On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units： high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which ...On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units： high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high-grade metamorphism, but the latter is less than 1% and crops out in the upper part with low-grade metamorphism. The high-TiO2 unit is dominated by tholeiitic lavas showing high rare earth element （REE） contents （~REE = 83.4-180.8 pg/ g）, high light/heavy REE （LREE/HREE） ratios （LREE/HREE=2.17-5.85） and weak negative Eu anomaly （Eu=0.79-1.01）. Its trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, showing within-plate basalt affinities. In contrast, the low-TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies, indicating typical arc or continental arc signature. Chondrite-normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid-oceanic basalt. Therefore, we suggest that these lavas should be formed in a back-arc basin setting. Sr-Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ~1144 Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic-island basalt-like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid-oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift-related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.展开更多
基金support by the National Natural Science Foundation of China(Grant Nos40872061)National Science Fundamental Project(Grant No 2006BAB01A11)Land and Resources Survey Project of China(Grant Nos1212010611804,1212010610319)
文摘On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units： high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high-grade metamorphism, but the latter is less than 1% and crops out in the upper part with low-grade metamorphism. The high-TiO2 unit is dominated by tholeiitic lavas showing high rare earth element （REE） contents （~REE = 83.4-180.8 pg/ g）, high light/heavy REE （LREE/HREE） ratios （LREE/HREE=2.17-5.85） and weak negative Eu anomaly （Eu=0.79-1.01）. Its trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, showing within-plate basalt affinities. In contrast, the low-TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies, indicating typical arc or continental arc signature. Chondrite-normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid-oceanic basalt. Therefore, we suggest that these lavas should be formed in a back-arc basin setting. Sr-Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ~1144 Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic-island basalt-like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid-oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift-related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.
