Apatite Fission Track Thermochronology of Granite from the Xiazhuang Uranium Ore Field,South China:Implications for Exhumation History and Ore Preservation

Xiazhuang uranium ore field,located in the southern part of the Nanling Metallogenic Belt,is considered one of the largest granite-related U regions in South China.In this paper,we contribute new apatite fission track data and thermal history modeling to constrain the exhumation history and evaluate preservation potential of the Xiazhuang Uranium ore field.Nine Triassic outcrop granite samples collected from different locations of Xiazhuang Uranium ore field yield AFT ages ranging from 43 to 24 Ma with similar mean confined fission track lengths ranging from 11.8±2.0 to 12.9±1.9μm and Dpar values between 1.01 and 1.51μm.The robustness time-temperature reconstructions of samples from the hanging wall of Huangpi fault show that the Xiazhuang Uranium ore field experienced a time of monotonous and slow cooling starting from middle Paleocene to middle Miocene(~60-10 Ma),followed by relatively rapid exhumation in the late Miocene(~10-5 Ma)and nearly thermal stability in the Pliocene-Quaternary(~5-0 Ma).The amount of exhumation after U mineralization since the Middle Paleogene was estimated as~4.3±1.8 km according to the integrated thermal history model.Previous studies indicate that the ore-forming ages of U deposits in the Xiazhuang ore field are mainly before Middle Paleocene and the mineralization depths are more than 4.4±1.2 km.Therefore,the exhumation history since middle Paleocene plays important roles in the preservation of the Xiazhuang Uranium ore field.

Apatite Fission Track Evidence of Uplift Cooling in the Qiangtang Basin and Constraints on the Tibetan Plateau Uplift

The Qiangtang basin is located in the central Tibetan Plateau. This basin has an important structural position, and further study of its tectonic and thermal histories has great significance for understanding the evolution of the Tibetan Plateau and the hydrocarbon potential of marine carbonates in the basin. This study focuses on low temperature thermochronology and in particular conducted apatite fission track analysis. Under constraints provided by the geological background, the thermal history in different tectonic units is characterized by the degree of annealing of samples, and the timing of major （uplift-erosion related） cooling episodes is inferred. The cooling history in the Qiangtang basin can be divided into two distinct episodes. The first stage is mainly from the late Early Cretaceous to the Late Cretaceous （69.8 Ma to 108.7 Ma）, while the second is mainly from the Middle- Late Eocene to the late Miocene （10.3 Ma to 44.4 Ma）. The first cooling episode records the uplift of strata in the central Qiangtang basin caused by continued convergent extrusion after the Bangong- Nujiang ocean closed. The second episode can be further divided into three periods, which are respectively 10.3 Ma, 22.6-26.1 Ma and 30.8-44.4 Ma. The late Oligocene-early Miocene （22.6-26.1 Ma） is the main cooling period. The distribution and times of the earlier uplift-related cooling show that the effect of extrusion after the collision between Eurasian plate and India plate obviously influenced the Qiangtang basin at 44.4 Ma. The Qiangtang basin underwent compression and started to be uplifted from the middle-late Eocene to the early Oligocene （45.0-30.8 Ma）. Subsequently, a large-scale and intensive uplift process occurred during the late Oligocene to early Miocene （26.1-22.6 Ma） and the basin continued to undergo compression and uplift up to the late Miocene （10.3 Ma）. Thus, uplift-erosion in the Qiangtang basin was intensive from 44.5 Ma to about 10 Ma. The timing of cooling in the second episode shows that the uplift of the Qiangtang basin was caused by the strong compression after the collision of the Indian plate and Eurasian plate. On the whole, the new apatite fission-track data from the Qiangtang basin show that the Tibetan Plateau started to extrude and uplift during 45-30.8 Ma. The main period of uplift and formation of the Tibetan Plateau took place about 22.6-26.1 Ma, and uplift and extrusion continued until the late Miocene （10.3 Ma）.

Late Mesozoic-Cenozoic Exhumation of the Northern Hexi Corridor：Constrained by Apatite Fission Track Ages of the Longshoushan

The apatite fission track（AFT） ages and thermal modeling of the Longshoushan and deformation along the northern Hexi Corridor on the northern side of the Qinghai-Tibetan Plateau show that the Longshoushan along the northern corridor had experienced important multi-stage exhumations during the Late Mesozoic and Cenozoic. The AFT ages of 7 samples range from 31.9 Ma to 111.8 Ma.Thermal modeling of the AFT ages of the samples shows that the Longshoushan experienced significant exhumation during the Late Cretaceous to the Early Cenozoic（-130-25 Ma）. The Late Cretaceous exhumation of the Longshoushan may have resulted from the continuous compression between the Lhasa and Qiangtang blocks and the flat slab subduction of the Neo-Tethys oceanic plate, which affected wide regions across the Qinghai-Tibetan Plateau. During the Early Cenozoic, the Longshoushan still experienced exhumation, but this process was caused by the Indian-Eurasian collision. Since this time,the Longshoushan was in a stable stage for approximately 20 Ma and experienced erosion. Since -5 Ma,obvious tectonic deformation occurred along the entire northern Hexi Corridor, which has also been reported from the peripheral regions of the Qinghai-Tibetan Plateau, especially in the Qilianshan and northeastern margin of the plateau. The AFT ages and the Late Cenozoic deformation of the northern Hexi Corridor all indicate that the present northern boundary of the Qinghai-Tibetan Plateau is situated along the northern Hexi Corridor.

The Exhumation History of North Qaidam Thrust Belt Constrained by Apatite Fission Track Thermochronology:Implication for the Evolution of the Tibetan Plateau

Determining the spatio-temporal distribution of the deformation tied to the India-Eurasian convergence and the impact of pre-existing weaknesses on the Cenozoic crustal deformation is significant for understanding how the convergence between India and Eurasia contributed to the development of the Tibetan Plateau. The exhumation history of the northeastern Tibetan Plateau was addressed in this research using a new apatite fission track （AFT） study in the North Qaidam thrust belt （NQTB）. Three granite samples collected from the Qaidam Shan pluton in the north tied to the Qaidam Shan thrust, with AFT ages clustering in the Eocene to Miocene. The other thirteen samples obtained from the Luliang Shan and Yuka plutons in the south related to the Luliang Shan thrust and they have showed predominantly the Cretaceous AFT ages. Related thermal history modeling based on grain ages and track lengths indicates rapid cooling events during the Eocene-early Oligocene and since late Miocene within the Qaidam Shan, in contrast to those in the Cretaceous and since the Oligocene-Miocene in the Luliang Shan and Yuka region. The results, combined with published the Cretaceous thermochronological ages in the Qaidam Shan region, suggest that the NQTB had undergo rapid exhumation during the accretions along the southern Asian Andean-type margin prior to the India-Eurasian collision. The Cenozoic deformation initially took place in the North Qaidam thrust belt by the Eocene, which is consistent with the recent claim that the deformation of the northeastern Tibetan Plateau initiated in the Eocene as a response to continental collision between India and Eurasia. The immediate deformation responding to the collision is tentatively attributed to the preexisting weaknesses of the lithosphere, and therefore the deformation of the northeastern Tibetan Plateau should be regarded as a boundary-condition-dependent process.

Fission Track Thermochronology Evidence for the Cretaceous and Paleogene Tectonic Event of Nyainrong Microcontinent, Tibet

Fission track dating was applied to analyze the 20 samples from Nyainrong microcontinent, and we obtained 20 apatite and 15 zircon fission track ages. The results show single population grain ages with a single mean age and associated central ages mainly ranging from 108±7Ma to 35±4Ma.Their mean track lengths are 12.2-13.9 μm with a single peak. Zircon fission track age range from 78±3 Ma to 117±4 Ma. The results represented the two tectonic uplift events in the study area, namely the Cretaceous and Paleogene periods. According to thermal history modeling results, uplifting rates of two tectonic events is 0.31-0.1 mm/a and 0.07-0.04 mm/a respectively. Combined with field condition and study results, it is suggested that the Cretaceous tectonic uplift event was related to the closure ocean basin caused by Qaingtang-Lhasa collision, and the Paleogene tectonic uplift event was related to the south to thrust system caused by Indo-Asian collision.

Apatite fission track thermochronology in the Kuluketage and Aksu areas,NW China:Implication for tectonic evolution of the northern Tarim

Tarim Precambrian bedrocks are well exposed in the Kuluketage and Aksu areas, where twenty four samples were taken to reveal the denudation history of the northern Tarim Craton. Apatite fission track dating and thermal history modeling suggest that the northern Tarim experienced multi-stage cooling events which were assumed to be associated with the distant effects of the Cimmerian orogeny and India-Eurasia collision in the past. But the first episode of exhumation in the northern Tarim, occurring in the mid-Permian to Triassic, is here suggested to be induced by docking of the Tarim Craton and final amalgamation of the Central Asian Orogenic Belt. The cooling event at ca. 170 Ma may be triggered by the Qiangtang-Eurasia collision. Widespread Cretaceous exhumation could be linked with docking of the Lhasa terrane in the late Jurassic. Cenozoic reheating and recooling likely occurred because of the northpropagating stress, however, this has not affected the northern Tarim much because the Tarim is characterized by rigid block-like motion.

Meso-Cenozoic Tectonic Events Recorded by Apatite Fission Track in the Northern Longmen-Micang Mountains Region

There is a cross-cutting relationship between the E-W trending structures and the NE- trending structures in the northern Longmen-Micang Mountains region, which reflects possible regional tectonic transition and migration. Apatite fission track （AFT） analyses of 15 samples collected from this area yield apparent ages varying from 30.3±4.2 Ma to 111.7±9.0 Ma and confined-track-lengths ranging from 10.6±0.3 pm to 12.4±0.1 μm. Four specific groups were identified on the basis of the Track Age Spectrum Calculation （TASC） patterns, i.e., 143-112 Ma, 93.6-88 Ma, 42-40 Ma and -25.6 Ma. These age groups correspond to the spatial distributions of datasets and may represent four tectonic events. Together with the regional deformation patterns, the four age groups are interpreted to indicate tectonic superposition, transition and migration during the Meso-Cenozoic with the following possible order： （1） the Micang Mountains belt was dominated by the E-W trending structure during 143-112 Ma; （2） the contraction of the Longmen Mountains belt from the NW to the SE during 93.6-88 Ma led to the superposition of the NE-trending structures over the E-W trendinding structures; （3） dextral strike-slip shear dominated the Longmen Mountains belt at 42-40 Ma; （4） westward migration of the active tectonic belt occurred from 93.6-25.6 Ma in a break-back sequence in the northern Longmen Mountains belt. The Late Cenozoic tectonics in the northern Longmen Mountains belt are characterized by the dextral strike-slip shear and the occurrence of westward break-back sequence of deformations. As a result, north-south differences in deformations along the Longmen Mountains belt were intensified since the Miocene time and strains were mainly accumulated in the hinterland of the Longmen Mountains instead of being propagated to the foreland basin.

Fission Track Dating of Authigenic Quartz in Red Weathering Crusts of Carbonate Rocks in Guizhou Province

The Cenozoic evolution history of Guizhou Province, which is located on the southeastern flank of the Qinghai-Tibet Plateau, is unclear because of the lack of sedimentation records. The red weathering crusts widespread on the Yunnan-Guizhou Plateau may bear critical information about their evolution history. This work firstly determined the ages of four red weathering crusts in eastern, central and northern Guizhou. The material used in fission track dating is well-crystallized quartz occurring in many in-situ weathering crusts of carbonate rocks. The results showed that the fission track ages of quartz vary over a wide range from 1 to 25 Ma in the four profiles, significantly younger than the ages of the Triassic and Cambrian parent rocks. In combination with the evolution history of the regional geology during the period from 25 to 1 Ma, the ages of quartz can exclude the possibility that the origin of quartz has nothing to do with primary clastic minerals in parent rocks, authigenesis during diagenesis and hydrothermal precipitation or roplacement by volcanic activities. It is deduced that the well-crystallized quartz was precipitated from Si-rich weathering fluids during the weathering process of carbonate rocks. The recorded ages of quartz from the four profiles are consistent with the episodes of the planation surfaces on the Qinghai-Tibet Plateau, the forming stages of red soil in the tropics of South China, the tectonically stable periods in Guizhou, and the ages of weathering in other parts of the world during the Cenozoic era. That is to say, the ages of authigenic quartz dated by the fission track method are well feasible and credible.

Latest Triassic to Early Jurassic Thrusting and Exhumation in the Southern Ordos Basin, North China: Evidence from LA-ICP-MS-based Apatite Fission Track Thermochronology

The contractional structures in the southern Ordos Basin recorded critical evidence for the interaction between Ordos Basin and Qinling Orogenic Collage. In this study, we performed apatite fission track （AFT） thermochronology to unravel the timing of thrusting and exhumation for the Laolongshan-Shengrenqiao Fault （LSF） in the southern Ordos Basin. The AFT ages from opposite sides of the LSF reveal a significant latest Triassic to Early Jurassic time-temperature discontinuity across this structure. Thermal modeling reveals at the latest Triassic to Early Jurassic, a ~50~C difference in temperature between opposite sides of the LSF currently exposed at the surface. This discontinuity is best interpreted by an episode of thrusting and exhumation of the LSF with -1.7 km of net vertical displacement during the latest Triassic to Early Jurassic. These results, when combined with earlier thermochronological studies, stratigraphic contact relationship and tectono-sedimentary evolution, suggest that the southern Ordos Basin experienced coeval intense tectonic contraction and developed a north-vergent fold-and-thrust belt. Moreover, the southern Ordos Basin experienced a multi-stage differential exhumation during Mesozoic, including the latest Triassic to Early Jurassic and Late Jurassic to earliest Cretaceous thrust-driven exhumation as well as the Late Cretaceous overall exhumation. Specifically, the two thrust-driven exhumation events were related to tectonic stress propagation derived from the latest Triassic to Early Jurassic continued compression from Qinling Orogenic Collage and the Late Jurassic to earliest Cretaceous intracontinental orogeny of QinUng Orogenic Collage, respectively. By contrast, the Late Cretaceous overall exhumation event was related to the collision of an exotic terrain with the eastern margin of continental China at -100 Ma.

Fission Track Geochronology of Xiaonanchuan Pluton and the Morphotectonic Evolution of Eastern Kunlun since Late Miocene

Apatite fission track （AFT） thermochronology of seven samples from the Xiaonanchuan （小南川） pluton in the Kunlun （昆仑） pass area was carried out, for the purpose of determining the timing of cooling and the relation between the exhumation and the morphotectonic processes. The AFT ages yield low denudation rates of 0. 020--0. 035 mm/a during the late Miocene, which correspond to a stable geomorphic and weak tectonic uplifting environment. The low denudation rates can be considered as the approximate tectonic uplifting rates. The AFT geochronology shows puroxysmully rapid cooling since the Pliocene and an apparent material unroofing of more than 3 km in the Xiaonanchuan area. This was not the result of simple denudation. The rapid cooling was coupled with the intensive orogeny since the Pliocene, which was driven by tectonic uplifting. The accelerated relief building was accompanied by a series of faulting, which caused the basin and the valley formation and sinking. The space pattern of the AFT ages also shows differential uplifting, which decreases northwardly. This trend is supported by the regional AFT data, which indicate that the exhumation decreases northwardly in eastern Kunlun. This trend also exists in cast-west orientation from the western Kunlun range to the eastern. The uplif- ting trend is also supported by gcomorphic characteristics including the elevation and the relief differences well as the distribution of the Late Cenozoic volcanism.

Late Mesozoic Thermotectonic Evolution of the Jueluotage Range,Eastern Xinjiang,Northwest China:Evidence from Apatite Fission Track Data

Although many authors have emphasized the Cenozoic history of deformation, exhumation and cooling in the Tiaushan area related to the India-Asia collision, very little is known about the Mesozoic history of compression and uplift within the Tianshan. In order to obtain information about the Mesozoic exhumation history and processes of cooling in eastern Tianshan, fission track methods on apatite were used. Sampling was made in the Jueluotage Range. Three samples （Z001-Z003） were taken from granite in borehole ZK6301 of Yandong pluton; the ages range from 97.0 to 87.6 Ma that are much younger than the pluton age which was dated by U-Pb zircon at 334±2 Ma. Two samples in northern piedmont of the Jueluotage Range were collected from Jurassic strata in Dikaner （DK001） and Dananhu （D001） whose ages are 91.5 and 93.4 Ma respectively. The average apparent exhumation rate is 0.039 nun/a calculated by extrapolation on the basis of Yandong samples, indicating an extremely slow exhumation in the Jueluotage Range since the Late Cretaceous. Two Jurassic samples reached the maximum depths after deposition and experienced the maximum temperatures of ca. 105 and 108℃ until the late Early Cretaceous before a period of cooling and exhumation occurred at 114 and 106 Ma.

Late Cretaceous-Cenozoic Multi-Stage Denudation at the Western Ordos Block： Constraints by the Apatite Fission Track Dating on the Langshan

The apatite fission track dating of samples from the Dabashan （i.e., the Langshan in the northeastern Alxa Block） by the laser ablation method and their thermal history modeling of AFT ages are conducted in this study. The obtained results and lines of geological evidence in the study region indicate that the Langshan has experienced complicated tectonic-thermal events during the the Late Cretaceous-Cenozoic. Firstly, it experienced a tectonic-thermal event in the Late Cretaceous （-90-70 Ma）. The event had little relation with the oblique subduction of the Izanagi Plate along the eastern Eurasian Plate, but was related to the Neo-Tethys subduction and compression between the Lhasa Block and Qiangtang Block. Secondly, it underwent the dextral slip faulting in the Eocene （-50-45 Ma）. The strike slip fault may develop in the same tectonic setting as sinistral slip faults in southern Mongolia and thrusts in West Qinling to the southwest Ordos Block in the same period, which is the remote far-field response to the India-Eurasia collision. Thirdly, the tectonic thermal event existed in the late Cenozoic （since -10 Ma）, thermal modeling shows that several samples began their denudation from upper region of partial annealing zone （PAZ）, and the denudation may have a great relationship with the growth of Qinghai-Tibetan Plateau to the northeast. In addition, the AFT ages of Langshan indicate that the main body of the Langshan may be an upper part of fossil PAZ of the Late Cretaceous （-70 Ma）. The fossil PAZ were destroyed and deformed by tectonic events repeatedly in the Cenozoic along with the denudation.

Mesozoic–Cenozoic Tectonic Evolution and Uplift in Pamir:Application of Fission Track Thermochronology

The Pamir Plateau can be divided into three secondary tectonic units from north to south:the North,the Middle and the South Pamir Blocks.The North Pamir Block belonged to the southern margin of Tarim-Karakum,thermochronological study of the Pamir structural intersection indicates that accretion of the Middle Pamir Block to the Eurasian Continental Margin and its subduction and collision with the North Pamir Block occurred in the Middle–Late Jurassic.Due to the Neo-Tethys closure in the Early Cretaceous,the South Pamir Block began to collide with the accretion(the Middle Pamir Block)of the Eurasian Continental Margin.Affected by the collision and continuous convergence between the Indian Plate and the Eurasian Plate since the Cenozoic,Pamir is in a multi-stage differential uplift process.During 56.1–48.5 Ma,North Pamir took the lead in uplifting,that is,the first rapid uplift in the Pamir region began there.The continuous compression and contraction of the Indian and Eurasian plates during 22.0–15.1 Ma forced the Pamir tectonic syntaxis to begin its overall uplift,i.e.Pamir began to enter the second rapid uplift stage in the Early Oligocene,which lasted until the Middle Miocene.During 14.6–8.5Ma,South Pamir was in a rapid uplift stage,while North Pamir was in a relatively stable state,showing asymmetry of tectonic deformation in the Pamir region in space.Since 6.5 Ma,Pamir began to rapidly uplift again.

Apatite Fission Track Ages in the Duolong Ore District and the Uplift Time of the Qiangtang Terrrane, Tibet

Objective Fission track （FT） analysis has developed into one of the most useful techniques throughout the geologic community to reconstruct low-temperature thermal historyof rocks over geological time （Reiners et al., 2005）. The FT method is based on the accumulation of narrow damage trails （i.e., fission tracks） in uranium-rich mineral grains （e.g., apatite, zircon, titanite） and natural glasses, which form as a result of spontaneous nuclear fission decay of 238U in nature. Apatite Fission Track （AFT） has been used in many places in Tibet to study the Qinghai- Tibetan Plateanuplifl-exhumation history. However, few AFT studies have been reported in the Duolong ore district. The Duolong ore district is one of the most important ore districts in the Bangong Co-Nujiang metallogenic belt, Tibet （Lin Bin et al., 2017） and the uplift-exhumation of the Duolong ore district is closely related to the evolution of the Qiangtang Basin. Therefore, AFT of the Duolong ore district will provide important information about the uplift-exhumation history of the Duolong ore district and the Qiangtang Basin.

Cenozoic Magmatism and Tectonic Framework of Western Yunnan,China：Constrained from Geochemistry,Sr-Nd-Pb Isotopes and Fission Track Dating

Western Yunnan is composed of several extruded continental microblocks that were generated by the oblique collision between the Indian and Asian continents during the Cenozoic.In this study,the magmatic and tectonic frameworks of western Yunnan in the Cenozoic were analyzed based on geochemistry,Sr-Nd-Pb isotopes,and apatite/zircon fission track dating.Magmatism during the Cenozoic in western Yunnan was then divided into three distinctive episodes：alkali granite rocks produced from 55 to 46 Ma were derived from the anatexis of crustal materials;bimodal igneous rocks formed between 37 and 24 Ma were possibly derived from an EMII mantle with a contribution from continental materials;and intermediate-basic volcanic rocks produced in the Tengchong microblocks since-16 Ma are considered to be generated by the partial melting of the upper mantle that was induced by the pulling apart of the dextral Gaoligong strike-slip fault system.Moreover,fission track analysis of apatite and zircon indicates that the regional crustal uplift in western Yunnan possibly began at -34 Ma,with accelerated annealing occurring at -24 Ma,-13 Ma,and -4 Ma.During the past24 Ma,the average denudation rate was -0.32 mm/yr for the faulted block controlled by the Chongshan-Lancang River fault.However,crustal uplift has been relatively gentle in places lacking influence from strike-slip shear zones,with an average denudation rate of-0.2 mm/yr.Combined with strike-slip shear and block rotation in the Cenozoic,the tectonic evolution of western Yunnan since -45Ma can thus be divided into four stages occurring at 45-37 Ma,37-24 Ma,24-13 Ma,and 13-0 Ma.

Mesozoic and Cenozoic uplift and exhumation of the Bogda Mountain,NW China: Evidence from apatite fission track analysis

Apatite fission track （AFT） analysis on samples collected from a Paleozoic series is used to constrain the cooling history of the Bogda Mountain, northwest China. AFT ages range from 136.2 to 85.6 Ma and are younger than rock depositional ages and the mean confined track lengths （11.0 13.2 μm） mostly showing unimodal distribution are shorten, indicating significant track-annealing. Thermal histories modeling based on the distribution of fission-track lengths combined with the regional geological data show that two rapid cooling phases occurred in the latest Jurassic-early Cretaceous and the Oligocene-Miocene. Those new data together with previous published data show that the AFT ages become younger from the southwest to northeast in the western Bogda Mountain and its adjacent areas. The fission-track ages of the southwest area are relatively older （〉100 Ma）, recording the earlier rapid uplift phase during the late Jurassic-Cretaceous, while the ages in the north pied- mont of the Bogda Mountain （namely the northeast part） are younger （〈60 Ma）, mainly reflecting the later rapid uplift phase in the Oligocene-Miocene. The trend of younger AFT ages towards the northeast might be explained by post-Cretaceous large-scale crustal tilting towards the southwest. In the thrust fault-dominated northern limbs of the Bogda Mountain, AFT ages reveal a discontinuous pattern with age-jumps across the major fault zones, showing a possible strata tilting across each thrust faults due to the thrust ramps during the Cenozoic. The two rapid uplift stages might be related to the accretion and collision in the southern margin of the Asian continent during the late Jurassic and late Cenozoic, respectively.

Transient fluid flow in the Binbei district of the Songliao Basin, China Evidence from apatite fission track thermochronology

The Songliao Basin is famous for the Daqing Oilfield, the biggest in China. However, no economic hydrocarbon reservoir has been found in the northeastern Binbei district. Its thermal history, which is of great importance for hydrocarbon generation and migration, is studied with apatite fission track （AFT） thermochronology. Samples with depositional ages of the late Cretaceous （-108-73 Ma） are analyzed. The AFT ages of the samples from reservoir rock （depositional age 〉 76.1 Ma） fall between the late Cretaceous （724-5 Ma） and the early Eocene （414-3 Ma） period, indicating their total annealing after deposition. In contrast, two samples from the main seals of the Qingshankou （depositional age 〉 89.3 Ma） and the Nenjiang Formation （depositional age 〉 73.0 Ma） are not annealed or partially annealed （AFT ages of 974-9 Ma and 704-4 Ma, respectively）. Because the maximum burial temperature （〈90 ℃） evidenced by low vitrinite reflectance （Ro〈0.7） is not high enough to account for the AFT total annealing （110-120 ℃）, the transient thermal effect arising from the syntectonic fluid flow between the late Cretaceous and the early Eocene is proposed. Transient thermal effects from fluid flow explains the indicated temperature discrepancies between the AFT thermometer and the Ro thermometer because the transient thermal effect from the fluid flow with a temperature high enough （110-120 ℃） to anneal the AFT thermometer does not last long enough （104-105 yrs.） for an enhancement of the Ro （minimum 106- 107 yrs. under the same temperature）. This indicates that dating thermal effect from fluid flow might be a new means to reconstruct the tectonic history. It also answers why the samples from the main seals are not annealed because the seals will prohibit fluid flow and supply good thermal insulation. The large-scale fluid flow in the Binbei district calls for a new idea to direct the hydrocarbon exploration.

Exhumation and Preservation of the Jinchuan Ni-Cu-PGE Deposit under the East Longshou Mountain Thermal Evolution,Revealed by Apatite Fission Track Thermochronology

Uplift and exhumation are important factors affecting the preservation of deposits.The anatomy of uplift-cooling evolution and exhumation in the East Longshou Mountain is of significant research value in understanding changes in the Jinchuan Ni-Cu-PGE deposit since its formation.This study uses apatite fission track(AFT)thermochronology to reconstruct the thermal history of the East Longshou Mountain,including the Jinchuan mine,revealing the uplift and exhumation history of the East Longshou Mountain and elucidating the preservation status of the Jinchuan deposit.The AFT ages in the East Longshou Mountain are distributed from 62.3±3.0 Ma to 214.7±14 Ma,with significant differences in ages in distinct areas,the central and pooled ages being consistent within the margin of error.Inverse thermal history models reveal two rapid cooling events associated with exhumation from the Early Jurassic to the Early Cretaceous(200–100 Ma)and since the Miocene(15–0 Ma),the former attributable to the far-afield response to the closure of the PaleoTethys Ocean and plate assembly at the southern margin of Eurasia,the latter associated with the initial India-Eurasia plate collision.A slow cooling event from the Early Cretaceous to the Miocene(100–15 Ma)is thought to be related to the arid environment in northwest China since the Cretaceous.These cooling events have diverse responses and cooling rates in different blocks of the East Longshou Mountain:the southwest and centre of which are mainly cooled over 200–120 Ma and 120–0 Ma,with cooling rates of~0.25 and~0.33°C/Ma(~1.25 and~0.33°C/Ma in the centre);the Jinchuan mine primarily cooled over 160–100 Ma,100–15 Ma and 15–0 Ma,with cooling rates of~1.33,~0.25 and~2.00°C/Ma.These differentiated coolings imply that the uplift of the East Longshou Mountain before the Miocene(~15 Ma)was integral.Strong uplift then occurred in the vicinity of the mining area,which is a critical period for the uplift of the Jinchuan deposit to the surface,meaning that the Jinchuan deposit was exposed no earlier than the Miocene(~15 Ma).Based on mineralization depth information obtained by previous researchers,in conjunction with the calculation and simulation results of this study,it can be seen that the bulk of the Jinchuan intrusion may still be preserved at depth.

Fission Track Method to Date Fault Activity—As Exemplified by the Three Gorges Dam Area of the Yangtze River Valley, China

The Xiannushan and Jiuwanxi are the two major fracture belts adjacent to the Three Gorges Dam Site. Apatites are selected from fracture clay and fracture\|cliff rock in the fracture belts and examined with the fission track dating method. Our study shows that the Xiannushan and Jiuwanxi fracture belts were tectonically active at 0.60±0.04Ma and 0.29±0.04Ma ago, respectively. Their ages are close to those obtained by the U\|series method.

MINERALIZATION AGES OF GOLD-HYDROTHERMAL DEPOSITS IN NORTHERN ZONE OF EASTERN KUNLUN MOUNTAINS BASED ON FISSION TRACK ANALYSIS

The age of mineralization in a mining area is a primary factor in various researches related to ore\|forming process. It is that the uncertainty of mineralization ages of gold ore deposits in northern zone of eastern Kunlun Mountains, Qinghai Province, restrains to probe the relationship of the deposits to the regional tectonic evolution. This paper documents the fission track method used to determine the ages of gold ore deposits in eastern Kunlun Mountains and considers the implication for the origin of the deposits.Eastern Kunlun Mountains is the northern part of the Qinghai—Tibet Plateau and is of three deep\|seated fault belts in about EW extension. This work mainly includes three gold ore districts. All of them, in the north of Mid\|Kunlun fault belt, belong to northern part of eastern Kunlun Mountains. The Yanjingou district, with geographical coordinate 96°00’E and 36°10’N, is located 60 km north of Hongqigou district . Both of them are large, typical tectonoalteration gold deposits and were formed in similar geological setting. Hongshuihe ore district is located 50 km east of Yanjingou district and includes tectonoalteration and magmatic cryptoexplosive gold deposits. Outcroped strata are dominantly Jinshuikou Group metamorphic rocks of Lower Proterozoic erathem. The occurrence area of igneous rocks, especially granitoid, accounts for about 90% in first two districts and become less in Hongshuihe district. The gold deposits occur in NW\|striking fault belts. The Rb\|Sr isochron age and K\|Ar isotopic age of Moyite relevant to the gold mineralization are respectively 228 25Ma and 207 1Ma. Rb\|Sr dating of diorite porphyrite is 209 09Ma. Sericite selected from Yanjingou orebody has 252 9Ma K\|Ar age. The ore in Hongqigou district has 197Ma K\|Ar age and 210Ma model age of Pb isotope of galena.