The Formation And Evolution Of Early Precambrian Continental Crust At Convergent Plate Margin:Geochemical Constraints From The Taihua Complex In The Southern Margin Of The North China Craton

The Early Precambrian(Archean to Paleoproterozoic)was the most important period for the growth of continental crust.The North China Craton(NCC)is one of the oldest cratons in the world.The Taihua Complex is widely exposed in the south-central margin of the NCC and records almost all of the important geological events in the Early Precambrian.Therefore,the Taihua Complex is an ideal target to study the formation and evolution of continental crust at convergent palte margins duing the Early Precambrian.This dissertation presents a comprehensive geochronological and geochemical study of the Neoarchean to Paleoproterozoic magmatic rocks and metasedimentary rocks from the Taihua Complex exposed in the Xiaoqinling area.The results not only provide constraints on the temporal framework and the origin of different rocks,but also reveal the tectonic history and the changes in the tectonic regime of the convergent plate margin.Futhermore,this study provides new insights into the formation and evolution of continental crust at convergent palte margins duing the Early Precambrian.Zircon U-Pb dating reveals two episodes of granitoid magmatisms at ca.2.65 Ga and ca.2.55-2.50 Ga.Sodic TTGs at ca.2.65 Ga are characterized by low K2O/Na2O ratios of<0.6,high Sr/Y and(La/Yb)N ratios,positive Eu anomalies,positive zircon εHf(t)values of 0.9 to 5.2,and zircon δ18O values of 6.3-6.5‰,suggesting their derivation from partial melting of the thickened oceanic crust.In contrast,K-rich granitoids at ca.2.65 Ga have higher K2O/Na2O ratios,low Sr/Y and(La/Yb)N ratios,negative Eu anomalies,mostly positive zircon εHf(t)values of-0.4 to 8.0,and high zircon δ18O values of 7.0-8.4‰.Young K-rich granitoids at 2.55-2.50 Ga show similarly high Sr/Y and(La/Yb)N ratios to the sodic TTGs at ca.2.65 Ga.All of these geochemical features suggest that these Neoarchean K-rich granitoids were derived from partial melting of the low-T seawater-hydrothermally altered oceanic basalt at different depths.Therefore,the sodic TTG and K-rich granitoids are linked to each other through both similarity and difference in their source compositions rather than magmatic processes such as partial melting and crystal fractionation.On the other hand,high-K granites at ca.2.55-2.50 Ga are highly siliceous,exhibit very high K2O/Na2O ratios of 4.0-7.9,and contain many relict zircon with U-Pb ages of ca.3.49-2.68 Ga,indicating their formation by reworking of the pre-existing continental crust.Using a filtered global geochemical database,it is found that the Neoarchean is the most important period for the growth of both sodic TTGs and K-rich granitoids.The Sr/Y and(La/Yb)N ratios of these granitoids also reach the maximum during this period,suggesting global-scale thickening of the oceanic crust due to warm plate convergence.Once the orogenic lithospheric mantle was foundered for thinning,the thickened oceanic crust experienced partial melting at different depths.This gave rise to the different types of granitoid for growth of the continental crust along convergent plate boundaries.Therefore,partial melting of the thickened oceanic crust at different depths is a viable mechanism for the compositional transition of continental crust in the Neoarchean.The storage timescales and thermal states of magma during the Archean are important factors affecting the growth and evolution of continental crust.However,few reliable estimates exist for these parameters in ancient magmatic systems.The present study provides constraints on Archean magma storage timescales by an in-situ analysis of Zr isotopes in zircon of 3.4 to 2.5 Ga together with modeling of disequilibrium crystal growth.The results suggest that fractional crystallization of zircon at isotopic equilibrium leads to small δ94Zr variations(<0.4‰),indicating that the observed δ94Zr variations from-0.6 to+0.4‰ are likely caused by disequilibrium crystal growth.There is a negative correlations between δ94Zr values and Zr/Hf ratios,suggesting that the fractional crystallization of zircon also plays a role in Zr isotope fractionation.The extent of isotopic disequilibrium requires zircon growth rates of 0.00070.0415μm/yr,which in turn yield(minimum)magma storage timescales of 5-193 kyr.Combined with evidence for zircon crystallization at high temperatures of 730-910℃,these long timescales imply protracted periods of "warm storage" for Archean magmas,in stark contrast with predominantly shorter "cold storage" conditions observed in modern systems.These results serve as independent evidence for higher geothermal gradients throughout the Archean,and the unique magmatic systems have cascading implications for the growth/emergence of continental crust,volcanic activity,and evolution of the Archean atmosphere.A tectono-magmatic lull at 2.4-2.2 Ga in the Early Paleoproterozoic is indicated by the global geological record,including decreases of magmatism and orogeny.However,its mechanism is unknown until now.The magmatic rocks in the Taihua Complex are widespread during this period,and this study provideds a detailed geochemical study of amphibolites,dioritic dikes and granitoids from the Taihua Complex.The amphibolites with a protolith age of 2.34 Ga show arc-like trace element distribution patterns with enrichment in LILE and LREE but depletion in HFSE relative to HREE,and depleted whole-rock Nd-Hf and zircon Hf isotope compositions.The amphibolites have lower Th/U ratios relative to N-MORB and Archean komatiite.All these geochemical features suggest that the amphibolites were produced by partial melting of enriched mantle domains that were generated through metasomatic reaction with subducting oceanic crust-derived fluids.The 2.27 Ga dioritic dikes also show arc-like trace element distribution patterns with enrichment in LILE and LREE but depletion in HFSE relative to HREE,and enriched Hf isotope compositions.The dioritic dikes have higher Th/Yb ratios relative to N-MORB and Archean komatiite.All these geochemical features indicate that the dioritic dikes were derived from partial melting of enriched mantle domains generated by metasomatic reaction with subducting sediment-derived melts.Compared to Archean mafic magmatic rocks,the forming temperature and pressure of Paleoproterozoic mafic magams are lower,but the T/P ratios are higher,suggesting the thinning of lithosphere mantle.The 2.33 Ga granitoids show high whole-rock Zr saturation temperatures and zircon-in-Ti temperatures,falling into the category of high temperature granites.Therefore,the Taihua Complex records a event of continental rifting in the early Paleoproterozoic.Combined with the detrital zircon Hf isotope composition,the high T/P metamorphism and lower rates of plate motion in the tectonomagmatic lull,it is suggested the global tectonic setting was controlled by extensional rifting.The age distribution of global detrital and magmatic zircons indicates that the tectonomagmatic lull is caused by the stabilization prior to breakup of Archean supercratons,which is as same as the zircon age lull after the aggregation of Nuan/Columbia and Rodinia.During the period of tectonomagmatic lull,the Supervaalbara supercraton is stable,but breakup of the Sclavia and Nunavutia supercratons would produce tectonomagmatic activities in some places including the Taihua Complex.The Wilson cycle is the essence of plate tectonics.The earliest complete Wilson cycle can truly represent the onset of plate tectonics.This study provideds a detailed U-Pb dating,Lu-Hf isotope and trace element analyses of detrital zircons in the Paleoproterozoic metasedimentary rocks from the Taihua Complex.The main U-Pb age peaks are 2.6-2.5 Ga,2.3 Ga,2.2 Ga and 1.9 Ga.The 2.6-2.5 Ga detrital zircons show positive εHf(t)values,indicating the growth of juvenile crust during the initial cratonization of the NCC.The 2.3 Ga detrital zircons have negative εHf(t)values,suggesting the reworking of continental crust during continental rifting.The 2.2 Ga detrital zircons show positive εHf(t)values and high U/Yb ratio,indicating their derivation from continental arc andesites above the oceanic subduction zone.Meanwhile,the 2.2 Ga detrital zircons have lower LREE/MREE,REE+Y and higher P than the older zircon grains,and the metasedimentary rocks contain a large amount of graphite,suggesting the addition of terrigenous sediments.These sediments would be oroganic-rich and reduced because of the 2.3 Ga Lomagundi-Jatuli event,resulting in the decrease of oxygen fugacity.The 1.9 Ga zircons are absent of zoning in CL images and show low Th/U ratios of<0.1 and high(U/Ce)/Th ratios of>1,suggesting their metamorphic origin due to continental collision in the late Paleoproterozoic.In summary,the detrial zircons completely record a complete Wilson cycle composed of 2.3 Ga continental rifting,2.2 Ga oceanic subduction and 1.9 Ga continental collision events in the Paleoproterozoic.In the late Paleoproterozoic,the extensive syn-collisional and post-collisional magmatism was developed in the southern margin of the NCC.This study provides a comprehensive geochemical study of pegmatites and granites of this period.The SiO2、Na2O and CaO contents of 1.82 Ga pegmatites and 1.82-1.78 Ga granites are similar,ruling out the possibility that the pegmatites are produced by high-degree crystal fractionation of granites.The pegmatites and granites were emplaced later than the major metamorphic events,so they are unlikely to be the products of syn-collisional magmatism due to roll-back of the subducting slab.Both pegmatites and granites have high Sm/Yb and Gd/Yb ratios,indicating that they are the products of postcollisional magmatism.However,all of these rocks show negative εHf(t)values and high δ18O values,indicating their derivation from partial melting of the ancient continental crust rather than the juvenile oceanic crust in the slab failure(breakoff)model.Due to gravitational instability,the thickened lithosphere caused by the continental collision would be foundered for thinning with subsequent upwelling of the underlying asthenosphere,leading to continental rifting along the convergent plate margin and melting of the overlying continental crust.Some felsic melts emplaced to form granites and some evolved melts formed pegmatites with highly fractiaonal crystallization.The lack of post-collisional mafic magmatism reflects the failure of continental rifting.It is the failed continental rifting that would have resulted in the amalgamation of different terranes into the North China Craton.