HR: 16:20h
AN: U44A-01    [Abstracts]
TI: Re-evaluation of a 2.7-Ga Mantle Event from U/Pb Zircon Age Distributions in Orogenic Granitic Rocks
AU: * Condie, K C
EM: kcondie@nmt.edu
AF: Dept of Earth & Environmental Science, New Mexico Tech, Socorro, NM 87801, United States
AB: During the late Archean, seven major episodes of orogenic granitoid magmatism occur at 2750, 2700, 2675, 2650, 2630, 2600 and 2550 Ma as recorded by U/Pb isotopic ages of single zircons from orogenic granitic rocks (largely TTG). Age peaks are defined by igneous zircon ages with < 5% discordancy and < 20 My uncertainty. Each episode is represented on four or more cratons and has an uncertainty generally less than 10 My. The 2750, 2700, 2675, and 2600 Ma peaks are also represented in the detrital zircon population. Only the 2700-Ma peak appears to be global in extent, and an average uncertainty of 5 My, it occurs on nine cratons (Baltica, North China, India, Yilgarn, Superior, Nain, Slave, Amazonia, Rae/Hearne). The 2600 and 2550-Ma peaks are widespread only in India, North China and Africa. The age distribution of ENd(T) values greater than zero falling within one epsilon unit of the mantle growth curve indicates that a large volume of juvenile continental crust was added at 2700 Ma with possible significant amounts also at 2750 and 2550 Ma. Although a mantle plume event at 2700 Ma is a viable model for the 2700-Ma granite-forming event, a single plume event does not seem capable of explaining the other six granitic episodes in the late Archean. A peak in relative komatiite abundance at 2700 Ma correlates well with the global 2700-Ma granitoid event. Other phenomena in the late Archean, which are less precisely dated, yet may correlate with the 2700-Ma granitoid event include: a negative excursion in δ¹³C of the organic reservoir (2750-2650 Ma),δ¹⁸O change in detrital zircons and in the composition of seawater (2700-2200 Ma); peak in Re/Os mantle depletion age (2900-2700 Ma); La/Yb peak in TTG (2750-2650 Ma); flood basalt eruption (Fortescue at 2740, Nunavut at 2710; and Ventersdorp at 2720); ENd(T) and Nb/Th changes in the mantle (2700-2100 Ma); and Ce and Eu anomalies in BIF/chert (2700 Ma).
DE: 1199 General or miscellaneous
DE: 1645 Solid Earth (1225)
DE: 1744 Tectonophysics
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 9619 Precambrian
SC: Union [U]
MN: 2007 Joint Assembly