| Citation: |
Yuan Tang, Ya-dong Qin, Xiao-dong Gong, Yao-yao Duan, Gang Chen, Hong-you Yao, Jun-xiong Liao, Shi-yong Liao, Dong-bing Wang, Bao-di Wang, 2020. Discovery of eclogites in Jinsha River suture zone, Gonjo County, eastern Tibet and its restriction on Paleo-Tethyan evolution, China Geology, 3, 83-103. doi: 10.31035/cg2020003
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Discovery of eclogites in Jinsha River suture zone, Gonjo County, eastern Tibet and its restriction on Paleo-Tethyan evolution
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Abstract
As the important component of the eastern Tethys tectonic region, the Jinsha River-Ailao Mountain suture zone is often considered to be an ophiolitic mélange belt. However, the P-T-t path and chronological framework of the metamorphic evolution in the collisional orogenetic process of this zone are still poorly understood owing to the lack of metamorphism research of symbolic high-pressure rocks. During a regional geological survey on a scale of 1∶50000 in Gonjo County, Tibet Autonomous Region involved in this paper, (retrograde) eclogites lenses of different scales were found in Jinsha River suture zone, eastern Tibet for the first time. The (retrograde) eclogites can be divided into garnet-albite-chlorite-actinolite schists and eclogites according to retrograde degrees. The mainly mineral components of eclogites include garnet (45%–50%), clinopyroxene (about 25%), and hornblende (5%–10%) primarily, and biotite, quartz, rutile, and muscovite secondarily. According to the data of electron probe micro analysis (EPMA), clinopyroxenes feature high content of Na2O (5.6%–6%) and corresponding jadeite (Jd) molecules of 37%–44%, and they fall within the omphacite region in Quad-Jd-Ae diagram. The temperature and pressure of the metamorphism at peak are P≈2.2–2.34 GPa and T≈622–688 °C respectively as measured with geobarometry Grt-Omp-Phe and geothermometer Grt-Omp. This will provide a new reference for the understanding of Paleo-Tethyan evolution. In this paper, two samples of eclogites were chosen for LA-ICP-MS zircon U-Pb dating and their 206Pb/238U ages obtained are 240 ± 3 Ma and 244 ± 1 Ma respectively. Furthermore, the zircons feature extremely low Th/U ratio (<0.01), extremely low content of Nb, Ta, and HREE, and invisible negative Eu anomaly. Therefore, the genesis of the zircons shall be eclogites-facies metamorphism, indicating that the 240–245 Ma determined in this paper shall be the age of eclogites-facies metamorphism and may represent the westward subduction-collision epoch of Paleo-Tethys Ocean located between Zhongza Block and Qamdo Block.
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Keywords:
- Eclogite /
- Omphacite /
- Jinsha River suture zone /
- Geological survey engineering /
- Qinghai-Tibet plateau /
- Eastern Tibet /
- China
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References
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Yuan Tang, Ya-dong Qin, Xiao-dong Gong, Yao-yao Duan, Gang Chen, Hong-you Yao, Jun-xiong Liao, Shi-yong Liao, Dong-bing Wang, Bao-di Wang, 2020. Discovery of eclogites in Jinsha River suture zone, Gonjo County, eastern Tibet and its restriction on Paleo-Tethyan evolution, China Geology, 3, 83-103. doi: 10.31035/cg2020003
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Figure 1.
Simplified tectonic map with age data, showing the Paleo-Tethyan system in Tibet and adjacent area (after Xu ZQ et al., 2015).
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Figure 2.
a–Geotectonic schematic map of Jinsha River suture zone (after Li XZ et al., 1999); b–generalized geological map of Jinsha River suture zone, Gonjo, Eastern Tibet (its location is shown in Fig. 2a).
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Figure 3.
Field occurrence characteristics of retrograde eclogites. a–d–retrograde eclogites occurring in the shape of lens; e–chlorite retrograded from garnet; f–white rim structure (yellow arrow) developed around a part of garnets; g–eclogites with low retrograde metamorphism.
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Figure 4.
Microscopic characteristics of garnet-albite-chlorite actinolite (cross-polarized light). a–residual garnets and small chlorite + albite + actinolite compose symplectites, and retrograde reaction rim; b–residual pyroxene. Grt–garnet, Chl–chlorite, Ab–albite, Act–actinolite, Cpx–clinopyroxene.
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Figure 5.
Microscopic characteristics of eclogites (a, c–cross-polarized light; b, d–plane-polarized light); cracks visible and fine mineral inclusions developing in garnet crystals; omphacite hypautomorphic columnar in shape and sub-amphibolized, with rutile inclusions visible inside garnet grains. Grt–garnet, Omp–omphacite, Amp–hornblende, Bt–biotite, Qz–quartz, Ms–muscovite, Ru–rutile, red arrows indicates blue-green hornblendes growing around the garnet.
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Figure 6.
SEM images of eclogites. a–garnet with radial cracks and phengite occurring among cracks in the fine foliaceous shape; b–worm-like fine grains generating omphacite rim owing to decompression and decomposition; c–rutile in xenomorphic granular shape; d–diagram of garnet components obtained by EPMA. Grt–garnet, Omp–omphacite, Phg–phengite, Ru–rutile.
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Figure 7.
a–Quad-Jd-Ae compositional diagram of clinopyroxene, showing all testing points falling within the omphacite region; b–characteristics of Raman spectra of clinopyroxene in the eclogites, having the characteristic spectral peaks of omphacite; c–end-member composition diagramof garnet, showing testing points falling within the area of class-C garnet; d–garnet composition diagram; e–f–hornblende composition diagram, showing testing pointsfalling within subclasses magnesiohornblende and achromaite (a, after Morimoto N et al., 1988; c, after Coleman RG et al., 1965; e–f, after Leake BE et al., 1997).
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Figure 8.
Partial CL images of zircons in eclogites and their hosting rocks.
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Figure 9.
a–U-Pb age of zircons in sample D0521-4; b–cumulative gaussian and histogram plots of zircons in sample D0521-4; c–U-Pb concordant diagrams of zircons in sample D0521-1; d–contents of Th and U and Th/U ratios of zircons in samples D0521-4 and D0521-1; e–f–REE chondrite normalized diagram of in situ single zircon in samples D0521-4 and D0521-1. Chondrite values are from Sun SS and McDonough WF, 1989.
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Figure 10.
a–b–U-Pb age concordia diagram of zircons in samples D01537-1 and D1537-2; c–content of Th and U and Th/U ratio of zircon in samples D01537-1 and D1537-2; d–chondrite-normalized REE patterns for in situ single zircon from sample D0521-4 and D0521-1. Chondrite values are from Sun SS and McDonough WF, 1989.
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Figure 11.
Content of Ta and Nb of zircons in the samples for LA-ICP-MS zircon U-Pb dating.