Paleo-Pacific subduction-accretion: Geochemical and geochronology constraints from the Raohe accretionary complex , NE China

The Raohe Complex forms the main part of the Nadanhada accretionary complex resulted from the westward subduction of Paleo-Pacific plate. The rocks assemblage and accretion process of the Raohe Complex will provide new constraints for subduction-accretion process of Paleo-Pacific plate beneath the Eurasia. Based on field geological investigation and large-scale geological map of Dadai area, we define that the Raohe Complex consists of pillow lava, cumulate gabbro, oceanic plate stratigraphy (OPS) , which form tectonic lenses distributing in the trench sedimentation, indicating its nature of tectonic melange, in addition, the Raohe Complex was intruded by granitic plutons, these evidence can provide important constraints for the rocks assemblage, accretion and final emplacement timing of the Raohe Complex. Geochemical data show that the Raohe basalts have ocean island basalts ( OIB) affinities. LA-ICPMS zircon U-Pb data define the formation ages of the basalt and gabbro are 166 +/- 2Ma and 214 +/- 5Ma, respectively, thus, there is a range in the formation age for the mafic-ultramafic from the Late Triassic to Middle Jurassic. A silty mudstone sample has maximum depositional age of 167 +/- 3Ma, different from the sandstone sample (133 +/- 4Ma) , which indicates the accretion timing of the Raohe Complex is from 167Ma to 133Ma, and their detrital ages can all be related to the Jiamusi Block and eastern segment of the Central Asian Orogenic Belt (CAOB), based on the Precambrian detrital and inherited ages occurring in the several adjacent blocks, we consider there might be Precambrian basement in NE China. In addition, we obtained forming ages of 126 +/- 1Ma and 105 +/- 2Ma from two monzonitic granite samples, indicating that the granitic plutons formed at two stages, the older age of 126 +/- 1Ma, together with an age of 133 +/- 4Ma from a sandstone sample, constrains the final emplacement of the Raohe Complex between 133 Ma and 126Ma. These evidences verify the westward subduction-accretion of the Paleo-Pacific plate between Middle Jurassic and Early Cretaceous, and provide important constraints for the tectonic evolution of Paleo-Pacific plate.