Following the collision between the Indian and Eurasian plates, the Cenozoic vol- canic activities are rather frequent in the Qiangtang area of northern Qinghai-Tibet Plateau. They can be divided into four series: alk...Following the collision between the Indian and Eurasian plates, the Cenozoic vol- canic activities are rather frequent in the Qiangtang area of northern Qinghai-Tibet Plateau. They can be divided into four series: alkaline basalt series, high-K calc-alkaline series, shoshonitic series and peralkaline potassic-ultrapotassic series. Geochemical data suggest that the magma sources of Cenozoic volcanic rocks have transferred from spinel lherzolite mantle in the early stage to garnet peridotite enriched mantle (EM2) in the later stage. The high Mg# number and extremely high Cr-Ni-Co abundance of high-K calc-alkaline and shoshonitic series andesites in the Qiangtang area indicate that the primary magma might be derived from subduction of conti- nent lithosphere from the Lhasa block. Incompatible element ratios of La/Rb, Zr/Rb, Rb/Nb, K/Nb, Pb/La and K/La of peralkaline potassic-ultrapotassic series lavas in northern Qinghai-Tibet Pla- teau are lower than island arc volcanic rocks and higher than and similar to oceanic island ba- salts. This signature indicates that the primary magma derive from a paleo-mantle wedge inter- fused by fluids derived from asthenosphere and/or subducted mantle lithosphere. But the above element ratios of ultrapotassic lavas in southern Tibet and ultrapotassic lamprophyres in eastern Tibet are higher than and similar to island arc volcanic rocks, which means that the primary magma sources contained a large quantity of crust contaminant from fluids and/or melts derived from subducted continent lithosphere. The studies result supports that the indian continental lithosphere has underthrust beneath Tibet to about the middle of the plateau, and Eurasian (Qaidam basin) mantle lithosphere has underthrust beneath the Qiangtang area of northern Tibet Plateau. In the paper we demonstrate further that the pulsing cycles of potassic-ultrapotassic volcanism of the Qinghai-Tibet Plateau result from an asthenospher pulsing upwelling caused by the intraplate subduction and the stress relaxation in hinterlan展开更多
The felsic volcanogenic tuffs named"green-bean rocks"(GBRs),characterized by a green or yellowish green color,are widely distributed in the western Yangtze platform and have a high lithium content(286-957 pp...The felsic volcanogenic tuffs named"green-bean rocks"(GBRs),characterized by a green or yellowish green color,are widely distributed in the western Yangtze platform and have a high lithium content(286-957 ppm).This paper studies the ages,origin and tectonic setting of the GBRs in the Sichuan basin on the western margin of the Yangtze platform through the whole-rock geochemistry and zircon trace elements by using U-Pb dating and Hf-O isotopes.The GBR samples from the Quxian and Beibei sections yielded zircon U-Pb ages of 245.5±1.8 Ma and 244.8±2.2 Ma.These samples can be used as the isochronous stratigraphic marker of the Early-Middle Triassic boundary(EMTB)for regional correlation.The whole-rock and zircon geochemistry,and zircon Hf-O isotopes exhibited S-type geochemical affinities with high positiveδ^(18)0 values(9.28‰-11.98‰),low negativeε_(Hf)(t)values(-13.87 to-6.79),and T_(DM)^(2)ages of 2150-1703 Ma,indicating that the lithium-rich GBRs were generated by the remelting of the pre-existing ancient Paleoproterozoic layer without mantle source contamination in the arcrelated/orogenic tectonic setting.The results of this study demonstrate that the lithium-rich GBRs in the western Yangtze platform were derived from arc volcanic eruptions along the Sanjiang orogen,triggered by the closure of the eastern Paleo-Tethys Ocean and the syn-collision between the continental Indochina and Yangtze blocks at ca.247 Ma.This was marked by a major shift from I-type magmas with intermediateε_(Hf)(t)values to S-type magmas with low negativeε_(Hf)(t)values.Collectively,our results provide new insights into the origin of the GBRs and decodes the closure of the eastern Paleo-Tethys.展开更多
文摘Following the collision between the Indian and Eurasian plates, the Cenozoic vol- canic activities are rather frequent in the Qiangtang area of northern Qinghai-Tibet Plateau. They can be divided into four series: alkaline basalt series, high-K calc-alkaline series, shoshonitic series and peralkaline potassic-ultrapotassic series. Geochemical data suggest that the magma sources of Cenozoic volcanic rocks have transferred from spinel lherzolite mantle in the early stage to garnet peridotite enriched mantle (EM2) in the later stage. The high Mg# number and extremely high Cr-Ni-Co abundance of high-K calc-alkaline and shoshonitic series andesites in the Qiangtang area indicate that the primary magma might be derived from subduction of conti- nent lithosphere from the Lhasa block. Incompatible element ratios of La/Rb, Zr/Rb, Rb/Nb, K/Nb, Pb/La and K/La of peralkaline potassic-ultrapotassic series lavas in northern Qinghai-Tibet Pla- teau are lower than island arc volcanic rocks and higher than and similar to oceanic island ba- salts. This signature indicates that the primary magma derive from a paleo-mantle wedge inter- fused by fluids derived from asthenosphere and/or subducted mantle lithosphere. But the above element ratios of ultrapotassic lavas in southern Tibet and ultrapotassic lamprophyres in eastern Tibet are higher than and similar to island arc volcanic rocks, which means that the primary magma sources contained a large quantity of crust contaminant from fluids and/or melts derived from subducted continent lithosphere. The studies result supports that the indian continental lithosphere has underthrust beneath Tibet to about the middle of the plateau, and Eurasian (Qaidam basin) mantle lithosphere has underthrust beneath the Qiangtang area of northern Tibet Plateau. In the paper we demonstrate further that the pulsing cycles of potassic-ultrapotassic volcanism of the Qinghai-Tibet Plateau result from an asthenospher pulsing upwelling caused by the intraplate subduction and the stress relaxation in hinterlan
基金the Geological Investigation Work project of China Geological Survey(Grant No.DD20190172)the National Key R&D Plan of China(Grant No.2017YFC0602806).
文摘The felsic volcanogenic tuffs named"green-bean rocks"(GBRs),characterized by a green or yellowish green color,are widely distributed in the western Yangtze platform and have a high lithium content(286-957 ppm).This paper studies the ages,origin and tectonic setting of the GBRs in the Sichuan basin on the western margin of the Yangtze platform through the whole-rock geochemistry and zircon trace elements by using U-Pb dating and Hf-O isotopes.The GBR samples from the Quxian and Beibei sections yielded zircon U-Pb ages of 245.5±1.8 Ma and 244.8±2.2 Ma.These samples can be used as the isochronous stratigraphic marker of the Early-Middle Triassic boundary(EMTB)for regional correlation.The whole-rock and zircon geochemistry,and zircon Hf-O isotopes exhibited S-type geochemical affinities with high positiveδ^(18)0 values(9.28‰-11.98‰),low negativeε_(Hf)(t)values(-13.87 to-6.79),and T_(DM)^(2)ages of 2150-1703 Ma,indicating that the lithium-rich GBRs were generated by the remelting of the pre-existing ancient Paleoproterozoic layer without mantle source contamination in the arcrelated/orogenic tectonic setting.The results of this study demonstrate that the lithium-rich GBRs in the western Yangtze platform were derived from arc volcanic eruptions along the Sanjiang orogen,triggered by the closure of the eastern Paleo-Tethys Ocean and the syn-collision between the continental Indochina and Yangtze blocks at ca.247 Ma.This was marked by a major shift from I-type magmas with intermediateε_(Hf)(t)values to S-type magmas with low negativeε_(Hf)(t)values.Collectively,our results provide new insights into the origin of the GBRs and decodes the closure of the eastern Paleo-Tethys.
