Maturation And Stabilization Of Ancient Crust:A View From Vertical Evolution In Yangtze Nucleus

Continental crust is the home to humanity and makes the Earth different from other planets.The maturation and stabilization process of ancient continental crust contribute to the knowledge of Earth history,natural resources,and the search for habitable exoplanets.But studying the continental crust is not straightforward.The poor exposure of the Archean crustal section and lack of effective techniques are in the way of exploring the old crustal archive.To understand the evolution of crustal architecture and chemistry,we place our focus on the Archean and Paleoproterozoic rocks in the Yangtze Craton,South China.The study area is in the Kongling Complex,where the most ancient rocks are exposed in the north margin of Yangtze Craton.We start with studying the Paleoproterozoic igneous rocks,including the A-type granites and mafic dykes.Their source rocks,xenolith rocks,and the wall rocks that they emplace,are all formed in the Archean era.These Archean rocks are found in various crustal levels,they are the keys to restore the continental crust architecture.In this contribution,we apply petrology,mineralogy,geochemistry,and crystal physics to study the representative Archean and Paleoproterozoic rock samples in the Yangtze Craton.Our focus is to investigate the vertical crustal structure and the chemical evolution in this craton during the Archean time.To answer these questions,our goals are:(1)studying the petrogenesis of Paleoproterozoic igneous rocks,like the A-type granites and mafic dykes;(2)identification of the source rock of A-type granites;(3)investigating the crustal assimilation between the mafic dykes and their crustal xenolith;(4)exploring the relationship of A-type granites,mafic dykes with their source,xenolith and the wall rocks with Archean age;(5)comparison of the crustal levels where the Archean rocks are distributed;(6)revealing the principles of how the continental crust of the Yangtze Craton evolves with time.Here are the basics of our findings:(1)At ¹.84-1.82 Ga,there is successive A-type granitic magmatism in the northern Yangtze Craton,including the granite porphyry and coarse-grained granite.They have zircon SHRIMP U-Pb ages of 1840 ± 17 Ma and 1821 ± 8 Ma,respectively.The granite porphyry contains phenocrysts of ¹5% K-feldspar,⁵% quartz,⁴% plagioclase,minor garnet(¹%),setting in fine-grained groundmass(⁷5%)of Kfeldspar,quartz,plagioclase,and biotite.The coarse-grained granite consists of ⁶0% K-feldspar,³0% quartz,⁵% biotite,and ⁵% plagioclase.The porphyries and the coarse-grained granites both show A-type granite affinity with high SiO₂(69.2–71.5 wt %,72.6–74.5 wt %),high Fe₂O₃^T/Mg O(21.5²7.7,5.9¹9.5)and 10⁴*Ga/Al ratios(3.6⁴.3,2.7³.5),enrichment in HFSEs(Th,U,Pb,Ga).The porphyries have higher total trace element and zirconium saturation temperature(mean 892 ?)than coarsegrained granites(mean 803 ?).Their respective ?_Hf(t)values are-17.6 ^-11.7 and-21.0 ^-14.8,the two-stage Hf model ages are 3.6³.3 Ga and 3.8³.4 Ga,both lying in the isotopic field of 2.9².8 Ga Kongling TTGs.The trace element modeling demonstrates the granites are formed by ².9-2.8 Ga tonalites via crustal anatexis and reworking process.It indicates this craton is under continuous extension during Paleoproterozoic.(2)In the Kongling Complex,the studied mafic dykes contain TTG xenolith from the deep continental crust.The mafic dykes are fine-grained diabase,with ⁴5% porphyritic plagioclase and pseudomorphs of amphibole after pyroxene(⁵5 %).Minor quartz xenocrysts(< 1 %)can be found at the boundary with the xenolith.The nearxenolith mafic dyke samples have higher SiO₂(49.79–54.30 wt %),P₂O₅(0.23–0.30 wt %)and lower Ca O(6.63–8.22 wt %),Mn O(0.14–0.17 wt %),K₂O(1.02–1.99 wt %)contents than samples away from xenoliths(SiO₂: 49.06–49.14 wt %,P₂O₅: 0.15–0.16 wt %,Ca O: 9.23–9.32 wt %,Mn O: 0.19–0.20 wt %,K₂O: 2.25–2.28 wt %).Nearxenolith mafic dyke samples show higher Th(2.7–5.7 ppm),U(0.39–0.84 ppm),Zr(111–200 ppm),Hf(3.2–5.1 ppm)contents,stronger negative Eu anomalies(?Eu = 0.78–0.95)and higher La _N/Yb _N ratios(7.31–9.98)than off-xenolith diabase(Th = 0.9,U = 0.22–0.27,Zr = 64–66,Hf = 2.1 ppm;?Eu = 0.92–0.96,La _N/Yb _N = 3.33–3.53).The off-xenolith sample has a primitive ?_Nd(t)value of-1.2,which is close to the mantle Nd isotopic signature,whereas the near-xenolith samples have much higher ?_Nd(t)values of-8.5 to-6.7.Combined with petrological observation,whole-rock geochemistry,and Nd isotope EC-AFC modeling,we find that the mafic magma has significantly assimilated the xenolith,indicating a crust-mantle interaction event.(3)The Yemadong mafic dyke zircons are only recovered from the samples near the xenolith.The U-Pb zircon date via laser ablation is 2009 ± 17 Ma(n = 34,MSWD = 1.2),except for one inherited Archean zircon(2890 ± 31 Ma).The zircon Lu-Hf isotopic tracing shows that the ?_Hf(t)value for 2.0 Ga zircons is at-19.0 to-10.3,with crust-like,subchondritic Hf isotopic feature.Importantly,the 2.0 Ga zircon Hf isotopes,the ?_Hf(t)values,are only slightly higher than those from the 2.9 Ga Kongling TTG rocks.It contrasts with other coeval mafic intrusions with mantle-related zircon Hf.Together with the crustal assimilation found in mafic magma and the xenolith,we suggest that these Paleoproterozoic zircons also result from crust-mantle interaction,in two possible ways.The first scenario is that the 2.0 Ga zircons are directly from preexisting crystallized rocks sourced from Kongling TTGs,in the deep crust level.These xenocrysts are captured by intruding mafic magma,registering the time of anatexis of TTG rocks,and the time of local high-grade metamorphism.It also means the mafic magma is crystallized after 2.0 Ga.Alternatively,because of the strong assimilation of ².9 Ga TTG xenolith in mafic dykes,the Zr and Si become saturated in the contaminated mafic magma,leading to zircon crystallization.In this case,the 2.0 Ga zircons inherit the xenolith Hf isotopic composition and represent the time of crustal assimilation,as well as the time of mafic magma emplacement.Based on the evidence of crustal assimilation,the second scenario is preferred in this study.Nevertheless,the ².0 Ga zircons are a result of crustal contamination and defining the lower time limit of mafic magma intrusion.The innovation of our research is built on these basic findings of A-type granite and mafic dykes in the Kongling Complex.We have revealed the relationship between these Paleoproterozoic rocks with Archean continental crustal materials in the deep level,by further exploring their source rock,the xenolith from depth,and the wall rocks in the shallow crust.The Archean rocks both belong to TTG like granitoids,and they are distributed at distinct crustal levels.Such phenomena enlighten us to investigate the vertical crustal structure of the Yangtze Craton in the Archean:(1)The studied Paleoproterozoic A-type granite is sourced from 2.9-2.8 Ga tonalite in the Kongling Complex.After modeling a wide spectrum of trace elements for available exposed TTG-related rocks in the Kongling Complex,and assuming the residual minerals from both major element modeling and the experimental petrology studies,we find that our A-type granite can only be the partial melting product of natural tonalite with the age of ².94 Ga.This conclusion can be supported by the zircon Hf tracing as well.The field observation shows that the ¹.84 Ga A-type granite porphyry is intruding into a ².87 Ga granitic gneiss.Together,it implies the continental crust changes from the component that forms the deep source rock of Atype granite,to the crustal material like the country rock on the surface.This is the first evidence that shows the composition of continental nuclei in the Yangtze Craton is stratified,from deeper,older tonalite to late-stage K-rich granites upwards.(2)The studied Kongling mafic dykes carry TTG xenolith from the deep continental crust,and intrude into the TTG wall rock at the surface.Both the TTG xenolith and wall rock are typical trondhjemite,with high SiO₂(65.02–70.70 wt %),Na2O(4.79–5.92 wt %),Al₂O₃(15.07–16.92 wt %)contents.The xenolith samples have slightly higher K₂O/Na2O(0.29–0.60)ratios and higher Fe₂O₃^T(1.85–2.89),Mg O(0.68–1.69)contents,than the wall rocks(K₂O/Na2 O = 0.24–0.25,Fe2O3 T = 1.65–1.85,Mg O = 0.68–0.80).Both trondhjemites have similar trace elements,like negative NbTa-Ti anomalies and enriched LREE patterns.The zircon dates from xenolith and wall rock are 2893 ± 18,2869 ± 21 Ma,the zircon 176Hf/177 Hf ratios are 0.280782-0.280904,0.280821-0.280998 with ?_Hf(t)ranges of-6.2 ^-1.3,-5.9 ^-2.3 respectively.The zircon Ti temperature is higher in xenolith(803 ± 65 ?)than the wall rock(761 ± 51 ?).Therefore,we conclude the xenolith and wall rock to the mafic dyke are the same ².9 Ga trondhjemite.But the inclusion and intruding relationship together indicate they are from different crustal depths.(3)To test whether the xenolith and wall rock are formed at different levels,we unprecedentedly study the residual inclusion pressure inside zircon host mineral.We choose the apatite inclusions that are deeply buried,far from free surfaces like crack or mineral surface,well-confined crystals,to compare their residual pressure.The xenolith and wall rock are of same whole-rock chemistry and zircon U-Pb age,their zircons have close Ti temperatures,the apatite inclusions are both the similar fluorapatites,and we observe random apatite orientations.Although both the xenolith and wall rock are the same ².9 Ga trondhjemite,the result reveals that they have systematically different inclusion pressures,at-0.60 ^-0.49 GPa and-0.79 ^-0.64 GPa,respectively.The implication is that the same ².9 Ga trondhjemite is found at different crustal depths with a measurable difference.Based on this,we propose that a trondhjemitic crust with noticeable thickness has already been formed in parts of the Yangtze Craton since Mesoarchean.This Archean trondhjemitic crust at depth in the Yangtze Craton is the indispensable foundation for ongoing crustal reworking and differentiation processes.(4)Apart from the vertical evolution,we have also investigated the Archean continental rocks from all parts of the Yangtze Craton.Data from widespread Archean rocks in the Yangtze Craton reveal that the trondhjemites(> 78%)dominate this craton before 2.9 Ga.Afterward,the chemistry of its continental crust gradually shifts to granitic rocks(> 34%).Building on our findings in the Kongling Complex,and the decrease of average Na2O/K2 O ratio in the Yangtze cratonic rocks,we propose that at the boundary of Mesoarchean and Neoarchean,the crust in the Yangtze Craton has evolved from thick and sodium-rich TTG rocks to granites in the shallower level,close to the modern Earth's continental crust.Such trends represent the chemical maturation during the cratonization process of the Yangtze Craton.In a nutshell,we have comprehensively studied the Paleoproterozoic rocks and the related Archean continental rocks in the Yangtze Craton.We discover that the subtle variations in age,chemistry,and crustal levels of the ancient continental rocks could be utilized to decipher early crustal evolution.Our exploratory work allows us to discover the temporal-spatial distribution of the ancient crustal materials for the very first time.These new results and findings revealed by our study would mark the advent of a chemically matured and vertically stabilized continental crust in the Yangtze Craton by the Mesoarchean.