U–Pb geochronology of Late Proterozoic rocks of the eastern Cobequid Highlands, Avalon Composite Terrane, Nova Scotia

Abstract

In the Cobequid Highlands of Nova Scotia, low-grade late Precambrian arc-related volcano-sedimentry rocks typical of the Avalon Composite Terrane overlie platformal metasedimentry rocks and are spatially associated with gneisses previously considered to be basement to both these units. U–Pb zircon dates of 580–587 Ma from an orthogneiss and an amphibolite are similar to the U–Pb zircon dates of 580–610 Ma from both syntectonic granites in ductile shear zones and high-level posttectonic plutons that intruded the Avalonian successions. Hence, the gneisses do not represent basement but are an integral part of the Avalonian orogenic cycle. The geochronological data indicate that penetrative fabrics in the gneisses, syntectonic granites, and volcano-sedimentary successions are penecontemporaneous (ca. 580–620 Ma) and not sequential, as previously interpreted. The gneisses have a metamorphic fabric (S1a), crystallized under amphibolite-facies conditions, and may represent the deeper roots of a late Precambrian magmatic arc. Fabrics within the deformed granite gneisses (S1b) are interpreted as reflecting crystallization within active ductile shear zones associated with intra-arc transtension and basin development. Fabrics in the volcano-sedimentary successions (S1c) are associated with deformation of the basin.

Neoproterozoic?Early Paleozoic evolution of peri-Gondwanan terranes: implications for Laurentia-Gondwana connections

“…However, its cessation was diachronous, terminating at ca. 590 Ma in New England (Kaye and Zartman 1980;Zartman 1985, 1992;Thompson et al 1996;Thompson and Bowring 2000), 600 Ma in southern New Brunswick (Bevier and Barr 1990;Barr et al 1994;Currie and McNicoll 1999), 605 Ma in mainland Nova Scotia (Doig et al 1991;Murphy et al 1997;, 575 Ma in southern Cape Breton Bevier et al 1993), 585 Ma in Newfoundland (Krogh et al 1988;O'Brien et al 1996), and 600 Ma in the British Isles (e.g., Tucker and Pharaoh 1991;Horµk 1993;Noble et al 1993).…”

A synopsis of the geology of the Cobequid Highlands, Nova Scotia

“…The fi rst two units occur principally as sheets tens to hundreds of metres thick within the Gamble Brook Formation, with structures indicating emplacement during shearing . Geochronology by 40 Ar/ 39 Ar on hornblende (Keppie et al 1990) and U-Pb on zircon (Doig et al 1991) showed that the Frog Lake gabbro/diorite is about 622 Ma and the Debert River granodiorite 609 -605 Ma. Poor quality U-Pb and Rb-Sr ages of ca.…”

“…Poor quality U-Pb and Rb-Sr ages of ca. 575 Ma (Doig et al 1991) have been obtained from the McCallum Settlement granite unit, which cuts the Debert River granodiorite. In addition, small bodies of the geochemically distinct Glen Road and Gully Brook granite units have been recognised in the Mount Thom area (Pe- .…”

“…They include the Great Village River Gneiss (Cullen 1984) and the "Mount Thom complex" (Meagher 1995), from which Gaudette et al (1984) reported a 934 ± 82 Rb/Sr isochron from granitic gneiss. Doig et al (1991) reported Late Neoproterozoic U-Pb zircon ages in the Great Village River Gneiss of 589 Ma from amphibolite, with a protolith interpreted as Folly River Formation on the basis of whole-rock chemistry, and 580 Ma from granitic gneiss, probably of McCallum Settlement granite protolith. Nearing et al (1996) dated amphibole from mylonite near the Rockland Brook fault as 601 Ma by 40 Ar/ 39 Ar.…”

Correlation of Neoproterozoic III sequences in the Avalon Composite Terrane of mainland Nova Scotia: tectonic implications

“…The basal contact of the Folly River Formation truncates a mylonitic fabric in the Gamble Brook Formation. A granite, thought to have in truded in the latest stages of the development of this mylo nitic fabric (Nance and Murphy, 1990), yielded aU-Pb zircon age of 605 ± 5 Ma (Doig et al, 1991). Nance and Murphy (1990) attributed the fabric development in the Gamble Brook Formation to sinistral, strike-slip movements during development of the basin into which the Folly River Forma tion was deposited.…”