LARGE-MAGNITUDE MIDDLE ORDOVICIAN VOLCANIC ASH FALLS IN NORTH-AMERICAAND EUROPE - DIMENSIONS, EMPLACEMENT AND POSTEMPLACEMENT CHARACTERISTICS

Middle Ordovician K-bentonites represent some of the largest known fal lout ash deposits in the Phanerozoic Era. They cover minimally 2.2 x 1 0(6) km(2) in eastern North America and 6.9 x 10(5) km(2) in northwest ern Europe, and represents the coeval accumulation of plinian and co-i gnimbrite ash on both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millb rig K-bentonites in North America and the Kinnekulle K-bentonite in no rthwestern Europe. The vents were located near the Laurentian margin o f Iapetus on an are or microplate undergoing collision with Laurentia. The volume of ash preserved in the stratigraphic record converted to dense rock equivalent (DRE) of silicic magma is minimally estimated to be 943 km(3) for the Deicke, 1509 km(3) for the Millbrig and 972 km(3 ) for the Kinnekulle. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km( 3). Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted and the ash is not preserved in the geologic record. The symmetry of the t hickness contours is suggestive that one or more ash clouds interactin g with equatorial stratospheric and tropospheric wind patterns dispers ed pyroclastic material to both the northwest and southeast in terms o f Ordovician paleogeography. Based on grain size measurements and thic kness/area(1/2) plots we conclude the three beds were each formed from co-ignimbrite or possibly phreatoplinian eruption columns. Analyses o f melt inclusions in primary quartz crystals indicate the parental mag ma contained approximately 4% dissolved water at the time of the erupt ion. This water provided the explosive energy during the initial gas t hrust phase. The implied fragmentation pressure on the magma would hav e reduced much of the ejecta to small particles, forming a deposit com posed largely of single crystals and glassy dust. Conversion of the as h to K-bentonite resulted in a mass loss of approximately 35%, mostly in the form of Si with lesser amounts of Na and K.