| This paper reports LA-ICP-MS U-Pb dating results of detrital zircons and the petrogeochemistry(the main elements and the trace elements)from the Hongshan and Heilonggong formations in the area of Yichun in the Lesser Xing,an Range of the east of Hei Longjiang province. In order to afresh their formation timing and discussing the provenance, and at that time the formations’ tectonic setting.1.The Deposition time of the Hongshan and Heilonggong FormationsMost of detrital zircons are euhedral-subhedral in shape from CL images, implying their magmatic origin. The dating results on magmatic zircons indicate that 42 detrital zircons from clayslate stones in the Hongshan Formation yield age populations of 747 Ma, 807 Ma,849 Ma, 903 Ma, 956 Ma,1167 Ma, and 1811 Ma, implying that the sedimentation of the Hongshan Formation could take place after 747 Ma. 97 detrital zircons from clayslate stone in the Heilonggong Formation yield from Yichun age populations of 700(loss of Pb) Ma, 805 Ma, 902 Ma, 1764 Ma, 2446 Ma, and 2467 Ma, but because of the losing Pb, it suggests that the sedimentation of the Heilonggong Formation could take place after 805 Ma. Combined with the detrital zircon dating results(wide occurrence of 561 Ma age population) from late Paleozoic strata in research area and adjacent area, we conclude that the formation ages of the Hongshan and Heilonggong formations are between 747 and 561 Ma and between 805 and 561 Ma, respectively; i.e., Neoproterozoic rather than the Late Paleozoic as previously believed. Based on the comparison between the age populations of detrital zircons from the two formations and geochronological data in the study area, it is suggested that the sediments of the two formations mainly sourced from the Neoproterozoic stones.2.The Rock Geochemical Characteristics of the Hongshan and Heilonggong FormationsThe argillite from Hongshan formation has Si O2=58.32%~64.56%,(Mg O+ TFe2O3)=7.18%~14.44%. It has the higher REE(∑REE =284.3×10-6~555.7×10-6),and the LREE enriched. The Eu has obviously negative anomalies(Eu/Eu*=0.56~0.66), similar to the rocks from island arc and/or active continental margin settings.The siltstone from Heilonggong formation has Si O2=59.74%~63.86%,( Mg O+ TFe2O3=7.4%~8.1%). The REE has a high numerical value(∑REE=191.2×10-6~214.9×10-6).And the average is 203.7×10-6. The numerical value of LREE is 165.9×10-6~187.3×10-6. The Eu has obviously negative anomalies(Eu/Eu*=0.62~0.69), similar to the rocks from island arc and/or active continental margin settings.3. The Provenance Characteristics of the Hongshan and Heilonggong FormationsFrom the CIA and ICV of the Hongshan and Heilonggong formations, it’s obviously that the source rocks have the medium maturity, and experienced a strong chemical weathering. The Th/U(2.76~4.06) of both stratums show the source rocks of Hongshan Formation has suffered the weak oxidation weathering, and the Heilonggong Formation has the stronger one. The Zr/Sc—Th/Sc ratios illustrate that both the source rocks didn’t have suffered the sedimentary cycle, it belonged to proximal deposit.According to the La/Th-Hf、Co/Th—La/S and La/Yb—∑REE diagrams, the type of resource rocks in Hongshan Formation, are mainly alkaline basalt,andesite, basic rocks. And Heilonggong Formation’s most are acidic rocks include the quartz-feldspathic rocks.4. The Tectonic evolution of Source Region in Hongshan and Heilonggong FormationsIn combination with the evidence of the U-Pb dating results of detrital zircons, the Neoproterozic stones take the high percentage of the two formations. It is suggested that the magma events provide the main source. So, in the Neoproterozoic era had the magma events in the northeast and the Inner Mongolia.Through the analysis of the global tectonic environment of the neoproterozoic, rock assemblages and lithogeochemical determine the structure of the Hongshan Formation tectonic evolution maybe an ocean island arc; The Heilonggong Formation imply an tectonic evolution continental island arc. |
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