关键词: 小兴安岭, 东风山群, 锆石U-Pb年代学, 物源, 同位素

Abstract:

LA-ICP-MS U-Pb dating of zircons from the Liangzihe Formation and Honglin Formation of the Dongfengshan Group in the Lesser Xing’an Mountains, eastern Heilongjiang Province, are presented in order to constrain their forming age and tectonic attribution. Most of zircons from the Dongfengshan Group are euhedral and subhedral in shape and display striped absorption or oscillatory zoning in CL images, implying their magmatic origin. The others show dark accretionary rim, implying their metamorphic origin. The dating results indicate that zircons from twomica schist in the Liangzihe Formation near the Dongfengshan iron ore deposit yielded age populations of 821 Ma, 1 286 Ma, 1 396 Ma, 1 504-1 595 Ma, 1 800 Ma, and 2 500 Ma; zircons from biotite quartz schist and metamorphic siltstone in the standard profile of the Liangzihe Formation yielded age populations of 425 Ma, 453 Ma, 485 Ma, 506 Ma and 288 Ma, 452 Ma, 486 Ma, 699 Ma, 801 Ma, 900 Ma, respectively; and zircons from carbon-bearing silty slate in the Honglin Formation yielded age populations of 275 Ma, 760 Ma, and 781 Ma; whereas zircons from granitic gneiss (a granitic intrusion intruding into carbon-bearing silty slate) in the Honglin Formation yielded age of 271 Ma. Based on isotopic dating results above and the regional magmatic events, we conclude that the sedimentary processes of the Liangzihe Formation near the Dongfengshan iron ore deposit could take place during the Late Neoproterozoic period, and it is the firstly discovered Precambrian terrane in the region; the biotite quartz schist and metamorphic siltstone near the standard profile of the Liangzihe Formation formed in late stage of Early Paleozoic and Late Paleozoic, respectively; and the sedimentary process of the Honglin Formation in the Dongfengshan Group should take place in the Late Palaeozoic period (275-271 Ma). The research results in the paper imply that the Dongfengshan Group in the Lesser Xing’an Range is composed of sedimentary rocks and metamorphic igneous rocks with different ages, its principal part formed in the Palaeozoic period, and the other strata formed in the Late Neoproterozoic period. The major material source of the Dongfengshan Group could be the Paleozoic geological bodies and minor Precambrian basement relic around the study area.

Key words: the Lesser Xing’an Mountains, Dongfengshan Group, zircon U-Pb dating, material source, isotope