Rock Association And Formation Time Of ’Proterozoic Strata’ In The Eastern Margin Of The Songnen-Zhangguangcailing Massif, NE China: Implications For Regional Tectonic Evolution

The Dongfengshan, Zhangguangcailing, and Tadong groups, previously believed asProterozoic, are selected as the research objects in present study. The depositional agesand tectonic nature of these strata are determined according to the detrital zirconLA-ICP-MS U-Pb dating and contact relationships between different terranes based on thedetailed field geological research. Besides, the formation ages and tectonic settings ofthese igneous rocks from so-called Proterozoic strata are determined according to thezircon LA-ICP-MS/SIMS U-Pb geochronology and major-and trace-element as well aszircon Hf isotope. These geochronological and geochemical data, together with regionaltectonic analysis, provide constraints on the Paleozoic-Early Mesozoic tectonic evolutionin the region.1. Depositional ages of the Dongfengshan, Zhangguangcailing and Tadong groupsDetrital zircon dating results show that the Dongfengshan Group have differentdepositional ages. The sedimentary process of the Liangzihe Formation near theDongfengshan iron ore-deposit could take place during821~752Ma. Whereas, thesedimentary processes of the Liangzihe Formation biotite quartz schist and metamorphicsiltstone near Liangzihe iron ore-deposit could take place during425~386Ma and288~271Ma, respectively. The sedimentary process of two-mica quartz schist collectedfrom Hualigou Formation, as the middle part of the Dongfengshan Group, could takeplace during425~386Ma. The Hongli Formation, in the top part of the DongfengshanGroup, mainly crops out in the northern Yichun and is composed mainly ofcarbon-bearing silty slate and granitic gneiss. According to the present study, thesedimentary process of the Honglin Formation should take palce in275~271Ma. Detailed field geological research, combined the detrital zircons dating results, suggest that threedetrital zircon samples from Zhenggou Formation, as the bottom part of theZhangguangcailing Group, deposited during226~211Ma,228~211Ma, and311~226Ma,respectively. Three detrital zircon samples from Hongguang Formation, as the middle partof the Zhangguangcailing Group, deposited during450~426Ma,426~262Ma, and262~219Ma, respectively. And the sedimentary process of biotite quartz schist collectedfrom Xinxing Formation, as the top part of the Zhangguangcailing Group, could take placeduring284~262Ma. The dating results also indicate that the Lalagou Formation of theTadong Group was deposited between450and426Ma, and the Zhudundian Formation ofthe Tadong Group was deposited551~450Ma and750~516Ma.2. Tectonic nature of the Dongfengshan, Zhangguangcailing and Tadong groupsDetailed field geological and petrological research suggest that the Dongfengshan,Zhangguangcailing and Tadong groups are consist of sedimentary and metamorphic rocks.The contact relationships among the different rocks, combined with the deitrital zirconU-Pb dating results, suggest that these so-called strata are not continous stratigraphicsequence of strata, but are actually a suite of tectonic mélanges that are made up of adiverse assortment of meta-sedimentary and volcanic rocks with ages that range fromNeoproterozoic to Early Mesozoic. Based on the youngest age for volcanic rocks withinthe tectonic mélange, and the age of deformation (174–184Ma) of the HeilongjiangComplex, we propose that the tectonic mélange formed during the Late Triassic–EarlyJurassic. A suite of Neoproterozoic terranes is present along the eastern margin of theSongnen–Zhangguangcai Range Massif (~821–752,~752–560, and~750–516Ma).Detrital zircons with ages of0.75–0.95Ga provided a large proportion of the clasticmaterial in the ‘Dongfengshan’,‘Zhangguangcailing’ and ‘Tadong’ groups, implying thata Neoproterozoic magmatic event occurred within or on the eastern margin of theSongnen-Zhangguangcai Range Massif. Paleo-Meso Proterozoic detrital zircons indicatethat several Paleoproterozoic rock masses may have existed at or close to the surface alongthe eastern margin of the Songnen–Zhangguangcai Range Massif.3. Temporal Features of the igneous rocks from Dongfengshan, Zhangguangcailingand Tadong groupsZircon U-Pb dating for igneous rocks from ‘Dongfengshan Group’, ‘Zhangguangcailing Group’ and ‘Tadong Group’ tectonic mélanges indicate that theseigneious rocks can be divided into eight stages, i.e., the Early Cambrian(~516Ma), theEarly Ordovician(~485Ma), Late Ordovician(~450Ma), Middle Silurian (~425Ma), LateCarboniferous (~317Ma), Early Permian (~282Ma), Late Permian (~256Ma), and LateTriassic (~217Ma).4. Tectonic settings of the igneous rocks in the ‘Proterozoic strata’The formation of the middle Cambrian tonalite is related to the partial melting of thesubducted oceanic plate in the early stage. The early-late Ordovician calc-alkaline igneousrocks suggest an active-continental margin on the eastern margin of theSongnen-Zhangguangcailing Range Massif, implying the westward subduction of anoceanic plate beneath the Zhangguangcai Range Massif. And the existence of the MiddleSilurian monzogranite implies that the collision and amalgamation of theSongnen-Zhangguangcai Range and Jiamusi massifs could take place in the middleSilurian. Late Carboniferous A-type rhyolite suggests a post-collisional extensionalenvironment in the region. The Early Permian granodiorite, combined with the coevalbimodal volcanic rocks from the eastern Harbin City, suggests an extensional environmentin the Songnen-Zhangguangcai Range Massif. The Late Permian igneous rocks arecomposed mainly of gabbrodiorite and monzogranite, implying a typical bimodal igneousassociation fromed in an extensional tectonic setting related to the amalgamation of theSongnen-Zhangguangcai Range-Jiamusi and the Khanka massifs. The Late Triassicbimodal volcanic rocks in the Zhangguangcai Range imply an extensional environmentrelated to the final closure of Paleo-Asian Ocean.