| Citation: |
Yu-zhen Fu, Zhi-ming Peng, Bao-di Wang, Guo-zhi Wang, Jing-feng Hu, Jun-lei Guan, Ji Zhang, Zhang Zhang, Yun-he Liu, Zou Hao, 2021. Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction, China Geology, 4, 111-125. doi: 10.31035/cg2021017
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Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction
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Abstract
High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M1), the peak eclogite facies (M2), the decompression blueschist facies (M3) and retrograde greenschist facies (M4). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.
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References
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Yu-zhen Fu, Zhi-ming Peng, Bao-di Wang, Guo-zhi Wang, Jing-feng Hu, Jun-lei Guan, Ji Zhang, Zhang Zhang, Yun-he Liu, Zou Hao, 2021. Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction, China Geology, 4, 111-125. doi: 10.31035/cg2021017
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Figure 1.
a–Tectonic location of the Changning-Menglian suture zone (after Wang BD et al., 2018); b–simplified geological map of the Langcang Group accretionary complex in southeast Tibetan Plateau Showing the outcroups of the buleschist and eclogites. The outcroups of the buleshist are from Zhang ZB et al., 2004; Wang F et al., 2016 and Fan WM et al., 2015.
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Figure 2.
Field occurrence characteristics of eclogites. a, b–beclogites occurring in the shape of lens in the Langcang Group accretionary complex; c, d–hand specimen of the eclogite samples investigated in this study. Grt–garnet; Gln–glaucophane; Omp–omphacite; Czo–clinozosite.
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Figure 3.
Photomicrographs and Backscattered-electron(BSE) images showing the textural relationship of glaucophane eclogite.
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Figure 7.
Chemical compositions of garnet, omphacite and amphibolite in the eclogite: a–teranry plot of (alm+sps)-grs-prp of garnet compositions (after Coleman R et al., 1965); b, d–compositional profile intersecting the center of the garnet porphyroblast in Fig. 5f and Fig. 6f showing the prograde growth both; c–ternay classification diagram for sodic clinopyroxenes after Morimoto N et al. (1988); e, f–chemical composition of amphibolite in the eclogite after Leake BE et al. (1997).
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Figure 5.
Element X-ray maps (a–e) and BSE image (f) of garnet in the glaucophane eclogite. The euhedral garnet crystal reveals a zonation and contains inclusions of omphacite (Omp-g), glaucophane (Gln-g), quartz (Qz) and rutile.
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Figure 4.
Microscopic characteristics of the actinolite-bearing eclogite. The eclogite with mineral assemblage of garnet (Grt) + omphacite (Omp) + actinolite (Act) + phengite (Ph) + clinozoisite (Czo), and chlorite (Chl) distributes surrounds garnet. The actinolite (Act) may be retrograded by the glaucophane (PPL).
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Figure 6.
Element X-ray maps (a–e) and BSE image (f) of garnet in the actinolite-bearing eclogite showing a zonation and poor inclusion.
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Figure 8.
Mineral assemblages and compositions during metamorphic evolutions.
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Figure 9.
P-T path for the eclogites in the Bangbing (blue arrow) and Mengku area (gray arrow). The bule diamond are P-T data estimated based on the geothermometer of Ravna K (2000) and geobarometer of Beyer C et al. (2015); the blue circles are the P-T data estimated based on the geothermometer of Ravna K (2000) and geobarometer of Ravna EJK and Terry MP (2004). The metamorphic facies and their abbreviations follow Liou JG et al. (2004) and Zhai QG et al. (2011b). Lws-EC–lawsonite-eclogite facies; Ep-EC–epidote-eclogite facies; Am-EC–amphibole- eclogite facies; BS–blueschist facies; EA–epidote amphibolite facies; AM–amphibolite facies; HGR–high granulite facies; GR–granulite facies; GS–greenschist facies.