HR: 0830h
AN: T31F-0892    [PDF]
TI: First Demonstration of Archean Accretionary Complex by Zircon Geochronology Instead of Index Microfossils
AU: * Kitajima, K
EM: saburo@geo.titech.ac.jp
AF: Dept. Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Ueno, Y
EM: ichiro@esa.c.u-tokyo.ac.jp
AF: Dept. Earth Science and Astronomy, Univeristy of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, 153-8902 Japan
AU: Hirata, T
EM: hrt1@geo.titech.ac.jp
AF: Dept. Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Dept. Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AB: The North Pole area (3.5 Ga) in the Pilbara Craton, Western Australia is one of the best regions in the Archean cratons to investigate accretion process at subduction zone, because this area had been subjected only to very low-grade metamorphism. A 1/5,000 scale mapping has been performed during the last 12 years in the North Pole area. The mapped area is divided into five units bounded by layer-parallel thrusts: Units-I, -II, -III, -IV and -V in ascending order. Greenstones in these units are divided into MORB-type (Unit-I and -II) and OIB-type (Unit-III, -IV and -V) units by oceanic plate stratigraphy (OPS) and geochemistry of the greenstones. The Unit-I and -II, the best exposed, are composed of 19 and 22 tectonic slices with similar lithostratigraphy, bounded by layer-parallel thrusts. Duplex structures are developed at cm- to mappable scale in these units. The duplex structure and OPS indicates that the North Pole area is an Archean accretionary complex. The sense of shear with horizontal shortening is top-to-the-east which indicates westward subduction. The reconstructed lithostratigraphy of each tectonic slice in MORB-type units reveals a simple pattern, from basaltic and pillowed greenstone, through bedded chert layer to mafic sedimentary rocks on the top, in ascending order in the same unit. This lithostratigraphy is quite similar to the Phanerozoic OPS, except for the mafic sedimentary rocks which suggest that the accretionary complex was formed in the intra-oceanic environment comparable to the present-day western Pacific Ocean. The five-fold pile nappe structure of granite-greenstone complex in the North Pole region indicates that the structural top is the oldest accreted unit, whereas the bottom is the youngest. This is opposite against the rift basin model proposed by previous workers. To confirm the geologic relations, we separated zircons from thin acidic tuff layers in bedded cherts. We used a laser ablation ICP-MS for zircon U-Pb dating combined with cathode luminescence image for checking igneous textures of zircons. The zircons from the Unit-III indicate age of 3463$\pm$34 and 3434$\pm$31 Ma, and from rhyolite in the Unit-IV, which is structurally near-top unit, show the oldest age of 3660$\pm$50 Ma in this area. We cannot obtain zircon age from lower two units (Unit-I and -II), but Pb-model age (3400 Ma) of galena has been reported from Unit-I (Richards et al., 1981). This age is consistent with age trend suggested by our zircon geochronology data. The granite intruding into these units indicates the youngest age of 3393$\pm$45 Ma. Granite age is consistent with lower intersect age of rhyolite from the Unit-IV. Thus, the younging trend of Unit from top-sitting Unit-IV to Unit-I is present, indicating that the North Pole area has grown toward structurally downward with time, as expected by field occurrence. This is the first demonstration that Archean greenstone belt is composed of accretionary complex. The break-through of the Phanerozoic accretionary complex in Japan has come from radiolarian geochronology, combined with OPS and detailed field mapping.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8010 Fractures and faults
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting