The Tectonic Setting Of Early Cretaceous Volcanic Rocks In The Northern Da Hinggan Mountains

The Mesozoic volcanic rocks in the northern Da Hinggan Mountains are an important part of the giant volcanic rocks on the continental margin of East Asia and they have been the hotspots of many scholars for a long time.Based on Ministry of Science and Technology Fund for Public Welfare Industry“Research of Deep Structure and Petrophysical properties in Da Hinggan Mountains”,this paper studies in petrology,isotopic chronology and geochemistry of trachyte and rhyolite combination which are widely distributed in the Suolun aera and Genhe aera of Northern Da Hinggan Mountains.These research reveals its formation time and the relationship between volcanic rocks and adjacent normal sedimentary,then concludes its tectonic setting.Ziron LA-ICP-MS U-Pb dating results show that the intermediate-acid volcanic rocks combination has the same formation time,ziron U-Pb age of upper rhyolite and lower trachyte is 131-130 Ma,corresponding to the Hauterivian period of the Early Cretaceous in age.Comparing the age with the fossil ages in the surrounding sedimentary stratas reveals that the volcanic rocks are continuously evolved in the depositional age of the upper and lower normal sedimentary stratas.The fossil age of the Nantun Formation under the volcanic rocks is the Valanginian period of the Early Cretaceous,corresponding to the numerical age of 139-132 Ma and that of the Damoguaihe-Yimin Formation above it is the late Hauterivian-Barremian period of the Early Cretaceous,corresponding to the numerical age of 129-125 Ma.These evidence reveals that the north part of the Da Hinggan Mountains suffered a successive evolution from sedimentation to volcanism to sedimentation during the Early Cretaceous(139-125Ma),reflecting the evolution characteristics of the extensional basins.The volcanic rocks with the age of 131-130 Ma represents a regional eruption event and also it has an important significance for dividing stratigraphic sequence in the Early Cretaceous.The geochemical characteristics of the whole rock show that the volcanic rocks have similar geochemical characteristics characterized by high silicon,relatively enriched alkaline components and significant depleted magnesium.The volcanic rocks have Si O2=57.38 % ~76.13 %,Na2O=2.24 % ~4.17 %,K2O=3.06 % ~5.10 %,and K2O+Na2O=6.18%~7.86%.Also they are characterized by enrichment of large ion lithophile elements(such as Rb,K and U)and depletion of high field strengh elements(such as Nb,Ti and P).The rhyolite has the characteristics of type A granite,which indicates that it is formed in extensional tectonic environment.Combined with the couped relationship among Da Hinggan Mountains,Songliao Basin and regional tectonic setting analysis of the aera,the volcanic eruption time of ± 130 Ma obtained in this paper is in the Da Hinggan Mountains from the rapid uplift to the slow uplift,and the Songliao basin changed from rapid settlement to slow sedimentation,which indicates that the formation of widely distributed alkaline volcanic rock combination is closely related to the evolution of the basin and mountain system in the Da Hinggan Mountains-Songliao Basin.In the view of Monggolia-Okhotsk ocean in the west aera of Da Hinggan Mountains has closed in the early Cretaceous,the eastern part of the northeastern is in the background of paleo-Pacific plate transforming from forward subduction to fast oblique subduction in the eastern margin of Northeast Asia,this paper believes that the formation of volcanic rocks of the 130 Ma age in the Da Hinggan Mountains are related to the deep magma effect and the extensional tectonic action because of paleo-Pacific plate transforming from forward subduction to fast oblique subduction in the eastern margin of Northeast Asia.The volcanic rocks are widely distributed and they have regional stratigraphic division and comparison significance.