HR: 0800h
AN: T41F-1302    [Abstracts]
TI: Grain-size Reduction of Feldspar in Middle Greenschist-facies Shear Zone
AU: * Ree, J
EM: reejh@korea.ac.kr
AF: Dept. of Earth & Environmental Sciences, Korea University, Anam-dong, Seongbuk-gu, Seoul, 136-701 Korea, Republic of
AU: Han, R
EM: rhhan@korea.ac.kr
AF: Dept. of Earth & Environmental Sciences, Korea University, Anam-dong, Seongbuk-gu, Seoul, 136-701 Korea, Republic of
AU: Yoo, S
EM: activeft@korea.ac.kr
AF: Dept. of Earth & Environmental Sciences, Korea University, Anam-dong, Seongbuk-gu, Seoul, 136-701 Korea, Republic of
AB: Deformation behavior of feldspar grains in granitic rocks is important in assessing crustal strength especially when they constitute a load-bearing framework. Grain-size reduction of feldspar can be achieved by fracturing and albite neocrystallization at greenschist facies condition, and by myrmekite formation, reaction and dissolution together with rotation recrystallization at epidote-amphibolite and higher-grade condition. Change in dominant deformation mechanism can occur with grain-size reduction. We report that grain-size reduction of feldspar is induced by a combination of fracturing, strain-related myrmekite formation and albite replacement in granitic mylonite from the middle greenschist facies Yecheon shear zone, South Korea. In granitic mylonite (K-feldspar 40%; plagioclase 34%; quartz 23%; muscovite 2%), feldspar grains deform by fracturing, mechanical twining and kinking while quartz grains deform by dislocation glide with recovery process mainly by subgrain formation and rotation recrystallization. High strained margins of K-feldspar porphyroclasts are characterized by intact or fractured myrmekite with fracture filling by muscovite. Along intragranular shear fractures of K-feldspar porphyroclasts, replacement (or neocrystallization) of fine-grained albite occurs whereas their intragranular extension fractures and pressure shadow are filled by fine-grained quartz and K-feldspar with minor amount of chlorite and muscovite. On the other hand, high strained margins and intragranular shear fractures of plagioclase porphyroclasts are replaced by muscovite and quartz while their intragranular extension fractures and pressure shadow are filled by fine-grained K-feldspar, muscovite and quartz. In the highly deformed portion of the shear zone, fine-grained plagioclase (46%), quartz (32%) and muscovite (20%) form monomineralic layers although plagioclase-rich layers contain minor amount of muscovite and quartz. If grain-size reduction of feldspar in brittle-ductile transition regime is accomplished by strain-related myrmekite formation and replacement in addition to fracturing, the crustal strength is presumably lower than assuming feldspar fracturing only.
DE: 8030 Microstructures
DE: 8159 Rheology--crust and lithosphere
DE: 5104 Fracture and flow
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting