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Zircon U-Pb geochronology, intraplate extensional environment and genesis of Mesozoic volcanic rocks in Zhenglan Banner area, Inner Mongolia, China

LIU Zhe;XUE Huaimin;CAO Guangyue;Institute of Geology,Chinese Academy of Geological Sciences;  
Mesozoic volcanic rocks are widely exposed in Zhenglan Banner of southeast Inner Mongolia. The volcanic rocks mainly consist of rhyolite, trachyte, porphyroclastic lava, ignimbrite, and obsidian, in which the porphyroclastic lava is the most developed.According to the results of the zircon LA-ICP-MS dating, the U-Pb zircon age of rhyolites is(141.4±0.7) Ma, that of felsitic mortar lava is(141.6±0.6) Ma and that of ignimbrite is(139.4±0.7) Ma. Within the error range, the formation ages of the three kinds of rocks are the same, which indicates that the time of the volcanic activity(from effusion to explosion and extrusion) lasted a short time. So they should be assigned to early Cretaceous Zhangjiakou Formation. Except for very few samples, the porphyroclastic lava that extruded in the late phase and the rhyolite-trachyte belonging to the early phase effusion facies are extremely consistent in the geochemical characteristics. They are rich in silicon and potassium, and poor in Ca, Mg and Al. So they could be classified as trachyte-rhyolite combination, and some samples have the features of pantellerite. They are enriched in Rb, Th, U and other largeion lithophile elements, and strongly depleted in Ba, Sr, Ti, P. The chondrite-normalized REE patterns show enrichment of light rare earth elements(LREE), strong negative Eu anomalies, and significant fractionation of LREE and HREE for the volcanic rocks(LaN/YbN=6.87-42.74, except for LQ-38). Compared with the early phase effusion facies lava, porphyroclastic lava has higher silicon and lower alkali, which shows that the magma chamber experienced obvious crystallization differentiation after effusion of the rhyolite and trachyte in the magma event. Then the magma with higher silicon and lower alkali extruded out of the earth's surface. The volcanic rocks of Zhenglan Banner area have the characteristics of A-type granites and possess the geochemical characteristics of A_1 type and A_1-A_2 transition granite. The Nb/Ta ratio of the extrusive porphyroclastic lava and effusive rhyolites is10.46-24.02, which is between the values of the crust and the mantle magma; Y/Nb, Ti/Yb, Ti/Zr also reflect the characteristics of crust-mantle mixing; Rb/Sr ratio is between 0.61-64.51(0.5), reflecting the characteristics of crust source. These data suggest that the volcanic rocks might have been produced in an extension environment of the transportation period of anorogenic and postorogenic activities, mainly being intraplate anorogenic activity. In addition, the volcanic rocks were probably derived from partial melting of the Precambrian lower crust which was strongly transformed by Mesozoic underplating and metamorphism.
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