Chemical Ages For The Multi-stage Metamorphism Of The Qinling Group At Taibai, Shaanxi Province

The Qinling Orogen, an orogenic belt between the North China and Yangtze Blocks andexperienced multi-stage complex evolution progress, is a key area for understanding geologicalevolution of the Chinese continent. In this study, monazite inclusions enclosed inpolymetamorphic mineral assemblages of the Qinling Group were dated in situ using electronmicroprobe chemical dating method, by which the chronology framework of the earlymetamorphic history of the group was reconstructed, and gave new chronology constraints on thereconstruction of the tectonic framework and evolution of the Qinling Orogen Belt.The Qinling Group, a metamorphic core complex of the North Qinling, is a medium tohigh-grade metamorphic complex, and its main rock types contain pelitic metamorphic rocks,quartzofeldspathic metamorphic rocks, calcareous metamorphic rocks and basic metamorphicrocks. Pelitic metamorphic rocks are distributed at the middle section and the horizon near theupper marble of the group, and the main rock types include various mica-schistes andmica-gneisses, especially biotite schist. Quartzofeldspathie metamorphic rocks are distributed atthe lower section and the lower horizon of the middle section of the group, and include biotite-plagioclase gneiss, garnetiferous biotite-plagioclase gneiss, sillimanite-biotite-quartz schist,two-mica quartz schist, and so on. Basic metamorphic rocks are mainly amphibolite, biotiteamphibolite and epidote amphibolite, and distributed both in three sections of the Qinling Group.Protolith restoration of the Qinling Group at Taibai county shows that: the protoliths ofpelitic-felsic metamorphic rocks are largely Al-rich clay rocks, lithic sandstones and wackeswith little feldspar sandstones and quartz sandstones, which reveal the character of the proximalsedimentary deposit, and the depositional environment is active continental margin; the protolithsof basic metamorphic rocks are mainly basic volcanic rocks with minor dolomitic wackes, thebasic volcanic rocks belong to tholeiite series, and its tectonic setting discrimination indicatesthat they are mainly within-plate basalts and show the features of oceanic island basalts; theprotoliths of calcareous metamorphic rocks are parametamorphic rocks.The texture, structure and mineral generation of the pelitic metamorphic rock from theQinling Group are quite complicated and display a complex metamorphic course. Theparagenetic association of minerals can divided to three generation: the first generation is St+Ky +Gt+Bi+Ms+Q, the second generation is columnar Sil+Gt+St(diablast)+Bi+Pl+Q, andthe third generation is hairlike Sil+And+Ms.Monazites enclosed in multi-stage metamorphic mineral assemblages of pelitic metamorphicrocks of the Qinling Group were dated using electron microprobe chemical dating. The isochronage of monazite inclusions in the first generation staurolite is 520±23 Ma (2σ) with MSWD=0.59, and the isochron age of monazites enclosed in the second generation of minerals andcoexisting with the third generation of minerals is 435±9 Ma (2σ) with MSWD=0.87.Two metamorphic ages were obtained from a same sample of the Qinling Group usingmonazite chemical dating. The ages of monazites enclosed in the second generation of mineralsare indistinguishable from these coexisting with the third generation of minerals, this suggeststhat: (1) the monazites coexisting with these two generations of minerals were formed during themineral growth of the second generation; or (2) the time interval between the growths of thesetwo generations of minerals was too short to be detected by the dating method. The age of 520±23 Ma is obtained from monazites enclosed in the first generation staurolite which themselves areenclosed in the second generation garnets. The U-Th-Pb system in these monazite grains has veryhigh chance to survive the subsequent thermal events. We therefore interpret the age of 520±23Ma represents a significant thermal event.The first generation mineral association of the Qinling Group was developed at～520Ma.Noticeably, the ages of～500Ma tectonothermal activity is also recorded in UHP eclogite and HPgranulite facies metamorphic and magmatic rocks on the north and south sides of the QinlingGroup and Qaidam-Altun. The existence of UHP paragneisses indicates that continents hadsubducted and collided at that time. In this study, the～500 Ma metamorphic event occurred in theQinling Group and the Central Orogenic Belt was attributed to the Late Pan-Africanmetamorphism, to emphasize the consistency of this event with the subduction-accretion event atthe margin of the Gondwanaland.Previous researches suggest that: the North Qinling is a microcontinent formed on the oldoceanic crust of the northern side of the Yangtze Block and was a part of the Yangtze Blockbefore the Neoproterozoic, and belonged to Gondwana structural domain with the Yangtze Blockand its surrounding oceanic crust. Paleobiologic and paleogeographic evidences also indicate thatthe Tarim Block and the Yangtze Block were together and belonged to Eastern Gondwanalandwith some other microcontinents distributed at eastern side of Tarim such as Qaidam duringSinian to Early Paleozoic. Therefore, it can be presumed that: the Noah Qinling-Qaidammicrocontinents were subducted and collided with Gondwanaland during the late Pan-African,the other Chinese continents which have no late Pan-African event were possibly far away fromthis location at the same time, following this event, the North Qinling-Qaidam microcontinentsbroke up and moved into the Proto-Tethys Ocean, and finally assembled with other Chinesecontinents followed the closing of the Proto-Tethys Ocean during Middle Ordovician to Silurian.