| The North China Craton?NCC?,as the oldest cratonic block in China,is generally considered as the result of the amalgamation of several microblocks,but the timing,mechanism and tectonic processes of the collisions between microblocks are still unclear.The Trans-North China Orogen?TNCO?is a key area for investigating the early Precambrian geological history of the NCC.The Taihua Complex and the Dengfeng Complex are the most important Archean basement rocks in the southern segment of the TNCO,which record important information about the early Precambrian tectonic evolution in the NCC.This thesis presents detail zircon SIMS U-Pb dating,in-situ zircon Hf isotope,whole-rock major and trace elements and Hf-Nd isotopes for the representative rocks from the Taihua Complex in Lushan area and the Dengfeng Complex in Songshan area,with aims to investigate their magma sources,petrogenetic processes and related tectonic settings,outline the spatial and temporal distribution of the early Precambrian magmatic activities and metamorphism in the area,provide further insights into the crustal evolution of the southern margin of the NCC and offer new perspectives on the early Precambrian evolution of the NCC.?1?The spatial and temporal evolution of the early Precambrian magmatic activities andrelated tectonic settings recorded by the Taihua Complex in Lushan areaThe Taihua Complex in Lushan area is mainly composed of tonalite-trondhjemite-granodiorite?TTG?gneiss and amphibolite.Zircon SIMS U-Pb dating results show four episodes of magmatic activities in the Late Mesoarchean-Early Neoarchean?2.88–2.87Ga;2.87–2.81 Ga;2.81–2.78 Ga and 2.78–2.72 Ga?and at least two stages of metamorphism?2.79–2.77 Ga and 1.94–1.92 Ga?.Late Mesoarchean amphibolites?2.88–2.87 Ga?and TTG gneisses?2.87–2.81 Ga?constitute the ancient core of Lushan area,while the Early Neoarchean amphibolites?2.81–2.78 Ga?and TTG gneisses?2.78–2.72 Ga?distribute as a peripheral belt of the Late Mesoarchean core.The amphibolites at 2.88–2.87 Ga?Amp-1?are enriched in LREE and LILE with negative Nb-Ta-Ti anomalies,indicating their affinities with arc magmatic rocks.The TTG gneisses at 2.87–2.81 Ga?TTG-1?have low MgO,Cr and Ni contents and positive whole-rock?Hf?t?and?Nd?t?values,and their protoliths were probably generated by partial melting of thickened lower crust.The amphibolites at 2.81–2.79 Ga?Amp-2?are characterized by flat REE patterns with negative Nb-Ta-Ti anomalies on the spidergram,which are interpreted as back-arc basin basalt formed in a back-arc basin.The TTG gneisses at 2.78–2.72 Ga are dominant TTG-2 with relatively low REE contents and notable positive Eu anomalies,and minor TTG-1 with high REE contents and weak Eu anomalies.The TTG-2 gneisses have positive whole-rock?Hf?t?and zircon?Hf?t?values,interpreted as the result of partial melting of juvenile thickened lower crust and the consequent plagioclase accumulation.The coeval TTG-1 gneisses have relatively high MgO,Cr and Ni contents,and show variable whole-rock?Hf?t?and?Nd?t?values,which were likely formed from partial melting of thickened lower crust with the participation of basaltic magmas.Therefore,the Taihua Complex in Lushan area reflects the Late Mesoarchean-Early Neoarchean convergent plate margin accomplished by the processes of subduction,arc-continent collision and post-collisional extension.?2?The spatial and temporal evolution of the early Precambrian magmatic activities andrelated tectonic settings recorded by the Dengfeng Complex in Songshan areaThe Dengfeng Complex in Songshan area mainly consists of Late Neoarchean TTG gneisses,metadiorites and amphibolites,exhibiting the features of a granite-greenstone terrane.The Sanhuangzhai amphibolite records the oldest magmatism?2658±4 Ma?in the region,which is enriched in LREE and LILE with negative Nb,Ta,Ti anomalies and positive initial?Hf?t?and?Nd?t?values,suggested to be the remnant of an arc-related juvenile mafic crust.The Shipaihe amphibolite?2492±7Ma?at the end of early Neoarchean is characterized by flat REE patterns,slightly negative Nb,Ta,Ti anomalies on the spidergram and positive initial?Hf?t?and?Nd?t?values,which is interpreted as large-degree partial melting of metasomatised mantle wedge in a back-arc setting.Zircon U-Pb dating shows two episodes of TTG magmatism?2.57–2.55 Ga and 2.55–2.50 Ga?within the Dengfeng Complex.All TTG gneisses have high SiO2and Na2O,but low Mg#,Cr,Ni,[Yb]N and Y with high[La/Yb]N and Sr/Y.The early TTG gneiss?2.57–2.55 Ga?in the eastern Songshan area has variable whole-rock Hf-Nd isotopes(?Hf?t?=-2.14–4.64;?Nd?t?=0.11–3.35;respectively),and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source.But the later TTG gneiss?2.55–2.50 Ga?in the western Songshan area has depleted whole-rock Hf-Nd isotopes(?Hf?t?=5.59–6.08;?Nd?t?=2.42–3.86;respectively),suggesting an overall juvenile source,and its protolith is interpreted as the partial melt of flat-subducting oceanic crust.The Shipaihe metadiorite is characterized by high Mg#,Cr,Ni,Sr,Ba,highly fractionated REE patterns and strong negative Nb,Ta and Ti anomalies,resembling to sanukitoid suite,which is interpreted as the partial melt of a mantle wedge metasomatised by subduction-derived fluids/melts.Integrated geochronology and geochemical data of the Dengfeng Complex in Songshan area reveal the Late Neoarchean crustal accretion process from subduction to arc-continental collision?accretionary orogeny?followed by post-collisional extension?back-arc basin?,implying an active continental margin arc environment in the southern segment of the TNCO.?3?The early Precambrian tectonic evolution in the southern segment of the TNCOThe spatial and temporal distribution of the early Precambrian magmatic activities in the southern segment of the TNCO shows a trend of being younger from the eastern and western parts to the central area.The oldest magmatic activities?Late Mesoarchean-Early Neoarchean?were recorded by the Taihua Complex in Lushan area in the east and the northwestern Huashan-Xiaoqinling area in the west.The Dengfeng Complex in Songshan area and the Taihua Complex in the northern Huashan-Xiaoqinling area record the Late Neoarchean magmatic activities,and the Taihua Complex in Xiong'er area and the southeastern Huashan-Xiaoqinling area record two episodes of the Paleoproterozoic magmatic activities.In the southern segment of the TNCO,there is a compressional evolution from subduction to arc-continental collision in the Late Mesoarchean-Early Neoarchean,and a tectonic transformation from compression to extension in the Early Paleoproterozoic,with abundant nearly coeval mafic and felsic magmatism.In the Late Paleoproterozoic,there are two episodes of metamorphism?1.97–1.91 Ga and 1.87–1.82 Ga?in the southern segment of the TNCO.The early metamorphism?1.97–1.91 Ga?is significantly stronger than the late one and would be related to a north-south collisional orogenic process,while the late one?1.87–1.82 Ga?was due to a west-east collisional orogenic process belonging to the final amalgamation between the Eastern and Western Blocks.?4?The early Precambrian crustal growth in the southern margin of the NCCBoth the Taihua Complex in Lushan area and the Dengfeng Complex in Songshan area show overall depleted whole-rock Hf-Nd and zircon Hf isotopes,suggesting that the Mesoarchean-Late Neoarchean is an important epoch of crustal growth in the southern margin of the North China Craton.The crustal growth is a lateral accretion in an active continental margin accomplished dominantly by subduction and arc-continent collision.However,the Taihua Complex in Xiong'er and Huashan-Xiaoqinling areas have relatively low and variable whole-rock Hf-Nd and zircon Hf isotopes,indicating that the Paleoproterozoic is an important epoch of crustal reworking with minor crustal growth in the southern margin of the NCC. |
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