Understanding the differences in mechanical properties and damage characteristics of granitoid under high temperatures is crucial for exploring deep geothermal resources. This study analyzes the evolution of the acoustic emission (AE) characteristics and mechanical parameters of granodiorite and granite after heating and water cooling by uniaxial compression and variable-angle shear tests under different temperature gradients. We identify their changes in mesostructure and mineral composition with electron probe microanalysis and scanning electron microscopy. Results show that these two hot dry rocks have similar diagenetic minerals and microstructure, but show significantly different mechanical and acoustic characteristics, and even opposing evolution trends in a certain temperature range. At the temperatures ranging from 100 °C to 500 °C, the compressive and shear mechanical properties of granodiorite switch repeatedly between weakening and strengthening, and those of granite show a continuous weakening trend. At 600 °C, both rocks exhibit a deterioration of mechanical properties. The damage mode of granite is characterized by initiating at low stress, exponential evolutionary activity, and intensified energy release. In contrast, granodiorite exhibits the characteristics of initiating at high stress, volatile evolutionary activity, and intermittent energy release, due to its more stable microstructure and fewer thermal defects compared to granite. As the temperature increases, the initiation and propagation of secondary cracks in granodiorite are suppressed to a certain extent, and the seismicity and brittleness are enhanced. The subtle differences in grain size, microscopic heterogeneity, and mineral composition of the two hot dry rocks determine the different acoustic-mechanical characteristics under heating and cooling, and the evolution trends with temperature. These findings are of great significance for the scientific and efficient construction of rock mass engineering by rationally utilizing different rock strata properties.

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Keywords: Hot dry rock, Acoustic emission, Mechanical properties, High temperature, Damage