Rb/Sr AGE AND GEOLOGICAL SETTING OF THE HOLYROOD GRANITE, SOUTHEAST NEWFOUNDLAND

Abstract

The Precambrian Holyrood granite of southeastern Newfoundland is of particular geological interest, because of its tectonic position on the easternmost exposed flank of the Appalachian mobile belt. Although previously considered to have been emplaced during the Grenville orogeny, this granite yields a Rb/Sr isochron age of 574 ± 11 million years (m.y.). Tectonic and sedimentary events that followed the granite emplacement, and that preceded the deposition of nonconformably overlying fossiliferous Lower Cambrian strata, are believed to have required at least 15 m.y. Consequently, a tentative maximum age of 560 ± 11 m.y. is proposed for the base of the Cambrian in this region.

Precise U–Pb zircon dates from the Avalon Terrane in Newfoundland

“…They suggested that the similarity of this date to that of the Swift Current Granite indicated a genetic link between them. They also pointed out that its similarity with the 590 f 11 Ma date for the Holyrood Granite (McCartney et al 1966) provides evidence for coeval plutonic and volcanic activity throughout the Avalon Terrane. We agree with this suggestion, although their data regress to 597: : : Ma and are not colinear, and caution is advised in using these dates.…”

“…to 597 f 42 Ma (Fairbairn 1965in McCartney et al 1966, giving a recalculated age of 580 f 42 Ma. Stukas and Reynolds (1976), using 40Ar139Ar incremental heating, reported an age "in the vicinity of 800 Ma" for feldspars from a rhyolitic ash-flow tuff near the western flank of the Harbour Main Group.…”

Nick Rast and the recognition of the Avalonian Arc

“…Newfoundland was considered to lack equivalents of the gneiss complexplatformal metasedimentary rock association, but had comparable molasse-type and CambrianOrdovician successions, and numerous volcanic and flyschoid sedimentary assemblages (e.g., Love Cove, Harbour Main, Connecting Point, and Conception groups) that considered correlative with the Coldbrook Group. But in Newfoundland, the volcanic assemblages were, in addition, locally intruded by the Holyrood granite of known Precambrian age (574AE11 Ma, Rb/Sr, McCartney et al, 1966;609AE11 Ma, Rb/Sr, Frith and Poole, 1972), and the molasse-type facies were known to be associated with younger, more alkaline volcanic rocks (Bull Arm Formation; e.g., Hughes and Bru¨ckner, 1971). …”