Zircon U-Pb Ages And Zircon Hf Isotopic Composition Of Volcanic Rocks In The Northern Songliao Basin

Songliao Basin belongs to the Circum Pacific tectonic domain in eastern China,which is the largest Mesozoic Continental petroliferous basin in the Northeast ofChina. The volcanic rocks in the basin are the important parts of the late MesozoicMagmatic rocks, whose formation age and genesis are closely related to theother parts of eastern China. The late Jurassic-early Cretaceous volcanism in easternChina is so intense that it plays an important part in the Mesozoic basin. In recentyears, some scholars have investigated the Mesozoic volcanic rocks in Songliao Basindetailed, but the high-precision isotopic data is still in short supply, which leads to thechronology and stratum contrast of the volcanic rocks in great difficulty anduncertainty, and it restricts the research to a large extent. The forming time, genesisand tectonic setting of the volcanic rocks have important significance not only for theSongliao Basin’s essence but also for the exploration development. The thesis isfocused on the study of chronology, geochemistry and Hf isotope of zircon, in order todiscuss the forming time, geodynamic evolution and the restrict to the Songliao Basin.1Chronology of the volcanic rocks in the northern Songliao BasinZircon U-Pb isotopic chronology is tested for the volcanic rocks in the northernSongliao Basin using the method of LA-ICPMS. The samples’206Pb/238U ages arefrom104Ma to120Ma, which are much different from the previous research results,and provided new ideas. The zircon chronological data displays that the forming timeof the Yingcheng Formation volcanic rocks in the northern Songliao Basin is from the Albian to the Aptian in the early Cretaceous.2Yingcheng Formation and Huoshiling Formation stratigraphic comparisonRen11Well and Suishen1Well in Renmin area were defined as K1h2andesitespreviously, yet all of the isotopic ages are less than12oMa, which were the same asthe Yingcheng Formation volcanic rocks in JiuTai, so these volcanic rocks shouldbelong to Yingcheng Formation. More than draw, Yingcheng Formation is a suite ofrock association including not only the acid ones, but also the andesites and basalts.Therefore, people should not only consider rock type but also inquire about thechronological data and typical sections in stratigraphic division, and draw reasonableconclusions.3tectonic setting of the Mesozoic volcanic rocks in the Songliao BasinA suite of volcanic intermediate rocks to acidites，mainly acidic eruptive rocks，was widely distributed in the northern Songliao Basin during the early Cretaceous.The volcanic activities began with a small amount of intermediate rocks and finishedwith massive acidites. Outcrop section study shows that the early Cretaceousvolcanism erupted in two stages, which are corresponding with1stmember and3rdmember of Yingcheng Formation respectively. The first stage is characterized withrhyolites and the second with Basaltic trachyandesites.Geochemical study shows that the early Cretaceous volcanic rocks belonged tocalc-alkaline, metaluminous to weakly peraluminous series, high∑REE and richerin LREE than HREE. The rhyolites are rich in such elements as Rb, Th, Zr, etc., andhave negative anomalies of Sr, Ba, P, Ti, etc., obvious negative Eu anomalies, act asthe “A-type” rhyolitic characteristic. The volcanic rocks in Songliao Basin in EarlyCretaceous formed in the extensional tectonic environment.Late Mesozoic volcanism shows a characteristic of belt-spreading and migratingwith the temporal and spatial variation in the northeastern Asia, it can be divided tofour belts from Mongolia-Okhotsk suture zone to southeast Pacific:160-140Ma,130-120Ma,100Ma and80Ma, which shows that it could be related to the subduction of the Pacific Plate. Mesozoic volcanic rocks in Songliao Basin are part of theMesozoic volcanism in East China, the Early Cretaceous volcanic rocks in SongliaoBasin and Northeast China have experienced the double peaking magmatism anddeveloped under the extensional tectonic environment, which implied that theMesozoic volcanic rocks in Songliao Basin should be related to an extensionfollowing the subduction of the paleo-Pacific Plate, namely Songliao is a extensionalbasin under the subduction of the paleo-Pacific Plate tectonic environment.4Hf isotope of the volcanic rocks and the restrict of crustal growth in Xing’anling-Mon golian Orogenic BeltThe176Hf/177Hf ratios of the three samples in Songliao Basin are comparativelyhigh(0.282741~0.282962, the average result is0.282867±0.000015), all of the εHf(t)have high plus results(+5.79±0.85、+6.00±0.78and+4.62~+7.30, the highest is+9.16).The model ages of Hf isotope centre at789-1494Ma, this indicates that thesource of the rocks is the new crust which growth from the depleted mantle in themiddle-late proterozoic, they melted in late Mesozoic and formed the rocks. SongliaoBasin lies in the east of Xing’anling-Mongolian Orogenic Belt, the volcanic rocks inthe northern Songliao Basin has high plus εHf(t) with model ages from0.8to1.5Ga,which proves that Songliao Basin should belongs to the Xing’anling-MongolianOrogenic tectonic evolution. It provides an important isotopic constraint on the crustalgrowth in Xing’anling-Mongolian Orogenic Belt in recent years.