Age, Geochemistry And Petrogenesis Of The Yingcheng Formation From Changling Depression In The Songliao Basin

In recent years, it has made an important progress in geochronology and geochemistry.However, the research mainly concentrated in the northern part of Songliao Basin and thesoutheast uplift area, there is still no systematic research results of the geochronology andgeochemistry for the volcanic rocks from the Yingcheng Formation from changlingdepression in the Songliao Basin, which restricts the stratum contrast in the area of theMesozoic and exploration development of oil and gas. Meanwhile, the petrogenesis ofvolcanic rocks from the Yingcheng Formation in the Songliao Basin still has disputes. Thethesis through the studies of zircon U-Pb chronology, geochemistry，in order to provide thebasis for accurate set of the volcanic rocks from the Yingcheng Formation from changlingdepression and provide evidence to discuss the genesis of the Mesozoic volcanic rocks inSongliao Basin.1. Chronology of the volcanic rocks of the Yingcheng Formation from Changlingdepression in Songliao BasinZircon U-Pb isotopic chronology is tested for the eight samples(YS1、YS3-1、YS4-1、YS5-2、YS6-2、YS7、YS201-1、SN102-1) of the Yingcheng Formation from Changlingdepression in southern Songliao Basin using the method of LA-ICPMS. The acidic volcanicrocks of the Yingcheng Formation from Changling depression, formed at Early Cretaceouswith zircon U–Pb ages ranging between102and112Ma.2. Petrology and Geochemistry of the volcanic rocks of the Yingcheng Formation fromChangling depression in Songliao BasinThe volcanic rocks of the Yingcheng Formation from Changling depression in southern Songliao Basin are mainly rhyolite, and chemical compositions of these rocks show that theyall belong to alkaline series.The SiO2contents of the rhyolite range from68.98%to76.76%, Al2O3contents rangefrom10.45%and14.85%, rhyolite are rich in alkali (K2O+Na2O=6.85~9.95%), CaOcontent are from0.08%to1.02%. NK/A is0.73to1.06, The chondrite-normalized rare earthelement patterns are characterized by light rare earth elements enrichment,∑REEabundances range from362.1to629.03ppm, chondrite normalized (La/Yb)Nvalues of6.99–10.38and δEu values of0.16~0.65. Primitive mantle-normalized multi-element patterns arecharacterized by relative enrichments in K, Rb, and LREE but depletions in Ba and Sr, as wellas P and Ti. However, they exhibit little or no depletion in Nb and Ta.3. Petrogenesis of the volcanic rocks of the Yingcheng Formation from Changlingdepression in Songliao BasinChemical compositions of these rocks show that they all belong to rhyolite. Thegeochemical study suggests that volcanic rocks are rich in silicon, alkalis, poor in aluminum,calcium, magnesium, which close to the average composition of the world A-type granites.NK/A is0.73to1.06, and the FeOT/MgO ratio of volcanic value is high (varying from10.37to45.82). All these characters indicate rhyolite from Changling depression is A-type granite.The volcanic rocks of the Yingcheng Formation from Changling depression exhibit littleor no depletion in Nb and Ta, Y/Nb ratio (0.6to0.90) and Yb/Ta ratio (0.95to1.49) are low,Nb/Ta varies between15.29and17.92(average16.78), closer to the primitive mantle values(17.5) but significantly higher than the Nb/Ta of the crust ratios (11). The averages of Zr/Hfand Nb/U are44.51and47.14, close to the Zr/Hf (44.6±2.3) and Nb/U (47±10) of theocean island basalts. Account to the Nb-Y-Ce and Nb-Y-3Ga ternary diagrams, the volcanicrocks of the Yingcheng Formation from Changling depression show an affinity with theA1-type granite.The ratios of trace element suggest that volcanic rock has the characteristics of themantle, which indicates the rhyolite formed by partial melting of juvenile crust. rhyolitebelongs to low-Sr and high-Yb type and some belongs to very low-Sr and high-Yb type. It isformed in a lower pressure environment (<0.8or1.0Ga) at the depth of normal crust by the partial melting of the juvenile crustal (20-30km).4. Tectonic significance of the volcanic rocks of the Yingcheng Formation fromChangling depression in Songliao BasinThe geochemical characteristics of rhyolite are similar to A-type granite, and the basaltsin the area have a similar REE patterns to ocean island basalts and continental rift basalts,which suggest the volcanic rocks in Songliao Basin formed in the extensional tectonicenvironment. The rhyolites in the area show an affinity with the A1-type granite, whichsuggest that volcanic rocks formed in the continental rift, mantle plume, or hot environment.From the morphology of volcanic in the Songliao Basin, we can exclude that the volcanicrocks in the area formed in the mantle plume or hot environments. As typical case of basinand range coupled, Songliao Basin and Daxinganling are similar to the Basin and Rangeprovince of the western United States, which impling Songliao Basin formed in theextensional environment, which may be related to subduction of ancient Pacific.