The age and tectonic environment of the rhyolitic rocks on the western side of Wuyi Mountain,South China |
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摘 要: | During the geological survey of the metamorphic rocks in Xingning-Wuhua region on the western side of Wuyi Mountain, South China, we discovered the Neoproterozoic rhyolite and rhyolitic greywacke for the first time that outcrop in the Proterozoic metamorphic rocks near Jingnan Town of Xingning County, eastern Guangdong Province. A systematic research on petrology, geochemistry and geochronology of rhyolitic rocks was conducted to understand their tectonic setting and formation age. The Jingnan rhyolite is interbedded with a coeval greywacke, with a total thickness of 60 m; both rhyolite and greywacke display a similar folding and metamorphic pattern. Meta-rhyolite consists of groundmass and phenocrystals including sanidine, orthoclase, and quartz with distinct undulose extinction; the groundmass has been recrystallized into fine-grain feldspar, quartz and sericite aggregation. Meta-greywacke is composed of crystallinoclastic grains (sanidine, orthoclase, quartz and oligoclase) and clay groundmass. Zircon grains used for the SHRIMP U-Pb analysis are light brown-colored and euhedral or subeuhedral. Dating data suggest two age groups; eight grains of magmatype zircon with an idiomorphic form yield an age of 972±8 Ma, and the other seven weakly corroded grains of zircon with euhedral to subeuhedral shape construct an average age of 1097±11 Ma, which were captured from older rocks by an uplifting magma, implying that a late Mesoproterozoic basement exists in the Nanling region. In addition, one Paleoproterozoic age, 2035±11 Ma, is obtained from a rounded detrital zircon, indicating that a Paleoproterozoic thermal event took place in the South China. Geochemically, the Jingnan rhyolitic rocks are characterized by high K2O content, intermediate Al2O3 content, with the ACNK value 0.98―1.11, and belonging to high-K alkaline series. They are rich in ΣREE, Rb, Th and Ce, depleted in Ba, Sr, Eu, Ti, P and Nb-Ta, and with moderate negative Eu and Sr anomalies. These features indicate that the Jingnan volcanic rocks have an affinity of continental arc that is similar to those of acid volcanic rocks in the SE-China Coastal Region, in other words, a Neoproterozoic tectonomagma event might have taken place in the western Wuyi region, leading to an eruption of high-K calc-alkaline granitic magma.
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The age and tectonic environment of the rhyolitic rocks on the western side of Wuyi Mountain,South China |
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Authors: | LiangShu Shu Ping Deng JinHai Yu YanBin Wang ShaoYong Jiang |
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Institution: | (1) State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, 210093, China;(2) Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China;(3) Center of Iron-Probe in Beijing, Beijing, 100037, China |
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Abstract: | During the geological survey of the metamorphic rocks in Xingning-Wuhua region on the western side of Wuyi Mountain, South
China, we discovered the Neoproterozoic rhyolite and rhyolitic greywacke for the first time that outcrop in the Proterozoic
metamorphic rocks near Jingnan Town of Xingning County, eastern Guangdong Province. A systematic research on petrology, geochemistry
and geochronology of rhyolitic rocks was conducted to understand their tectonic setting and formation age. The Jingnan rhyolite
is interbedded with a coeval greywacke, with a total thickness of 60 m; both rhyolite and greywacke display a similar folding
and metamorphic pattern. Meta-rhyolite consists of groundmass and phenocrystals including sanidine, orthoclase, and quartz
with distinct undulose extinction; the groundmass has been re-crystallized into fine-grain feldspar, quartz and sericite aggregation.
Meta-greywacke is composed of crystallinoclastic grains (sanidine, orthoclase, quartz and oligoclase) and clay groundmass.
Zircon grains used for the SHRIMP U-Pb analysis are light brown-colored and euhedral or sub-euhedral. Dating data suggest
two age groups; eight grains of magma-type zircon with an idiomorphic form yield an age of 972±8 Ma, and the other seven weakly
corroded grains of zircon with euhedral to subeuhedral shape construct an average age of 1097±11 Ma, which were captured from
older rocks by an uplifting magma, implying that a late Mesoproterozoic basement exists in the Nanling region. In addition,
one Paleoproterozoic age, 2035±11 Ma, is obtained from a rounded detrital zircon, indicating that a Paleoproterozoic thermal
event took place in the South China. Geochemically, the Jingnan rhyolitic rocks are characterized by high K2O content, intermediate Al2O3 content, with the ACNK value 0.98–1.11, and belonging to high-K alkaline series. They are rich in ΣREE, Rb, Th and Ce, depleted
in Ba, Sr, Eu, Ti, P and Nb-Ta, and with moderate negative Eu and Sr anomalies. These features indicate that the Jingnan volcanic
rocks have an affinity of continental arc that is similar to those of acid volcanic rocks in the SE-China Coastal Region,
in other words, a Neoproterozoic tectono-magma event might have taken place in the western Wuyi region, leading to an eruption
of high-K calc-alkaline granitic magma.
Supported by the National Natural Science Foundation of China (Grant Nos. 40634022, 40221301, 40572118, 40672125 and 40132010) |
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Keywords: | rhyolite calc-alkaline series early stage of Neoproterozoic western Wuyi Mountain South China |
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