Zircon Geochronology Constraints On The Age And Nature Of "precambrian Metamorphic Rocks" And Lower Paleozoic In The Xing’an Block Of Northeast China

The Central Asian Orogenic Belt (CAOB) is one of the largest orogenic belts in theworld and its formation is closely related to the evolution of the Paleo-Asian Ocean. Alarge number of tectonic units, including subduction accretionary rocks, magmatic arcs,micro–continents, seafloor plateaux, and oceanic islands now form the complex tectonicframework of the CAOB. The key to understanding the orogenic formation of the CAOBis to constrain the age and nature of the micro–continents (or continental blocks) in theCAOB, the timing and mechanisms of their convergence, and hence the characteristics ofPhanerozoic crustal growth. The Great Xing’an Range in northeast China is located in theeastern part of the CAOB and is an important area to study the formation of the CAOB.From north to south, the Great Xing’an Range is divided into the Erguna, Xing’an, andSongliao blocks. Controversy exists regarding the timing and formation of themicro-continents and the Early Palaeozoic tectonic evolution in the Xing’an block. Due tothe lack of systematic chronology, the formation time of the most Precambrianmetamorphic rocks and the Lower Paleozoic remain blank in the Xing’an block. As such,we have conducted a laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS) U–Pb dating study of zircons from the Precambrian metamorphic rocksand the Lower Paleozoic in the Xing’an Block.Detrital zircons from phyllites of the Xinkailing Group in Duobaoshan yieldpopulations of~1505,~810, and~485Ma, with the youngest peak constraining itsdepositional age to be <485Ma. Zircons from amphibolitic gneisses of the XinkailingGroup in Nenjiang have magmatic ages of309Ma. Mylonitic granites of the XinkailingGroup in Nenjiang have zircon magmatic ages of164Ma. Detrital zircons from two-micaquartz schists of the Luomahu Group in the Galashan Forest yield~2419,~1789,~801,~536,~480, and~420Ma, with the youngest peak indicating its depositional age is <420Ma. Detrital zircons from mylonitized sericite–chlorite schist of the Ergunhe Formation inTaerqi yield populations of982–948,~519, and~410Ma, with the youngest peakdemonstrating that its depositional age is <410Ma. The meta-sedimentary rock of theWolegen Group was formed not earlier than the Early Ordovician as attested by the peak age of480Ma of the youngest population of zircons. The meta-volcanic rock of theWolegen Group was formed at431Ma, belonging to the Early Silurian.Based on these results and published data, these zircon ages for a range oflithologies show that the metamorphic rocks formed during the Phanerozoic. We concludethat there is no evidence of Precambrian metamorphic basement in the Xing’an block. Insummary, the age data indicate that: a unified Precambrian metamorphic basement maynot exist in the Xing’an region, but probably will be that there are some micro-continentalblocks exist in local areas.Dacite of the Duobaoshan Formation in Duobaoshan have zircon magmatic ages of485Ma, belonging to the Early Ordovician. Lithic tuffs of the Handaqi Formation inHandaqi have zircon magmatic ages of463Ma, belonging to the Middle Ordovician. Thesericite-quartz schist of the Duobaoshan Formation in Yiminhe was formed not earlierthan the Middle Ordovician as attested by the peak age of463Ma of the youngestpopulation of zircons. The two-mica chlorite quartz schists of the Upper Ordovician inHandaqi were formed not earlier than the Middle Ordovician as attested by the peak ageof470Ma of the youngest population of zircons. The tuffaceous siltstone of theBashilixiaohe Formation in Handaqi was formed not earlier than the Early Silurian asattested by the peak age of432Ma of the youngest population of zircons. Themylonitized tuffaceous fine sandstone of the Niqiuhe Formation in Xiaosuoerqi wasformed not earlier than the Late Ordovician as attested by the peak age of447Ma of theyoungest population of zircons.The detrital zircon age of the Ordovician-Middle Devonian in the Xing’an blockare dominated by Early Palaeozoic, followed by Neoproterozoic and Paleoproterozoic,with few Middle Proterozoic and Archean. Compared with the dating statistical results ofthe Precambrian-Early Palaeozoic and the detrital zircon age distribution curve of theNeoproterozoic in the Erguna and Songliao blocks, the age composition of LowerPaleozoic in the Xing’an Block is consistent with that in the Erguna Block. And they areall lack of the Middle Proterozoic age. The absence of the Neoproterozoic granite andPaleoproterozoic rocks in the Songliao block, and the high proportion of MiddleProterozoic age in the Neoproterozoic sedimentary in the Songliao block, showing thatthe Songliao block is different from the Erguna and Xing’an blocks. And the Ergunablock should be a main source area of the Lower Paleozoic sediments in the Xing’anblock. There is a lot of Silurian and Devonian age peaks in the detrital zircons from theSilurian and Devonian in the Xing’an block respectively, due to the absence of themagmatic event in the Erguna block, indicating the Early Paleozoic volcanic activity inthe Xing’an block was also an important source area. The above characteristics suggestedthat the Early Paleozoic marine basin in the Xing’an region is a continental margin basin,with the single direction terrigenous supply, also proved the Huanyu ophiolite was alreadyin place in the Early Ordovician. The Early-Middle Ordovician age peak in the detrital zircons from the Ordovician-Middle Devonian in the Xing’an block is consistent with the formation age of Early-Middle Ordovician magmatic arc and the Early Ordovician porphyry Cu deposit in theXing’an region, showing that Xing’an block is an active continental margin in EarlyPaleozoic. The Xing’an block may not have a uniform Precambrian metamorphicbasement, which formed in the plate subduction and collage based on the eastern edge ofthe Erguna block during the early Early Paleozoic.