2013 Vol. 24, No. 4
It is a puzzle to determine metallogenetic ages in the world. This article uses zircon fission track (FT) dating to probe the mineralizing epochs of Wulonggou (五龙沟) gold ore deposits in eastern Kunlun (昆仑) Mountains. Total of six zircon FT ages have been obtained and can be divided into groups of 235–216 and 197–181 Ma, as well as 162 and 124 Ma, revealing multiple epochs of metallogenetic processes took place in Wulonggou area, mainly first two age groups. The mineralizing ages become lower from northeast Yanjingou (岩金沟) to southwest Hongqigou (红旗沟). The second epoch of 197–181 Ma is first achieved by this work. It is shown that the FT ages consist with other isotopic data and reflect different regional thermotectonic events.
Apatite fission-track dating and thermal-history modeling were carried out on samples from the Dabashan (大巴山), a fold-thrust belt, northeast of the Sichuan (四川) Basin and east of the Tibetan Plateau. A first cooling event in the Late Cretaceous is followed by a prolonged period of thermal stability with exhumation rates of ≤0.025 mm/a, as determined from age vs. elevation relationships. The preservation of age vs. elevations relationships and the lack of distinct age changes across tectonic structures indicate that the Dabashan fold-thrust belt formed prior to the Late Cretaceous, consistent with the current view of Triassic–Early Cretaceous shortening. Relatively short mean track lengths (~12 μm) indicate that the samples remained in the partial annealing zone for a prolonged time. The knick points in the best-fitting temperature-time models suggest that the onset of late-stage accelerated cooling commenced at ≤11 Ma. Related exhumation rates are 0.3–0.2 mm/a assuming geothermal gradients of 20 and 30 ℃/km. We speculate that this late-stage event results from eastward growth of the Tibetan Plateau and overstepping of the Sichuan Basin, it is likely responsible for the youthful morphology of the Dabashan.
The western Qinling (秦岭) orogenic belt is one of the outermost ranges in the northeastern Tibetan Plateau. Its tectonic uplift history is therefore essential to insight on the evolution history of the plateau. However, the timing of deformation and uplift history is still poorly known. Fortunately, its Cenozoic orogenic history is recorded in an excellent synorogenic sedimentary sequence exposed in the Tianshui (天水) Basin, the northeastern foot of western Qinling. According to sedimentary-tectonic analysis of the Yaodian (尧店) and Lamashan (喇嘛山) sections based on the previous magnetostratigraphy studies, we speculated that two stages (occurred at 9.2–7.4 and ~3.6 Ma) of variation in depositional facies were attributed to the uplift and deformation of the western Qinling, and the modern structure geomorphic frame of the northeastern Tibet formed after 2.6 Ma. Furthermore, four stages of active processes along the western Qinling occurred at 49–41, 34–27, 25–19 and ~13 Ma, are deciphered from an integrated detrital apatite fission-track data of the Ganquan (甘泉), Yaodian main sections and seven small ones. The former two are represents the exhumation episodes triggered by tectonism and the others attributed to the volcanic signals.
The north-trending Liupan Shan (六盘山) is an important tectonic boundary between the Tibetan Plateau and the Ordos platform. The Late Cenozoic red earth deposits of the Liupan Shan record its tectonic history and environmental effects. In this article we report a new Late Cenozoic red earth section from an intermontane basin in the southern part of the Liupan Shan. Lithofacies analysis, paleomagnetic and fission-track chronologies, and paleocurrent analysis have been employed to identify the tectonic uplift events of the Liupan Shan. Based on the age constraints of mammal fossils, the paleomagnetic polarity zones of the Huating (华亭) Section can be approximately correlated with the standard polarity zones that lie between C3An.2n and C5n.1n of the Geomagnetic Polarity Timescale; the bottom age of this section is approximately 10 Ma. Based on this and the previous studies, we infer that a tectonic event commenced in the southern Liupan Shan in this interval between 8.3 and 8.7 Ma, accompanied by a remarkable increase in sediment accumulation rate. Field observations, fission-track dating, determinations of grainsize frequency distributions and the vertebrate fossils found there suggest that the red earth deposits were reworked by water and mainly transported by fluvial-alluvial processes from the adjacent area.
Alkali-rich dykes of the Late Permian in Ulungur (乌伦古) area are representative products of granitic magmatism in the evolution of the Paleozoic orogenic belt in the East Junggar (准噶尔), North Xinjiang (新疆). We selected two representative samples for geochemical analysis (major and trace elements), and twenty-two zircon grains for zircon dating. Isoplot (ver3.0) was used to calculate isotopic age and make concordia diagrams. This study shows that the trachy porphyry dykes, featuring low concentration of Al2O3, CaO, MgO and high alkali contents, are metaluminous alkaline rock and belong to A-type granitoids. The dykes have low concentration of the REE and incompatible elements, and the REE patterns show clear negative Eu anomalies (δEu=0.74–0.58), enriched LREE(LREE/HREE>4, (La/Yb)N=5.97–4.63) and undifferentiated HREEs. Similar normalized REE and incompatible element patterns are also showed in the dykes from Yemaquan (野马泉) and granites from Ulungur, suggesting that they are possibly originated from the same source and formed in the same tectonic environment, but the trachy porphyry dykes are more evolved. The age of the trachy porphyry dykes is 255.3±2.4 Ma, which is probably the crystallization time of the trachy porphyry. The dykes formed in late-orogenic phase of post-collision process or within-plate environment, which suggested that the trachy porphyry dykes possibly crystallized in the transition period during which the tectonic setting changed from post-collision to within-plate environment. So we consider that the age when the post-collision ended and the crustal cratonization begun in the East Junggar is 255Ma, Late Permian.
The Liaohe (辽河) depression is an important part of the Bohai (渤海) Bay Basin, and the Bohai Bay Basin located in the center of lithospheric destruction and thinning in the eastern North China Craton. The North China lithospheric thinning activities have been verified from evidences of structural geology, petrology, geochemistry and geophysics, but there are still some controversies on their timing, mechanism and controlling factors. The sedimentary basin is a thin-bedded geologic unit with a limited distribution in the upper lithosphere, and its formation and evolution represent the shallow response of the deep geodynamic process. Therefore, its thermal evolution is closely related to the deep dynamic conditions. In this article, the Mesozoic–Cenozoic thermal history of the Liaohe depression is reconstructed using the vitrinite reflectance and apatite fission track data. Meanwhile “thermal” lithospheric thicknesses in the Mesozoic and Cenozoic are calculated using the geothermic method on the basis of the above thermal history results. The results show that the Liaohe depression has undergone the Cretaceous and Paleogene heat flow peaks, 81 and 83 mW/m2, respectively, corresponding to two strong rift movements. Accordingly, the depression has experienced two different levels of thinning processes in the Creta-ceous and the Paleogene since the Mesozoic, and the “thermal” lithospheric thicknesses were 60 and 50 km, respectively. This may reveal that the depression has experienced two largescale destructions. The work may provide valuable geothermal evidence for initial revealing the time, process and stage of the lithospheric thinning in the Liaohe depression.
The Huangling (黄陵) massif is an important area to understand the tectonic evolution of the northern Middle Yangtze Block. Integrating previously published thermochronology data with new zircon and apatite fission track, and apatite (U-Th)/He thermochronometry, the Meso–Cenozoic exhumation history of the Huangling massif has been quantitatively studied. Based on the data and the time-temperature thermal history modelling results, the exhumation process of the Huangling massif can be divided into four stages: the slow cooling stage during 200–150 Ma; the rapid cooling event at ca. 150–80 Ma; a period of relative thermal stability during ca. 80–40 Ma, and an increase in cooling thereafter. Two rapid cooling/exhumation indicate two tectonic events in the northern Middle Yangtze Block. The rapid exhumation between ca. 150–80 Ma is likely related to a wide range Cretaceous intracontinental reactive and deformation in the eastern China. The accelerated cooling after ca. 40–30 Ma may result from a farfield effect of the India-Asia collision.
Genetic type of basement granite from volcanic arc in the north of West Burma Block is S-type granites, which developed in volcanic arc of convergent plate margins. The results yield a group of weighted mean 206Pb/238U ages at 102±0.81 Ma (MSWD=0.23), which show similarity to 93.7±3.4 Ma in the northern part of sampling points and 105±2 Ma in the southern part of sampling points, indicating continuous development of volcanic arc in the north of West Burma Block and subsequent granitic intrusion of late Early Cretaceous. The apatite fission track age of the samples is 22.72±3 Ma, thermal history modeling reveals that the volcanic arc in the north of West Burma Block went through two main stages in the process of uplift-cooling since Cenozoic: rapid uplifting and cooling from Late Oligocene to Early Miocene (29±1 to 20±1 Ma) and slow uplifting and cooling since Early Pliocene (4.2±1 Ma).
The Qilian (祁连) Mountain is an active orogenic belt located at the northeastern margin of the Tibetan Plateau. During the process of continuous convergence between Indian and Eurasian plates, the Qilian Mountain grow correspondingly by means of reaction of old faults and generation of new ones. Here we present apatite fission-track data along a river profile crossing three minor fault (the Minle (民乐)-Damaying (大马营) fault, the Huangcheng (皇城)-Taerzhuang (塔尔庄) fault and the Kangningqiao (康宁桥) fault) which compose the North Qilian fault (NQF) to test the timing and pat-terns of the fault activities. Apatite fission-track (AFT) results indicate that these minor faults expe-rienced two active phases in the Cretaceous and the Oligocene–Miocene. Further research indicate that the initiation timing of faulting became younger northward in both active periods and the later phase probably more active than the former phase. These tectonic activities might be highly related to the docking of the Lhasa Block to the south in the Cretaceous and uplift and expansion of the Tibetan Pla-teau in the Cenozoic.
Two 40Ar/39Ar ages and six fission track ages from monzonite and the Jurassic–Cretaceous rocks provide new geochronologic constraints on the timing of uplifting events in the north margin of the Luxi (鲁西) rise, eastern China. 40Ar/39Ar age 111.1±2.4 and 111.2±2.5 Ma of biotite and K-feldspar sampled from the monzonite may record the cooling age at 300 and 150–300 ℃, respectively. Fission track ages of zircon and apatite from the monzonite changing from 75±7 to 40±3 Ma record the cooling age at 250 and 120 ℃, respectively. The apatite from the Jurassic–Cretaceous sandstone and volcanic rocks yielded a different T-t path. The results indicate that there are two phases 111–46.9 and 13.4 (6.5)–0 Ma of rapid uplifting happened to the north margin of the Luxi rise; the first one is a tilted uplift from north to south in Zibo (淄博) during 111–46.9 Ma and in the south in Mengshan (蒙山) during 70–40 Ma; the second one is a tilted uplift from south to north in Mengshan during 32–20 Ma, and in turn in Taishan (泰山) and Zibo during 23–20 and 13.4–0 or 6.5–0 Ma, respectively. The aging coincidence between magmatism and tectonic uplifting implies there are two phases of uplifting induced by large scale extension and lithospheric thinning.
Geochronology of hydrocarbon accumulation and reconstruction is an important and challenging subject of petroleum geology in multi-cycle superimposed basin. By fluid inclusion (FI) analysis combined with the apatite fission track (AFT) modeling thermal path of the FI host rock, a case study of constraining the hydrocarbon accumulation and reconstruction periods of the Permian reservoirs in Northeast Ordos Basin (OB) has been conducted. The FI homogenization temperatures of the oil-gas-bearing sandstone coresamples from the Lower to Upper Permian reservoirs statistically present two groups with a low and a high temperature peaks in ranges of 90–78 and 125–118 ℃, respectively, corresponding to 2-stage primary hydrocarbon accumulations of the Early–Middle Mesozoic. Besides, there exists another group with a medium-high peak temperature of 98 ℃ in the Upper Permian reservoirs, corresponding to the hydrocarbon reconstruction and destruction of the Late Mesozoic to Cenozoic. According to the projected ages of the FI peak temperatures on the AFT thermal path of the FI host rocks, it is further induced that there experienced 3-stage hydrocarbon accumulations in the Permian reservoirs during the Mesozoic to Cenozoic. These are: (1) all the various primary hydrocarbon Permian reservoirs together developed 2-stage accumulations of 162–153 and 140–128 Ma in agreement with the multicycle subsidence burial heating process of the basin; (2) the Upper Permian reservoirs further experienced 1-stage secondary hydrocarbon accumulation and reconstruction at ca. 30 Ma in coincidence with a critical tectonic conversion of the slow to rapid uplifting process from the Late Cretaceous to Neogene.
By means of the vitrinite reflectance and U-Th/He dating of apatite and zircon in cutting samples from the T3x–K1j formations in the Yuanba (元坝) area of Northeast Sichuan (四川) Basin, a correlation has been established between the He-derived age and depth/temperature in this area assuming helium closure temperature of apatite in this area being 95 ℃. Mesozoic strata (T3x–K1j) experienced helium closure temperature of apatite approaching 95 ℃, but didn’t reach the helium closure temperature of zircon (ca. 170–190 ℃) although some reached the highest palaeogeothermal temperature of about 170 ℃. The Mesozoic strata in the Yuanba area experienced an important uplift and denudation during Paleogene–Neogene periods (0.2–36.4 Ma), the erosion rate being about 109.9 m/Ma. The K1j Formation and overlying strata experienced a maximum denudation loss of about 4 000 m. Geotemperatures gradually fell to the helium closure temperature of apatite and then fell further to the current temperature. The thermal evolution history of this area indicates that the maximum palaeogeothermal temperature of Mesozoic strata was close to 170–190 ℃, prior to the strata being uplifted. During the period between 176 and 36 Ma, the palaeogeothermal temperature fell to 95–170 ℃, and after 36 Ma, it continued to fall to the present geotemperature of less than 95 ℃
Three types of granitoids were recognized in Tuerkubantao (土尔库班套) district, located on the northwestern margin of the Junggar Block. The zircon U-Pb age of the three types of granitoids suggests that they formed in different orogenic stage in response to the convergence and collision be-tween Siberian and Kazakhstan plates. Biotite granite is similar to the volcanic arc granites in composi-tion, with zircon U-Pb age of 382.8±2.5 and 355.6±2.4 Ma. The former is crystallizing age of the volcanic arc granite, while the later record the time of hydrothermal alteration induced by a later magmatism. The granite is severe altered and its zircon grain get a lower intercept age of 324.4±7.3 Ma, which is possibly coursed by magmatism in syn-collision. The rhyolite with composition similar to A-type gra-nites in adjacent area, crystallized at 295.9±1.4 Ma, is possibly the extrusive phase of A-type granite formed in the post-orogenic tectonic setting. The granitoids in Tuerkubantao area record the evolution of the northwestern Junggar orogen. The area has been in convergence setting from Middle to Late Devonian, and stepped into syn-collisional tectonic setting in Early Carboniferous, finally evolved into the post-orogenic tectonic stage since Early Permian.
There are very few studies on the granites formed in Late Permian in Hainan (海南) Island. Petrology, geochronology and geochemistry research on Late Permian granites in Chahe (叉河) area, Hainan Island will be reported in this article. The results show that, Chahe granites formed in 251 Ma, are riched in SiO2, alkali, Fe2O3 while depleted in MgO and CaO, with low Sr and high Yb, and have all the features consistent with A-type granite. The forming of Chahe granites may suggest that Hainan Island was already in post-orogenic setting in Late Permian.
This article reports 21 AFT (apatite fission track) data from the West Shandong (山东) rise (WSR) and Jiyang (济阳) depression, and mainly studies their Cenozoic uplifting/subsidence history and the relationship between them. Furthermore, we improve our insights into the Bohai Bay Basin (BBB). Our AFT analysis and AFT T-t modeling indicates that the WSR was uplifted at ca. 65 Ma with apparent uplift rate of 0.019 mm/a; it underwent two relatively rapid uplifting events at 43–33 and 16–0 Ma with rates of 0.097 and 0.052 mm/a, respectively. Meanwhile, the Jiyang depression subsided at rate of 0.032 mm/a at 52–43 Ma, and the rate increased to 0.13 mm/a at ca. 42–33 Ma; finally the subsidence rate increased to 0.053 mm/a in 16–0 Ma. They all underwent a uplift in time of 23–16 Ma with rate of 0.04–0.07 mm/a. A careful comparison shows that the Cenozoic uplifting of the WSR coupled well with the subsidence of the Jiyang depression. Our research also suggests that the uplift- basin coupling events are part of the couplings between the Bohai Bay Basin and its peripheral mountains. This intraplate mountain-basin coupling is a reflection of global tectonic events.
Track fading is a basic phenomenon in track science and has been the source of information in geosciences. This article summarizes the knowledge of track fading and gives some examples of successful applications of track fading in archaeology, tectonics and geothermal chronology in China. The applications of track fading are classified into 5 modes: (1) mode of complete fading (annealing); (2) mode of partial fading; (3) use of the dependence of track fading on time and temperature; (4) use of the differences of track fading between coexisting minerals; and (5) use of fading-reduced track length. Track fading mechanisms hints that scientists in geothermal chronology should adopt microprobes for quantitative elemental analysis to determine the detailed chemical compositions of each mineral grain or at least of the grains from each position of geological structures in order that one becomes well aware of the relation between the track fading behavior and chemical compositions of the mineral used.
The traditional paleotopographic explanation of mountain belts from low-temperature thermochronology is based on the simulation of low-temperature age data, the main defect of which is that we cannot make comparisons at the same time between the samples. In this article, we intend to make paleotopographic reconstruction on the basis of thermal history modelling. We first digitalize the thermal history curve and take contemporary temperatures for comparison, then reconstruct the paleotopography according to the distribution of the samples’ temperatures. The finite difference method is used to solve the diffusion equation for heat in the reconstruction process. Our paleotopography reconstruction method can restore the topography over time. However, due to the nature of thermal history modeling and the noise in the data, the accuracy of this reconstruction is currently limited.
On the basis of previous research achievements of measuring the solid state nuclear track in apatite by thermal analysis method, the author further proposes the research program to measure the energy deposited by the solid state nuclear track contained in zircon, sphene, epidote, apatite and other samples, in order to study the geological age and geothermal history. Compared with the measurement of nuclear track density by etching method, this one does not need to conduct so many processing programs for samples, but can improve the measurement accuracy.
Here we report a novel mass spectrometry measurement system (MSHE4) developed at State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences for quantifying helium-4 contents of geological mineral samples. The MSHE4 system consists of seven units, a high vacuum level generating unit, a gas purified unit, a laser heating unit, a pneumatic valve unit, an automatic control unit, a gas standard unit and a quadropole mass spectrometer unit. The unique software package, developed using LabVIEW for embedded system, allows users to control the full measurement sequences. Results from a test of the Durango apatite sample are used to illustrate the performance of the new MSHE4. We show that the constructed MSHE4 system is able to accurately measure helium-4 contents of apatite mineral, which in turn can be used to quantify the low temperature thermochronology of geological minerals in combination with measurements of uranium and thorium by LA-ICP-MS. The configuration of the MSHE4 system has following advantages: smaller volume of pipeline for gas purification, easier operation of graphic user interface for measurements, and more compact design of sample holder for single grain measurements. The separate step-heating unit can be mounted to the system smoothly. The MSHE4 system can be used for measurement of noble gases from minerals, and in turn thermochronology applications for geochemists.