| Northeast China is located in the eastern Central Asia orogenic belt, experienceda complex evolution history. During the period of the Paleozoic, it is mainlycontrolled by Paleo-Asian Ocean tectonic system; after the Mesozoic, NrotheastChina is mainly influenced by the circum-Pacific tectonic domain andMongolia-Okhotsk tectonic domain. The study area is located in Helong city Yanbianarea, the eastern of the Jilin Province, in the east of northern margin of North ChinaCraton, between Sino-Korean ancient plate and Khanka massif, and it is theintersections of the Paleo-Asian Ocean tectonic domain and the Pacific tectonicdomain. The vegetation cover is serious, and the exposed outcrops are lessly, in theare, so the boundary of the pluton is not clear, the information of distribution andspatial relation of the pluton can not be observed directly. Therefore, it is possible toextract the geological information of surface and underground and to confirm theinformation which we can not observe directly, such as the boundary, strcture andspatial distribution of pluton. In this paper, we do some researches for Shasongdingzipluton, including petrography, geochronology, geochemical characteristics, and thepetrogenesis and tectonic setting, as well as provide new basis for regional geology.Through the study, we obtain the following understanding:1. The geochronology of Shasongdingzi plutonLA-ICP-MS zircon U-Pb dating results of Shasongdingzi pluton show that theweighted mean age is175.6±1.1Ma, at the early Jurassic, which can represent theemplacement age of the pluton.2. The geological characteristics of Shasongdingzi plutonShasongdingzi pluton is located in the west of the study area with north-southtrending. Its center phase is gray fine-grained gabbro, and marginal facies is dark grayfine-grained gabbro, the main lithology is gabbro. In this paper, we do1:1million high-precision ground magnetic survey in the vicinty of Shasongdingzi plution, fourdifferent types of magnetic anomaly are depending on the susceptibility of differentlithologies, delineated magnetic anomaly of Shasongdingzi pluton. According to thedifferences of susceptibility and magnetic measurements of rock, there are fourdifferent types of rocks near this area: the area with high magnetic anomalies(CH)may be ultrabasic rocks, mainly caused by hornblende pyroxenite; relief-lowmagnetic anormalies(CL) may be acidic rocks, which is caused by biotite feldspargranite; moderate magnetic anomalies(CM) may be mafic rocks, mainly caused bygabbro; while the magnetic of jumping low magnetic anomaly(CJ) is stronger thanacid rock, and weaker than mafic rock, is intermediate felsic rock, based on the resultsof geological mapping, it is caused by biotite quartz diorite. And according to the0°,45°,90°,135°level first derivative about the extremely extending200meterson magnetic anomaly data, we concluded that the fracture structure in the study areais in four directions. According to the vertical second derivative zero chart ofextremely extending200meters, determine the boundary of the pluton and thedistribution of the pluton. Depending on the calclation results of magnetizationextremely unusual upward continuation of100m,200m,500m,1000m,2000m,5000m,combined with the pluton outcrops in the form, to understand the spatial form of thepluton.3. The geochemical characteristics of Shasongdingzi plutonShasongdingzi pluton has low Si and high Mg, w(SiO2)=42.5%~46.3%,Mg#=51~68;w(Al2O3)=17.1%~29.3%;w(CaO)=11.2%~15.2%;w(FeOt)=3.15%~14.05%; A/CNK=0.75～0.96, with the characteristics of quasi-aluminum rock. The rareearth element(REE) contents are low in Shasongdingzi pluton, and chondrite-normalized rare earth element patterns indicate that the rock is characterized byrelatively enrichment in LREEs and depletion in HREEs, and weak positive Euanomalies; the rock is enriched in large ion lithophile elements(Cs, K, Sr), depleted inhigh field strength elements(Nb, Ta), with high Sr.4. Petrogenesis and tectonic settingGeochemical characteristics of Shasongdingzi pluton show the characteristics ofmagic magmatite which are formed by the subduction environment. Shasongdingzipluton in this paper has lower Nb,P2O5,TiO2, suggesting that its primary magma is notaffected by metasomatism of melt. Moreover, the value of Mg#(51~68) is high, and it also has high Cr, Sc, Co, and the ratio of Rb/Sr in samples is0.01~0.03, has the samecharacteristics with the Rb/Sr of mantle source. These geochemical characteristicsindicate that the characteristics of primitive magma of Shasongdingzi pluton are samewith mantle derived magma. Therefore, Shasongdingzi pluton magmas originate frompartial melting of mantle wedge metasomatized by subduction fluid, and in theprocess of rising has not been contaminated by crust. Combined with trace elementdiscrimination diagrams and the tectonic setting, we conclude that the tectonic settingof Shasongdingzi pluton is formed in the active continental margin under subductionof paleo-pacific plate. |
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