Petrogenesis Of The Late Paleozoic To Early Mesozoic Granitic From The Chifeng Region And Their Tectonic Implication

This thesis conducts a systematic study on the petrology,zircon U-Pb geochronology and whole-rock geochemistry of the late Paleozoic to early Mesozoic granitic rocks in the Chifeng area.The age of granitic rocks from Paleozoic to early Mesozoic in this area was determined by using zircon LA-ICPMS U-Pb isotope analyses.Based on the major elements and trace element geochemical characteristics of granites,the magmatic origin and tectonic setting of granitic rocks are discussed.Based on these geochemical and geochronogical data as well as regional geological investigations,we finally discussed the late Paleozoic to early Mesozoic tectonic evolution history of the study area.1.Magmatism periods from the Paleozoic to early Mesozoic in Chifeng area.In this study,we report new zircon LA-ICPMS U-Pb data of Late Paleozoic to early Mesozoic granitic rocks in the Chifeng region.Zircons within the granite samples are generally euhedral to subhedral in shape.In CL images,they display striped absorption and fine-scale oscillatory zoning.Combined with the high Th/U ratio,it is suggested that they are of magmatic origin.The results of dating indicate that the late Paleozoic to early Mesozoic magmatism in the study area can be further divided into seven periods: Late Silurian?419 Ma?,Early Carboniferous?358³51 Ma?,Early Permian?294²84 Ma?,Middle Permian?269²60 Ma?,Late Permian-Early Triassic?256²47 Ma?,Middle Triassic?244²43 Ma?,Late Triassic?232²30 Ma?.2.Arc--continent collision environment in the Late SilurianThe Late Silurian monzogranite samples in the Chifeng area are rich in silicon,slightly rich in aluminum and high in K2_O.All of the samples are enriched in light rare earth elements?LREEs?,depleted in heavy rare earth elements?HREEs?and have medium negative Eu anomalies.These data indicate that the samples are high-K calc-alkaline I-type granitic rocks.Besides,these rocks are enriched in large ion lithophile elements?LILEs?and depleted in high field strength elements?HFSEs?,with low Sr contents.These characteristics imply the monzogranites formed in a relatively shallow crustal depth at low pressure conditions.Therefore,the Late Silurian granites were formed in an island arc environment setting.At the Late Silurian,the northern margin of the North China Craton was developed in the process of the disappearance of the Bainaimiao Island arc.3.Active continental margin environment in the Early CarboniferousThe Early Carboniferous granitic rocks in this study region were only exposed in the Yuanbaoshan and Haladaokou towns of the eastern Chifeng.They are dominated by granodiorites with low silicon and aluminum,high total alkaline,and slightly negative Eu anomalies.The granodiorites of this period belong to aluminum-unsaturated,high-K calc-alkaline series rocks,and were emplaced in an active continental margin setting.4.Active continental margin environment in the Early PermianThe Early Permian magmatic activities in the Chifeng area are represented by the Jianshanzi pluton and Mingshan pluton,which are mainly composed of monzogranite and syenogranite,which are high in SiO₂ and K2_O,and belong to high-K calc-alkaline series intrusive rocks.Additionally,these granites are characterized by enrichments in LREEs and LILEs,and depletions in HFSEs.Based on these geochemical data,we propose that the formation of the Early Permian granites in an active continental margin setting closely related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Craton.5.Continent-Continent Collision in the Middle PermianThe Middle Permian granitic rocks in the Chifeng region are represented by the alkali-rich and high-K calc-alkaline series monzogranites.The geochemical characteristics show that the monzogranites are high in silicon,aluminum,alkali,and K.In addition,the chondrite-normalized REE patterns invariably show a relative enrichment in LREEs with middle negative Eu anomalies.They are classified as peraluminous granites owing to their A/CNK values of 1.08-1.23.Therefore,the monzogranites are aluminum-saturated,high-K calc-alkaline I-type granites.According to the Y-Nb diagram,Yb+Ta-Rb diagram and R₁-R₂ diagram,the monzogranites belong to the syn-collision granites,implying that the formation of the Middle Permian monzogranites is related to the collision between the Siberian Craton and the northern margin of the North China Craton.6.Closure of the Paleo-Asian Ocean and Post-collision extension in the Late Permian to Early TriassicThe Late Permian to Early Triassic granites are widespread in the Chifeng area,which range from monzogranite,granodiorite to syenogranite.The Late Permian granites are high in silica and alkalis?8.40-8.86 wt.%?,with Na2_O/K2_O ratios ranging from 0.94 to 1.19.These granites are aluminum-unsaturated,alkali-rich,high-K calc-alkaline rocks.Their Chondrite-normalized REE patterns are typical right-leaning "v" patterns,with significant fractionation of LREEs and HREEs and negative Eu anomalies.In the primary-mantle-normalized spidergrams,all the samples show relative enrichments in Zr,Hf,and LREEs?e.g.,Sr,Ba?.The A/CNK and NK/A values range from 0.95 to 1.05 and 0.89 to 94?all of which are more than 0.85?,respectively,which indicates that the granites are medium alumina,and have the characteristics of A-type granites.Besides,corundum molecules appeared in CIPW standard minerals.The Early Triassic syenogranites belong to high-K calc-alkaline series,with high silica,alkalis,and K2_O as well as middle negative Eu anomalies,showing metaluminous to weakly peraluminous characteristics.In summary,the Late Permian-Early Triassic granites in the Chifeng area are a set of high-K calc-alkaline series of A-type granites and I-type granites.The typical bimodal rock assemblage is formed by combining the basic rocks co-existing in the same period.Accordingly,it is suggested that the Chifeng area was developed in an extensional environment during the Late Permian-Early Triassic,which is related to the slab break-off after the collision between the Siberian and North China cratons.In combination with the continental sedimentary environment of the upper and middle levels of the Upper Permian Linxi Formation,these data indicate that the Paleo-Asian Ocean has been completely closed at this time.7.Continuous collision orogeny in the Middle TriassicThe Middle Triassic magmatic activity of the Chifeng area is represented by the porphyry granodiorites.The petrology and geochemistry characteristics indicate that the granodiorites are generally medium Si-bearing,middle in aluminum,and low in total alkali contents.The values of Na2_O/K2_O and A/CNK range from 1.30 to 1.38 and 0.92 to 0.97,respectively.Geochemically,they belong to quasi-aluminite,medium alkali,and high-K calc-alkaline I-type granites.The diagram of R₁-R₂ shows that they are late orogenic granites.Based on the granite characteristics and regional data,we proposed that the Chifeng area is still in a compressional orogenic environment during the Middle Triassic after the collision between the Siberian and North China cratons.Meanwhile,the near east-west ductile shear zone appeared in the granodiorite pluton also indicates the existence of compressive tectonic stress in the north and south direction of the rock mass.8.Extensional transition from late orogenic period to post-orogenic period in the Late TriassicThe Late Triassic granitic rocks are composed of granodiorite and monzogranite.The granodiorites are middle in Si,Al and low in total alkali contents.The A/CNK values range from 0.95 to 1.01,showing that they belong to weakly peraluminous high-K calc-alkaline series.Besides,the granodiorites are enriched in LILEs and HFSEs.The diagram of R₁-R₂ shows that they are late orogenic granites.The monzogranites are characterized by high SiO₂,alkali,and K2_O,A/CNK values range from 1.10 to 1.18,suggesting that these granites belong to strong peraluminous,alkali-rich and high-K calc-alkaline series rocks.The existence of the Late Triassic bimodal rock assemblage as well as the alkaline complex indicates that they formed in the lithospheric extensional environment after the collision orogeny of the North China plate and the Xingmeng orogenic belt.