| Citation: |
MIAO Yu, WEI Shaogang, Lü Xiaochun, SONG Wenting, WAN Liwen. Geochemical characteristics and formation environment of metamorphic rocks in Etouchang Formation of Middle Proterozoic Lower Kunyang Group in northeastern Yunnan[J]. Geological Bulletin of China, 2020, 39(10): 1538-1548.
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Geochemical characteristics and formation environment of metamorphic rocks in Etouchang Formation of Middle Proterozoic Lower Kunyang Group in northeastern Yunnan
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Abstract
As an important part of the metamorphic fold basement on the western margin of the Yangtze block, the light metamorphic rock series of Kunyang Group has attracted much attention from experts for its formation and evolution mechanism.Taking the metamorphic rocks of Etouchang Formation, the representative stratum of Kunyang Group, as an example, this paper constrains the protolith type, provenance, sedimentation and tectonic environment background of the metamorphic rocks in the aspects of rock symbiotic association, petrographic markers and petrogeochemical.The micropaleontology and isotopic age of the metamorphic rocks indicate that the diagenetic age of the metamorphic rocks belongs to Late Mesoproterozoic, and the sedimentary facies and petrographic shows the characteristics of normal marine sedimentary.Comprehensive petrogeochemical analysis shows that the original rocks of metamorphic rocks were composed of graywacke and subgraywacke with some mudstone components, which were formed in the oxidative sedimentary environment of the shallow-littoral facies.The rocks experienced moderate weathering and leaching with lower rock maturity and leaching with lower rock maturity.The sediments mainly came from the tectonic active area of the upper crust, with the characteristics of single cycle sedimentary products in the active structural belt and experienced a certain degree of intracrustal geochemical differentiation.The main source of the sediments was the continental island arc type low aluminum TTG series volcanic rocks, with the intervention of part of the upper crust terrigenous materials.Furthermore, petrogeochemical characteristics also show that the metamorphic protolith mainly deposited in the passive continental margin tectonic environment.The authors hold that the metamorphic protoliths were probably formed in the residual basin in the late closure stage of Kunyang marine rift basin.
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References
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MIAO Yu, WEI Shaogang, Lü Xiaochun, SONG Wenting, WAN Liwen. Geochemical characteristics and formation environment of metamorphic rocks in Etouchang Formation of Middle Proterozoic Lower Kunyang Group in northeastern Yunnan[J]. Geological Bulletin of China, 2020, 39(10): 1538-1548.
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Figure 1.
Geological and structural sketch map of the study area
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Figure 2.
PM025 metamorphic rocks section of the Etouchang Formation in the study area
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Figure 3.
Field outcrop(a) and microphotographs(b, c) for metamorphic rocks of Etouchang Formation in the study area
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Figure 4.
Chondrite-normalized REE patterns(a)and PAAS-normalized trace elements spidergrams(b) of metamorphic rocks in the study area
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Figure 5.
Protolith restoration diagrams of the metamorphic rocks in the study area
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Figure 6.
Diagram of Mishjin
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Figure 7.
Trace elements discrimination plots of the redox environment for metamorphic rocks in the study area
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Figure 8.
Hf-La/Th diagram of the sediment provenance for metamorphic rocks in the study area
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Figure 9.
Th-Hf-Co diagram of the sediment provenance for metamorphic rocks in the study area
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Figure 10.
Diagrams of the tectonic environment of metamorphic rocks in the study area