Abstract
Oval caves have recently been discovered in the Bashuihe granite pluton of Laoshan Mountain, China. Oval caves typically occur in alkaline granites. This study conducted microthermometry and stable isotope analysis of quartz inclusions from oval caves and host rocks from the Bashuihe pluton to reconstruct the diagenetic evolutionary history of the Laoshan area. The temperature measurement results indicated a homogenisation temperature range from 162.5 to 261.6°C (mean 203.9°C), a salinity range of 2.1–8.3 wt% (mean 5.07 wt%), and a density range of 0.8–0.98 g/cm3 (mean 0.90 g/cm3), indicating a low-temperature, low-salinity, and low-density fluid. The emplacement depth ranged from 2.73 km to 4.43 km, indicating medium-shallow granite. A hydrogen and oxygen isotope analysis ( δ D = − 83.58 – − 67.17 , δ 18 O H 2 O = 0.83 – 0.39 ) revealed that the diagenetic fluids of the Bashuihe pluton represented a mixed hydrothermal solution composed of meteoric water and magmatic water. The results of a whole rock, H–O isotopes, rare earth element, and high field strength element analysis on the Laoshan alkali granites suggest significant hydrothermal activity in the late stage of magmatism. Primary oval caves in the Bashuihe pluton most likely evolved in the following sequence: fluid was enriched in the late diagenetic stage, diagenetic minerals crystallised under low temperature and pressure conditions, the crystallisation rate accelerated, and the magma condensed rapidly. Moreover, the increase in magma fluid enabled the movement and convergence of fluid. The accumulated fluid and volatiles occupied more space, and rapid magma condensation trapped the accumulated fluid and volatiles in the pluton, forming the oval granite cave. This research provides a crucial theoretical reference for the development and utilisation of underground space and engineering buildings in granite regions.
Highlights
The granite landforms are complex and diverse [1, 2], because they are in different forming environments and transformation of complex tectonics, climate, and other environmental factors in the later period [3, 4]
In the hydrogen-oxygen isotope diagram of diagenetic fluid (Figure 5), it can be seen that all the sample points fall outside the region of metamorphic water and magmatic water and are tilted towards the meteoric line, indicating that the fluid inclusion is a mixture of magmatic water and meteoric water or groundwater related to meteoric water
This study attempted to reconstruct the diagenetic evolution of granites in the Laoshan area using microthermometry, stable isotopes, and a whole-rock analysis
Summary
The granite landforms are complex and diverse [1, 2], because they are in different forming environments and transformation of complex tectonics, climate, and other environmental factors in the later period [3, 4] It is the result of the interaction of multiple factors of geological internal and external forces. The superior geographical location and unique geological and geomorphological characteristics of the Laoshan area are typical of granite landforms in eastern China [7]. These granites are popular research areas for many geologists [5, 8, 9]. Previous studies have established the age [10, 11], geochemical characteristics [12, 13], and magmatic structural background of the Laoshan granite [14, 15]
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