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Paper No. 5
Presentation Time: 2:35 PM


CANNON, William F., US Geological Survey, 12201 Sunrise Valley Dr, MS 954, Reston, VA 20192-0001, SCHULZ, K.J., U.S. Geological Survey, Reston, VA 20192, BELKIN, Harvey E., U.S. Geol Survey, 956 National Center, Reston, VA 20192, ADDISON, William D., Kakabeka Falls, ON P0T 1WO, Canada, BRUMPTON, Gregory R., 211 Henry St, Thunder Bay, ON P7E 4Y7 and JIRSA, Mark A., Minnesota Geological Survey, Univ of Minnesota, 2642 University Ave, St. Paul, MN 55114,

Accretionary lapilli are a common component in the 1850 Ma Sudbury impact layer in the Lake Superior region between 450 and 800 km from the impact site near Sudbury, Ontario. Lapilli are as large as 2.5 cm in diameter and may display up to four cycles of inner coarse-grained to outer finer-grained laminae. Mineral, rock, or devitrified glass fragments form the cores of some lapilli and appear to have been the nucleus for accretion of finer particles that make up most of the lapilli. Angular quartz grains occur in all lapilli and are the most common particles in some. Rare quartz grains display relict shock-metamorphic planar deformation features. Dolomite is also common at many localities and is largely in recrystallized rhombs. The extent to which dolomite is a primary component of these lapilli is not clear. At most localities, lapilli are widely dispersed or grouped in irregular clusters in a matrix of finer-grained clastic material. Less commonly, they are concentrated in distinct layers a few tens of centimeters thick and range from sparse to tightly packed.

Chemical analyses of lapilli from three sites show variable major element compositions reflecting variable carbonate and silica content. All three samples have very low Na2O contents (≤ 0.12 wt.%) and high K20/Na2O ratios (9.4-19.4). The lapilli have variable trace element abundances, but show similar chondrite-normalized trace element patterns with enriched light REE (La/Sm = 5-8), relatively flat heavy REE (Gd/Yb = 1.7-2.5), and negative Ba, Nb, Ta, Sr, P, and Ti anomalies. These patterns are similar to those in melt spherules from the impact layer, suggesting that the lapilli composition includes a significant component of impact melt or materials from the same source as the melt.

Accretionary lapilli are believed to have formed in the atmosphere as wet accumulations of dust in the presence of liquid water vapor and thus are an indicator of a low temperature environment where they formed. However, lapilli in the impact layer are found in very high energy ground surge breccias and ballistic deposits, and have been reworked into tsunamites. The survival of most lapilli as undeformed, unbroken, and unabraded spheres suggests that they became cohesive and indurated during or very rapidly after formation and prior to deposition.

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