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<p>In the Cabo Ortegal Complex, NW Iberia, the subduction-related Variscan (ca. 390&#xa0;Ma) high-pressure granulites of the Bacariza Formation (Cambrian protolith age, ca. 500&#xa0;Ma) commonly show banding/regions defined by varying mineral abundance of amphibole and plagioclase-quartz that overprint high-pressure high-temperature (HP-HT) garnet and Na-rich diopsidic clinopyroxene, implying retrogression-related replacements triggered by the infiltration of H<sub>2</sub>O-bearing fluids. Regions with varied amphibole+plagioclase and garnet+clinopyroxene abundances are similar in bulk-rock composition, however, denoting insignificant metasomatism during the post-peak infiltration of H<sub>2</sub>O-fluid. On the contrary, bands rich in carbonate (Mg-calcite) denote substantial changes in bulk-rock composition relative to the original magmatic protoliths. This study presents petrographic and geochemical data and thermodynamic modelling of metamorphosed banded metasomatic carbonate-bearing mafic granulites of the Bacariza Formation. The textural-mineral assemblage development of the rocks made of varied amounts of garnet, Na diopside, calcite, epidote, scapolite, plagioclase, amphibole, rutile, titanite and phengite, plus relict K-feldspar and chlorite, denotes pre-metamorphic CaO-K<sub>2</sub>O-CO<sub>2</sub>-SO<sub>3</sub>-H<sub>2</sub>O metasomatic transformations to varied extent and subsequent roughly isochemical granulitic HP–HT metamorphism characterized by garnet + Na diopside+calcite+epidote+titanite+SO<sub>4</sub>-CO<sub>3</sub>-rich scapolite±plagioclase in the more carbonated bands. U-Pb dating of texturally-peak titanite yielded 387.3 ± 1.4/5.8&#xa0;Ma, interpreted as a cooling age near the peak of metamorphism, while dating of texturally-peak epidote yielded 373.2 ± 3.7 / 6.6&#xa0;Ma of uncertain significance. Subsequently, these assemblages did not experience significant retrogression during exhumation and cooling, implying a limited influx of fluid. Hence, the localized metasomatism of the Bacariza Formation occurred prior to early Devonian Variscan HP-HT metamorphism due to infiltration of aqueous CO<sub>2</sub>-SO<sub>3</sub>-Ca-K rich fluids during the Cambrian volcanic-arc stage that formed the protoliths of this Formation or after its accretion to the margin of Gondwana, perhaps during the Ordovician rifting stage that affected this margin.</p>

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High-pressure granulitic metamorphism of pre-Variscan metasomatic mafic rocks (Cabo Ortegal, NW Iberia)

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  • 发表日期：

    2026年5月26日

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  • A. Garcia-Casco,
  • I. Novo-Fernández,
  • N. Pujol-Solà,
  • A. Beranoaguirre,
  • J. A. Proenza,
  • R. Arenas

摘要

In the Cabo Ortegal Complex, NW Iberia, the subduction-related Variscan (ca. 390 Ma) high-pressure granulites of the Bacariza Formation (Cambrian protolith age, ca. 500 Ma) commonly show banding/regions defined by varying mineral abundance of amphibole and plagioclase-quartz that overprint high-pressure high-temperature (HP-HT) garnet and Na-rich diopsidic clinopyroxene, implying retrogression-related replacements triggered by the infiltration of H2O-bearing fluids. Regions with varied amphibole+plagioclase and garnet+clinopyroxene abundances are similar in bulk-rock composition, however, denoting insignificant metasomatism during the post-peak infiltration of H2O-fluid. On the contrary, bands rich in carbonate (Mg-calcite) denote substantial changes in bulk-rock composition relative to the original magmatic protoliths. This study presents petrographic and geochemical data and thermodynamic modelling of metamorphosed banded metasomatic carbonate-bearing mafic granulites of the Bacariza Formation. The textural-mineral assemblage development of the rocks made of varied amounts of garnet, Na diopside, calcite, epidote, scapolite, plagioclase, amphibole, rutile, titanite and phengite, plus relict K-feldspar and chlorite, denotes pre-metamorphic CaO-K2O-CO2-SO3-H2O metasomatic transformations to varied extent and subsequent roughly isochemical granulitic HP–HT metamorphism characterized by garnet + Na diopside+calcite+epidote+titanite+SO4-CO3-rich scapolite±plagioclase in the more carbonated bands. U-Pb dating of texturally-peak titanite yielded 387.3 ± 1.4/5.8 Ma, interpreted as a cooling age near the peak of metamorphism, while dating of texturally-peak epidote yielded 373.2 ± 3.7 / 6.6 Ma of uncertain significance. Subsequently, these assemblages did not experience significant retrogression during exhumation and cooling, implying a limited influx of fluid. Hence, the localized metasomatism of the Bacariza Formation occurred prior to early Devonian Variscan HP-HT metamorphism due to infiltration of aqueous CO2-SO3-Ca-K rich fluids during the Cambrian volcanic-arc stage that formed the protoliths of this Formation or after its accretion to the margin of Gondwana, perhaps during the Ordovician rifting stage that affected this margin.