Early Cretaceous Volcanic Event In The Northern Songliao Basin And Its Geodynamics

Late Mesozoic tectonic-magmatism event is widespread throughout northeastern Asia, which is characterized with large-scale volcanic belts and many faulted-depressions from NE China to mid-eastern Mongolia. As the direct product of magmatism during the process of tectonic evolution, volcanic rocks are the important clue in discussing the geological events and geodynamic evolution. This dissertation is focused on the study about the characteristics and geodynamic mechanism of the volcanic rocks from the northern Songliao Basin and its basin-margin outcrop. The main conclusions are as follows:(1) A 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 finished with massive acidites. Outcrop section study shows that the early Cretaceous volcanism erupted in two stages, which are corresponding with 1^st member and 3^rd member of Yingcheng Formation respectively. The first stage is characterized with rhyolites and the second with Basaltic trachyandesites.(2) Geochronological study through zircon SHRIMP of the volcanic rocks displays that the main volcanic eruption occured between 108 Ma～116 Ma in the northern Songliao Basin, and mainly concentrated between 111 Ma～113 Ma.The volcanic rocks in both Shengping gas-field and Xingcheng gas-field formed synchronously, which sustained from the Aptian to the early Albian in the early Cretaceous. The volcanic strata in those two fields are thought to be divided into 3^rd member of Yingcheng Formation. Based on former studies and the SHRIMP-age of the volcanic rocks, the geological time of Yingcheng Formation is considered from Hauterivian to Albian in the the early Cretaceous. Several inheritable magmatic zircons appearing different ages implied that NE China experienced multiple tectonic-thermal events in Mesozoic.(3) Geochemical study shows that the early Cretacesous volcanic rocks belonged to subalkaline and para-pelite, as high∑REE and richer in LREE than HREE. The rhyolites are rich in such elements as Rb, Th, Zr, etc., and have obvious negative anomalies of Sr, Ba, P, Ti, Eu, etc., while the andesites have a notable loss of those elements, and the characteristics of dacites are between them. The loss of Eu, Ba, P and Ti in rhyolites indicates that the melts like intermediate intermediate rocks experienced a plagioclase-apatite-tutabuiferite predominant fractionation. All the above geochemical results imply that an evolved affinity existed between the intermediate and acidic rocks.(4) Sr and Nd isotopic analysis exhibits that the early Cretaceous rocks in the Songliao Basin have the low content of Sr and high content of Nd data. The positiveε_Nd and low value of the initial ⁸⁷Sr/⁸⁶Sr ratio indicate they were derived from a similar mantle source and contaminated by the crust in different level during the period of ascending and evolving. The model ages of Nd suggest that the source crust of the volcanic rock was formed in Neo-Proterozoic Era.(5) Late Mesozoic volcanism shows a characteristic of belt-spreading and migrating with the temporal and spatial variation in the northeastern Asia. It is suggested that the late Jurassic volcanism might have a relationship with the closure of the Okhotsk Ocean and the Cretaceous volcanic activities was quite possible to be controlled by the delamination and roll-back of the paleo-Pacific subduction. The early Cretaceous volcanic rocks characterized with A-type rhyolites and developed under a post-orogenic and within-plate tectonic environment. Those are attributed to lithospheric thinning during the late Mesozoic. The lithosphere of the Daxing'an ranges and its westward part are believed to have mainly experienced an extensional lithospheric thinning and subsequently their thickness reduced just a little in scale. However, the part to east of the Daxing'an Ranges, especially under the Songliao Basin, is thought to have undergone both extensional and thermal-chemical erosion lithospheric thinning, and its thickness decreased dramatically. Inhomogeneous lithospheric thinning may be the vital reason for the different evolution of basins which locate in the two sides of Daxing'an Ranges.