Study On Fission Track Method And The Thermochronology Of The Yunkaidashan

This dissertation is composed of the study on fission track method and application fission track method, combined with 40Ar/39Ar(K-Ar) method to study the Indosinian tectonic events and Meso-Cenozoic uplifting and exhumation of the Yunkaidashan.Firstly, the present status and developing tendencies about the principle, analytical methods, and experimental techniques of fission track dating and fission track annealing kinetics are introduced. Then combined fission track method with some experiences and knowledges learned by the author in the long-term work in fission track analysis, the existing problems and possibly solving approaches about analytical methods and experimental techniques of fission track dating are discussed. Next, the application of fission track thermochronology in geology are briefly introduced.Secondly, a case on the thermochronology of the Yunkaidashan is studied. The Yunkaidashan, situated in the southern edge of South China block, closely adjacent to Indochina block and westward linked to Qinzhou-Fangcheng trough, is the ideal region which is applicated to study the Meso-Cenozoic tectonic evolution of Indochina and South China continental blocks. This paper is combined fission track with 40Ar/39Ar thermochronology to study the Indosinian deformational events and the Meso-Cenozoic uplifting and exhumation of the Yunkaidashan, in the meanwhile, to offer new constraints and elicitations for further understanding the united dynamics mechanism of deformation in the Indochina and South China blocks, and Meso-Cenozoic geotectonic evolution of South China block.The representative samples of various-type rocks (mylonite, migmatite and granite) in the Yunkaidashan were selected to study fission track thermochronology, and four representative mylonite samples from them were selected to study 40Ar/39Ar isotopic dating. And the measured fission track and 40Ar/39Ar data are combined with available 40Ar/39Ar (K-Ar) ages, the main conclusions are obtained as follows:1. The principal zircon fission track ages range from 97.4Ma to 133.0Ma, and their main peak age is 125 Ma±, another peak age is 105 Ma±, indicating that there exists an uplifting and cooling event of the Yunkaidashan in early Cretaceous, and apatite fission track ages range from 43.5Ma to 68.4Ma, indicating that there existsanother uplifting and cooling event in the Paleogene. Average lengths of apatite confined fission tracks range from 12.60 μ m to 14.36 μ m, standard errors of most samples are more than 2 μ m, and distributions of apatite fission track lengths are broad and unimodal. Combined with geologic background of the Yunkaidashan , fission track ages are considered as cooling age of uplifting and exhumation of the Yunkaidashan.2. Grain ages and track length distributions of apatite samples were selected, with the help of AFTsolve software, temperature-time evolvement curves and modeling fission track lengths are obtained. Peak value of modeling track lengths range from 12 μm to 15 μm, which show rather broad and unimodal distributions, and the modeling track lengths distributions are basically identical to the measured data.3. Assuming that the closure temperatures of zircon and apatite fission track are 200 ℃ and 120℃, respectively, the average surface temperature of the Yunkaidashan is considered to be 20℃, and paleogeothermal gradient to be 35℃/km, zircon and apatite (named "the mineral pair") fission track ages are used to calculate the cooling and uplifting rate, and it can be elicited that from late Mesozoic to the Paleogene, the average cooling rate of rocks is Vc≈1.33℃/Ma, and the average uplifting rate of the Yunkaidashan is Vu≈0.04mm/a, and its uplifting amplitude is over 2km; since the Paleogene, the average cooling rate of rocks is Vc≈1.82℃/Ma, and the average uplifting rate of the Yunkaidashan is Vu≈0.05mm/a; since late Mesozoic, uplifting amplitude of the Yunkaidashan is more than 5km, and the exhumation and unroofing thickness is over 4km, indicating that the Yunkaidashan have undergone rather intensive uplifting and exhumation process since late Mesozoic, and the uplifting of the Yunkaidashan since the Paleogene may be greater than that from late Mesozoic to the Paleogene. And during late Mesozoic, the northwestern and southeastern Yunkaidashan have earlier initiated time of uplifting or higher paleographic pattern than that of northeastern and southwestern Yunkaidashan.4. The characteristics of all four 40Ar -39Ar plateau date for biotite manifest that they do not undergo apparently thermal interference, and show considerably identical character. 40Ar/39Ar ages are restricted between 209Ma and 226Ma, indicating that these ages should be a tectono-thermal event age, and represent the age of deformational event in Indosinian epoch. The four 40Ar/39Ar ages combined with partial available 40Ar/39Ar (or K-Ar) ages indicates that strong tectonic-metamorphic-magmatic events of the Yunkaidashan occurs in Indosinian epoch.5. Zircon and apatite fission track thcrmochronology are combined with 40Ar/39Ar thermochronology of the Yunkaidashan, it can be elicited T-t cooling curve of the Yunkaidashan. From late Triassic to early Cretaceous, average cooling rate of samples is about 1.0℃/Ma, from early Cretaceous to the Paleogene, average cooling rate of samples is about 1.33℃/Ma, and since the Paleogene, average cooling rate of samples is about 1.82℃/Ma, all of these show average cooling rate of rocks becomes great step by step.6. The accelerated uplifting in the Yunkaidashan since 25-30Ma modeling from grain ages and track length distributions of apatite samples may be related to the post-effect of the Indian plate subduction and collision with Europe-Asia plate or lateral extrusion of the Indochina block along Ailaoshan-Red River shear system in the Cenozoic.To sum up, the fission track and 40Ar/39Ar thermochronology offers good data for constraining the pattern of tectonic landform, the history of uplift and cooling, and further understanding the uplifting and exhumation process and the mechanism of uplift process in tectono-magmatic belt of South China block during the Meso-Cenozoic.