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<p>Apatite is a key accessory phase in metamorphic rocks, capable of recording mineral growth, deformation, and fluid-rock interaction across a wide range of metamorphic conditions. To evaluate the mechanisms of apatite growth and deformation, we present an integrated microstructural and geochemical study of apatite in high-pressure granulite- to lower amphibolite-facies rocks from the Eastern Himalayan Syntaxis, Tibetan Plateau. Cathodoluminescence imaging, electron backscatter diffraction, electron probe microanalysis, and in situ laser ablation inductively coupled plasma mass spectrometry trace element data were combined to assess apatite growth histories and post-crystallization modifications. In high-pressure granulite-facies samples, apatite grains display strong shape-preferred orientation (SPO) and significant intragranular deformation but lack a crystallographic preferred orientation (CPO), consistent with pre-peak metamorphism growth and experienced peak metamorphic conditions. Medium-pressure granulite-facies apatite shows oscillatory zoning, weak SPO and CPO, and limited deformation, indicating post-tectonic growth from compositionally homogeneous fluids. Apatite from upper amphibolite-facies samples preserves both strong SPO and CPO, with only minor deformation features, suggesting syn-tectonic crystallization during foliation development. In lower amphibolite-facies rocks, apatite is characterized by core-mantle structures, weak SPO, but CPO subparallel to stretching lineation, and variable trace element patterns, consistent with episodic growth during fluid-mediated dissolution–precipitation under retrograde conditions. Systematic variations in apatite chemistry—including decreasing Sr/Y, ΣLREE, and (La/Yb)<sub>N</sub>, and increasing Th/U and F/Cl with decreasing metamorphic grade—reflect the influence of metamorphic fluid composition, mineral reaction pathways, and elemental partition among mineral phases at corresponding metamorphic conditions. Despite differences in deformation and growth history, consistent correlations between texture and composition suggest localized, short-range fluid-mediated element redistribution. Our results demonstrate that multiple generations of apatite can be distinguished through integrated microstructural and geochemical datasets, providing a robust archive of pressure–temperature (<i>P–T</i>) conditions and fluid evolution in metamorphic terranes.</p>

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Growth and deformation of apatite across metamorphic facies during collisional orogenesis

  • 期刊论文

  • 发表日期：

    2026年1月6日

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  • Yanyan Zhang,
  • Zhongbao Zhao,
  • Wentao Cao,
  • Jinxue Du,
  • Zuolin Tian,
  • Paul D. Bons,
  • Xuxuan Ma,
  • Xiaohong Mao,
  • Fenghua Liang,
  • Zhiyong Zhu,
  • Haibing Li,
  • Guocan Wang

摘要

Apatite is a key accessory phase in metamorphic rocks, capable of recording mineral growth, deformation, and fluid-rock interaction across a wide range of metamorphic conditions. To evaluate the mechanisms of apatite growth and deformation, we present an integrated microstructural and geochemical study of apatite in high-pressure granulite- to lower amphibolite-facies rocks from the Eastern Himalayan Syntaxis, Tibetan Plateau. Cathodoluminescence imaging, electron backscatter diffraction, electron probe microanalysis, and in situ laser ablation inductively coupled plasma mass spectrometry trace element data were combined to assess apatite growth histories and post-crystallization modifications. In high-pressure granulite-facies samples, apatite grains display strong shape-preferred orientation (SPO) and significant intragranular deformation but lack a crystallographic preferred orientation (CPO), consistent with pre-peak metamorphism growth and experienced peak metamorphic conditions. Medium-pressure granulite-facies apatite shows oscillatory zoning, weak SPO and CPO, and limited deformation, indicating post-tectonic growth from compositionally homogeneous fluids. Apatite from upper amphibolite-facies samples preserves both strong SPO and CPO, with only minor deformation features, suggesting syn-tectonic crystallization during foliation development. In lower amphibolite-facies rocks, apatite is characterized by core-mantle structures, weak SPO, but CPO subparallel to stretching lineation, and variable trace element patterns, consistent with episodic growth during fluid-mediated dissolution–precipitation under retrograde conditions. Systematic variations in apatite chemistry—including decreasing Sr/Y, ΣLREE, and (La/Yb)N, and increasing Th/U and F/Cl with decreasing metamorphic grade—reflect the influence of metamorphic fluid composition, mineral reaction pathways, and elemental partition among mineral phases at corresponding metamorphic conditions. Despite differences in deformation and growth history, consistent correlations between texture and composition suggest localized, short-range fluid-mediated element redistribution. Our results demonstrate that multiple generations of apatite can be distinguished through integrated microstructural and geochemical datasets, providing a robust archive of pressure–temperature (P–T) conditions and fluid evolution in metamorphic terranes.