The Elastic Wave Velocities of the Granitoids from Eastern Junggar of Xinjiang at High Temperature and High Pressure and Their Geological Constraints on the Crustal Structure

Abstract

AbstractThe compressional (VP) and shear wave velocities (VS) of typical granitoids (alkali‐feldspar granite, alkaline granite, granodiorite, monzogranite and quartz diorite) from Kalamaili granite belt and Yemaquan body in Eastern Junggar, Xinjiang were experimentally measured at pressures up to 1.0 GPa (room temperature) and temperatures up to 700°C (1.0 GPa) by ultrasonic reflection‐transmission method. The results show that both VP and VS of the granitoids present a linear velocity increase with the pressure at room temperature from 0.4∼1.0 GPa, which is interpreted in terms of closure of the microcracks in rocks under this pressure range. The VP and VS of the granitoids are 5.79∼6.84 km·s−1 and 3.26∼3.85 km·s−1 respectively at room temperature and 1.0 GPa. On the basis of the linear relationship between pressure with VP and VS, we derived the pressure derivatives of VP and VS as 0.1568∼0.4078 km/(s·GPa) and 0.0722∼0.3271 km/(s·GPa) respectively, and the reference velocities VP0 and VS0 are 5.62∼6.47 km·s−1 and 3.15∼3.75 km·s−1 respectively. At 1.0 GPa, VP and VS of the granitoids show a linear decrease with temperature from room temperature to 700°C, and temperature derivatives of velocities are (−3.41∼−4.96) × 10−4 km/(s · °C) and (−0.88∼−3.22) × 10−4 km/(s · °C), respectively. Taking advantage of derivatives of velocities and reference velocities of the granitoids obtained from our experiments, and combining regional geothermal resources of East Junggar area, VP and VS‐depth profiles were calculated. From the comparison of velocity‐depth profiles with geophysical exploring results, we infer that the VP and VS of the alkali‐feldspar granite, alkaline granite, monzogranite and part of granodiorite in Eastern Junggar of Xinjiang are coherent well with the velocity of the upper crust, and the average Poisson's ratio of several rocks are also consistent with the Poisson's ratio of the upper crust. Therefore, we suggest that these kinds of rocks are important parts of the upper crust in this region. In addition, the VP and VS of the quartz diorite are both in accord with the middle crust, suggesting that it may be one kind of rocks in the middle crust of this region.