A Triassic-Jurassic westward scissor-like subduction history of the Mudanjiang Ocean and amalgamation of the Jiamusi Block in NE China: Constraints from whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes of the Zhangguangcai Range granitoids

NE China recorded the key tectonic evolution history of the Eurasian Plate from the Paleozoic-Mesozoic
collisional formation of the Central Asian Orogenic Belt to the Mesozoic subduction of the Paleo-Pacific Ocean.
To better understand this tectonic transition, it is crucial to constrain the time and pattern of the initial subduc-
tion of the Paleo-Pacific Ocean. Recently, someresearchers proposed that theMudanjiang Ocean existed between
the Songnen and Jiamusi blockswas part of the Paleo-Pacific Ocean. Here, through geochemical and geochrono-
logical studies on the widespread granitoids in the Lesser Xing'an-Zhangguangcai Range in the eastern Songnen
Block, we verify that these magmatic rocks show volcanic arc affinity with increased mantle contribution from
east to thewest of the range, likely related to a flattening subduction of theMudanjiang Ocean. In addition, a uni-
versal westward younging trend for over 70 Myr can be observed for the granitoids throughout the Lesser
Xing'an-ZhangguangcaiRange, indicating a long-lastingsubductionof theMudanjiangOcean.More interestingly,
the oldest ages of the granitoids in the east display a northward younging trend from275Ma to 218Ma, suggest-
ing that the subduction of the Mudanjiang Ocean had been initiated at latest by 275 Ma in the south and then
progressively expanded to the north. Based on these observations, we proposed a new tectonic evolution
model for theMudanjiang Ocean, i.e., a Triassic-Jurassicwestward scissor-like subduction and closure, to contrib-
ute to the understanding of the early subduction of the Paleo-PacificOcean