Geochemical Characteristics And Tectonic Setting Of Early Mesozoic Granites In Dongning Area

Dongning area, which is located within the eastern part of Heilongjiang province and the most eastern part of the Central Asian Orogenic Belt, form the southwestern continent margine of Khanka block with Yanbian area in Jinlin province and Russian Primorye, and develop geologic unit since late Paleozoic mainly. In the investigate region, immense volumes of granitic rocks emplaced during the Phanerozoic. This paper made a precise dating for the granodiorite of Taiping range pluton in the area, by U-Pb isotopic dating on zircon, and obtain 201±1Ma, which represent the age of the intrusion of this plutons. The age of Taiping range pluton is consistent with the ages of the 199±8Ma/206±4Ma of Qishierdingzi pluton, the 203±2/201±10Ma of Dahuanggou pluton and the 214±10Ma of Naozhigou Pluton. The new chronology framework of plutons in Dongning-Yanbian area has changed the previous viewpoint that all of these plutons formed during Late Permian-Early Triassic and reflect a magmatothermal event in the Dongning-Yanbian area at the end of late Triassic.This paper research the petrological and geochemical characters amply on the Taiping range pluton and Qishierdingzi pluton. Combined with the previous datums on Naozhigou pluton and Dahuangou pluton, the late Triassic granites in Dongning-Yanbian are mainly composed of granodiorite and monzogranite, and some quartz diorite and K-feldspar granite. These granitoid rocks are hypidiomo- rphic granular or prophyaceous texture and massive structure. Many mafic magma enclaves are contained in the granite, and the characters of the shape, the texture, the minerals of the enclaves reflect the granitic magma is formed by mixing of high temperature basic magma and relative low temperature felsic magma. The petrologic geochemistry characters show these granites rich in CaO and Na2O, their Rittmann index(δ)=0.77- 2.78, < 3, A/CNK range from 0.84 to 1.25, most are less than 1.1,and ANK are more than 1, so all they are calc-alkaline metaluminous granites. These granites belong to medium K calc-alkaline series, and have a tendence that evolving into high-K calc-alkaline series, all they belong to I-typed granite. The primary mantle normalized spider diagrams of trace elements shows that these granites are rich in LILE (Rb, Th, K, La et al.), and the enrichment become more and more obvious with the evolvement of magmas, deplete Sr, P, Ti and the HFSE (Nb, Ta, Zr, Hf et al.) relatively, Ba show a slightly depletion compared with Rb and Th. Combined with the variety of main elements, the trace elements and REE variation indicate that the granitic magma has occurred fractional crystallisation during the magmatic evolution. The fractionating phase are hornblende and biotite during the early stage, and is plagioclase in the late stage, and some accessoy minerals (apatite and/or titanite) occurred fractional crystalli- zation also. The isotopic characters of low Isr and highεNd(t) of granites are similar with Phanerozoic granites in CAOB, and someone's Isr are close to the Isr of ocean tholeiite, which indicate the contribution of oceanic crust to the formation of granitic magma. Through the comparation on petrologic, mineral assemble and geochemistry, these granites are similar with the ACG(ATG) and parts of KCG, which are eatablished by Barbarin(1999) and their formation are related to the subduction of oceanic crust. Using the tectonic discrimination diagram to judge the setting of the granites formation, all samples of the studied plutons fall within the field pre-plate collision and syn-collision granite of R1-R2 diagram (Batchelor et al.,1985), which show these granites have a relation with the subduction of oceanic crust.The research on stratigraphy and paleontology, paleomagnetic, petrology and tectonic indicate that the paleo-Asian ocean closed along the west Lamulun river- Changchun- Yanji and the south block and north block have occurred intense convergence at the end of Permian. It become a integer at late Triassic. The medium K cale-alkaline granite that form at the active continental margin at Late Triassic in Dongning-Yanbian and the early Jurassic high K calc-alkaline granite in Zhangguangcai range, which form at post-collision uplift set, form a changing tendence of composition. It reflect that there is a subduction of oceanic crust to the Northeast China at that time. It show that the paleo-Pacific plate has started to subduct under the Northeast China at the end of late Triassic.