論文 - 芳野 極
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H Yamamoto, T Yoshino
LITHOS 43 ( 4 ) 219 - 234 1998年9月
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記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ELSEVIER SCIENCE BV
A deep-level crustal section of the Cretaceous Kohistan are is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene + clinopyroxene + plagioclase +/- quartz [1]): and garnet-clinopyroxene granulite (garnet + clinopyroxene + plagioclase + quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet-clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet-clinopyroxene granulite. Garnet-clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende +/- garnet + plagioclase + quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression-dehydration reaction: hornblende + orthopyroxene + plagioclase = garnet + clinopyroxene + quartz + H2O up arrow. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet + clinopyroxene + plagioclase + H2O = hornblende + quartz and plagioclase + hornblende + H2O = zoisite + chlorite + quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet + hornblende + clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal-chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet-clinopyroxene granulite, The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening, through the development of the Kohistan are and the subsequent KKohistan-Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision. (C) 1998 Elsevier Science B.V. All rights reserved.
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T. Yoshino, H. Yamamoto, T. Okudaira, M. Toriumi
Journal of Metamorphic Geology 16 ( 6 ) 729 - 748 1998年
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記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Blackwell Publishing Inc.
The lower-crustal rocks of the Kohistan complex (northern Pakistan) are mostly composed of metabasic rocks such as pyroxene granulites, garnet granulites and amphibolites. We have investigated P-T trajectories of the relic two-pyroxene granulites, which are the protolith of the amphibolites within the Kamila amphibolite belt. Aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core. The significant amount of Al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film-like quartz as a product at grain boundaries between plagioclase and clinopyroxene. Distinct Al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim, and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in agreement with the zoning profiles of Ca-Tschermaks and jadeite components inferred from variations of Al, Na, Ti and Fe3+ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geothermobarometry based on Al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The peak of granulite facies metamorphism occurred at conditions of about 800°C and 800-1100 MPa. These prograde P-T paths represent a crustal thickening process of the Kohistan arc during the Early to Middle Cretaceous. The crustal thickening of the Kohistan arc was caused by accretion of basaltic magma at mid-crustal depths.