Episodic Ordovician-Silurian plutonism in the Topsails igneous terrane, western Newfoundland

Abstract

The Topsails igneous terrane of western Newfoundland contains several intrusive and volcanic suites underlain and separated by screens of older intrusive rocks. The heterogeneous Hungry Mountain complex yielded U-Pb zircon upper and lower intercept ages of 2090 ± 75 Ma and 467 ± 8 Ma, demonstrating a significant inherited component of Aphebian age, while an adjacent suite of relatively massive granodioritic to granitic rocks yielded a slightly discordant U-Pb zircon age of 460 ± 10 Ma. The 438 ± 8 Ma age of the Rainy Lake complex, a suite of island arc type intrusive rocks, suggests it forms part of a Silurian magmatic episode, which also included Springdale Group bimodal volcanics (429 ± 4 Ma), and peralkaline granite and subvolcanic porphyries which intrude the Springdale Group (429 ± 3 Ma and 427 ±3 Ma, respectively). Most igneous units contain a slight component of inherited zircon, but initial 87Sr/86Sr ratios (average 0·704) are similar to calculated ‘Bulk Earth’ values at this time.Available data suggest that the Topsails terrane formed an oceanic tract with active volcanic island arcs when obduction commenced in early Ordovician time. The subsequent magmatic history, including the major but short-lived early Silurian magmatism, can be directly or indirectly related to obduction processes, including over-riding of the Topsails terrane by ophiolitic allochthons. There is no evidence of any Acadian (Devonian) igneous activity in the Topsails terrane.

Geochemistry of post-tectonic mafic intrusions in the Central Gneiss Terrane of southwestern Newfoundland

“…Middle Ordovician radiometric ages have been obtained from both gneisses and granitoid intrusions (Stevens et ah, 1982;Dunning and Chorlton, 1985). North of the Little Grand Lake fault, undeformed early Silurian A-type granites intruded plutons of Ordovician age (Whalen et al, 1987) and little metamorphosed Ordovician volcano-sedimentary suites of ophiolitic affinity. This region has been thrust southward along the Little Grand Lake fault (Whalen and Currie, 1983;van Berkel and Currie, 1988;Currie and Piasecki, in press).…”

“…(1988) reported an age of 431±2 Ma (U-Pb on zircon) for the Main Gut mafic complex. The age of the other mafic complexes is not definitely known, but on the basis of similar undeformed character, similar to identical pe trography, and lack of known igneous rocks younger than Silu rian in this part of Newfoundland (Whalen et al, 1987), they are assumed to be of similar late Ordovician to Silurian age. In sharp contrast to the undeformed Ordovician to Silurian mafic rocks, voluminous Silurian deformed granitoid rocks outcrop only a few kilometres to the southeast across the Victoria River fault (Dunning et al, 1988).…”

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

“…Cross-cutting plutonic rocks presumably of the Hodges Hill Complex show that thrusting ended in the New Bay Pond area by Late Silurian-Early Devonian. Further south in the Buchans area, thrusts involve 473 + 3/-2 Ma and 462+4/ -2 Ma rocks (Whalen et al, 1987;Dunning et al, 1987), and a 429+3 Ma pluton cuts rocks of one thrust sheet (Whalen et al, 1987). It is unknown, however, whether or not this pluton is allochthonous.…”

“…Several of the Silurian subaerial/shallow marine overlap sequences in central Newfoundland (the circa 430-420 Ma Botwood and Springdale Groups: Williams, 1967;Chandler et al, 1987;Whalen et al, 1987;Elliott et al, 1991) have been interpreted as deposits of pull-apart basins related to this Silurian dextral strike-slip fault system (Kusky et al, 1987;Buchan and Hodych, 1992), and alternatively as subaerial thrust-related deposits (Karlstrom, 1982;Karlstrom et al, 1982Karlstrom et al, , 1983b. Problems with the thrust-related interpretation include the observation that these sequences are almost invariably bounded by strike-slip faults, not thrust faults, and such an abundance of volcanic rocks as is found in these basins is atypical of thrust-related foredeeps.…”

Age of high-grade gneisses south of Grand Lake, Newfoundland

“…2, van Berkel and Currie, 1988). The Notre Dame Subzone exhibits lowgrade mafic and felsic volcanics of lower to mid-Ordovician age and locally ophiolitic affinities (Glover and Buchans Groups, Nowlan and Thurlow, 1987), unfoliated mid-Ordo vician tonalitic to granitic plutons (-460 Ma, Whalen et al, 1987), and a highly distinctive suite of Silurian A-type granites (Whalen et al, 1987;Whalen and Currie, 1990). To the east, the Lloyds River Fault truncates the Central Gneiss Subzone against the Exploits Subzone of the Dunnage Zone (Fig.…”