Geochemistry Of The Yangtze Continental Nucleus And Its Anatectic Granitoids

Investigation of the growth and evolution of Precambrian crust is a key to understanding of continental dynamics and developing of the plate tectonic theory.The Kongling Complex, which is the oldest basement of the Yangtze Block,and the Neoproterozoic Huangling Batholith are located in the Three Gorge area,Yichang,Hubei Province.In this thesis,a detailed study of major and trace element analyses,zircon U-Pb dating,Rb-Sr and Sm-Nd isotope analyses,mineral Oxygen isotope analyses and newly-developed zircon Lu-Hf isotope analyses was carried out for different rocks from the Kongling Complex,the Huangling Batholith and the Liantuo Formation.The results not only place geochemical constraints on the genesis of these rocks,but also provide insight into the Yangtze basement evolution and the Neoproterozoic geodynamics of South China.Our study is of great implications for the continental crust growth,differentiation and evolution,and petrogenetic mechanism of granites(especially TTG rocks).The Kongling Complex consists of basement rocks,supracrustal rocks and some marie rocks occurring as lenses,boudins or layers.The basement rocks are mainly TTG rocks with minor granites.The chemical compositions of metsedimentary rocks varied significantly and are controlled by the source materials.Three groups of ages are obtained from zircon U-Pb dating on seven basement rocks,including migmatites,gneisses and granites.The first group represents the time of magmatic activity at 2.90-3.00 Ga except one granite with an age of 2.85 Ga.The second group is obtained on the inherited cores of some migmatites and gneisses, ranging from 3.12 Ga to 3.35 Ga.The third group at about 1.98 Ga represent the time of migmatization,which is obtained on the zircon overgrowth of a migmatite.Zircon U-Pb dating is carried out for two metasedimentary rocks of the Kongling Complex,yielding two metamorphic ages of 1.95 Ga and 1.98 Ga.A similar age of 1.94 Ga is obtained by zircon U-Pb dating on an amphibolite.These ages are close to the time of migmatization,indicating an episode of tectonothermal event at 1.97±0.03 Ga,consistent with contemporaneous global events.It is this event that causes accretion of the Yangtze continental nucleus even cratonization.The O isotope compositions of basement rocks are similar to typical Archean TTG rocks,with most zirconδ¹⁸O values lower than 6‰.The O isotopic ratios for the metasedimentary rocks are obviously higher,consistent with its sedimentary sources.Zircon Lu-Hf isotopes are measured for the three groups of zircons from migmatites,gneisses and granites.The～2.9 Ga grains from different rocks have similar Hf isotope compositions,with ε_Hf(t)values of-2.1 to -5.2 and Hf model ages of 3.4-3.5 Ga.The Hf isotopic ratios obtained on 13 inherited cores are lower than the～2.9 Ga grains.The weighted mean of theirε_Hf(t) values is -1.6 and corresponding Hf model age is 3.55 Ga.The Hf isotopic data suggests that the growth of Yangtze continental crust had begun as early as 3.5 Ga.Hf isotopic analysis was carried out for one Paleoproterozoic zircon overgrowth rim,yielding anε_Hf(t)value of-21.7. Zircons from the two metasedimentary rocks have similar Hf isotope compositions,ε_Hf(t) values ranging from -13.3 to -1.0 and weighted mean values being -6.4 and -6.5, respectively.The amphibolite has a wide range of Hf isotopic ratios,reflecting the effect of metamorphism.The migmatites,gneisses and granites of the Kongling Complex have similar Rb-Sr and Sm-Nd isotope compositions.Theε_Nd(t)values calculated to 2.9 Ga are all around zero,from -1.9 to +2.6.The Nd isotopic ratios of the metasedimentary rocks varied greatly and are controlled by the source materials.The amphibolite and gabbro haveε_Nd(t)values(t= 1.97 Ga)of-4.3 and +4.4,respectively.The study on the Kongling Complex suggests that the initial growth of.Yangtze continental crust started no later than 3.5 Ga and suffered reworking in the Paleoproterozoic after the growth in the Archean.The Huangling Batholith is subdivided into four rock suites,namely the Huanglingmiao trondhjemite-granodiorite suite,the Sandouping quartz diorite-tonalite suite,the Dalaoling monzodiorite-monzogranite suite,and the Xiaofeng composite suite including mafic-felsic dykes and associated epigranite.Except the Xiaofeng Suite,most rocks from the other three suites are characterized by high Na and Sr,low Y and HREE,depletion of Nb,Ta,similar to typical TTG rocks.However,different rock suites show systematical differences in chemical compositions.The Huanglingmiao suite has higher Sr/Y and La/Yb ratios,but lower Nb/La ratios than the Sandouping and Dalaoling suite.Two granites of the Xiaofeng suite have zircon U-Pb age of 797±5 Ma and 799±2 Ma,respectively.Two dykes,felsic and mafic in composition,have ages of 806±12 Ma and 806±4 Ma,respectively.Thus all rocks in the Xiaofeng Suite formed simultaneously at about 800 Ma.Zircon U-Pb dating on six granites from the Huanglingmiao suite,yielding ages from 821 Ma to 800 Ma,with a weighted mean of 815±7 Ma,and Paleoproterozoic to Archean ages on the inherited cores which constrains the ages of source rocks.Two Sandouping tonalites both gave two groups of ages,803±11 Ma and 919±15 Ma,810±15 Ma and 911±14 Ma.The younger age represents the crystallization age of the Sandouping Suite,whereas the older age reflects the possible influence of Grenvillian orogen.A Dalaoling tonalite show a zircon U-Pb age of 817±22 Ma.Therefore, four rock suites of the Huangling Batholith are formed in the period of 800-820 Ma.Mineral O isotope analyses show that most rocks from the Huangling Batholith have O isotope compositions similar to typical I-type granite and only small-scaled high-temperature water-rock interactions happened during the intrusion of dykes into the granitoids.Four suites of the Huangling Batholith generally have similar Sr isotope compositions with initial Sr isotopic ratios of 0.705-0.708.ε_Nd(t)values for each rock suite are,-6.4 to -10.4 for the Xiaofeng suite,-9.3 to -21.3 for the Huangling suite,-2.1 to -9.2 for the Sandouping suite, and -4.4 for a Dalaoling tonalite.Zircon Lu-Hf isotope analyses gaveε_Hf(t)values of -8.3～-12.9 for the Xiaofeng suite,-10.5～-13.1 for the Sandouping suite,and -6.1 for the Dalaoling tonalite.The Neoproterozoic co-magmatic zircons and ancient inherited cores have contrasting Hf isotopic compositions,withε_Hf(t)values of-10.3～-24.1 for co-magmatic zircons and significantly lower Hf isotopic ratios for the inherited cores in spite of the alteration by Neoproterozoic magmatism.The lowestε_Hf(t)values are similar to Archean rocks of the Kongling Complex.These suggest that the Huangling Batholith is the anatexis product of ancient crust at 800-820 Ma.The ancient crust includes the Archean rocks underlying Kongling TTGs and Paleoproterozoic amphibolites.The Archean rocks were molten at great depth with garnet in the residual and the melting depth of Paleoproterozoic rocks was relatively shallower with amphibole as a major residual phase.The source materials of the Huangling Batholith are Archean and Paleoproterozoic crusts,suggesting that it was not the product of mantle plume or oceanic crust subduction,but connected with ancient orogen tectonic collapse as stated in the plate-rift hypothesis.This has significant implications for TTG petrogenesis,I-type granite origin and continental crust evolution.The Liantuo Formation with deposition time at about 750 Ma,unconformably overlies on the Huangling Batholith.Zircon U-Pb dating was carried out for 106 detrital zircons from a Liantuo sandstone,39 grains of which were further analyzed for their Lu-Hf isotopes.Four populations of ages are recognized,namely＞3.0Ga,～2.95Ga,～1.95 Ga and 820-750 Ma.The 3.8 Ga zircon with anε_Hf(t)value of -0.8 and a model age of 4.0 Ga represents the oldest crustal relict in the Yangtze Block.The 3.3 Ga zircons have positiveε_Hf(t)values as high as 4.2,providing compelling evidence for growth of juvenile crust.All the zircons have Archean Hf model ages,with prominent peaks at 3.2 to 3.6 Ga,indicating an important period of crustal growth.The other three zircon populations at～2.95 Ga,～1.95 Ga and 820-750 Ma have negativeε_Hf(t)values and consistent Archean Hf model ages,suggesting multi-stage episodic reworking of Archean crust.The youngest age is～750 Ma,close to the deposition time of the Liantuo Formation,indicating rapid recycling of supracrustal materials in a rift basin,possibly associated with breakup of the supercontinent Rodinia.