”

“…Based on the presence of metasedimentary, metavolcanic and granitic rocks, Fletcher (1881Fletcher ( , 1984, Ferguson (1946), Ferguson and Weeks (1950), and Keppie (2000) correlated the rocks at Cape Porcupine with the metamorphic and granitic rocks exposed in the area now known as the Bras d'Or terrane of adjacent Cape Breton Island (Fig. 1).…”