| The lower crust flow of the southern margin of Tibetan plateau is an hot topic in the international geology community and its flow direction is still controversial although a number of researches has been taken. This dissertation presents geology geochronology, whole rock geochemistry, whole rock Sr-Nd-Pb isotope, and zircon Hf isotope of mineralized porphyries sampled from the southern Tibetan plateau. Combining with geophysical data. It is concluded in this dissertation that the lower crustal flow moves from south to north beneath the Tibetan plateau in Miocene. The diagenetic mineral sources of the mineralized porphyries are analyzed and also for their genetic mechanism and indications to the background. Based on zircon fission tracks of5smaples and apatite fission tracks of4samples from the Cho Oyu mountain in Tibet, the uplift rates of this mountain at different periods are proposed, which further constrains evolution history of the south Tibetan detachment system (STDS) in Miocene. This study reveals the effect of thermo-upwelling extension response to lower crustal flow process in Miocene on the southern margin in Tibet.A large number of geophysical studies indicated that a low-velocity and low-resistivity layer widely develop in the southern Tibet plateau, the distribution of this layer is consistent with that of both high heat flow in the lower crust and the scope of the seismogenic layers, indicating that the melt layer within crust is widely developed in the southern part of Tibet plateau. The moho layer of the southern Tibet plateau and the Ganges exhibit mirrors with their topography. The lower crust of the Ganges basin is very thin, and the upper crust is thicker. On the contrary, the crust beneath the Tibetan plateau is very thick, especially the lower crust. This difference is consistent with a cycle of substance between the Tibetan plateau and the Ganges basin:the surface material of the Tibetan plateau are transported to the Ganges basin by erosion exposing the mid-lower crust, and at the meanwhile, the crust of the Ganges basin thickens. On the other, lower crust material beneath the Ganges basin is melted and moved towards the Tibetan plateau as ductile flows. This transportation of masses between the Tibetan plateau and the Ganges basin yield thinning lower crustal beneath the Ganges basin and thickening lower crust beneath the Tibetan plateau. The north-south trending normal faults in the southern Tibet determine the occurrence of the surface hot springs, whose locations show excellent agreement with the wide range of low-velocity and the low-resistivity layers, indicating that the partial melting portions of the crust layer form different flow channels that control the distribution of superficial earthquakes and hot springs.Granulites facie or granulize xenoliths from the lower crust mainly occur at the core of the Himalaya mostly occur in the form of lenticular, lumpish, bedded units within gneiss and quartzite. These facies and xenoliths feature strong strain zones and have tectonic features like mylonitic foliations and tectonic foliations. The minerals of granulize, including orthopyroxene, clinopyroxene, plagioclase and amphibole, show a strong plastic deformation with micro-structural characteristics, with a solid-state rheological characteristic. The age of high-pressure granulize is17Ma, indicating that its formation was synchronic to the large-scale thrusting, extensional detachment, and the emplacement of leucogranite within the Himalaya. Strong plastic flow, large strain, and localization strain occur within the strong strain zone, forming a smooth layer stretch ductile shear zones, the penetrative stretching lineation, fine-grained mineral particles, and exhibiting steady flow characteristics. The metamorphism, magmatism in Himalayan are same tectonic process, which are related with the crustal extension and ductile flow. This dissertation focused on the Chongjiang and Zhunuo mining area located at the southern margin of the Gangdese. High angle normal faults are the major tectonic features within the mining areas, which can be divided into two groups:the earlier E-W normal faults usually occur alongr vallies and form typical grabens, such as those occur at Zhelapuqu and Kangshapuqu; the more recent faults are S-N and NNW in distribution, such as those forming the Meiquzangbu and Mupuqu graben. Among the graben in this area, the S-N and NNW trending graben are larger in scale. The interaction segments of the E-W and S-N faults are ore-forming areas. The ore-bearing porphyry in this area usually occurs as irregular-shaped dikes on the surface, which are gray to gray-white in color exhibiting massive structure and porphyritic-like texture. The ore bodies mostly occur in the adamellite biotite granite porphyry that are located at the border of the porphyry and ambient rocks. The altered minerals in this area include many types such as beresitization, carbonatization, and kaolinization, in which beresitization and silicification are closely related with mineralization.The zircon U-Pb age test was conducted for both the two samples got from the Chongjiang mining area and three samples from the Zhunuo mining area. The test results revealed magmatic crystallization occurred between14.8-15.3Ma in Miocene. The zircons are colorless to lightgray with a good prismatic crystals. The zircon particles range in size from50to400μm. The length and width ratios are1.5:1to4:1. CL image clearly show the rhythm vibration anneals within the zircons, which are relatively narrow. This reveals the zircons were magmatically derived and then crystallized slowly in a low-temperature environment. The host rocks belong to the granodiorite-granite suite in composition, with SiO2of63.37-74.59%, A12O3of12.80-16.27%, CaO of0.63-4.13%, K2O of0.03%-5.34%, Na2O of2.84-5.98%, and with high Sr/Y of11-166, high (La/Yb)N value of22-83, and Eu/Eu*value of0.6-1.0, indicating the host rocks belongs to the Adak granite. Meanwhile, The Adakitic granite porphyry in Gangdese contains a large quantity of Mg#(31.70-57.91) with an average value of44.98, indicating that a higher Mg#value reveals contamination from the mantle materials. Moreover, The Gangdese ore-bearing Adak porphyry is rich in large ion incompatible elements such as Rb, K, U, Th and Pb, but poor in high field-strength elements such as Nb, Ta and Ti. Moreover, it is also depleted in heavy rare earth element Yb and has a deficient of Eu. These characteristics indicate residual garnets remained in the magma source. The zircon Hf values of Chongjiang samples are obviously different from that of Zhunuo samples:Chongjiang rock zircon ε Hf (t) value ranges from+0.5to+5.1, Zhunuo rock zircon ε Hf (t) value changes from-6.9to-0.1, indicating they may have come from partial melting of the Indian lower crust. Our results are well consistent with that of Xu et al.(2010). In addition, The Sr-Nd isotopes reflect that the magmatic source of the rocks were a mixture of the Yarlung Tsangpo ophiolite and Ihasa block, Pb isotopes indicate that the magmatic source elements include a mixture of the Yarlung Tsangpo and Himylaya crust. Through Sr-Nd-Pb isotopes tracing, it can be concluded that material of Gangdese ore-bearing Adakite porphyry originated from the Indian continental crust, the Ihasa block, and the Yarlung Zangbo River ophiolite.We use fission track experiments to study the geological activity for the South Tibetan Detachment System (STDS) based on5zircon samples and4apatite samples taken from the Cho Oyu in the Tibetan Himalayan. The age range for the zircons is11.2-17.1Ma and that of the apatites is12.4-14.3Ma. After integrating the age-closure temperatures measured for these samples, we find the crustal cooling rate about the Lhotse detachment fault along the Cho Oyu was increased gradually during the Miocene:1) during17.1-15.2Ma the crustal cooling rate was approximately37.8℃/Ma;2) during15.2-13.5Ma the average crustal cooling rate was-82.4℃/Ma, and the tectonic activity was the most intense-14.3Ma;3) during13.5-12.4Ma the crust cooling rate during that time was up to100℃/Ma. The tectonic history of the cooling crust suggests a rapid uplift in Miocene of the southern margin of the Tibetan plateau, which responses to lower crust flow process.Therefore, based on geochronology and geochemistry of the ore-bearing porphyry in the southern margin of Gangdese, it is suggest that the adakitic porphyry is result of partial melting of the thickened crust beneath the Gangdese, and the sources include the Lhasa continental crust, Indian continental crust and the Yarlung Zangbo River ophiolite. These materials, especially Indian continental crust, concentrated at the Gangdese is probably caused by the lower crustal flow from south to north in the southern Tibetan Plateau. |
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