HR: 1340h
AN: V23C-1557    [Abstracts]
TI: A Reconnaissance Study of Rutile from the Barberton Greenstone Belt, South Africa: Implications for a Possible new Archean Impact Layer
AU: * Kohl, I E
EM: ikohl1@lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Byerly, G R
EM: glbyer@lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Lowe, D R
EM: drlowe@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States
AB: The 3.2 to 3.5 Ga Barberton greenstone belt (BGB) is composed of a remarkably well-preserved assemblage of volcanic and sedimentary rocks deposited over a 300-myr period. Rutile is a common minor phase produced by diagenesis and or metamorphism of the ultramafic and mafic volcanic rocks of the BGB. We examined rutile from 6 units in the greenstone belt to determine their potential as either petrogenetic or geochronologic indicators. In all but one unit the rutile occurs as relatively fine-grained acicular clusters with high SiO2 (0.5 wt%) and low Zr (< 0.03 wt%). They have low concentrations of Pb, dominated by Pb 204, which do not yield a meaningful age determination. This rutile appears to have formed under diagenetic or lower greenschist facies conditions in lava flows or primary pyroclastic deposits of basaltic and komatiitic composition. The one sample where meaningful geochronologic information was obtained is a tuffaceous sandstone in the 3.3 Ga Mendon Formation that we informally call the "Green Sand" unit. This 3-4m thick sandstone lies about 1m above a previously unrecognized impact layer and it may represent detrital material derived from the impact crater or its subsequent uplift. This unit contains a remarkably diverse population of zircons, many of them rounded detrital grains, ranging in age from 3.3 to over 3.7 Ga. Rutile within this unit display ages that range from 3.2 to 3.4 Ga and have compositions that also distinguish them from those found in other BGB units. They are large yellow-orange to red single grains with distinctive polysynthetic twinning. They have high Zr, up to 9000 ppm, suggestive of formation temperatures above 1000C -- well above temperatures associated with known metamorphism of the BGB or surrounding granitoids, and possibly another indication of an impact origin. The rutile has high Cr concentrations, indicative of derivation from mafic and ultramafic protoliths. Zr/Nb ratios correlate positively with Pb 207/206 measurement error. Zr/Nb ratios of <1 yield triple digit age errors (up to 300 Ma), while ratios of >1 yielded single and low double digit errors. We believe the Zr/Nb ratio could be one criterion for identifying rutile suitable for Pb 207/206 geochronology. Development of a geochronometer that could be used with mafic and ultramafic rocks would be invaluable to greenstone belt studies.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 3690 Field relationships (1090, 8486)
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 8425 Effusive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting