This paper reports the systematic study on petrology, geochemistry, LA ICPMS zircons U-Pb dating, and in situ Hf isotope geology of the four plutons in the central-southern Jiangxi Province, an important part of the S...This paper reports the systematic study on petrology, geochemistry, LA ICPMS zircons U-Pb dating, and in situ Hf isotope geology of the four plutons in the central-southern Jiangxi Province, an important part of the South China Block. In the outcrops, rocks are gradually changed from wall rock （slate or schist） to pluton （gneissic granite）; some residual blocks of sandy rock occur in the margin of pluton, and the foliations of residual blocks are parallel to those of both wail rock and gneissic granite. The thin-section observations show that the four plutons contain peraluminous minerals such as muscovite and sillimanite. The flattened and elongated feldspar and quartz grains are often visible in the gneissic granite, parallel to direction of lineation, suggesting that the granitic rock were subjected to a strong ductile sheafing. Geochemically, the A/CNK values from 13 granitic samples are between 1.03 and 1.37 with an average of 1.16, indicating that the granites are of strongly peraluminous plutons. The REE compositions of the 13 samples are similar, showing higher EREE contents, with enrichment in LREEs, depletion in Eu and REE patterns with relative LREE-enrichment and negligible Eu anomalies. They show enrichment in Rb, Th, U and depletion in Ba, Sr, Nb, Ti, belonging to a low Ba-Sr type of granite. Thus, the four bodies should be derived from the same magmatic source. Zircons used as U-Pb dating mostly exhibit euhedral shape and high Th/U values from 0.52 to 1.54 with an average of 1.08, suggesting that most zircons are of magmatic genesis. The zircons from four plutons yielded rather similar 206pb/238U vs. 207Tpb/235U concordia ages： 436.1±5.7 Ma for the Tangwan granite, 440.6±4 Ma for the Jiekou gneissic granite, 435.9±6.2 Ma for the Dongbao gneissic granite, and 441.9±3.1 Ma for the Jinxi K-granite, respectively, corresponding to Silurian Llandovery. Several xenocrysts yielded U-Pb ages around 700 Ma, implying that a breakup event took place during Neoproterozoic in the South China Block. In situ Lu-Hf isotopic analysis shows that all εHf（t） values of zircons are negative and have two-stage Hf model ages （TDM2） from 1.4 to 3.6 Ga, indicating that the Silurian granitic magma came from the re- cycle of Meso-Paleoproterozoic basement and even partly Archean rocks, and had not been effected by mantle magma. Re- searches on regional geology suggest that an intracontinental tectono-magmatic event took place during the early Paleozoic in the study areas, which is characterized by folding and thrusting, leading to crustal shortening and thickening, up to 20 km thickness. The high geothermal temperature from thickening crust and accumulation of producing high-heat radioactive elements gradually softened crustal rocks and caused a partial melting, forming peraluminous granitic magma. Under the post-orogenic extensional and de-pressure condition, these granitic magma rose and was emplaced in the upper crust, leading to development of S-type plutons展开更多
Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, w...Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, western Guangxi. High non-terrigenous SiO2 contents （Sinon_ter/Sibulk（%）〉 80%） and pure chert components （〉 70%） indicate a large extent of silicifi- cation in the Upper Paleozoic cherts, except for the Upper Devonian-Lower Carboniferous Luzhai Formation cherts, which have lower non-terrigenous SiO2 contents （avg. 71.8%） and pure chert components （40%-70%）. The Al/（AI＋Fe＋Mn） ratios and Feter/Febulk（%） values of samples from the lowest horizon of the Pingen Formation are 0.05-0.26, 13.1%-14.5%, respec- tively, indicating hydrothermal origins. All other samples show high Al/（Al＋Fe＋Mn） ratios （0.39±0.81） and high Feter/Febulk（%） values （23.1%-186.8%）, indicating non-hydrothermal origins. The Pingen Formation and Liujiang Formation cherts show slightly-moderately negative Ce anomalies （0.71±0.07, 0.81±0.08, respectively） and higher Y/Ho ratios （33.49±1.27, 36.10±2.05, respectively） than PAAS. This suggests that these cherts were deposited in the open marine basin, rather than in the intracontinental rift basin as previously assumed. The Luzhai Formation cherts may be deposited near the seamount or sea- floor plateaus with no negative Ce anomalies （1.09±0.07） and no significant Y-Ho fractionation （Y/Ho=28.60±1.25）. The Nandan Formation and Sidazhai Formation cherts were deposited in the open-ocean basin with moderately negative Ce anom- alies （0.67±0.08, 0.73±0.11, respectively） and high Y/Ho ratios （36.01±1.00, 32.00±2.25, respectively）. On the basis of our studies about cherts, we conclude that the Youjiang Basin originated as part of the Paleo-Tethys that controlled the deposition- al environments of cherts during late Paleozoic. The rift of the Youjiang Basin had occurred at least since the Early-Middle Devonian. The basin had a trend of evolving into an open-ocean basin during the Early-Middle Permian.展开更多
基金supported by National Natural Science Foundation of China (Grant Nos. 40634022, 40972132)State Key Laboratory for Mineral Deposits Research of Nanjing University (Grant No. 2008-I-01)
文摘This paper reports the systematic study on petrology, geochemistry, LA ICPMS zircons U-Pb dating, and in situ Hf isotope geology of the four plutons in the central-southern Jiangxi Province, an important part of the South China Block. In the outcrops, rocks are gradually changed from wall rock （slate or schist） to pluton （gneissic granite）; some residual blocks of sandy rock occur in the margin of pluton, and the foliations of residual blocks are parallel to those of both wail rock and gneissic granite. The thin-section observations show that the four plutons contain peraluminous minerals such as muscovite and sillimanite. The flattened and elongated feldspar and quartz grains are often visible in the gneissic granite, parallel to direction of lineation, suggesting that the granitic rock were subjected to a strong ductile sheafing. Geochemically, the A/CNK values from 13 granitic samples are between 1.03 and 1.37 with an average of 1.16, indicating that the granites are of strongly peraluminous plutons. The REE compositions of the 13 samples are similar, showing higher EREE contents, with enrichment in LREEs, depletion in Eu and REE patterns with relative LREE-enrichment and negligible Eu anomalies. They show enrichment in Rb, Th, U and depletion in Ba, Sr, Nb, Ti, belonging to a low Ba-Sr type of granite. Thus, the four bodies should be derived from the same magmatic source. Zircons used as U-Pb dating mostly exhibit euhedral shape and high Th/U values from 0.52 to 1.54 with an average of 1.08, suggesting that most zircons are of magmatic genesis. The zircons from four plutons yielded rather similar 206pb/238U vs. 207Tpb/235U concordia ages： 436.1±5.7 Ma for the Tangwan granite, 440.6±4 Ma for the Jiekou gneissic granite, 435.9±6.2 Ma for the Dongbao gneissic granite, and 441.9±3.1 Ma for the Jinxi K-granite, respectively, corresponding to Silurian Llandovery. Several xenocrysts yielded U-Pb ages around 700 Ma, implying that a breakup event took place during Neoproterozoic in the South China Block. In situ Lu-Hf isotopic analysis shows that all εHf（t） values of zircons are negative and have two-stage Hf model ages （TDM2） from 1.4 to 3.6 Ga, indicating that the Silurian granitic magma came from the re- cycle of Meso-Paleoproterozoic basement and even partly Archean rocks, and had not been effected by mantle magma. Re- searches on regional geology suggest that an intracontinental tectono-magmatic event took place during the early Paleozoic in the study areas, which is characterized by folding and thrusting, leading to crustal shortening and thickening, up to 20 km thickness. The high geothermal temperature from thickening crust and accumulation of producing high-heat radioactive elements gradually softened crustal rocks and caused a partial melting, forming peraluminous granitic magma. Under the post-orogenic extensional and de-pressure condition, these granitic magma rose and was emplaced in the upper crust, leading to development of S-type plutons
基金supported by National Natural Science Foundation of China(Grant Nos.40972078and40921062)"111Project"(Grant No.B08030)Fundamental Research Funds for the Central Universities,China University of Geosciences(Wuhan)
文摘Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, western Guangxi. High non-terrigenous SiO2 contents （Sinon_ter/Sibulk（%）〉 80%） and pure chert components （〉 70%） indicate a large extent of silicifi- cation in the Upper Paleozoic cherts, except for the Upper Devonian-Lower Carboniferous Luzhai Formation cherts, which have lower non-terrigenous SiO2 contents （avg. 71.8%） and pure chert components （40%-70%）. The Al/（AI＋Fe＋Mn） ratios and Feter/Febulk（%） values of samples from the lowest horizon of the Pingen Formation are 0.05-0.26, 13.1%-14.5%, respec- tively, indicating hydrothermal origins. All other samples show high Al/（Al＋Fe＋Mn） ratios （0.39±0.81） and high Feter/Febulk（%） values （23.1%-186.8%）, indicating non-hydrothermal origins. The Pingen Formation and Liujiang Formation cherts show slightly-moderately negative Ce anomalies （0.71±0.07, 0.81±0.08, respectively） and higher Y/Ho ratios （33.49±1.27, 36.10±2.05, respectively） than PAAS. This suggests that these cherts were deposited in the open marine basin, rather than in the intracontinental rift basin as previously assumed. The Luzhai Formation cherts may be deposited near the seamount or sea- floor plateaus with no negative Ce anomalies （1.09±0.07） and no significant Y-Ho fractionation （Y/Ho=28.60±1.25）. The Nandan Formation and Sidazhai Formation cherts were deposited in the open-ocean basin with moderately negative Ce anom- alies （0.67±0.08, 0.73±0.11, respectively） and high Y/Ho ratios （36.01±1.00, 32.00±2.25, respectively）. On the basis of our studies about cherts, we conclude that the Youjiang Basin originated as part of the Paleo-Tethys that controlled the deposition- al environments of cherts during late Paleozoic. The rift of the Youjiang Basin had occurred at least since the Early-Middle Devonian. The basin had a trend of evolving into an open-ocean basin during the Early-Middle Permian.
