Formation And Evolution Of Cratons

Archean cratons are the ancient and stable nucleus of the continental crust. These cratons recorded the early formation and evolution history of the continental crust, which is closely related to ore formations, climate change and biological evolution. The Yangtze Craton and North China Craton are the two most important Archean cratons in the eastern Asia. Both the early formation and evolution history of the Yangtze Craton and cratonic destruction of the eastern North China Craton are research hotspots in recent years. This thesis studied the mineralogy, U-Pb geochronology and trace and isotopic geochemistry of (1) 14 granitic gneisses from the central Kongling Terrane in the Yangtze Craton and (2) one mafic garnet granulite xenolith from the Early Cretaceous high-Mg dioritic porphyry in the Xu-Huai region from the southeastern margin of the North China Craton. The achievements are as follows:(1) Systematic comparison of SIMS and LA-ICP-MS U-Pb dating methods on the same spots of ancient natural zircons. The beam size of SIMS is 20 μm×15 μm with ablation depth <2 μm. The laser spot size of LA-ICP-MS is 32 μmin diameter with ablation depth >20μm. The results show that in most cases, the SIMS and LA-ICP-MS ages agree with each other within ±2% difference (i.e.,<70 Ma) for the 3.4-3.3 Ga zircons. For younger zircons, the age difference is less than ±5%. Exceptions are some discordant ages, which can be attributed to ablation of different domains by LA-ICP-MS. The agreement is remarkable, considering that LA-ICP-MS ablated a larger volume of zircon, which may possess heterogeneity on the surface and at the depth even for the same domain.(2) Discovery of the oldest rocks currently known in the southern China:two 3.45 Ga granitic gneisses from the Bianyuchigou area in the Kongling Terrane of the Yangtze Craton. SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4,3.3,2.9,2.7, and 2.0 Ga for both orthogneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4 Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical 176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-1CP-MS. Taken together, zircons from the two samples with 98-102% age concordance gave weighted average SIMS ages of 3434.3±9.6 Ma (2σ, MSWD=13, n=8) for Group A,3446.0±8.8 Ma (2σ, MSWD=10.7, n= 15) for Group B, and 3479±26 Ma (2σ, MSWD=0.49, n=2) for Group C. Groups A and B together yield an upper intercept age of 3457±14 Ma (2σ, MSWD=0.85, n=23). The LA-1CP-MS data yield weighted average ages of 3442±19 Ma (2σ, MSWD=0.17, n=7) for Group A and 3435±11 Ma (2σ,MSWD=0.44, n=16) for Group B. They yield an upper intercept age of 3443±13 Ma (2a, MSWD=0.63, n=23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their 2σ age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrane by ca.150 Ma.(3) Analysis of the whole-rock major and trace element and zircon Lu-Hf-O isotopic compositions of the two 3.45 Ga granitic gneisses. The results revealed the nature of the source regions for 3.45 Ga granitic magmatism and the genesis of younger zircons. Both samples have high SiO2 and K2O contents, belonging to potassium granites. They have low and fractionated REE contents, depleted HREE, and weak negative Eu anomalies. The 3.4 Ga zircons show near chondritic εHf (t) (-0.7±1.0.2a, MSWD=1.14, n=8), which is below the coeval value of the depleted mantle. This indicates that the granitic magma may have contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1-6.4%o) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4 Ga zircons vary from 3.9 to 3.6 Ga with a weighted average of 3703±27 Ma (2σ, MSWD=1.05, n=7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8 Ga. Among the younger age groups, the 3.3 Ga zircons exhibit 176Hf/177Hf (t) and δ18O values that are comparable to the 3.4 Ga zircons, suggesting that they were altered from the 3.4 Ga zircons. The 2.9 and 2.7 Ga zircons in both samples are rare and magmatic. Their 176Hf/177Hf (t) ratios are distinct from the 3.4 Ga zircons, indicating different sources. These two age groups are consistent with the 2.9 Ga trondhjemitic-tonalitic-granodioritic and the 2.7 Ga A-type granitic magmatism in the Kongling Terrane. The 2.0 Ga metamorphic zircons, regardless of being concordant or discordant, have 176Hf/177Hf (t) ratios overlapping those of the 2.7 Ga zircons, suggesting a common source. In contrast, the δ18O values of 2.0 Ga zircons are strongly variable and positively correlated with their age concordance. The low δ18O values (down to 3.1%o) requires interaction with hydrothermal fluids. These results suggest that at least some of the 2.0 Ga zircons were likely to have been altered from the 2.7 Ga zircons by hydrothermal fluids.(4) Analysis of the whole-rock major and trace element and zircon U-Pb-Lu-Hf isotopic compositions of the other 12 Ga granitic gneisses. The results revealed five granitoid magmatic events:3.3 Ga,2.8 Ga,2.7-2.6 Ga, and 2.0 Ga. The 3.3 Ga samples have high Na2O and low K2O contents, belonging to trondhjemites, whereas the rest samples have both high Na20 and high K2O contents. The two 3.3 Ga samples only have one generation of magmatic zircons, with crystallization ages of 3298±24 Ma (2σ, MSWD=3.4, n=21) and 3292±33 Ma (2σ, MSWD= 4.3, n=17), respectively, which confirmed the existence of 3.3 Ga rocks in the Yangtze Craton. Their whole rocks have low LaN/YbN ratios (7.6-16.2), non-depleted HREE, and negative Eu anomalies (Eu/Eu*=0.63-0.91). Additionally, both samples have high FeOtotal, MgO and transitional metal elements (e.g., V, Cr, Ni, Cu, and Zn). The εHf (t) values of 3.3 Ga zircons range from -1.5 and +1.0, being close to the chondritic values, and the corresponding TDM2 ages are 3.73-3.57 Ga, indicating >3.6 Ga source rocks. The other granitic gneisses have whole-rock compositions similar the 3.45 Ga granitic gneisses, all of which have enrich LILE, depleted HFSE, low and less fractionated REE, variably depleted HREE, and weak Eu anomalies. The magmatic zircons in these granitic gneisses mostly have negative εHf (t) values and their TDM2 ages are as high as 3.8-3.7 Ga, indicating Eoarchean source rocks. These geochemical features suggest derivation of these granitic magmas from mafic Eoarchean continental crust. Minor positive εHf (t) values in the 2.7-2.6 Ga zircons also suggest addition of juvenile crustal materials.(5) Summary of the spatial and temporal distribution of Paleoarchean-Paleoproterozoic magmatic events in the Kongling Terrane. The Paleoarchean-Paleoproterozoic magmatism in the Kongling Terrane is episodic in time, including four major episodes:3.45 Ga,3.3-3.2 Ga, 3.0-2.85 Ga, and 2.7-2.6 Ga. Also present are several less significant magmatic episodes, e.g.,3.1 Ga,2.8 Ga,2.4 Ga,2.0 Ga, and 1.85 Ga. These magmatic activities may have been accompanied by coeval metamorphism, e.g.,3.45 Ga (granulite-facies).3.2 Ga,2.74-2.72 Ga (amphibolite-facies),2.5 Ga (amphibolite-facies), and 2.0 Ga (high-pressure granulite-facies). In space, the eastern Kongling Terrane consists of predominant 2.7-2.6 Ga granitic gneisses and 2.9 Ga TTG gneisses, with minor 2.0 Ga granitic gneisses. The central part of the Kongling Terrane is dominated by 2.7-2.6 Ga granitic gneisses, with minor occurrences of 2.8 Ga and 2.4 Ga granitic gneisses and rare 3.45-3.2 Ga TTG-granitic gneisses. The western Kongling Terrane mainly consists of 2.95-2.85 Ga TTG gneisses, with minor 3.0-2.9 Ga metabasalts,2.0 Ga S-type granites, and 1.85 Ga A-type granites. Besides,1.85 Ga mafic dykes are widespread in the whole Kongling Terrane.(6) Petrogenesis of two contrasting types (A- and I-type) of 2.7-2.6 Ga granitic gneisses in the Kongling Terrane. The two types of granitic gneisses differ from each other in field occurrences, whole-rock geochemistry, and zircon Lu-Hf isotopic compositions. The A-type granitic gneisses occur in the eastern Kongling Terrane and have high Zr-saturation temperatures (1010-1112℃) and abundant zircons with positive εHf (t) values (up to the coeval depleted-mantle value), which suggest a mantle origin for the A-type magmatism. On one hand, the whole rocks of the A-type granitic gneisses have high SiO2 and K2O, low Al2O3 and Cao. high REE, and strong negative Eu anomalies (Eu/Eu*=0.22-0.35), indicating significant amounts of fractional crystallization of plagioclase. The extremely low Mg# (19-23) and low Cr and Ni concentrations suggest that ferromagnesian mineral (e.g., pyroxene and amphibole) may have also been fractionated. The depleted Sr and P relative to nearby REE suggests fractionation of apatite. On the other hand, minor magmatic zircons therein have negative εHf (t) values with TDM2 ages of >3.5 Ga. features that are similar to the crystallization basement of the Kongling Terrane. This indicates crustal assimilation. In summary, the above features suggest that the A-type granitic magmatism in the Kongling Terrane was produced by differentiation of mantle-derived magmas with crustal assimilations. This fits one of the typical genetic models for A-type granites. The coeval I-type granitic gneisses occur in both the eastern and central parts of the Kongling Terrane. Their magma temperatures are only 740-841 ℃. Besides, they have high Mg# (30-55), low REE contents, slightly depleted HREE and weak Eu anomalies, suggesting a "middle-pressure" garnet amphibolite-facies metabasaltic source region. Their magmatic zircons have mostly negative εHf (t) value. Together with minor occurrences of 3.4-3.1 Ga inherited zircons, they suggest ancient continental crustal source rocks. Spatially, the magma temperatures and zircon f (t) values of I-type granitic gneisses show decreasing trends from the eastern to the central Kongling Terrane, which suggest the genesis of I-type granitic magmas by melting of ancient country rocks during the emplacement of high-temperature mantle-derived magmas. The high-temperature A-type granitic magmas (and their associated mantle-derived magmas) not only provided heat for reworking of the ancient continental crust, but also led to crustal growth. The 2.7-2.6 Ga magmatism in the Yangtze Craton could be related the global 2.7 Ga mantle plume events.(7) A possible connection between the Yangtze Craton and the most ancient Archean supercraton (Vaalbara). The Vaalbara supercraton, which comprises the Pilbara Craton in the northwestern Australia and the Kaapvaal Craton in the southern Africa, was suggested to have existed from 3.47 Ga to 2.7 Ga. Its existence is corroborated by lithographic sequences, mafic dyke swarms, igneous activities, and paleomagnetic data of the Pilbara and Kaapvaal Cratons. Both cratons were well constrained in their formation and evolution. They experienced a series of Archean granitic magmatic activities. For instance, the Pilbara Craton has five major Archean granitic magmatic episodes:3.53-3.43 Ga,3.35-3.29 Ga,3.27-3.24 Ga,2.99-2.93 Ga, and 2.89-2.83 Ga. A compilation of geochronological studies in the Kongling Terrane suggests similar Archean granitic magmatism to the Pilbara and Kaapvaal Cratons (including the Middle Zone of the Limpopo Belt, northern Kaapvaal Craton). Such similarities suggest a possible connection for the Yangtze Craton to the oldest supercraton (Vaalbara) in the world.(8) Confirmation of existence of the Triassic thickened lower crust in the southeastern margin of the North China Craton, and reconstruction of the pressure-temperature-time history of this lower crust during Triassic continental collision. The mafic garnet granulite xenolith from the Xu-Huai region has two types of titanite:granular and coronary (overgrowth on rutile). The coronary titanite is clearly a secondary product of rutile decomposition. The granular titanite exhibits zonation in U-Pb age and chemical composition. Petrographic and geochemical evidence suggests that the zonation may be formed by thermal diffusion and later fluid-assisted recrystallization. Occurrences of granular titanite between garnet grains point to a pressure of>10 kbar, while inclusions of rutile inside granular titanite rims imply that the pressure might have reached 15 kbar. Granular titanite cores give U-Pb ages of 237-241 Ma and Zr-temperatures of 794-831℃ at 10 kbar and 850-892℃ at 15 kbar, indicating high-pressure granulite-facies metamorphism. Together with previous P-T estimates of coeval eclogite-facies xenoliths (800-1060℃ and>15 kbar at 200 Ma), a geotherm of above 60 mW m-2 is implied.(9) Delamination of >20 km lower continental crust in the southeastern margin of the North China Craton during Jurassic-Cretaceous. The Xu-Huai xenoliths that were entrained by the Early Cretaceous dioritic magmas are dominated by mafic high-pressure lithologies such as garnet granulite, garnet clinopyroxenite and eclogite, indicating a thickened dense lower crust (>50 km) of the North China Craton in the Triassic. In great contrast, lower crustal xenoliths from the Quaternary basalts from the nearby Nushan area in the southeastern North China Craton are characterized by garnet-free granulite xenoliths without eclogite-facies xenoliths but with abundant intermediate granulite xenoliths. Accordingly, the crust-mantle boundary was suggested to be at only 30 km during Quaternary eruption of the Nushan basalts. The large contrast in mineralogy between Xu-Huai and Nushan lower crustal xenoliths indicates>20 km removal of the lower crust along the southeastern margin of the North China Craton. The Triassic geotherm (>60 mW m-2) of the lower crust beneath the North China Craton is similar to that of the Kohistan arc, which has preserved a 12-km-thick dense lower crust, but significantly cooler than the geotherm of the Talkeetna arc, where most of the dense lower crust has been foundered. By comparison with the Kohistan and Talkeetna arc crusts, it is proposed that the dense lower crust of the North China Craton was not hot enough to get foundered in the Triassic. Foundering must have occurred in the Jurassic-Cretaceous in order to explain the present-day seismic velocity structure characterized by a sharp Moho, overall slow velocities in the lower crust, and a thin crustal thickness in the Xu-Huai area and other parts of the eastern North China Craton. It is also suggested that the Jurassic-Cretaceous foundering was related to the Pacific subduction. On one hand, the Jurassic subduction may have further thickened the southeastern margin of the North China Craton prior to Cretaceous extension, leading to greater instability of the lower crust. On the other hand, the subduction-related magma provided heat and water that weakened the lower crust, resulting in the ultimate foundering.