Neoproterozoic Tectonic Evolution of the Wushi Area,NW Tarim Craton

Author:Liu Yuanzhi

Supervisor:Bin of characters on a seal carved in relief

Database:Doctor

Degree Year:2018

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Pages:172

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The Wushi-Aksu area is an important Precambrian uplift in the NW Tarim Craton.Well-preserved Neoproterozoic metamorphic and magmatic rocks and sedimentary sequences have been documented in this area,including the well-known Aksu blueschist terrane which is one of the well-preserved and well-substantiated Precambrian blueschist terranes in the world.These Neoproterozoic records provide crucial information in investigating the tectonic evolution of the Tarim Craton in the Rodinia-Gondwana supercontinent cycle.Predecessors have done a lot of researches in this area,but great controversies still exist.In addition,all researches on the blueschist terrane were performed on the outcrop of the Aksu area,and little attention has been paid to the blueschist terrane in the Wushi area.Detailed field study,systematic petrological observation,geochemistry research,zircon U-Pb dating and 40Ar/39Ar chronology analyses are carried out in this study for the Aksu group from the Wushi area and other related Precambrian units.In this thesis,we fill the gap in the study of the blueschist terrane in the Wushi area,and the following findings are obtained.(1).Deformations of the blueschist terrane in the Wushi area are subdivided,and comparison of deformations between blueschist terranes from the Aksu and Wushi area have been madeLike the Aksu area,four phases of fold deformation can be identified in the blueschist terrane of the Wushi area.The first phase was contemporary with the blueschist-facies metamorphism(754 Ma)and produced numerous foliations(S1)and lineations(L1)throughout the blueschist terrane,and lineations(L1)were less developed in comparison with the blueschist terrane of the Aksu area.The second phase was a little later than the first,and the axial planes of the second phase folds intersect those of the first phase at large angles.The third phase(ca.750-745 Ma)resulted in a series of NE-SW trending broad folds which were missing in the Wushi area,and NW-SE trending parallel faults which were well developed both in the Aksu and Wushi area.A regional anticline was formed in the fourth phase during Mesozoic-Cenozoic.(2).Protolith identification of mafic schists from the Wushi area are performed via geochemical analyses,and the age of protolith of mafic schists is constrained through U-Pb datingProtolith of mafic schists from the Aksu and Wushi area are both Oceanic crust basalts,and bulk-rock geochemical analyses reveal that some of the protolith have characteristics similar to modern EMORB and others have characteristics similar to modern N-MORB.U-Pb data of the captured zircons in mafic schists from the Wushi area,of which protolith have characteristics similar to modern NMORB,yield a youngest peak age of 763 Ma,which means the age of these protolith is younger than ca.763 Ma.In addition,mafic schists from the Aksu area,of which protolith have characteristics similar to modern EMORB,yielded a protolith age of 890 ± 23 Ma,suggesting protoliths of mafic schists in the Wushi-Aksu area are complex of oceanic crust basalts with different compositions and ages.(3).Age of protolith of metasedimentary rocks from the Wushi area is constrained and the Spatiotemporal relationship between the Aksu Group and the Qiaoenbrak Formation is discussedThere are probably no sedimentary contacts between the Aksu Group and the Qiaoenbrak Formatioa Protolith of metasedimentary rocks from the Wushi area was probably formed between ca.790-761 Ma,and the Qiaoenbrak Formation was deposited between ca.780-744 Ma.Detrital zircon spectra show that there are slight differences in Neoproterozoic zircon compositions between metasedimentary rocks from the Aksu and Wushi area.Geochemistry and detrital zircon analysis show that the provenance of the Aksu Group metasedimentary rocks is very similar to that of the Qiaoenbrak Formation,and protolith of the Aksu Group metasedimentary rocks and the Qiaoenbrak Formation were probably both deposited in an arc-related tectonic setting(fore-arc setting)in the northwestern margin of the Tarim Craton.4.We constrain ages of the mafic dykes intruding the blueschist-bearing Aksu Group and mafic rocks of the Sugetbrak FormationAmphiboles from a mafic dyke crosscutting the blueschist-bearing Aksu Group yield a 40Ar/39Ar plateau age of 744.5 ± 2.8 Ma,which can represent the intrusion age of the Aksu mafic dyke.The pyroxene aliquot from mafic rocks of the Sugetbrak Formation yield a mini-plateau age of 521.0 ± 2.1 Ma and a mini-inverse isochron age of 520.6 ± 2.1 Ma,which also constrain the minimum age of mafic rocks.5.Combining with previo us researches,tectonic implications of Neoproterozoic tectonic-thermal events in the Wushi-Aksu area are discussedAs an important component of both Rodinia and Gondwana supercontinent,the Tarim Craton was amalgmated to the Rodinia supercontinent on its southern edge during ca.1000-900 Ma,and the northern margin experienced circum-Rodinia subduction until at least ca.750 Ma,which generated the Aksu Blueschist Terrane.The 744.5 ± 2.8 Ma mafic dykes probably indicate a post-subduction extension environment as results of retreat of the c ircum-Rodinia subduction system Mafic rocks of the Sugetbrak Formation probably record a rifting events that separated the Tarim Craton and NW Australia within the Gondwana supercontinent。