Geology, geochemistry and geochronology of the Springdale Group, an early Silurian Caldera in central Newfoundland

Coyle, Marylou (1990) Geology, geochemistry and geochronology of the Springdale Group, an early Silurian Caldera in central Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Volcanic-sedimentary facies and structural relationships of the Silurian Springdale group in west central Newfoundland are indicative of a large collapse caldera, with an area of more than 2,000 km². Basaltic flows, andesite flows and pyroclastics rocks, silicic ash-flow tuffs, high -silica rhyolite domes, and volcanically derived debris-flows and breccias and mesobreccias, fluviatile red sandstones and conglomerates make up the group Five new uranium/lead zircon dates provided in this study show for the lowermost ash-flow tuff of the group an age of 432.4 + 1.7/-1.4 Ma, the Burnt Berry rhyolite dome of 430.8 ± 2 Ma, and the topmost Indian River ash-flow tuff of 425 ± 3 Ma, as well the age of 427 ± 2 Ma for both the King's Point complex and the Cape St. John Group. -- The Springdale Group is bounded on the east and west by up-faulted basement rocks which include gnesisses, amphibolites and pillow lavas derived from Lower Ordovician volcanic rocks, and in the northwest unconformably overlies the equivalent less metamorphosed Lower Ordovician submarine volcanics. These margins are intruded by cogenetic and younger granitoid rocks. The volcanic rocks form a calc-alkaline series, although gaps in silica content between 52 to 56%, 67 to 68%, and 73 to 74% separate them into four groups, basalts, andesites-dacites, rhyolites, and high-silica rhyolites. -- The high-silica rhyolites are chemically comparable to melts thought to form the upper parts of large layered silicic magma chambers of epicontinental regions. Such an environment is also suggested by the large area of the Springdale caldera, and the probability that it is one of at least five calderas which make up a large Silurian volcanic field. An epicontinental tectonothermal environment for central Newfoundland in Silurian-Devonian times is readily explained by the fact that this magmatic activity followed a period of destruction and closure of the Lower Paleozoic Iapetus Ocean, with trapped heat and basaltic magma causing large scale crustal melting in an overall transpressional tectonic regime.

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