Characteristics And Significance Of Volcanic Rocks From Meiietu Formation In Keyouzhongqi Area, Inner Mongolia

Immense Mesozoic volcanic rocks are found in the eastern part of XingmengOrogenic Belt in Da-Hinggan Mountains with complicated rock types, which form themain part of Mesozoic volcanic rock belt of East China. As one of the major types ofigneous rocks in Da-Hinggan Mountains, Meiletu Formation andesitic rocks give away to better understand the petrogenesis and tectonic background of Da-HingganMountains igneous rocks.According to the detailed field investigation in Keyouzhongqi area, it can beconfirmed that the rock association of Meiletu Formation is andesite, basalt-andesiteand andesitic breccia tuff lava, interbedded with andesitic breccia tuff, andesiticbreccia ignimbrite, dacitic breccia tuff lava, dacitic breccia tuff and basalt. MeiletuFormation is the youngest stratum in the studying area covering other strata with anangular unconformity. Its occurence is flat,with the dip angles, below10°. Fortopography and geomorphology, Meiletu Formation always constitute the high andsteep hills which is significantly different from other formation units. According tothe LA-ICP-MS U-Pb isotopic dating of zircons, the forming age of MeiletuFormation is123-125Ma, and thus it is the product of volcanic eruption in the middleof Early Cretaceous.The studies of zircon Hf isotope and geochemistry of Meiletu Formationandesites indicate that they belong to the sub-alkaline series. Most of the samples areweakly peraluminous and the rest are peraluminous. The average values of Ta/Hf, Nb/Ta, Ta/Yb and Sm/Nd are similar to the chondrites and primitive mantle. Thesamples are characterized by relatively low SiO2and moderate Mg#, which are closeto the characteristics of the mantle. And the strong fractionation of LREE and HREE,apparently enrichment in LILE such as Rb, K, and depletion in HFSE such as Nb, Taand Ti all show similar characteristics to continental island andesite. The values ofSiO2, Na2O and K2O and the ratio of K2O and Na2O of andesites in the studying areaare within the range of potassic igneous analogous to subalkalic series high-K rocksas Ge reported (1999), which suggest that the magma of the andesites was derivedfrom the enriched mantle added the water and LILE from the subduction zone. Thehigh positive value of εHf(t)(+7.79±0.26) implying the depletion of176/177Hf isotope,combined with the characteristic of enrichment in LILE, it implies that the sourceregion of Meiletu Formation andesites is newly generated enriched-mantle.The average content of TiO2of Meiletu Formation andesites is0.92%, and theaverage ratio of Ba/La, La/Nb, Th/Ta, Ta/Hf are4,2.4,7.07, and0.15, respectively,which suggest that the andesties was formed in the extensional environment related tothe subduction. Taking several fractors into consideration, such as the distributiondirection of Mesozoic volcanic rocks, chronology results, geophysical data andamalgamated accretionary complexes well-developed in the eastern part of Asiancontinent in Jurassic to Cretaceous, the tectonic environment of Meiletu Formationandesites in Keyouzhongqi area is the intra-continental extension related to thesubduction of Pacific Plate.