Precambrian Crustal Evolution Of The Chinese Tianshan Belt

The Central Asian Orogenic Belt(CAOB)stretches from Urals via Kazakhstan-Kyrghyztan Tianshan to the Chinese Tianshan and to the Pacific.It is separated by the Siberian Craton and European Craton to the north and the Tarim Craton,South China Craton as well as the North China Craton to the south.As one of the largest Phanerozoic accretionary orogenic belts on earth,the CAOB was formed by continuous accretions of different Precambrian continental fragments(microcontinents),island arcs,seamounts and oceanic plateaus.The origin and Precambrian crustal evolution of the Precambrian microcontinents involved in the CAOB are two vital unresolved problems,which provide insights into early history of the CAOB and in turn impose constraints on reconstructing its tectonic evolution.In the southern part of the CAOB,the Chinese Central Tianshan Block(CTB)and the Yili Block(YB)are two key elements to solve these problems.In this dissertation,we present new petrology,whole-rock geochemistry,LA-ICPMS and SHRIMP zircon U-Pb ages and Hf-in-zircon isotopic data for the Meso-to Neoproterozoic granitic gneisses and sedimentary rocks in both the CTB and the YB.The results provide new constraints on not only the emplacement ages of the studied rocks and their petrognesis but also the early crustal evolution of the CTB and the YB and their links with the supercontinental cycles.In combination with magmatism,sediments,metamorphism and structual geology studies of the CTB,YB and other blocks,we suggest a close tectonic affinity between the CTB and the YB.The CTB is composed of augen-gneisses,banded gneisses and mylonitic gneisses.The augen-gneiss was emplaced at 902-930 Ma,whereas the banded and mylonitic gneiesses were cystaliazed at 806 Ma and 896 Ma,respectively.In the western part of the CTB,the augen-gneisses and banded gneisses have high Al2O3 and K2 O,low CaO and Na2 O contents,and thus exhibit high aluminium saturation index(ASI)(1.09-2.05).They are enriched in light rare earth elements(LREE),display relatively flat heavy rare earth element(HREE)patterns with negative Eu anomalies,and are characterized by pronounced depletions in Nb,Ta,Ti and Sr and strong positive Pb anomalies.Zircons from these gneisses yielded negative ?Hf(t)values(-10.87 to 0).Together with Mesoproterozoic to Neoproterozoic inherited zircons(0.88-1.41 Ga)and relatively old crustal model ages(1.52-2.47 Ga),the geochemical characteristics indicate that these gneisses were derived from partial melting of the metapelite and graywacke.In the eastern part of the CTB,the augen-and the mylotnic gneisses also have high Al2O3 and K2 O.The gneisses have Cr and Ni contents similar to those of the typical S-type granite in the Lachlan belt.They also have REE patterns comparable to the upper continental crust.These rocks exhibit evolved zircon ?Hf(t)values(-9.0 to +1.6).The existences of muscovite and Paleo-Mesoproterozoic inherited zircon grains(1.25-2.21 Ga)and the geochemical characteristics indicate that protoliths of these gneisses are S-type granites produced by partial melting of metasedimentary rocks of the Paleo-Mesoproterozoic basement in the CTB.These results,compiled with published geochronological and hafuium data,suggest that the CTB experienced three episodes of magmatism,namely at 1.40-1.46 Ga,0.81-1.01 Ga and 0.73-0.79 Ga.The CTB was characterized by the crustal growth in the Mesoproterozoic(ca.1.40 Ga),whereas was featured by the crustal reworking in the Neoproterozoic(0.73-1.01 Ga).The oldest rocks in the YB consist of fine-grained graintic gneisses,augen-gneisses and mylonitic gneisses(888-1329 Ma).Compiled with published geochronological and hafuium data,it is suggested that these gneisses in the YB have similar geochemical chareacteristics to those of the CTB.This suggests that these gneisses were derived from partial melting of metasedimentary rocks of the Paleo-Mesoproterozoic basement in the YB.Three epsiodes of the magmatic events have recorded in the YB at 1.33 Ga,0.85-0.99 Ga and 0.76 Ga.The YB is featured by crustal reworking in the Neoproterozoic(0.76-0.99 Ga).Our study suggests that the oldest Xingxingxia and Kawabulake formations in the CTB were deposited at 940-1095 Ma and 932-960 Ma,respectively,rather than Paleoproterozoic.Detrital zircon grains from these two formations have similar age distributions,with two dominant peaks at 0.95-1.0 Ga and 1.20-2.00 Ga(peak at 1.40-1.60 Ga)and a minor peak at 2.60 Ga.Their similar age peaks and comparable Hf isotopic features suggest that meta-sediments of the Xingxingxia and Kawabulake formations share a similar sedimentary provenance.According to the zircon morphologies,age peaks and Hf isotopic data,the early Neoproterozoic detrital zircon grains were mainly local-derived,whereas the Paleo-Mesoproterozoic grains were originated from both the CTB and the YB.In contrast,the 2.60 Ga detrital zircon grains were allochthonous.These results suggest that the CTB has recorded two synchronous crustal growth and reworking events at ca.1.70-2.00 Ga and 1.30-1.60 Ga,respectively,and it was then dominantly characterized by crustal reworking event at the early Neoproterozoic(0.95-1.00 Ga).The Wenquan Group is the oldest formation in the YB.Detrital zircon studies suggest that the lower and the upper Wenquan groups were deposited at 845-900 Ma and 857-880 Ma,respectively,rather than Paleoproterozoic.The lower and the upper Wenquan groups have similar age peaks at 0.88-0.95 Ga,1.10-2.00 Ga(peak at 1.40-1.60 Ga)and a weak peak at 2.60 Ga.Among them,the early Neoproterozoic detrital zircon grains were derived from the YB itself.The Paleo-Mesoproterozoic grains have a provenance from both the YB and the CTB,while the 2.60 Ga detrital zircon grains were originated from extotic blocks.Smilarly,two synchronous crustal growth and reworking events were identified in the YB(1.70-1.80 Ga and 1.30-1.60 Ga).After the last growth and reworking event,continuous crustal reworking took place in the YB untill the early Neoproterozoic.Based on above results,we conclude that the Chinese Central Tianshan Block and the Yili Block have no Archean basement,but Paleoproterozoic crustal materials probably exist in these two blocks(1.80-2.00 Ga).Their ancient nucleus were probably formed in the Mesoproterozoic(ca.1.3-1.40 Ga)and were intensively reworked and finally formed a united crystalline basement in the early Neoproterozoic(0.81-1.01 Ga).Our studies indicate that two phases of synchronous crustal growth and reworking happened in the Chinese Central Tianshan Block at late Paleoproterozoic(1.70-2.00 Ga)and early Mesoproterozoic(1.30-1.60 Ga),and then changed into continuous crustal reworking until the Mid-Neoproterozoic(0.81-1.01 Ga).As to the Yili Block,it has a similar crustal evolution to the CTB.After that,these two blocks were in a continental rifting or extensional setting in the Mid-Neoproterozoic(0.73-0.79 Ga).We suggest that the Mesoproterozoic crustal growth and the early Neoproterozoic crustal reworking were likely related to the breakup of the Columbia(ca.1.33-1.46 Ga)and the assembly of the Rodinia(0.81-1.01 Ga),respectively.According to the systematic comparisons on the magmatism,deformation,metamorphism and detrital zircon ages between the Chinese Central Tianshan Block and the Yili Block and as well as others surroungding blocks,we conclude that the CTB and the YB have a close tectonic affinity in the Precambrian.However,these two blocks were not once a part of the the Tarim Craton,Siberian Craton,North China Craton,South China Craton,Australia Craton and Amazonian Craon.Instead,they were most likely located near the Laurentia and European Craton in the Neoproterozoic(0.90-1.00 Ga)and probably originated from the European Craton.