Late Permian medium-pressure metamorphism in the eastern Songnen Massif, eastern Central Asian Orogenic Belt (NE China): Implications for the final closure of the Paleo-Asian Ocean

摘要

The nature of late Paleozoic metamorphism in the eastern Central Asian Orogenic Belt (CAOB) is controversial. This paper reports mineral chemistry, petrological, and zircon and titanite U-Pb age data for representative rocks in the Dongfengshan Group from the eastern margin of the Songnen Massif. We use these data to determine the rock associations, P-T conditions, and timing of metamorphism. The P-T conditions were estimated from 54 mineral pairs in representative rocks, including garnet-biotite schist, garnet-two-mica schist, garnet-biotite-plagioclase gneiss, and garnet amphibolite, and indicate the Dongfengshan Group experienced metamorphism at P = 5.5-7.1 kbar and T = 592-630 degrees C (i.e., a geothermal gradient ranging from 25 to 30 degrees C/ km) under medium-pressure lower amphibolite-facies conditions. The biotite-plagioclase gneiss and garnet amphibolite yielded titanite U-Pb ages of 251 and 260 Ma, respectively, which are interpreted to represent the regional metamorphic age (260-251 Ma). The consistency of these dates and ages of metamorphic rims in zircons from a biotite plagiogneiss (252 Ma) confirm that medium-pressure metamorphism occurred in the late Permian rather than the Neoproterozoic, as assumed previously. Based on metamorphic, magmatic, stratigraphic, and paleontological constraints, as well as the regional tectonic history, we propose that late Permian medium-pressure metamorphism in the Dongfengshan Group occurred during the collision and amalgamation of the Siberian and North China cratons following the final closure of the eastern Paleo-Asian Ocean.