| Citation: |
Yuan Tang, Yu-ping Liu, Peng Wang, Wen-qing Tang, Ya-dong Qin, Xiao-dong Gong, Dong-bing Wang, Bao-di Wang, 2021. A new understanding of Demala Group complex in Chayu Area, southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating, China Geology, 4, 77-94. doi: 10.31035/cg2021021
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A new understanding of Demala Group complex in Chayu Area, southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating
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Abstract
The Chayu area is located at the southeastern margin of the Qinghai-Tibet Plateau. This region was considered to be in the southeastward extension of the Lhasa Block, bounded by Nujiang suture zone in the north and Yarlung Zangbo suture zone in the south. The Demala Group complex, a set of high-grade metamorphic gneisses widely distributed in the Chayu area, is known as the Precambrian metamorphic basement of the Lhasa Block in the area. According to field-based investigations and microstructure analysis, the Demala Group complex is considered to mainly consist of banded biotite plagiogneisses, biotite quartzofeldspathic gneiss, granitic gneiss, amphibolite, mica schist, and quartz schist, with many leucogranite veins. The zircon U-Pb ages of two granitic gneiss samples are 205 ± 1 Ma and 218 ± 1 Ma, respectively, representing the ages of their protoliths. The zircons from two biotite plagiogneisses samples show core-rim structures. The U-Pb ages of the cores are mainly 644–446 Ma, 1213–865 Ma, and 1780–1400 Ma, reflecting the age characteristics of clastic zircons during sedimentation of the original rocks. The U-Pb ages of the rims are from 203 ± 2 Ma to 190 ± 1 Ma, which represent the age of metamorphism. The zircon U-Pb ages of one sample taken from the leucogranite veins that cut through granitic gneiss foliation range from 24 Ma to 22 Ma, interpreted as the age of the anatexis in the Demala Group complex. Biotite and muscovite separates were selected from the granitic gneiss, banded gneiss, and leucogranite veins for 40Ar/39Ar dating. The plateau ages of three muscovite samples are 16.56 ± 0.21 Ma, 16.90 ± 0.21 Ma, and 23.40 ± 0.31 Ma, and the plateau ages of four biotite samples are 16.70 ± 0.24 Ma, 16.14 ± 0.19 Ma, 15.88 ± 0.20 Ma, and 14.39 ± 0.20 Ma. The mica Ar-Ar ages can reveal the exhumation and cooling history of the Demala Group complex. Combined with the previous research results of the Demala Group complex, the authors refer that the Demala Group complex should be a set of metamorphic complex. The complex includes not only Precambrian basement metamorphic rock series, but also Paleozoic sedimentary rock and Mesozoic granitic rock. Based on the deformation characteristics, the authors concluded that two stages of the metamorphism and deformation can be revealed in the Demala Group complex since the Mesozoic, namely Late Triassic-Early Jurassic (203–190 Ma) and Oligocene–Miocene (24–14 Ma). The early stage of metamorphism (ranging from 203–190 Ma) was related to the Late Triassic tectono-magmatism in the area. The anatexis and uplifting-exhumation of the later stage (24–14 Ma) were related to the shearing of the Jiali strike-slip fault zone. The Miocene structures are response to the large-scale southeastward escape of crustal materials and block rotation in Southeast Tibet after India-Eurasia collision.
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References
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Yuan Tang, Yu-ping Liu, Peng Wang, Wen-qing Tang, Ya-dong Qin, Xiao-dong Gong, Dong-bing Wang, Bao-di Wang, 2021. A new understanding of Demala Group complex in Chayu Area, southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating, China Geology, 4, 77-94. doi: 10.31035/cg2021021
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Figure 1.
a−Schematic tectonic map of the Himalayas and southern Tibet (modified from Lee HY et al., 2003). b−Generalized geological map of Chayu area.
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Figure 2.
Outcrop structural characteristics of the Demala Group complex.
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Figure 3.
Microstructural characteristics of gneiss in the Demala Group complex; Fs−feldspar, Qz−quartz, Mus−muscovite, Bi−biotite, BGR−bulging recrystallized feldspar.
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Figure 4.
Microstructural characteristics of the Demala Group complex; a−mica schist, b−quartz schist, c−amphibolite, d−biotiteleptynite; Fs−feldspar, Qz−quartz, Mus−muscovite, Bi−biotite, Am-amphibole.
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Figure 5.
Cathodoluminescence (CL) images of representative zircons from the Sample 17CY01. a−zircons with wide rim dated 206Pb/238U age around 203 Ma; b−zircons with narrow rim. Circles with data indicate spots and ages of LA-ICP-MS dating. The diameters of the circles are 32μm.
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Figure 6.
a−Zircon U-Pb concordia diagrams of Sample 17CY01; b−content of Th and U and Th/U ratios of zircons from Sample 17CY01; c−e−zircon U-Pb concordia diagrams of samples17CY09-1 and 17CY09-2; f−content of Th and U and Th/U ratios of zircons from samples 17CY09-1 and 17CY09-2.
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Figure 7.
Cathodoluminescence (CL) images of representative zircons from the Sample 17CY09-1. a−zircons dated 206Pb/238U age around 190 Ma; b−zircons dated 206Pb/238U age around 205 Ma; c−zircons dated 206Pb/238U age older than 300 Ma. Circles with data indicate spots and ages of LA-ICP-MS dating. The diameters of the circles are 32μm.
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Figure 8.
Cathodoluminescence (CL) images of representative zircons from the samples 17CY09-2, 17CY05-1 and 17CY05-2. Circles with data indicate spots and ages of LA-ICP-MS dating. The diameters of the circles are 32 μm.
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Figure 9.
a−Zircon U-Pb concordia diagrams and 206Pb/238U weighted mean age of Sample 17CY05-1; b−zircon U-Pb concordia diagrams and cumulative Gaussian plus histogram plots of Sample 17CY05-2.
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Figure 10.
40Ar/39Ar plateau age and36Ar/40Ar-39Ar/40Ar isochron age of biotite and muscovite from samples in the Demala Group complex. WMA–weighted mean age; MSWD–mean square of weighted deviates.
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Figure 11.
40Ar/39Ar plateau age and36Ar/40Ar-39Ar/40Ar isochron age of biotite and muscovite from samples in the Demala Group complex. WMA–weightedmean age; MSWD–mean square of weighted deviates.
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Figure 12.
Frequency plot diagram of the detrital zircon U-Pb ages of the Demala Group complex.