Formation And Tectonic Implications Of The Raohe Accretionary Complex In Nadanhada Terrane

The Nadanhada Terrane,located along the eastern margin of Eurasia,belongs to the circum-Pacific tectonic domain.The Raohe accretionary complex forms the main part of the Nadanhada Terrane,its rocks assemblage and accretion process will provide new constraints for subduction-accretion process of Paleo-Pacific plate beneath the Eurasia.This thesis conducts systematic researches on the petrology,geochronology,whole-rock geochemistry of the Raohe accretionary complex,which defines the rocks assemblage,accretion and emplacement timing of the Raohe accretionary complex and constrains the subduction-accretion timing of the Paleo-Pacific plate in Northeast(NE)China.Based on field geological investigation and large-scale geological map of Dadai area,we define the Raohe accretionary complex as a suit of mélange with typical block-in-matrix structure,the rocks consist mainly of limestone,chert,siliceous shale,cumulate gabbro,pillow lava,which form tectonic lenses distributing in matrix that consists of conglomerate,greywacke,silty mudstone,and the Raohe accretionary complex was intruded by granitic plutons.Geochemical data show that the Raohe basalts have ocean island basalts(OIB)affinities,together with OIB-type gabbro,pyroxenite and plagioclasite,indicate that the mafic-ultramafic rocks in the Raohe accretionary complex is a suit of OIB-type igneous rocks.Zircon U-Pb data define the formation ages of the basalt(14JH-42)and gabbro(18HLJ-50)are 167 ± 1 Ma and 214 ± 5 Ma,respectively,thus,there is a range in the formation ages for the mafic-ultramafic rocks from the Late Triassic to Middle Jurassic.The silty mudstone(14JH-44)and greywacke(18HLJ-48)samples were collected from the matrix of Raohe accretionary complex and have maximum depositional ages of167 ± 3 Ma and 133 ± 4 Ma,which indicates the accretion timing of the Raohe accretionary complex is from Middle Jurassic to Early Cretaceous,and the formation ages of matrix display an eastward younging trend coinciding with the westward subduction of Paleo-Pacific plate.The detrital zircons from the matrix have peak ages of 139 Ma,173 Ma,264 Ma,364 Ma,502 Ma,782 Ma,1878 Ma and 2503 Ma and are mainly derived from OIB-type igneous rocks in the Raohe accretionary complex and adjacent Jiamusi-Khanka block.In addition,the granitic plutons have formed between 126 Ma and?100 Ma,together with an age of 133 ± 4 Ma from a greywacke sample,constrains the final emplacement of the Raohe accretionary complex between 133 Ma and 126 Ma.These evidences verify the westward subduction-accretion of the Paleo-Pacific plate between Middle Jurassic and Early Cretaceous.The subduction-accretion of Paleo-Pacific plate in NE China took place between Late Triassic and Early Cretaceous and can be divided into two stages:(1)during Late Triassic to Early Jurassic,as a result of the westward subduction of Paleo-Pacific plate,the Jiamusi Block was collided with Songliao Block,following by the emplacement of the Jilin-Heilongjiang high-pressure metamorphic belt and Yuejinshan Complex along the western and eastern margins of the Jiamusi Block;(2)between Early Jurassic and Early Cretaceous,the Raohe accretionary complex was formed as a result of the subduction of Paleo-Pacific plate.In addition,due to the termination of Paleo-Asian ocean tectonic domain in NE China took place on Early Triassic or Middle Triassic,we consider that the transition from Paleo-Asian Ocean to circum-Pacific tectonic domains may initiate on Late Triassic.