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

Abstract

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