HR: 1340h
AN: V13E-0589    [Abstracts]
TI: Petrology of Archean greenstone in Wyoming Province - Part 1: Thermal effects of Louis Lake batholith on Archean greenstones at South Pass, in southern Wind River Range-
AU: * Takeuchi, Y
EM: yoh-take-21@moegi.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Yuasa, T
EM: taisukeyuasa@fuji.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Adachi, T
EM: t-adatchi@fuji.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Ogasawara, Y
EM: yoshi777@waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AB: South Pass Area is one of the greenstone belts in Wyoming Province. This area is famous for sequence of greenstones and iron mine ``Atlantic Mine". BIFs form a part of the sequence. Archean greenstones occur along the later Archean batholith (Louis Lake batholith). This greenstone belt locates to the east of the batholith and extends south and north direction. This is composed of four units from west to east; ultramafic to mafic rock unit, BIF-bearing pelitic rock unit, mafic rock unit, and metavolcanic - metagraywacke unit. These units are parallel to a contact between this greenstone belt and Louis Lake batholith. BIFs also lie along the contact about 0.3 km east. Several previous studies suggested that these greenstones were metamorphosed under conditions of amphibolite, and locally greenschist. However we discovered a lot of ``actinolite-facies" rocks that exposed in vicinity to high- grade garnet-bearing schists within 1 km. Therefore, it is necessary to clear the metamorphic grade and metamorphic history of the Archean greenstones in the South Pass Area. Actinolite is dominant constituent mineral in greenstones in far side (more than 1 km) of the Louis Lake batholith. We called these rocks ``actinolite-facies" rocks. Actinolite-facies rocks have a mineral assemblage of Act + Ab. Albite grains were mostly replaced by epidote. Actinolite grains have thin bluish-green hornblende rims like corona texture. Evidence for volcanic textures was observed. Pillow lava structure occurs about 3 km away from the Louis Lake batholith. A unit of pillow lava structure (7.5 cm _~ 20 cm, rim 0.5 cm width) is composed of pale green core and thin dark gray rim (chilled margin of pillow structure). The core assemblage is Ep + Chl + Qtz + Carb. The thin rim is composed of euhedral epidote, very fine-grained chlorite, and quartz. A volcanic texture was confirmed in this pillow lava structure. Albite grains preserve a relic intergranular texture in the part of these rocks. Relic volcanic augite grains occur in actinolite grains in metabasalt showing dolerite textures. These augite grains are anhedral and up to 0.5 mm in diameter. Garnet-bearing iron rich schists suggesting upper-amphibolite facies occur in pelitic unit about up to 0.3 km from the batholith. These schists have a mineral assemblage of Alm + Gru + Hbl + Chl + Qtz _ Bt and occur with BIFs. Iron oxides in BIFs in pelitic unit are mainly composed of magnetite and slightly maghemite. Actinolite-facies rocks and Garnet-bearing schists have a gap in metamorphic condition. Actinolite-facies rocks occur relatively far from the Louis Lake batholith. Garnet-bearing schists occur in pelitic unit within 0.3 km from the batholith. This local heterogeneity of metamorphic condition suggests that Archean greenstones in the South Pass Area were metamorphosed under "actinolite-stability" condition. Subsequently to this regional metamorphism, these Archean greenstones were undergone thermal effects of the Louis Lake batholith. Thermal effects are noticeable only along the contact. In relatively far side of the contact, only hornblende coronas surround actinolite grains. Changing of oxidation-reduction environment by thermal effects after precipitation may form magnetite in BIFs. Many relic volcanic textures such as pillow structure, augite grains, and intergranular texture of basalt were preserved. We expect to get more information of source rocks of Archean greenstones in the South Pass Area.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3660 Metamorphic petrology
DE: 8400 VOLCANOLOGY
DE: 9623 Archean
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005