New SHRIMP U-Pb zircon ages and CHIME monazite ages from South Harris granulites, Lewisian Complex, NW Scotland: Implications for two stages of zircon formation during Palaeoproterozoic UHT metamorphism

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• 作者：Sotaro Baba^a ; ^{baba@edu.u-ryukyu.ac.jp} ; Daniel J. Dunkley^b ; ^d ; Tomokazu Hokada^b ; Kenji Horie^b ; Kazuhiro Suzuki^c ; Kazuyuki Shiraishi^b
• 关键词：SHRIMP U&ndash ; Pb age dating ; CHIME monazite ages ; Lewisian Complex ; South Harris ; UHT metamorphism
• 刊名：Precambrian Research
• 出版年：2012
• 期刊代码：75_03019268
• 类别：ear
• 出版时间：April, 2012
• 卷：200-203
• 期：Complete
• 页码：104-128
• 文件大小：4096 K

摘要

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age dating was applied to several types of granulites and gneisses from the Lewisian Complex in South Harris, NW Scotland, to clarify the timing of Palaeoproterozoic tectono-thermal events. For comparison, monazite was dated using the chemical Th-U-total Pb isochron method (CHIME). The obtained U-Pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole-rock compositions of analysed samples, the zircon internal texture based on Th/U ratios, and the monazite CHIME ages. Zircons in three samples, characterised by zoned overgrowth rims, sector-zoned grains, and structureless high-U cores, yielded concordant ages of approximately 1900 Ma (1909 卤 3 Ma, 1899 卤 10, and 1888 卤 15 Ma, respectively). The age of 1899 卤 10 Ma, obtained from melanocratic granulite (garnet and kyanite/sillimanite: 76-91 vol.%), is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite/sillimanite. Therefore, the age of 鈭?900 Ma is proposed to represent the timing of early metamorphism, close to the thermal peak (M1). In the leucocratic gneiss (sample 93921-7), the age population at 1868 卤 35 Ma obtained from low-U and low-Th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt (M3), whereas the high-U cores represent the timing of zircon growth and recrystallisation prior to anatexis. Monazites from two leucocratic gneisses yield a relatively narrow range of CHIME ages: 1858 卤 7 Ma and 1840 卤 7 Ma. These ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the U-Th-Pb system. These results suggest that the timing of zircon formation was controlled by metamorphic processes, such as restite formation and crystallisation from partial melt.