The Nadingcuo volcanic rock suite is the most volmninous Cenozoic volcanic suite in the southern Qiangtang area of the northern Tibetan Plateau. These high-K calc-alkaline volcanic rocks were formed between 36 and 34 ...The Nadingcuo volcanic rock suite is the most volmninous Cenozoic volcanic suite in the southern Qiangtang area of the northern Tibetan Plateau. These high-K calc-alkaline volcanic rocks were formed between 36 and 34 Ma, characterized by high Mg# values, high concentrations of TiO2 and P205, 87Sr/S6Sr ratios of 0. 704682--0. 706 112, and aNd（t） values of - 1.2 to 1.6. There is a lack of reasonable explanations for sour- cing and origin of magmas that formed the rocks with high Mg# values and TiO2 and P2O5 enrichments, which makes the previous research results still controversial. This study reviews the geochemical characteristics of Nadingcuo volcanic rocks and the data we have newly found in our fieldwork. We give some new interpretation to the magmatic evolution of the basaltic magmas in the discussed area dominated by fractional crystallization. The geochemistry of trachyandesite and trachyte units in the studied area is indicative of formation from mantle- derived magmas that mixed with crustal materials. The high values of Mg# and TiO2 and P205 enrichment in these units are evident to show the mixing between mantle-derived magmas with -30-40 wt.% rhyolitic melt or assimilation of a similar amount of felsic rocks. The geochemistry of basaltic rocks in the area also suggests that the Nadingcuo basalts may have been derived from an ocean island basalt （OIB） -type source that contained and was mixed with ancient mantle wedge derived material, indicating that a 36-34 Ma asthenospheric upwelling e- vent in the Qiangtang area may relate to the northward subduction of Indian lithospheric mantle and the south- ward subduction of Asian lithospherie mantle. This upwelling of asthenospherie material was centered in the southern Qiangtang area between 36 and 34 Ma, while the northward movement of the Indian Craton caused this upwelling mantle flow to continuously migrate northward, resulting in the current centering of this upwelling in the Hoh Xil-Kunlun region.展开更多
By using petrological,isotope chronological,and geochemical methods,the authors studied the volcanic rocks in the studied area,mainly including dacites and trachytes. The results show that they formed during the late ...By using petrological,isotope chronological,and geochemical methods,the authors studied the volcanic rocks in the studied area,mainly including dacites and trachytes. The results show that they formed during the late Early Cretaceous. Geochemically,the volcanic rocks are relatively enriched in large-ion lithophile elements( Rb,K,and Th) and depleted in high field strength elements( Nb,Ta,and Ti),and rich in light rare earth elements,and depleted in heavy rare earth elements. The fact indicates that the main body of the volcanic rocks in the Qushenla Formation was derived from the partial melting of lower crust. The lithological assemblages are characterized by continental high-K calc-alkaline and shoshonitic series,suggesting that the southward-subducting oceanic slab in southern Bangong Lake had break off and that the Bangong Lake-Nujiang Ocean had closed before 107 Ma. The main dynamic mechanisms for the genesis of this set of intermediate-acidic volcanic rocks were upwelling of the asthenosphere and partial melting of the lower crust caused by slab break-off.展开更多
基金Supported by project of National Natural Science Foundation of China(41172056)
文摘The Nadingcuo volcanic rock suite is the most volmninous Cenozoic volcanic suite in the southern Qiangtang area of the northern Tibetan Plateau. These high-K calc-alkaline volcanic rocks were formed between 36 and 34 Ma, characterized by high Mg# values, high concentrations of TiO2 and P205, 87Sr/S6Sr ratios of 0. 704682--0. 706 112, and aNd（t） values of - 1.2 to 1.6. There is a lack of reasonable explanations for sour- cing and origin of magmas that formed the rocks with high Mg# values and TiO2 and P2O5 enrichments, which makes the previous research results still controversial. This study reviews the geochemical characteristics of Nadingcuo volcanic rocks and the data we have newly found in our fieldwork. We give some new interpretation to the magmatic evolution of the basaltic magmas in the discussed area dominated by fractional crystallization. The geochemistry of trachyandesite and trachyte units in the studied area is indicative of formation from mantle- derived magmas that mixed with crustal materials. The high values of Mg# and TiO2 and P205 enrichment in these units are evident to show the mixing between mantle-derived magmas with -30-40 wt.% rhyolitic melt or assimilation of a similar amount of felsic rocks. The geochemistry of basaltic rocks in the area also suggests that the Nadingcuo basalts may have been derived from an ocean island basalt （OIB） -type source that contained and was mixed with ancient mantle wedge derived material, indicating that a 36-34 Ma asthenospheric upwelling e- vent in the Qiangtang area may relate to the northward subduction of Indian lithospheric mantle and the south- ward subduction of Asian lithospherie mantle. This upwelling of asthenospherie material was centered in the southern Qiangtang area between 36 and 34 Ma, while the northward movement of the Indian Craton caused this upwelling mantle flow to continuously migrate northward, resulting in the current centering of this upwelling in the Hoh Xil-Kunlun region.
基金Supported by project of National Natural Science Foundation of China(No.41172056)
文摘By using petrological,isotope chronological,and geochemical methods,the authors studied the volcanic rocks in the studied area,mainly including dacites and trachytes. The results show that they formed during the late Early Cretaceous. Geochemically,the volcanic rocks are relatively enriched in large-ion lithophile elements( Rb,K,and Th) and depleted in high field strength elements( Nb,Ta,and Ti),and rich in light rare earth elements,and depleted in heavy rare earth elements. The fact indicates that the main body of the volcanic rocks in the Qushenla Formation was derived from the partial melting of lower crust. The lithological assemblages are characterized by continental high-K calc-alkaline and shoshonitic series,suggesting that the southward-subducting oceanic slab in southern Bangong Lake had break off and that the Bangong Lake-Nujiang Ocean had closed before 107 Ma. The main dynamic mechanisms for the genesis of this set of intermediate-acidic volcanic rocks were upwelling of the asthenosphere and partial melting of the lower crust caused by slab break-off.
