A Cambrian island arc in Iapetus: geochronology and geochemistry of the Lake Ambrose volcanic belt, Newfoundland Appalachians

Abstract

The Lake Ambrose volcanic belt (LAVB) outcrops as a 45 km long northeast-trending belt of mafic and felsic volcanic rocks along the eastern side of the Victoria Lake Group in southcentral Newfoundland. It comprises roughly equal proportions of mafic pillow basalt and high silica rhyolite, locally interbedded with epiclastic turbidites. Volcanic rocks have been metamorphosed in the greenschist facies and are extensively carbonatized.U-Pb (zircon) dates from rhyolite at two, widely separated localities give identical ages of 513 + 2 Ma (Upper Cambrian), and this is interpreted as the eruptive age of the volcanic sequence. Primitive arc and low-K tholeiites can be recognized on the basis of major and trace element geochemistry, ranging from LREE-depleted to LREE-enriched. Geochemical variation between mafic volcanic types is interpreted predominantly to reflect contrasts in source characteristics and degree of partial melting; some variation within each geochemical type attributable to fractional crystallization can be recognized. Detailed examination of some samples indicates that the heavy REE and related elements have locally been mobile, probably as a result of carbonate complexing.The LAVB is the oldest well-dated island arc sequence in Newfoundland, and perhaps in the Appalachian-Caledonian Orogen. Its age requires modification of widely held models for the tectonic history of central Newfoundland. It is older than the oldest known ophiolite, demonstrating that arc volcanism was extant before the generation of the oldest known oceanic crust in this part of Iapetus. It further demonstrates that there was a maximum of approximately 30 Ma between the rift-drift transition which initiated Iapetus, and the initiation^ of subduction. This suggests that the oceanic sequences preserved in Newfoundland represent a series of arcs and back arc basins marginal to the main Iapetus Ocean, and brings into question whether the Appalachian accreted terranes contain any remnants of normal mid-ocean ridge type Iapetan crust.

Geology and U-Pb geochronology of the Annidale Group, southern New Brunswick: Lower Ordovician volcanic and sedimentary rocks formed near the southeastern margin of lapetus Ocean

“…The Annidale Group could represent one of many arcs that developed near the southern margin of the Iapetus Ocean. This interpretation is supported by workers in New foundland (e.g., Dunning et al, 1991) who have established the presence of at least one, and consider the possibly of several, Lower Paleozoic arcs and back-arc basins marginal to the Iapetus Ocean. As in Newfoundland, the development of the southeastern side of Iapetus in New Brunswick may have been more complex than previously suggested (e.g., .…”

“…If it is assumed that the volcanic-sedimentary rocks in the Annidale Group developed on sialic crust, then two types of basement are most likely: (1) an older arc like the Cambrian arc in Newfoundland (Dunning et al, 1991) or (2) Avalon crust. Apparent inheritance in the zircons of sample 2, which provides a possible age of source rocks in the vicinity of 1000 Ma, favors the latter possibility because rocks of the this age may occur in the Avalon Terrane (Nance, 1987).…”

Late Neoproterozoic Avalonian and related peri-Gondwanan rocks of the Newfoundland Appalachians

“…540 Ma) events. During the Ordovician, the oceanward edge rocks exhibit evidence of interaction with Iapetan crust, either along or offshore the eastern proto-Atlantic margin (e.g., Dunning and S. J. O'Brien, 1989;Dunning et al, 1991;, and record an early Paleozoic history unlike that seen elsewhere along the belt. Their magmatic and tectonothermal history indicates interaction of Avalonian and Iapetan rocks before, during, and following Ordovician (Penobscot) orogenesis on the eastern margin of the Iapetus Ocean Tucker et al, 1994).…”

“…It is possible that some deformation of the lowgrade basement was synchronous with the Early Ordovician bimodal magmatism and related metamorphism. Emplacement of the plutons is coeval with a major period of arc magmatism recorded in the peri-Gondwanan successions of the Dunnage Zone (cf., Evans et al, 1990;Dunning et al, 1991).…”

Tectonic implications of early silurian thrust imbrication of the northern exploits subzone, Central Newfoundland

“…Traditional models of Appalachian orogenesis have recently been challenged by paleomagnetic. paleontologic, geochronologic and structural data indicating a more complex sequence of events, in which several different arc sequences developed at the same time but in different places, and were later juxtaposed during closure of the Iapetus ocean (van der Pluijm, 1986;Dunning et al, 1991;van der Pluijm et al, 1993;de Roo and van Staal, 1994;van Staal and de Roo, 1996;Dalziel et al, 1994). In this study, we examine the structural setting of a group of volcanic rocks in the Exploits Subzone of central Newfoundland's Dunnage Zone that have proven critical in determining the ages of magmatism and the relative positions of the different volcanic sequences of central Newfoundland.…”