Early Paleozoic metamorphic rocks of the Erguna block in the Great Xing'an Range, NE China: Evidence for the timing of magmatic and metamorphic events and their tectonic implications

摘要

The Erguna block is a crustal unit located along the eastern margin of the Central Asian Orogenic Belt (CAOB) in the far north of China. It contains a variety of khondalitic rocks that include sillimanite- and garnet-bearing gneiss, biotite-plagioclase gneiss and carbonate, in association with hornblende-plagioclase gneiss and a variety of granitic orthogneisses. These rocks are collectively referred to as the Mohe Complex and make up the basement rocks in this poorly exposed terrane. Four samples were analyzed for this study, including two samples of biotite-plagioclase gneiss, one sample of garnet-sillimanite gneiss, and a sample of hornblende-plagioclase gneiss. All samples provide evidence of metamorphism in the form of discrete zircon grains or metamorphic rims around detrital cores and they record remarkably consistent ~(206)Pb/~(238)U ages between 495±2Ma and 497±6Ma. Detrital zircon cores with oscillatory zoning from the two biotite-plagioclase gneiss samples yield magmatic ages that range from 608±8Ma to 1015±12Ma, whereas those in the garnet-sillimanite gneiss range from 678±8 to 1373±17Ma. These results establish that a range of Meso- to Neoproterozoic magmatic rocks were present in the source area and contributed to the sedimentary protoliths. The youngest zircon core with an age of 608±8Ma limits the maximum possible age of deposition, indicating that the rocks of the Mohe Complex cannot be Late Archean to Early Proterozoic as previously considered. The ~500Ma age for the granulite facies metamorphic event in the Mohe Complex is identical to that recorded in the Mashan Complex of the Jiamusi block and the Hutou Complex of the Khanka block, >1000km to the south-southeast, implying that an extensive early Paleozoic khondalite belt extended across much of the eastern CAOB. Rocks of similar age and metamorphic grade have also been recorded from the Sayang-Baikal Orogen along the southern margin of the Siberia Craton, >1000km to the west. It is unclear at present whether these two belts are directly related but, whatever the relationship, it is evident that extensive tracts of khondalitic rocks were present in the eastern CAOB. They were subsequently affected by high-grade metamorphism in the Early Paleozoic, forming collisional accretionary terranes that were accreted during the Late Pan-African global event.