The timing of continental collision between India and Asia has been controversial for a long time because of the difficulty in screening isotopic ages for different types of tectonothermal event along the convergent c...The timing of continental collision between India and Asia has been controversial for a long time because of the difficulty in screening isotopic ages for different types of tectonothermal event along the convergent continental boundary. After distinguishing the collisional orogeny from the precollisional accretionary orogeny and the postcollisional rifting orogeny, an age range of 55 ± 10 Ma is obtained to mark the collisional orogeny in the Early Cenozoic rather than throughout the Cenozoic. This age range provides the resolution to the timing of tectonic reactivation not only for reworking of the marginal arc systems in the Early Cenozoic but also for overprinting of granulite facies metamorphism on eclogites in the Late Cenozoic. In particular, superimposition of the rifting orogeny on both accretionary and collisional orogens in the Late Cenozoic is the key to the reactivation of both Gangdese and Himalayan orogens for contemporaneous metamorphism and magmatism at high thermal gradients. Therefore, rise of the plateau may be caused by underplating of the asthenospheric mantle for rifting orogeny in the composite Himalayan–Tibetan orogens after foundering of their roots in the Late Cenozoic.展开更多
The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subdu...The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams展开更多
The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late ...The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.展开更多
基金supported by the National Key Basic Research Program of China (2015CB856100)the National Natural Science Foundation of China (41590620)
文摘The timing of continental collision between India and Asia has been controversial for a long time because of the difficulty in screening isotopic ages for different types of tectonothermal event along the convergent continental boundary. After distinguishing the collisional orogeny from the precollisional accretionary orogeny and the postcollisional rifting orogeny, an age range of 55 ± 10 Ma is obtained to mark the collisional orogeny in the Early Cenozoic rather than throughout the Cenozoic. This age range provides the resolution to the timing of tectonic reactivation not only for reworking of the marginal arc systems in the Early Cenozoic but also for overprinting of granulite facies metamorphism on eclogites in the Late Cenozoic. In particular, superimposition of the rifting orogeny on both accretionary and collisional orogens in the Late Cenozoic is the key to the reactivation of both Gangdese and Himalayan orogens for contemporaneous metamorphism and magmatism at high thermal gradients. Therefore, rise of the plateau may be caused by underplating of the asthenospheric mantle for rifting orogeny in the composite Himalayan–Tibetan orogens after foundering of their roots in the Late Cenozoic.
基金supported by the China Postdoctoral Science Foundation(M2017612220)the Shandong Province Natural Science Foundation(Doctoral Funds,ZR2017BD033)
文摘The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams
基金financially supported by the Programme of the China Geological Survey (No.1212011120608, No.1212011220907)the National Key Projects for Basic Research of China (No.2009CB421002, No.2011CB403102)+2 种基金NSF of China (No. 40672044)Program for Changjiang Scholars, Innovative Research Team in University (No.IRT1083)111 project (No.B07011)
文摘The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.
