| The investigated region in this thesis is located at the southeastern margin of the North China Craton (NCC) close to the southern segment of the Tan-Lu Fault, covering the Bengbu-Fengyang (including Wuhe Complex and the Mesozoic intrusions in the Bengbu Uplift) and Xuzhou-Suzhou region. The investigated objects are early Precambrian metamorphic basement rocks (represented by Wuhu Complex), representative intrusion (Laoshan gneissic granite) in Bengbu Uplift, various kinds of deep-seated xenoliths in the Mesozoic Jiagou dioritic porphyry in the Xuzhou-Suzhou region. Combinded analytical methods including petrology, geochronology, elemental and isotopic geochemistry have been used. At the some time, with the aim to better compare and discuss the Neoarchaean (2.5-2.8Ga) crustal accretion of the NCC, the recently published high-quality zircon Hf isotopic data of the metamorphic basement and sedimentary cover rocks have also been reviewed. The main research subjects in this contribution include:(1) zircon Hf isotopic compositions of the NCC metamorphic basement and sedimentary cover rocks have been collected, and the obtained data have been used to explore the NCC crustal growth in Neoarchaean, and to discuss their tectonic significations;(2) the Laoshan intrusion within the Bengbu Uplift have been carried out for zircon dating in order to determine its formation timing and its source materials;(3) geochronology, elemental and isotopic geochemistry, and petrology investigations have been undertaken on the meta-basic rocks from the metamorphic basement (i.e., Wuhe Group) to date its formation and metamorphic timings, and further to investigate the petrogenesis of the Mesozoic intrusions in the Bengbu Uplift by using the obtained data of metamorphic basement rocks;(4) mafic granulites entrained in the Nushan basalts, Mesozoic intrusions in the Bengbu Uplift, and the Wuhe Complex have been firstly integrated to test the recently proposed melt-relict model;(5) various kinds of deep-seated xenoliths entrained in the Mesozoic Jiagou dioritic porphyry within the Suzhou region have been collected, and conducted for elemental and isotopic geochemistry, petrology and geochronology analyses with the aim to investigate their formation and metamorphic timings, and their petrognensis. The main results and conclusion can be addressed in the following.1. Late Neoarchean(2.5-2.8Ga) crustal accretion of the NCC and its geological implicationsIn contrast to other cratons in the world which are characterized by main crustal accretion growth in the timing of2.7-2.8Ga, the NCC crustal growth revealed by zircon Hf isotope occurred both at2.7-2.8Ga and2.5-2.6Ga. Besides, the Neoarchean growth of the NCC also displays marked regional differences. Specially, the Eastern Block (EB) are featured by abundant early and middle Archaean crustal growth, predominant2.7-2.8Ga but minor2.5-2.6Ga crustal accretion, no notable Paleoproterozoic crustal accretion (mainly characterized by reworking of ancient crustal materials); whereas the Western (Yinshan) Block (WB) is characterized by both～2.7Ga and Paleoproterozoic crustal growths with sparse early and middle Archaean zircon ages. Additionally, the Trans-North China Craton (TNCO) also show many notable differences in the northern and southern segments, and the northern segment are characterized by both2.7-2.8Ga and2.5-2.6Ga crustal growths with minor early and middle Archaean zircon ages, while the southern segment of the TNCO are featured by wide crustal growth timing ranging from2.5to2.9Ga. There are also some differences between the northern and southern segments of the TNCO, like the sedimentary cover and the Re-Os isotopic features of mantle xenoliths.Thus, the compiled zircon Hf isotopic data suggest that it is reasonable to tectonically divide the NCC metamorphic basement into three different blocks, i.e., Eastern and Western Blocks and TNCO. However, the data also indicate that the TNCO is more complex than previously suggested, and the southern segment is not the simple southern extension of the TNCO. The TNCO could experience multistage of subduction (～2.5Ga,～2.1Ga and～1.85Ga), as recently proposed by some authors. In terms of zircon U-Pb ages and Hf isotopic data, it is strongly proposed that the NCC in the Neoarchaean formed in a combined mantle-plume and magmatic arc setting, an important tectonic transition period in the global as documented in the other cratons of the world.2. Zircon dating for the Laoshan intrusion in Bengbu Uplift and their constraints on magma sourcesThe representative Laoshan intrusion has been dated by SHRIMP zircon U-Pb method with the aim to date its formation timing and to explore its magma source materials. By zircon CL images, three types of zircons have been identified, namely, inherited magmatic zircon cores and metamorphic zircon mantles, and magmatic zircon rims. The zircon rims yield concordant～159Ma age, which is considered to be the emplacement timing of the Laoshan intrusion, rather than late Neoarchaean or Paleoproterozoic formation age as previously suggested. The inherited zircons yield～673Ma concordant age, and the metamorphic mantles exhibit Triassic metamorphic ages. The inherited magmatic core and metamorphic mantle ages are in good agreement with the protolith and metamorphic ages of the Dabie-Sulu UHP metamorphic rocks, respectively. Thus, it is proposed that the Laoshan intrusion emplaced in the Mesozoic and sourced from the subdcuted South China Block (SCB). The Laoshan intrusion represents one of the earliest stage Mesozoic magmatism in the Bengbu Uplift. Generally, there are two criteria to identify the sources of the Mesozoic granites from the studied region, the inherited zircon ages and Pb isotopic compositions. The analytical results suggest that Pb isotope composition is not an effective index (see the next). What’s more, more and more data suggest that Neoproterozoic magmatism and zircon ages (>850Ma) are also documented in the NCC especially in the southern segment or inner of the Eastern Block. So, the criteria of inherited zircon ages is also not as important as before. It is noted here that the Neoproterozoic zircons in the NCC are generally>850Ma, slightly older than those from YC basement and the protolith ages of the Dabie-Sulu UHP metamorphic rocks (mostly being <850Ma with a age peak at～750Ma), to some degree, the criteria of Neoproterozoic zircon ages is still practical. It is strongly suggested here that the simultaneous occurrence of both Neoproterozoic magmatic zircon and Triassic metamorphic zircon ages is the best index for identifing magma sources.3. The petrology, geochronology, geochemistry studies on the meta-basic rocks from the Wuhe Group and its constraints on petrogenesis of the Mesozoic magmatismBengbu Uplift consists of Precambrian Wuhe Group and Mesozoic intrusions. Two occurrences of meta-basic rocks, i.e., the garnet amphibolite occurring as bands or blocks within the impure marbles (in the Jiuhuashan within Fengyang region), the garnet granulite and plagioclase amphibolite occurring as blocks or lenses within the Mesozoic intrusions (in Mejia and Tushan in Huaiyuan region), have been collected and carried out for chronology, petrology, and geochemistry studies. Coupled petrology and zirconology including CL imaging, mineral inclusions, trace element and SHRIMP dating indicate that the meta-basic rocks experienced high pressure (HP) granulite facies metamorphism at the timing of-1.85Ga. This is the first report on Paleoproterozoic HP granulite facies metamorphism at the southeastern margin of the NCC, which is ascribed to the mantle upwelling related to mantle plume provided abundant heat source and resulted in crustal thickening by accretion of basaltic magma for the development of HP granulite facies metamorphism in the lower crust beneath the studied region. Although no magmatic/inherited zircons have been found in meta-basic rocks, yet it is supportive that the most metamorphic basement rocks of the Wuhe Group formed during late Neoarchean in terms of many late Neoarchean inherited zircons found in the Mesozoic intrusions and in the Neoproterozoic mafic dykes and the previously reported～2.5Ga TTG gneiss. However, considering that～2.1Ga Mopanshan/Shimenshan and Zhuangzili K-feldspar granites experienced deformation and metamorphism, it is reasonable to presume that the K-feldspar granites also represent a portion of metamorphic basement, and thus some of the metamorphic basement rocks formed in the Paleoproterozoic.Main and trace element data of the meta-basic rocks show that their protoliths are island-arc basalts (IAB), indicating that they formed in a magmatic arc setting at the late Neoarchean. Detailed geochemical analyses and plots also suggest that the meta-basic rocks of the Wuhe Group can be well compared with the granulite terrains at the northern margin of the NCC, thus the meta-basic rocks can be considered to represent ancient granulite terrains. Isotopic data suggest that the Rb-Sr isotopes have been mobile or were disturbed during the later processes, while Nd isotope data show their NCC terrain affinity. Interestingly, however, the meta-basic rocks show both high and low radiogenic Pb isotope compositions, i.e., the Fengyang and Huaiyuan meta-basic rocks show distinct high and low radiogenic Pb isotopic compositions, which are higher than and equal to the range of NCC terrain. This discovery is of great importance:firstly, the previous Pb isotope criteria distinguishing NCC basement from YC basement should be revised or abandoned, at least for the southeastern margin of the NCC; secondly, it has important implications for investigating "Pb paradox"The obtained geochemistry data in combination with previously published results of Mesozoic intrusions in Bengbu Uplift and Nushan granulite xenoliths suggest that the melt-relict model can explain the petrogenesis of the Mesozoic intrusions in the studied region, i.e., some Mesozoic intrusions (melt) in the Bengbu Uplift are sourced from the melting of the meta-basic rocks of the Wuhe Complex.. However, the granulite xenoliths with relict origin is lack/rare at the southeastern margin of the NCC. The reasons that lead to the lack of this kind of granulite xenoliths are listed in the following. Firstly, the not abundant research, up to now the research degree in the northern margin is far more than in the southeastern margin, especially for the mafic granulite xenoliths. Secondly, the melting degree at the southeastern margin is relatively lower than that at the northern margin. Lastly, as a matter of fact, minor samples (like sample NS201from Huang et al.,2004) can be regarded to be relict. Moreover, to some degree, as a result of felsic melt extraction, the dense residue could founder into the underlying mantle.In any rate, this is the first study to link the petrogenesis of metamorphic basement, granulite xenolith, and Mesozoic granites at the southeastern margin of the NCC on the basis of geochemistry data, which can better boost the research between different kinds of rocks in the NCC.4. The studies on the deep-seated xenoliths entrained in the Mesozoic Jiagou dioritic porphyry, northern Anhui Province Combined petrology, geochronology and geochemistry suggest that the collected deep-seated xenoliths can be classified into four types, i.e.,(1) Paleozoic pyroxenite xenoliths (Group1),(2) Precambrian lower-crust xenoliths (Groups2,3,4). In order to better discuss the petrogenesis of various xenoliths, two sub-sections have been addressed in the following.Paleozoic pyroxenite xenolithsGroup1samples represent the Paleozoic pyroxenite xenoliths, including spinel-bearing garnet clinopyroxenite, phlogopite clinopyroxente, spinel pyroxenite. Many tiny garnet exsolutions occurring in the clinopyroxene and garnet coronae surrounding spinel suggest that they were originally sourced from the mantle. Some zircons are abstracted from one of the pyroxenite xenolith. Zircon SHRIMP U-Pb dating and trace element analyses suggest that they formed at-393Ma, which is the first report of Paleozoic magmatism at the southeastern margin of the NCC. Four inherited zircons yield concordant middle to late Proterozoic U-Pb ages, resembling the age features of North Qinling Group/Basement. Sr-Nd-Pb isotope and zircon trace element data suggest that the origin of studied pyroxenite xenoliths resemble that of pyroxenite xenoliths/veins entrained in the Hannuoba basalts at the northern margin of the NCC. Thus, it is proposed that the pyroxenite xenoliths are mainly sourced enriched mantle (EM) which was metasomated/contaminated by melts from marine sediments and subducting continental crust, and one of the metasomatic events occurred in the Paleozoic. Interestingly, some authors have also reported that the lithospheric mantle and deep crust of the NCC beneath Xinyang region have been influenced by the Paleozoic subduction. The combined results imply that the Paleozoic North Qinling-Tongbai Orogen could be eastward extension to the southeastern margin of the NCC, which are covered by sedimentary strata and the Mesozoic basin sediments.Precambrian lower-crustal rocksThe main types of deep-seated xenoliths are basic gneiss, garnet granulite, garnet amphibolite, hornblendite with minor garnet-bearing felsic gneisses (intermediate-acid gneisses). Petrological and geochemical data suggest that they are of magmatic protoliths. In terms of petrology, and to some degree geochronology data, the Precambrian deep-seated xenoliths can be classified into three groups, i.e., Groups2,3and4. The samples in Group2are meta-basic rocks including garnet granulite, hornblendite, basic gneiss, garnet amphibolite, they formed at～2.5Ga and experienced1.80-1.90Ga HP granulite facies metamorphism, resembling the chronological features of the meta-basic rocks from the Wuhe Group. While minor samples (like sample07JG34, Group2) formed at late Neoarchean but suffered HP granulite-facies metamorphism at～2.1Ga. Geochemical data show that the protoliths of the Group2are mainly basalts with minor gabbros (like sample07JG06), and in the primitive mantle-normalized trace element and corresponding diagrams they show the features of island-arc basalts (IAB), some of which exhibit the geochemical compositions of Nb-enriched basalt (NEB) and high-Nb basalt (HNB). The above results together suggest that they formed in a magmatic-arc (or island arc) setting during the Late Archean, consistent with the conclusion from the Wuhe Group, implying an important stage of subduction-accretion at the southeastern margin of the NCC during the late Archean.Group4comprise two samples including garnet-bearing basic and granitic gneisses, which are formed at2.55-2.65Ga and subsequently experienced2.48-2.49Ga granulite-facies metamorphism. Geochemical data indicate that they resemble the features of island-arc basalt (IAB) and volcanic arc granite (VAG), implying that they also formed in magmatic arc setting, in common with the Group2xenoliths. However, different from the Group2, the Group4has no documents of retrograded overprints of petrology and geochronology. The results imply that the Groups2and4represent the lower and upper part of the lower continental crust (LCC), and both formed in the late Archean magmatic-arc setting.The two samples in Group3are garnet-bearing gneisses (intermediate-acid gneisses). Combined zircon mineral inclusion and SHRIMP U-Pb dating show that they emplaced at～2.1Ga and suffered1.80-1.90Ga HP granulite facies metamorphism. The protoliths of the two samples are dacite and Mg-andesite (MA), respectively, with adakitic affinity, which are characterized by high Sr/Y and (La/Yb)N ratios and low HREE contents. In combination with Sr-Nd-Pb isotopic data, the dacite can be considered to represent the subducting oceanic slab involving minor or no marine sediments, and the Mg-andesite (MA) can be regarded to be the melting of mantle metasomated by the melt from the subducting slab and its overlying marine sediments. These data confirm another stage of subduction-accretion at～2.1Ga at the southeastern margin of the NCC, which is the first report of the early Proterozoic subduction in the studied region.Elemental and Sr-Nd-Pb isotopic geochemical data of the Precambrian xenoliths suggest that the Rb-Sr isotopic system of the deep-seated xenoliths was open during the later alteration or was disturbed by the metamorphism or metamorphic overprint, while Sm-Nd isotopic system is more stable and suitable for tracing. Groups2and4xenoliths show consistent Nd-Hf isotopic compositions, with2.7-2.8Ga Nd-Hf age peak, implying their NCC affinity. While the high and positive initial Nd isotopic data of dacite (acid gneisses) approaching the coeval depleted mantle (DM) values (Group3, formed in the～2.1Ga) suggests that it was sourced from the melting of the subducting oceanic slab, and the slighter lower initial Nd value and～2.5Ga model age of andesite (intermediate gneisses) was derived from the melting of mantle which is metasomated by melt from oceanic slab and its overlying sediments. Similar to the meta-basic rocks from the Wuhe Group, the Precambrian xenoliths also display both high (Groups2and3) and low (Group4) radiogenic Pb isotopic compositions. Several lines of evidence including Th/Yb vs. Ba/La plot and～2.1Ga Pb-Pb "isochron" together suggest that the high radiogenic Pb isotopic compositions of the deep-seated xenoliths and metamorphic basement rocks are sourced from the melt metasomatism from the marine sediments related to the～2.1Ga slab subduction, bearing important geological implications for explaining "Pb paradox" of the Earth. |
PDF Full Text Request