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<p>The Jijal Complex in the Kohistan region of northern Pakistan consists of ultramafic to mafic plutonic rocks, including dunite, pyroxenite, garnet-bearing hornblendite (meta-hornblendite), and garnet gabbro. Chromite mineralization occurs in medium to thick chromite-bearing layers within the ultramafic rocks, primarily in dunite. The sequence of lithologies (from dunite through pyroxenite to garnet-bearing hornblendite and garnet gabbro) is interpreted to represent a fractionation sequence derived from the crystallization of mafic–ultramafic magma. Chromite deposits in the region have been extensively documented and mined. These deposits, along with their host ultramafic rocks, formed during the continuous convergence of the Indian and Asian plates. Field investigations reveal that chromite occurs as lenses, layers, or disseminated ore deposits, with disseminated deposits (defined here as chromite constituting &lt; 20% modal abundance) occurring alongside massive layers (&gt; 50% chromite). Petrographic data indicate that the chromite grains are typically medium-grained, with euhedral to subhedral textures, and may contain primary olivine inclusions, with serpentine occurring along fractures due to hydrothermal alteration. The field and microscopic data suggest that these rocks are ultramafic cumulates derived from arc-related tholeiitic magma, which possibly formed at pressures exceeding 9&#xa0;kbar on the southern edge of the Kohistan Island Arc (KIA), as indicated by the stable coexistence of garnet and clinopyroxene in ultramafic cumulates, the presence of high-Mg garnet compositions consistent with high-pressure crystallization (e.g., garnet pyroxenite assemblages), exsolution textures in clinopyroxene suggesting crystallization at elevated pressures, and the absence of plagioclase in garnet-bearing domains, consistent with experimental phase equilibria constraining garnet stability to pressures above ~ 9 kbar (Green and Ringwood, Contrib Mineral Petrol 15(1):103–190, 1967). This interpretation is further supported by previous studies documenting garnet stability fields and mineral chemistry in the Jijal Complex (Yamamoto and Yoshino, Isl Arc 7(3):330–341, 1998; Khan et al., Geol Soc London Spec Publ 74(1):123–138, 1993). Small ultramafic bodies also occur as conformable layers and emplaced masses within garnet granulites. Chromite compositions are typically homogeneous within the ore deposits, whereas chromite grains in dunite and pyroxenite display varying compositions.</p>

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Geotectonic Setting of the Dubair Nallah Ophiolite, Kohistan Region, Pakistan: Insights from Geology and Petrography

  • 期刊论文

  • 发表日期：

    2025年10月17日

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  • Johar Ali,
  • Mohammad Ali Rajabzadeh,
  • Sina Asadi,
  • Muhammad Saleem Mughal,
  • Mirza Nasir Ali

摘要

The Jijal Complex in the Kohistan region of northern Pakistan consists of ultramafic to mafic plutonic rocks, including dunite, pyroxenite, garnet-bearing hornblendite (meta-hornblendite), and garnet gabbro. Chromite mineralization occurs in medium to thick chromite-bearing layers within the ultramafic rocks, primarily in dunite. The sequence of lithologies (from dunite through pyroxenite to garnet-bearing hornblendite and garnet gabbro) is interpreted to represent a fractionation sequence derived from the crystallization of mafic–ultramafic magma. Chromite deposits in the region have been extensively documented and mined. These deposits, along with their host ultramafic rocks, formed during the continuous convergence of the Indian and Asian plates. Field investigations reveal that chromite occurs as lenses, layers, or disseminated ore deposits, with disseminated deposits (defined here as chromite constituting < 20% modal abundance) occurring alongside massive layers (> 50% chromite). Petrographic data indicate that the chromite grains are typically medium-grained, with euhedral to subhedral textures, and may contain primary olivine inclusions, with serpentine occurring along fractures due to hydrothermal alteration. The field and microscopic data suggest that these rocks are ultramafic cumulates derived from arc-related tholeiitic magma, which possibly formed at pressures exceeding 9 kbar on the southern edge of the Kohistan Island Arc (KIA), as indicated by the stable coexistence of garnet and clinopyroxene in ultramafic cumulates, the presence of high-Mg garnet compositions consistent with high-pressure crystallization (e.g., garnet pyroxenite assemblages), exsolution textures in clinopyroxene suggesting crystallization at elevated pressures, and the absence of plagioclase in garnet-bearing domains, consistent with experimental phase equilibria constraining garnet stability to pressures above ~ 9 kbar (Green and Ringwood, Contrib Mineral Petrol 15(1):103–190, 1967). This interpretation is further supported by previous studies documenting garnet stability fields and mineral chemistry in the Jijal Complex (Yamamoto and Yoshino, Isl Arc 7(3):330–341, 1998; Khan et al., Geol Soc London Spec Publ 74(1):123–138, 1993). Small ultramafic bodies also occur as conformable layers and emplaced masses within garnet granulites. Chromite compositions are typically homogeneous within the ore deposits, whereas chromite grains in dunite and pyroxenite display varying compositions.