Zircon is one of the most commonly used accessory minerals rich in U and Th for(U-Th)/He dating system. Compared with apatite, zircon has a higher He closure temperature (~190℃), which gives it more advantages in solving the problem of source material and thermal history reconstruction in sedimentary basins. However, the crystals of zircons often have U and Th zoning development, with obvious differences in concentration. Even the standard sample of FCT(Fish Canyon Tuff)zircon which is widely used in (U-Th)/He dating has an average age dispersion of about 10%. In this study, the Alphachron He isotope mass spectrometer is used for laser melting of a batch of single grains of FCT zircon(11 grains)to determine their ⁴He content. The contents of U and Th of parent isotopes are accurately determined by automatic injection of Agilent 7900 ICP-MS and isotope diluent method. The Th/U ratios of the 10 FCT zircons calculated with (U-Th)/He average age in this paper range from 0.52 to 0.67, which are consistent with the Th/U ratios of 186 reported so far. According to the Th/U ratios of 189 FCT zircons published in the statistical literature, we found that only three of them had high Th/U ratios, namely, 1.12, 1.16 and 1.5, the other 186 FCT zircons(occupy>98%) had a Th/U ratio less than 1. Based on previous results and the 10 Th/U ratios measured in this paper, 196 FCT zircons have a normal Th/U ratio ranging from 0.27 to 1.00, with an average ratio of 0.56(n=196). Excluding one abnormally old age, the(U-Th)/He ages of the remaining FCT zircons in this study range from 26.61 to 31.91Ma, with a weighted mean age of (28.8±3.1)Ma (2SD, n=10), which is consistent with the mean age ((28.3±3.1)Ma, 2σ, n=127) or (28.29±2.6)Ma(2σ external error, 9.3%, n=114)obtained by several other international laboratories. This indicates that the zircon single particle(U-Th)/He dating process established by our laboratory is reliable. For the zircon samples with U, Th banding and concentration differences prevailing, determining the distribution of U, Th elements in the crystal prior to the (U-Th)/He experiment is essential for understanding effects of geometry and elemental zoning on nuclear recoil and diffusion and the interpretation of (U-Th)/He age data.

LA-ICP-MS(laser ablation-inductively coupled-mass spectrometry)has been recently used for rapid, accurate and precise U-Pb geochronology on zircon grains. In this paper, we adopted an Agilent 7900 quadrupole ICP-MS coupled with a Resolution M50-LR 193nm excimer laser system to establish integrated measurement procedures. Before analysis, the system is tuned to achieve sensitivities better than 30000 cps/s for ²³⁸ U with a 40μm spot size, at~3.5J/cm². Detailed parameters for laser system and ICP-MS are presented here. Then, we analyzed five reference zircons(91500, GJ-1, Plesovice, FCT, Penglai)with a wide range in age from~1064 to~4.4Ma. Two standard zircons, 91500 and GJ-1, are employed as external reference standards. Generally, second zircon standard is analyzed in an effort to ensure accuracy and evaluate reproducibility. A typical analysis sequence includes one international glass standard(NIST610), two external reference standards, five grains of unknown zircon with every eight ablations. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractional caused by ionization differences are corrected by external reference standard 91500 or GJ-1. Compared with 91500 and GJ-1, common Pb content of Plesovice, FCT, Penglai can't be ignored. Thus, we did common Pb correction for the above three standard zircons. The performance of the established procedure was assessed by analyzing zircon range in age from~1 064 to~4Ma. The results show that the ages of these five references are consistent with the ages of published studies with accuracy for three international references(91500, GJ-1, Plesovice)better than 3% and two young secondary references(FCT, Penglai)lower than 7% at the 2 sigma level, which indicates that our analytical procedure is reliable. For individual laser analysis, the uncertainties are mainly from three sources:Measurement error of isotope ratio, error of correction factors for instrument drift and element fractionation, and error of recommended age of external references. Compared to three international references, there are three extra uncertainties for young reference zircons, including:1)little radioactive isotopes closing to blank level increase the measurement error of isotope ratio; 2) effect of common lead becomes more significant;3) the nonhomogeneous samples couldn't match references well. Therefore, accuracy and precision of measurement depend on absolute age, content of common lead and matching degree between references and samples. In summary, the accuracy and precision obtained using the technique presented in this study are similar to those of other LA-ICP-MS laboratories.

(U-Th)/He isotopic dating has been developed very quickly in recent years, due to the recognition that the thermal history of rock at low temperature can be effective revealed by such dating method. In particular, He closure temperature in apatite (40~80℃) is very low, so apatite (U-Th)/He ages can reflect the thermal history information of the low-temperature stage, and have a good application prospect in the field of low-temperature thermal chronology. However, because of many influence factors and complicated measurement procedures, the development of apatite He dating in China remains in its early stage. In this study, a measurement procedure was established at the (U-Th)/He dating laboratory of Institute of Geology, China Earthquake Administration. We measured the daughter isotopic helium by diode laser heating four batches of a total seventy-five grains of Durango apatite in an Alphachron helium mass spectrometry system. Then the apatite grains were dissolved to precisely measure the concentration of parent nuclides (U, Th)using the solution isotope dilution method through an automatic sampling ICP-MS (Agilent 7900). Results show that the Th/U values of Durango apatite grains were in the range of 17.23 to 23.60, while all the 75ages were in the range of 28.61 to 34.51Ma with an average of (31.71±1.55)Ma (1σ), which are consistent with the international calibrated ages.

The Yabrai range-front fault is a normal fault,which is about 120km long,trends N60°E and distributes along the southeast margin of the Alashan block. In this paper,we focus on the geomorphology and kinematics of the Yabrai range-front fault,and discuss the implications of the fault for the regional tectonics.
This fault consists of three segments and the most active one is located in the southwest,which has a length of about 35km. The about 1～2m-high scarp,stretching almost the full segment,might be the result of the latest earthquake event. Fresh free surface indicates that the elapsed time of the last event should not be long.
The middle segment is about 31km in length. The results suggest that just a single fault is developed along the piedmont of the Yabrai Shan,and there is no evidence of recent activity on this fault. In contrast to the simple geometric structure of the middle segment,the northeast segment consists of several faults. The scarps of the most recent earthquake event,which are clear but discontinuous,are about 0.5～1.5m high and some are up to 2m. Although the scarps along the southwest and northeast segments of the fault are similar,it is difficult to suggest they are caused by the same earthquake without precise dating.
The seismic reflection profile suggests that the Yabrai range-front fault came into being as a normal fault in Cretaceous,when the Tibetan plateau did not emerge at that time. Therefore,we conclude that the Yabrai range-front fault is not the consequence of the Indo-Asian collision. But this region plays a great role in constraining the tectonic evolution of the Alashan block and therefore,the Tibetan plateau.