Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan Arc, northern Pakistan: dehydration and rehydration within deep arc crust

Hiroshi Yamamoto, Takashi Yoshino

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49 Citations (Scopus)

Abstract

A deep-level crustal section of the cretaceous Kohistan arc 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 = 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 garanet clinopyroxenite, garnetite, and hornblendite. Field relationa nd 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 contstraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan-Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.

Original languageEnglish
Pages (from-to)219-234
Number of pages16
JournalLithos
Volume43
Issue number4
DOIs
Publication statusPublished - Sep 1998
Externally publishedYes

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rehydration
Garnets
Dehydration
dehydration
garnet
replacement
granulite
clinopyroxene
crust
Quartz
hornblende
plagioclase
quartz
orthopyroxene
Rocks
pyroxene
granulite facies
Bearings (structural)
collision
rock

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

@article{493a42bd34ad4b80bfed80ff829b4499,
title = "Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan Arc, northern Pakistan: dehydration and rehydration within deep arc crust",
abstract = "A deep-level crustal section of the cretaceous Kohistan arc 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 = 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 garanet clinopyroxenite, garnetite, and hornblendite. Field relationa nd 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 contstraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan-Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.",
author = "Hiroshi Yamamoto and Takashi Yoshino",
year = "1998",
month = "9",
doi = "10.1016/S0024-4937(98)00014-0",
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volume = "43",
pages = "219--234",
journal = "Lithos",
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T1 - Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan Arc, northern Pakistan

T2 - dehydration and rehydration within deep arc crust

AU - Yamamoto, Hiroshi

AU - Yoshino, Takashi

PY - 1998/9

Y1 - 1998/9

N2 - A deep-level crustal section of the cretaceous Kohistan arc 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 = 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 garanet clinopyroxenite, garnetite, and hornblendite. Field relationa nd 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 contstraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan-Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.

AB - A deep-level crustal section of the cretaceous Kohistan arc 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 = 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 garanet clinopyroxenite, garnetite, and hornblendite. Field relationa nd 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 contstraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan-Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.

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