A MIDDLE ORDOVICIAN GIGANTIC ERUPTION AND ITS PALAEOGEOGRAPHIC IMPLICATIONS

The late Arenig (466±1.5 Ma) felsic volcanic rocks of the Flat Landing Brook Formation, Tetagouche Group of the Bathurst Mining Camp, northern New Brunswick, Canada, record evidence of a major eruption. This volcanic event resulted from the rifting of the Popelogan Arc, which went on to form the Tetagouche-Exploits back-arc basin. The extremely high volume of magma erupted may in part be the result of ridge-subduction beneath the rifting arc.

Although subsequent polyphase deformation prevents an accurate determination of the volume of magma erupted, statistical probability models and detailed U-Pb zircon geochronology indicate that the Flat Landing Brook volcano was at least of a similar magnitude and eruption rate as Toba, in Sumartra, the largest eruption in recent geological history.

The presence of thick felsic pyroclastic flow deposits (including welded ignimbrites), interbedded Surtseyan-style mafic tuffs and the rarity of sedimentary rocks, indicates that the Flat Landing Brook Formation was substantially very shallow marine to subaerial. Such eruptions would have large eruption columns and therefore would be expected to produce extensive distal ash deposits (bentonites). However, widespread, circa 466 Ma bentonites are not recognized in Laurentia. In contrast bentonites of this age are well represented in Gondwana (notably in the Pampeanas terrane of South America), and possibly also Baltica. It is not currently possible to test if the Flat Landing Brook volcano was responsible for any of these late Arenig Gondwanan bentonites as they are not constrained adequately chemically or geochronologically to exclude the other approximately coeval potential sources preserved in South Wales and the Brabant Massif, Belgium.

The apparent absence of Flat Landing Brook Formation related bentonites in Laurentia and potential correlatives in Gondwana suggests that the Ganderian microcontinent, on which this volcanism occurred, was still far removed from the Laurentian margin during the late Arenig, consistent with the paleomagnetic evidence. This assertion is further supported by the absence of the Deike or Millbrig K-bentonites in the Caradocian black shales that overlie the Flat Landing Brook Formation.