| Citation: |
Zheng-bin Gou, Bao-di Wang, Dong-bing Wang, Zhi-min Peng, 2021. High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication, China Geology, 4, 95-110. doi: 10.31035/cg2021010
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High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication
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Abstract
The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone (CMSZ) provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys. An integrated petrologic, phase equilibria modeling and geochronological study of the garnet amphibolites, southeast Tibetan Plateau, shows that the garnet amphibolites have a peak mineral assemblage of garnet, glaucophane, lawsonite, chlorite, rutile, phengite and quartz, and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495–515°C, indicating a cold geothermal gradient of about 240–260°C/GPa. Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies. The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies, and therefore the obtained zircon U-Pb age of 231 ± 1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence. The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded high-pressure eclogite-facies blueschist, instead of the previously proposed eclogites, and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.
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Zheng-bin Gou, Bao-di Wang, Dong-bing Wang, Zhi-min Peng, 2021. High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication, China Geology, 4, 95-110. doi: 10.31035/cg2021010
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Figure 1.
a–Geological map of the Sanjiang area in the southeastern Tibetan Plateau, with an inset map showing tectonic framework of southeastern Asia (modified from Wang HN et al, 2019b). N. Q.–North Qiangtang terrane; S. Q.–South Qiangtang terrane; CMSZ–Changning-Menglian suture zone; LSSZ-Longmu Co-Shuanghu suture zone; IBRS– Inthanon-Bentong-Raub suture zone; JASS–Jinshajiang-Ailaoshan-Song Ma suture. b–Sketch geological map of the Changning-Menglian orogen showing the location of the lawsonite-bearing eclogites and the studied samples (modified from Sun ZB et al, 2017a). 1–Quaternary sediments; 2–Jurassic sequences; 3–Upper Triassic sediments; 4–western Late Paleozoic metasedimentary rocks; 5–eastern Late Paleozoic metasedimentary rocks; 6–Early Paleozoic metamorphic complex (Lancang group); 7–Precambrian metamorphic basement; 8–Eocene syengranite; 9–Middle Triassic biotite monzonitic granite; 10–Permian granodiorite; 11–Tongchangjie ophiolitic mélange; 12–Wanhe ophiolitic mélange; 13–lithologic boundary; 14–unconformable boundary; 15–fault; 16–sample locations (red stars: this study; yellow stars: lawsonite-bearing eclogites).
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Figure 2.
Outcrop photographs and photomicrographs of the garnet amphibolites and metapelites from the Changning-Menglian suture zone. a–Elongated garnet amphibolites enclosed in metapelites. b–Blocks of garnet amphibolites. c–Garnet porphyroblast and matrix minerals of amphibole, plagioclase and titanite in garnet amphibolites; the porphyroblastic garnet is partially replaced by chlorite along its rim; sample 15SJ-28. d–Garnet porphyroblast with inclusions of quartz, lawsonite, epidote, amphibole and rutile in garnet amphibolites. The yellow line marked in garnet grain shows the location of zoning profile in Fig. 3b; sample 15SJ-28. (e) Medium-grained amphibole with core–rim structure and fine-grained amphibole–plagioclase symplectites in garnet amphibolites; sample 15SJ-28. f–Garnet porphyroblast in the matrix consisting of amphibole, plagioclase and titanite in garnet amphibolites; sample 15SJ-25. The subscripts “c” “r” “m” and “s” refer to the textural position “core” “rim” “matrix” and “symplectite”. The classification of amphiboles can be found in Figs. 4a, b. Mineral abbreviations are after Whitney DL and Evans BW (2010): Ab–albite; Act–actinolite; Amp–amphibole; Bt–biotite; Chl–chlorite; Coe–coesite; Cpx–clinopyroxene; Di– diopside; Ep–epidote; Gln–glaucophane; Grt–garnet; Hbl–hornblende; Ky–kyanite; Lws–lawsonite; Ms–muscovite; Omp–omphacite; Pl–plagioclase; Pg–paragonite; Qz–quartz; Rt–rutile; Ttn–titanite.
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Figure 3.
a–X-ray mappings of garnet porphyroblast in the garnet amphibolites; b–compositional profiles of garnet porphyroblast from the garnet amphibolites. XFe = Fe2+/ (Fe2+ + Mn + Mg + Ca), XCa, XMg and XMn defined accordingly.
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Figure 4.
a–Si (p.f.u.) vs. Mg/(Mg + Fe2+) and b–(Na + K)A vs. Mg/(Mg + Fe2+) diagrams of amphibole in the garnet amphibolite (after Leake BE et al., 1997). c–Ab–An–Or diagram of plagioclase in the garnet amphibolites. d–Mg + Fe2+ vs. Si (p.f.u.) diagram of phengite in the garnet amphibolites. The subscript “c” “r” and “s” refer to core, rim and symplectite.
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Figure 5.
a, b–P-T pseudosection for the garnet amphibolite sample 15SJ-28 calculated in the system MnNCKFMASHTO using the bulk-rock composition normalized based on XRF analysis: SiO2 = 56.70 (mol%), Al2O3 = 9.70, CaO = 5.31, MgO = 10.49, FeO = 10.25, K2O = 0.80, Na2O = 3.32, TiO2 = 1.75, MnO = 0.30, O = 1.38; c, d–P-T pseudosection calculated with an effective bulk composition generated by subtracting garnet core compositions from the above analyzed bulk-rock composition: SiO2 = 57.57 (mol%), Al2O3 = 9.47, CaO = 5.02, MgO = 10.88, FeO = 9.55, K2O = 0.84, Na2O = 3.51, TiO2 = 1.85, MnO = 0.01, O = 1.30. Tri-, quini-, hexa- and septi-variant fields are increasing heavily shaded. The pseudosections are contoured with isopleths of XMg and XCa in garnet and Si in phengite. The red- and white-filled circles represent intersections of XMg and XCa isopleths of garnet core and rim, respectively. The yellow lines with arrow represent the inferred prograde P-T paths. The mineral symbols are shown in Fig. 2
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Figure 6.
a–Cathodoluminescence images of zircon from the garnet amphibolite sample 15SJ-25, showing locations of analyzed spots and relevant ages (in Ma). b–chondrite-normalized REE patterns of zircons (data of chondrite after Sun SS and McDonough WF, 1989). c–U-Pb concordia diagram of zircons. d–diagram of zircon 206Pb/238U ages. The red and blue circles/lines refer to concordant and discordant dates, respectively. The orange circle/line indicates the date excluded from the weighted mean age calculation.
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Figure 7.
Summary of the metamorphic P-T-t path of the studied garnet amphibolites, the eclogites and blueschists from the Changning-Menglian and Longmu Co-Shuanghu suture zones. The major phase stability fields and boundaries of metamorphic facies follow Brown M (2009). The transition line of lawsonite and epidote is after Wei CJ and Clarke GL (2011), which is calculated based on the phase equilibria modeling of typical MORB composition. A–amphibolite facies; AE–amphibole eclogite facies; AEE–amphibole-epidote eclogite facies; ALE–amphibole-lawsonite eclogite facies; BS–blueschist; E-HPG–medium-temperature eclogite-high-pressure granulite metamorphism; G–granulite facies; GS–greenschist facies; LM–low grade metamorphism; D09, Dong YS and Li C, 2009; F15, Fan W et al., 2015; W16, Wang F et al., 2016; W19b, Wang HN et al., 2019b; Z06, Zhang KJ et al. (2006); Z09, Zhai QG et al., 2009; Z11, Zhai QG et al., 2011b; Z18, Zhang YX et al., 2018.