Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studyin...Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole-rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ-type peridotites. They exhibit typically U-shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two-stage evolution history is considered： Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%-20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo-Tethys, forming at middle Ordivician-upper Cambrian, and they may be the direct evidences for spreading of palo-Tethys.展开更多
The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing meta...The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.展开更多
Three radiolarian fauna,aged as the Middle-Late Triassic,Late Jurassic-Early Creta-ceous and Cretaceous radiolarian fauna,have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo sut...Three radiolarian fauna,aged as the Middle-Late Triassic,Late Jurassic-Early Creta-ceous and Cretaceous radiolarian fauna,have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo suture zone,southern Tibet,China.The average contents of SiO2 in the radiolarian cherts of the Middle-Late Triassic and Late Jurassic-Early Cretaceous are 90.24%and 92.58%respectively,with average ratios of Al/(Al+Fe+Mn)as 0.75 and 0.74 respectively,the average ratios of MnO/TiO2 as 0.36 and 1.24,the average ratios of Ce/Ce*as 1.15 and 1.03,and the average ratios of LaN/CeN as 0.85 and 0.93.These geochemical features indicate that both of them are biogenic,deposited in a continental margin basin.The SiO2 content of the Early Cretaceous radiolarian chert is 94.12%,with the ratio of Al/(Al+Fe+Mn)as 0.59,ratio of MnO/TiO2 as 4.30,ratio of Ce/Ce*as 0.60,ratio of LaN/CeN as 1.59,which imply that the chert is biogenic and was deposited in a pelagic basin.The Middle-Late Triassic association of the radiolarian chert and turbidites as well as their geochemical characteristics indicates the existence of a strong rifting marginal basin in the belt of the Yarlung Zangbo River then.The association of radiolarian chert and bedded basalt indicate an initial Tethyan ocean basin in southern Tibet during the Late Jurassic-Early Cretaceous.The early Cretaceous radiolarian chert coexisting with pillow basalt in top of the ophiolite suite represents sediments from the oceanic Tethyan basin.展开更多
The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side ...The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side of the suture, turbidites of the the Jiachala Formation fed largely from the Gangdese arc have long been considered as post-collisional foreland-basin deposits based on the reported occurrence of Paleocene-early Eocene dinoflagellate cysts and pollen assemblages. Because magmatic activity in the Gangdese arc continued through the Late Cretaceous and Paleogene, this scenario is incompatible with U-Pb ages of detrital zircons invariably older than the latest Cretaceous. To solve this conundrum, we carried out detailed stratigraphic, sedimentological, paleontological, and provenance analyses in the Gyangze and Sajia areas of southern Tibet,China. The Jiachala Formation consists of submarine fan deposits that lie in fault contact with the Zongzhuo Formation.Sandstone petrography together with U-Pb ages and Hf isotope ratios of detrital zircons indicate provenance from the Gangdese arc and central Lhasa terrane. Well preserved pollen or dinoflagellate cysts microfossils were not found in spite of careful research, and the youngest age obtained from zircon grain was ～84 Ma. Based on sedimentary facies, provenance analysis and tectonic position, we suggest that the Jiachala Formation was deposited during the Late Cretaceous(~88–84 Ma) in the trench formed along the southern edge of Asia during subduction of Neo-Tethyan oceanic lithosphere.展开更多
基金supported by the National Natural Science Foundation of China (Nos.40872146, 41072166, 41272240)the Project of China Geological Survey (No. 1212011086064,1212011221093)
文摘Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole-rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ-type peridotites. They exhibit typically U-shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two-stage evolution history is considered： Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%-20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo-Tethys, forming at middle Ordivician-upper Cambrian, and they may be the direct evidences for spreading of palo-Tethys.
文摘The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.
基金sponsored by Regional Geology Survey of Lhaze County Sheet(1:250000)(Grant No.19991300009221)the National Natural Science Foundation of China(Grant No.40372061).
文摘Three radiolarian fauna,aged as the Middle-Late Triassic,Late Jurassic-Early Creta-ceous and Cretaceous radiolarian fauna,have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo suture zone,southern Tibet,China.The average contents of SiO2 in the radiolarian cherts of the Middle-Late Triassic and Late Jurassic-Early Cretaceous are 90.24%and 92.58%respectively,with average ratios of Al/(Al+Fe+Mn)as 0.75 and 0.74 respectively,the average ratios of MnO/TiO2 as 0.36 and 1.24,the average ratios of Ce/Ce*as 1.15 and 1.03,and the average ratios of LaN/CeN as 0.85 and 0.93.These geochemical features indicate that both of them are biogenic,deposited in a continental margin basin.The SiO2 content of the Early Cretaceous radiolarian chert is 94.12%,with the ratio of Al/(Al+Fe+Mn)as 0.59,ratio of MnO/TiO2 as 4.30,ratio of Ce/Ce*as 0.60,ratio of LaN/CeN as 1.59,which imply that the chert is biogenic and was deposited in a pelagic basin.The Middle-Late Triassic association of the radiolarian chert and turbidites as well as their geochemical characteristics indicates the existence of a strong rifting marginal basin in the belt of the Yarlung Zangbo River then.The association of radiolarian chert and bedded basalt indicate an initial Tethyan ocean basin in southern Tibet during the Late Jurassic-Early Cretaceous.The early Cretaceous radiolarian chert coexisting with pillow basalt in top of the ophiolite suite represents sediments from the oceanic Tethyan basin.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41525007, 41602115)
文摘The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side of the suture, turbidites of the the Jiachala Formation fed largely from the Gangdese arc have long been considered as post-collisional foreland-basin deposits based on the reported occurrence of Paleocene-early Eocene dinoflagellate cysts and pollen assemblages. Because magmatic activity in the Gangdese arc continued through the Late Cretaceous and Paleogene, this scenario is incompatible with U-Pb ages of detrital zircons invariably older than the latest Cretaceous. To solve this conundrum, we carried out detailed stratigraphic, sedimentological, paleontological, and provenance analyses in the Gyangze and Sajia areas of southern Tibet,China. The Jiachala Formation consists of submarine fan deposits that lie in fault contact with the Zongzhuo Formation.Sandstone petrography together with U-Pb ages and Hf isotope ratios of detrital zircons indicate provenance from the Gangdese arc and central Lhasa terrane. Well preserved pollen or dinoflagellate cysts microfossils were not found in spite of careful research, and the youngest age obtained from zircon grain was ～84 Ma. Based on sedimentary facies, provenance analysis and tectonic position, we suggest that the Jiachala Formation was deposited during the Late Cretaceous(~88–84 Ma) in the trench formed along the southern edge of Asia during subduction of Neo-Tethyan oceanic lithosphere.
