| Dong’an is located in the eastern part of Heilongjiang province, the North Lesser Khingan Ranges. Tectonically, it is located in the eastern segment of the MongoliaHinggan Orogen Belt. The region underwent not only the evolution of the Paleo-Asian oceanic tectonic system in the Paleozoic, but also was overprinted and modified by the circum-Pacific tectonic system in the Mesozoic-Cenozoic. Because of the special tectonic position, the complicated evolution of geological structure, and the widespread Phanerozoic granite rocks, this area has drawn extensive attention by domestic and international scholars. This thesis take the widespread granite in Dong’an area as research object. Detailed studies of petrology, zircon LA-ICP-MS U-Pb geochronology, geochemistry and in situ zircon Hf isotopic analyses, are carried out on the basis of field geological investigation, in order to determine their formation time, and discuss their petrogenesis and tectonic setting. Through this research, we obtain the following conclusions.Based on emplacement relations and the U-Pb geochronology results, the granites of Dong’an area are the Early Jurassic in age, by the formation time order they are porphyroid monzonitic granite, fine-grained alkali-feldspar granite(184±2Ma), medium-coarse grained alkali-feldspar granite(185±1Ma) and granite aplite(178±1Ma). Meanwhile, the granite geochronology data show the evolution of granitic magma, zircon crystallization during emplacement process of magma(187~189Ma), crystallization of main granitic magma accompany formation of fine-grained alkali-feldspar granite(184±2Ma) and medium-coarse grained alkali-feldspar granite(185±1Ma), and the crystallization of the remainder magma accompany formation of granite aplite(178±1Ma).According to petrography and geochemistry of granites in the Dong’an area, the geochemistry compositions of granites are basically the same, although there are some differences in mineral content, texture and structure in granites. These ganites have a mineral assemblage of alkali feldspar+ quartz+ plagioclase+ biotite, without alkaline dark minerals. Geochemically, they are enriched Si( Si O2= 73.93%~ 78.30%) and alkali(especially K,K2O+ Na2O= 8.20%~ 8.91%, K2O/ Na2O= 1.17~1.68, K2O> Na2O), negative in Fe( Fe Ot= 0.25~ 0.97), Mg( Mg O= 0.06~ 0.28), Ca( Ca O= 026~ 0.46), Ti( Ti O2= 0.08~ 0.21), P(P2O5=0.01~0.04), weakly peraluminous( A/CNK= 1.01~1.17), higher differential degree(D.I.=93.94~97.51). Ti O2, Fe Ot, Mg O, P2O5 and Al2O3 show a negative correlation with Si O2. The abundance of REE is relatively low( ∑LREE=(46.96~117.39)×10-6). The fractionation between LREE and HREE are intense( ∑LREE/ ∑HREE= 4.28~18.83,(La/Yb)N=3.58~25.04) with the evident negntive anomaly of Eu(?Eu =0.21~0.78). They are enriched in LILEs such as Rb and K, HFSEs such as Th, U, Zr, Hf, and depleted in HFSEs such as Ta, Nb, P, Ti. The values of 10000Ga/Al, Zr+Nb+Ce+Y, Fe Ot, Fe Ot/Mg, AKI, and zircon saturation temperature(TZr) are 2.04~2.47, 88.68~222.15, 0.25~0.97, 2.66~6.18, 0.82~0.94, and 684~797℃, respectively, for these granites. The ratios of Rb/Sr, La/Nb, Th/Nb, Th/La are similer to those of crust rocks. In addition, the εHf(t) values of zircon in the granite aplite range from-1.8 to +5.4, with the two-stage Hf model ages(TDM2) varying from 1172 Ma to 1820 Ma.The granites in Dong’an area belong to the highly fractionated high-K calc-alkaline I-type granitic rocks. The granitic magma was derived from partial melting of Neoproterozoic- Mesoproterozoic accreted crust. Crystal fractionation is the main mechanisn during the process of magma evolution. These granites are similar to those from the active continental margin setting. Combined with the achievements of regional geological investigations, we conclude that these granites formed under a back-arc basin extensional environment related to the subduction-exhumation of the Paleo-Pacific Plate beneath the Eurasian continent. |
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