A Geochemical Study Of Neoproterozoic Rocks In The North Qinling Terrane And The Southern Margin Of The North China Craton

The Qinling orogen constitutes the geologic heart of China continent and links the North China Craton to the South China Block.Its basement is dominated by the North Qinling Terrane with Mesoproterozoic-Neoproterozoic basement rocks.However,there has been a dispute over its basement propertis and tectonic evolution because of the widespread influence of the Paleozoic orogeny and metamorphism.The North China Craton is one of the oldest cratons in the world.Due to the relative lack of late Mesoproterozoic and Neoproterozoic rocks,the Neoproterozoic tectonic evolution of the North China Craton is not constrained well.Therefore,the relationship between the North Qinling Terrane and the North China Craton has been controversial for a long time.This PhD dissertation presents a geochemical study of metasedimentary rocks from the Qinling group,Neoproterozoic metagranites from the North Qinling Terrane and Neoproterozoic magmatic rocks from the southern margin of North China Craton.The results are used to reveal the sedimentary sources and the tectonic framework of the North Qinling Terrane,to constrain the origin and tectonic setting of different types of Neoproterozoic magmatic rocks in the North Qinling Terrane and the southern margin of North China Craton,and to decipher the tectonic evolution of the North Qinling Terrane and the southern margin of North China Craton in the Neoproterozoic.In supercontinent reconstruction,for the position of microcontinents such as the North Qinling Terrane is commonly ignored.However,it is of great significance to constrain the position of these small Precambrian microcontinents.Because they may be an independent microcontinents connecting two continents or fragments teared off from the margin of continents.The North Qinling Terrane is a microcontinent between the North China Craton and South China Block.It was accreted to the southern margin of the North China Craton by a series of processes including oceanic subduction in the Neoproterozoic and continental collision in the Cambrian.But its Precambrian evolution history is not very clear.There are a large number of Mesoproterozoic to Neoproterozoic sedimentary and magmatic rocks exposed in the North Qinling Terrane,which provides the window to limit its tectonic evolution during the convergence and breakup of Rodinia supercontinent.We carried out whole-rock major and trace elements as well as zircon U-Pb ages and Lu-Hf isotopes for the metasedimentary rocks in the Qinling Group.The metasedimentary rocks show greywacke compositions with A/CNK ratios of 0.99～1.28.The trace elements show LILE and LREE enrichment,HFSE and HREE depletion with negative Eu anomalies,similar to the upper continental crust.Their metamorphic ages are ca.510～490 Ma,recording Paleozoic continental collision.The youngest detrital zircon U-Pb ages from three metasedimentary rocks are 1161 ± 54 Ma,954 ± 15 Ma,861 ±10 Ma,respectively.In combination with ages of granitic intrusions at 960 to 890 Ma in the target region,their deposition ages are constrained to be 1110-960 Ma,ca.960 Ma and ca.860 Ma,respectively.The cumulative proportions of the difference between the detrital zircon crystallization ages and the deposition ages for the sediments of ca.960 Ma and ca.860 Ma suggest a setting of plate convergence.Detrital zircon age spectrum yields two major populations at 1000-900 Ma and 1700-1400 Ma and three subordinate populations at 900-800 Ma,1300-1200 Ma and 1800-1700 Ma with rare Neoarchean zircon grains.Neoproterozoic detrital zircons mainly have negative εHf(t)values,consistent with Neoproterozoic granitoids in the North Qinling Terrane.In contrast,Mesoproterozoic detrital zircons mainly have positive εHf(t)values,similar to those in East Laurentia and East Greenland.This means that the major provenance of sediments in the Qinling Group was the North Qinling Terrane itself and East Laurentia,supporting the connection between the North Qinling Terrane and East Laurentia before the Rodinia assembly.In combination with the coeval strata in the southern margin of the North China Craton,we conclude that the North Qinling Terrane is a missing piece connecting the southern margin of the North China Craton with the northern part of East Laurentia in Rodinia in the early Neoproterozoic.The Neoproterozoic S-type granites in the North Qinling Terrane represent the anatectic product of the continental crust in the North Qinling Terrane.Zircon U-Pb dating shows that the magmatism of S-type granites lasts from 946 Ma to 893 Ma.Combined with previous data,we obtain that S-type granites in the North Qinling Terrane have their crystallization ages from 960 Ma to 890 Ma,which overlap the age of ca.890 Ma for I-type granites in the North Qinling Terrane.The S-type granites are calc-alkaline,peraluminous in lithochemistry.The trace elements show enrichment in LILE and LREE,depletion in HFSE and HREE,positive Pb anomalies and negative Eu anomalies,similar to the upper continent crust.Their whole-rock εNd(t)values are-4.98 to-2.99,whole-rock εHf(t)values are-3.69 to 4.00 and zircon average εHf(t)values are-2.79 to-0.10,exhibiting different degrees of whole-rock Hf-Nd isotope decoupling and discrepancy between whole-rock Hf and zircon Hf isotope compositions.And the zircon Hf isotopes in the granites with higher degrees of decoupling show a greater variation.The zircon Ti thermometry and zircon saturation thermometry yield crystallization temperatures of>720℃.Whole-rock Sr-Nd-Hf isotopes and trace elements compositions indicate that the S-type granites were produced by partial melting of the metasedimentary rocks in the Qinling Group.The granites are characterized by low Ba and high Rb/Sr ratios on the one hand and low Sr and high Rb/Sr ratios on the other hand,indicating that the crustal anatexis is caused by the dehydration breakdown of muscovite and biotite,respectively.The whole-rock Hf-Nd isotope decoupling cannot be explained by the incomplete dissolution of zircon and insufficient interference correction during the MC-ICPMS analysis in the laboratory and incongruent melting of apatite,monazite and garnet.Since the metasedimentary rocks of the Qingling Group do not show obvious whole-rock Hf-Nd isotope decoupling,the zircon sorting effect in the source is also unlikely.The incongruent melting of zircons and the effect of dissolved inherited zircons on local melts may be the cause for the whole-rock Hf-Nd isotope decoupling,the discrepancy between whole-rock Hf and zircon Hf isotopes,and the heterogeneity of zircon Hf isotopes.The ca.890 Ma S-type granites have higher magmatic temperatures than the 960-900 Ma S-type granites,and ca.890 Ma I-type granites are reported in the North Qinling Terrane.These observations indicate elevation of crustal heat flow for crustal anataxis,corresponding to the transition of the tectonic setting in post-collisional extension.In order to constrain the tectonic setting of the southern margin of the North China Craton in the Neoproterozoic,we studied the alkaline rocks from the Fangcheng area in the southern margin of North China Craton.The Fangcheng alkaline rocks are composed of silica-unsaturated nepheline syenite,silica-saturated quartz syenite and alkali feldspar syenite.The alkali-feldspar syenites have abnormally high K2O content with ultrapotassic-peraluminous character.Nepheline syenites and quartz syenites are alkaline-shoshonitic-metaluminous rocks.The abnormal K2O contents of alkali-feldspar syenite are the result of strong hydrothermal alteration in the later stage.The Fangcheng alkaline rocks show enrichment in LILE,HFSE and REE.The zircon U-Pb age of quartz syenite is 848.7± 5.9 Ma.The zircon REE pattern shows depletion in LREE and enrichment in HREE,positive Ce anomalies and negative Eu anomalies.The titanite U-Pb age of nepheline syenite is 837± 15 Ma with titanite enriched in LILE and LREE but depleted in HFSE and HREE.Combined with the previous age data,it is deduced that the crystallization ages of the Fangcheng alkaline rocks range from 870 Ma to 830 Ma.The zircon Ti thermometry,the zircon saturation thermometry and the feldspar ternary diagram yield magmatic temperatures of 900-1000℃ for the nepheline syenites and 800-900℃ for the quartz syenites.The Al2O3-in-Titanite barometry suggests that the emplacement pressures of nepheline syenite are about 246-285 MPa.The high oxygen fugacity(ΔFMQ+0.1 to ΔFMQ+2.2)indicates that the source of nepheline syenite and quartz syenite is of metasomatic origin.The nepheline syenites have εNd(t)values of-1.5 to-1.4 and εHf(t)values of-3.79 to-3.39,and the quartz syenites haveεNd(t)values of-4.8 to-4.4 and εHf(t)values of-6.28 to-5.97.Such enriched whole-rock Hf-Nd isotope compositions indicate their origination from the enriched sources.The nepheline syenites were produced by low-degree partial melting of the enriched source with fractional crystallization of the primary melt,while the quartz syenites were produced partial melting of the lower continental crust.Zircon SIMS oxygen isotopes and other mineral oxygen isotopes show that quartz syenite have lowδ18O values of 3.37 ro 4.57‰,which are probably inherited from low δ18O source.These alkaline rocks are the low-degree melting products of the lithosphere in the southern margin of the North China Craton,corresponding to the breakup of Rodinia,indicating the continental extensional setting.The ca.960 Ma and ca.860 Ma metasedimentary rocks in the Qinling Group suggest a setting of plate convergence.The occurrence of 960-890 Ma S-type granites indicates the continental thickening.The 890 Ma I-type granites and the relatively higher magmatic temperatures of S-type granites at ca.890 Ma suggest that the North Qinling Terrane was readily in the setting of post-collisional extension.During 870-830 Ma,the southern margin of North China Craton was in the setting of tectonic extension after the continental collision.