Eclogites exposed in the Kaghan Valley of Pakistan are divided petrologically into two groups as ... more Eclogites exposed in the Kaghan Valley of Pakistan are divided petrologically into two groups as high- pressure (HP) and ultrahigh-pressure (UHP) eclogites. Group I (HP) eclogites are composed of garnet + clinopyroxene + quartz + amphibole ± phengite ± epidote/allanite + rutile ± ilmenite with abundant zircon grains of large size (> 250 µm), considered as derived from a gabbroic protolith. Group II (UHP) eclogites are composed of garnet + clinopyroxene + coesite + epidote + phengite + amphibole + rutile/ilmenite with rare zircon grains of small size (< 50 µm), considered as the products of tholeiitic basalts. Whole-rock multi-isotope systems indicate considerable trace element mobility or fluid infiltration during subduction of the Indian plate under the Asian plate. Due to this mobility or fluid infiltration, the Sm-Nd, and Lu-Hf isotope systems do not define a geologically meaningful age. The very low Sm/Nd (< 0.6) and Lu/Hf (< 0.4) ratios of garnet and the variable tw...
Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the ... more Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the Kaghan Valley, Pakistan in this study. The highest metamorphic grades are recorded in the north of the valley, near the India–Asia collision boundary, in the form of high-pressure (HP: Group I) and ultrahigh-pressure (UHP: Group II) eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267 ± 2.4 Ma), which were emplaced along the northern margin of Indiawhen it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and −2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (−2.25 to−1.44‰) in the central, the best preserved (least retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7 ± 0.3‰. The unusually lowor negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during exhumation-related retrogression.
Geological observations in the Horoman area, south-central Hokkaido, show that the Horoman perido... more Geological observations in the Horoman area, south-central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east-dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top-to-the-west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top-to-the-east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.
The Himalayan Metamorphic Belt of the Kaghan Valley transect in northern Pakistan is a key area w... more The Himalayan Metamorphic Belt of the Kaghan Valley transect in northern Pakistan is a key area where rocks of the Indian Plate have been subducted to greater depths >100 km beneath the Asian Plate and experienced high- to ultrahigh-pressure metamorphism. At least three distinct metamorphic events can be recognized in the metamorphosed mafic rock units (eclogites and amphibolitized eclogites) of the Kaghan Valley: (i) prograde; (ii) peak; and (iii) retrograde stages. Chemical composition, zoning pattern and inclusions in garnet, and metamorphic reactions among major mineral assemblage (garnet + clinopyroxene + quartz + amphibole + epidote) show that core portions of garnet grew under the prograde stage, whereas most of the middle portions were developed or recrystallized under the high-pressure stage with P–T conditions at 2.3 ± 0.4 GPa and 766 ± 107°C. The rim portions of garnet in association with amphibole and presence of symplectites indicate amphibole-eclogite to amphibolite-facies (retrograde) stage with average P–T conditions at 1.5 ± 0.2 GPa and 710 ± 75°C. Zoning in garnet shows Ca-poor wider cores, Ca-rich thin middle portions, and chlorite and amphibole development along the outermost rims, suggesting prograde growth at cores and homogenous middle portions and retrogressive formation at rims during the amphibolite-facies stage. These features suggest that most of the garnet grew during subduction of the Indian Plate lithosphere before the India–Asia collision, whereas rim portions developed during rapid exhumation in the post-collision stage.
We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ult... more We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100 μm up to 500 μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6 ± 2.4 Ma (MSWD = 8.5), have higher U and Th contents with a Th/U ratio > 1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71 Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9 ± 1.2 Ma (MSWD = 4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.► The Himalayan HP and UHP eclogites contain two generation (igneous and metamorphic) of zircons. ► Magmatic zircons are oscillatory zoned, have higher U and Th contents (Th/U ratio > 1) formed in Permian (267 Ma). ► Metamorphic zircons are sector zoned, have lower U and Th contents (Th/U ratio < 0.04) formed in Eocene (45 Ma).
Eclogites exposed in the Kaghan Valley of Pakistan are divided petrologically into two groups as ... more Eclogites exposed in the Kaghan Valley of Pakistan are divided petrologically into two groups as high- pressure (HP) and ultrahigh-pressure (UHP) eclogites. Group I (HP) eclogites are composed of garnet + clinopyroxene + quartz + amphibole ± phengite ± epidote/allanite + rutile ± ilmenite with abundant zircon grains of large size (> 250 µm), considered as derived from a gabbroic protolith. Group II (UHP) eclogites are composed of garnet + clinopyroxene + coesite + epidote + phengite + amphibole + rutile/ilmenite with rare zircon grains of small size (< 50 µm), considered as the products of tholeiitic basalts. Whole-rock multi-isotope systems indicate considerable trace element mobility or fluid infiltration during subduction of the Indian plate under the Asian plate. Due to this mobility or fluid infiltration, the Sm-Nd, and Lu-Hf isotope systems do not define a geologically meaningful age. The very low Sm/Nd (< 0.6) and Lu/Hf (< 0.4) ratios of garnet and the variable tw...
Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the ... more Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the Kaghan Valley, Pakistan in this study. The highest metamorphic grades are recorded in the north of the valley, near the India–Asia collision boundary, in the form of high-pressure (HP: Group I) and ultrahigh-pressure (UHP: Group II) eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267 ± 2.4 Ma), which were emplaced along the northern margin of Indiawhen it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and −2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (−2.25 to−1.44‰) in the central, the best preserved (least retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7 ± 0.3‰. The unusually lowor negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during exhumation-related retrogression.
Geological observations in the Horoman area, south-central Hokkaido, show that the Horoman perido... more Geological observations in the Horoman area, south-central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east-dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top-to-the-west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top-to-the-east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.
The Himalayan Metamorphic Belt of the Kaghan Valley transect in northern Pakistan is a key area w... more The Himalayan Metamorphic Belt of the Kaghan Valley transect in northern Pakistan is a key area where rocks of the Indian Plate have been subducted to greater depths >100 km beneath the Asian Plate and experienced high- to ultrahigh-pressure metamorphism. At least three distinct metamorphic events can be recognized in the metamorphosed mafic rock units (eclogites and amphibolitized eclogites) of the Kaghan Valley: (i) prograde; (ii) peak; and (iii) retrograde stages. Chemical composition, zoning pattern and inclusions in garnet, and metamorphic reactions among major mineral assemblage (garnet + clinopyroxene + quartz + amphibole + epidote) show that core portions of garnet grew under the prograde stage, whereas most of the middle portions were developed or recrystallized under the high-pressure stage with P–T conditions at 2.3 ± 0.4 GPa and 766 ± 107°C. The rim portions of garnet in association with amphibole and presence of symplectites indicate amphibole-eclogite to amphibolite-facies (retrograde) stage with average P–T conditions at 1.5 ± 0.2 GPa and 710 ± 75°C. Zoning in garnet shows Ca-poor wider cores, Ca-rich thin middle portions, and chlorite and amphibole development along the outermost rims, suggesting prograde growth at cores and homogenous middle portions and retrogressive formation at rims during the amphibolite-facies stage. These features suggest that most of the garnet grew during subduction of the Indian Plate lithosphere before the India–Asia collision, whereas rim portions developed during rapid exhumation in the post-collision stage.
We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ult... more We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100 μm up to 500 μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6 ± 2.4 Ma (MSWD = 8.5), have higher U and Th contents with a Th/U ratio > 1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71 Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9 ± 1.2 Ma (MSWD = 4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.► The Himalayan HP and UHP eclogites contain two generation (igneous and metamorphic) of zircons. ► Magmatic zircons are oscillatory zoned, have higher U and Th contents (Th/U ratio > 1) formed in Permian (267 Ma). ► Metamorphic zircons are sector zoned, have lower U and Th contents (Th/U ratio < 0.04) formed in Eocene (45 Ma).
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eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267 ± 2.4 Ma), which were emplaced along the northern margin of Indiawhen it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and −2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (−2.25 to−1.44‰) in the central, the best preserved (least
retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7 ± 0.3‰. The unusually lowor negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration
of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during
exhumation-related retrogression.
eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267 ± 2.4 Ma), which were emplaced along the northern margin of Indiawhen it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and −2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (−2.25 to−1.44‰) in the central, the best preserved (least
retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7 ± 0.3‰. The unusually lowor negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration
of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during
exhumation-related retrogression.