K-ar, Rb-sr and Fission Track Geochronometry of the Bock's Brook Stock, Kluane Ranges, southwestern Yukon Territory
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The Bongor Basin in southern Chad is one of the Cretaceous–Paleogene rift basins developed on the Precambrian crystalline basement and has been confirmed as a petroliferous basin in the last decade. Less than 400 m of Cenozoic unconsolidated sediments are separated by an unconformity from an underlying section of Lower Cretaceous units, in turn separated by another unconformity from underlying Precambrian basement. In addition, there is a locally low-angle unconformity within the Cenozoic section. A synthesis of apatite fission-track analysis data in four wells from the basin reveals two cooling episodes from Late Cretaceous to early Paleocene (beginning between 75 and 60 Ma) and mid-Miocene, respectively. The results suggest that regionally synchronous cooling is a likely scenario. The first exhumation between 75 and 60 Ma affected the whole basin, and the magnitude of uplift and erosion was approximately 1100–1250 m across the whole basin. In contrast, the second exhumation during the Miocene affected mainly the northern part of the basin while the magnitude was weak and could not be detected in the southeast of the basin. Potential trapping structures, for example, fault blocks and synsedimentary anticlines, formed prior to and inverted anticlines as a result of the first cooling phase of exhumation (strong compressional inversion) and were available for hydrocarbon migration and accumulation during the main phase of hydrocarbon generation. The Miocene exhumation was less pronounced and had weak or no impact on the hydrocarbon generation and accumulation.
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Vitrinite reflectance
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Thermochronology
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Fission tracks are linear trails of intense radiation damage in the crystal structure of a mineral, produced by spontaneous fissioning of uranium-238 (238U) atoms. Detail information on the low-temperature thermal histories of rocks, below∼120 °C for tracks in apatite and below∼350 °C for zircon, can be provided by Fission-track (FT) analysis. The purpose of this article is to present apatite and zircon fission-track data, and U–Pb granite ages that provide information about the cooling histories of a rock which can be crucial in comprehending the exhumation episodes of the study area, in particular, and the region, in general. Granite samples were collected along the same vertical profile at different elevation, 178–944 m.a.s.l. These samples were used to determine Fission-Track and crystallization ages. HeFTy software was employed to interpret the cooling histories of the samples using forward and inverse models. The inverse model was an approach of reproducing the observed data, and it was carried out only for fission-track data from the apatite grains. And it was constructed after generating a number of forward models, where in each of these models the predicted apatite fission-track parameters were compared to the measured values. The apatite fission track (AFT) and zircon fission track (ZFT) data indicated expected age trends, i.e. the older ages at higher elevations and the younger ages at lower elevations. Similarly, the data shows that the apatite and zircon FT ages appear younger than the age of the rock crystallization. The U–Pb age in zircon consistently suggest the age of the granite is Late Triassic.
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Fission-track(FT) thermochronology is a low-temperature dating technique utilizing the damage property produced by uranium fission decay.As the advanced researches of annealing behavior,and laboratory annealing models,this technique,especially apatite fission-track,has become an important measure to constrain the thermal history of sedimentary basin.Among three main apatite fission-track annealing models proposed by Laslett et al.(1987),Crowley et al.(1991) and Ketcham et al.(1999),respectively,the last one is the most significant and widely-applied annealing model,which integrated the influence of time,temperature and chemical composition to various types of apatite,together using the c-axis projected track-length and Dpar.This apatite multi-kinetic fission-track annealing model by Ketcham et al.(1999) can be applied well to make thermal modeling,describing the temperature-time relation in the development of basin.Zircon FT,(U-Th)/He,apatite(U-Th)/He,Ro value,surface temperature,and stratigraphic age are all as constraints for fission-track thermal modeling,while the HeFTy(2009) is an excellent software for reconstruction of low-temperature thermal history.
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Thermochronology
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