Chlorine-rich potassic-ferro-pargasite with Cl-poor cummingtonite in mafic granulite from Austhovde, Lützow-Holm Complex, East Antarctica

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  • HIROI Yoshikuni
    Department of Earth Sciences, Chiba University National Institute of Polar Research
  • HOKADA Tomokazu
    National Institute of Polar Research Department of Polar Science, SOKENDAI
  • ADACHI Tatsuro
    Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University
  • KAMEI Atsushi
    Department of Earth Science, Interdisciplinary Faculty of Science and Engineering, Shimane University
  • SHIRAISHI Kazuyuki
    National Institute of Polar Research
  • MOTOYOSHI Yoichi
    National Institute of Polar Research

説明

<p>Blue-green potassic-ferro-chloro-pargasite containing up to 0.3 wt% BaO and 4.6 wt% Cl occurs together with Cl-poor cummingtonite in a mafic granulite composed mainly of olive-brown potassic-ferro-pargasite with subordinate orthopyroxene, Ba- and Cl-bearing biotite, calcic plagioclase, quartz, ilmenite, zircon, and Cl-rich fluorapatite. The mafic granulite is characterized by high bulk rock contents of large-ion lithophile elements such as K, Ba, Pb, and Rb. Mineral assemblages, compositions and textures suggest four stages of recrystallization to form and modify the potassic-ferro-pargasite. Stage 1 is the main granulite-facies metamorphism, whereas stages 2-4 correspond to local phenomena most probably caused by fluid infiltration along a fracture during cooling. Stage 2 includes biotite formation with concomitant compositional change of orthopyroxene and potassic-ferro-pargasite to more Fe-rich. Fluorapatite partially changed composition from Cl-rich to Cl-poor. Stage 3 was a more localized event including (1) increase of K, Cl, Al, and Fe and decrease of Si, Ti, and Mg in the potassic-ferro-pargasite, accompanied by the color change from olive-brown to blue-green, and (2) formation of Cl-poor cummingtonite and sodic plagioclase by replacement of preexisting potassic-ferro-pargasite. Stage 4 was a more localized phenomenon within blue-green potassic-ferro-pargasite. It may be expressed as a volatile-conserving reaction such as Cl-bearing potassic-ferro-pargasite → potassic-ferro-chloro-pargasite + Cl-poor cummingtonite. The mafic granulite appears not to have undergone partial melting even at stage 1 most probably because of the high Cl content of the dominant potassic-ferro-pargasite.</p>

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