Research On Characteristics And Genesis Of Large Granite Caves In The Laoshan Mountain

Laoshan granite is one of the representative granites in Eastern China.Its main lithology is alkaline granites.The rock mass generally develops geode structure,which is a typical geode alkaline granite.In recent years,the cause of the pit-like micro-geomorphology on the surface of the Laoshan granite has become the focus of debate on whether there is a Quaternary glaciation in eastern China.During the investigation in Laoshan area,our research team found more than 10 oval caves with a diameter of more than 1 m(this article calls them "Laoshan granite large caves").After research,it is believed that the pit-like micro-geomorphology is formed directly after the exposure of the Laoshan granite large caves or after a later transformation,but our knowledge of the caves is still deficiency.In this paper,through field geological survey,petrographic observation,morphological analysis,fluid inclusion petrographic observation,micro temperature measurement and H-O isotope analysis,etc.,we conducted an further study of the Laoshan granite cave,systematically summarized Characteristics and analyzed the cause of the cave,and further studied the formation process of the caves.Through field geological surveys,morphological analysis,petrographic studies,and observation of microscopic features,the Laoshan granite large caves have the following characteristics:The caves are distributed at different altitudes in different areas of Laoshan mountain.The shape of the cave is mellow,the whole shape is mostly ellipsoid,and a few are gourd-shaped connected by double ellipsoids.The cave openings are on the vertical joint surface.The cave is mainly developed in the Bashuihe rock mass,the lithology of which is geode alkaline granite,the mineral size of these rock masses is medium and fine.The microscopic features of the smooth rock surface,observed with the naked eye in the cave,indicate that it is untouched by external forces and belongs to the native surface of the rock.The weathering degree of the rock in the cave wall is relatively low,and the weathering degree of the rock in the cave is lower than that in the cave mouth.Comparing the Laoshan granite large caves with the common sea caves,carbonate caves,artificially excavated caves,wind eroded caves and caves formed by collapse,slippage or accumulation under the action of external forces,it can be determined that the Laoshan granite cave is a new granite cave.The study on the micro characteristics and weathering degree of the rock surface on the inner wall of the cave indicates that the Laoshan granite large caves is not formed by the external geological process after the granite is exposed,but the original cave of the granite.After confirming that the Laoshan granite large cave is a native granite cave,in order to further determine the primary cause and formation process of Laoshan granite large caves,a further study was conducted through fluid inclusion petrographic observation and micro temperature measurement and H-O isotope testing of the diagenetic environment of the Bashuihe rock mass.According to the petrographic observation of fluid inclusions,a large number of fluid inclusions rich in liquid phase of NaCl-H2O system are found in quartz minerals of Bashuihe rock mass.Microscopic temperature measurement results show that the uniform temperature of fluid inclusions is mainly concentrated at 160～240℃,the freezing temperature is mainly concentrated at-5～-2℃,the salinity of the fluid is mainly concentrated at 2～7wt%,and the density is mainly concentrated at 0.85～0.95 g/cm3,the distribution range of diagenesis depth calculated by the pressure of fluid inclusions is 2.73-4.43km,mainly concentrated in 3～3.9km.Analysis of the experimental data on H-O isotopes shows that the granite of the Bashihe rock mass is accompanied by strong hydrothermal activity during the rock formation process,and the hydrothermal fluid is composed of magmatic water mixed with external atmospheric precipitation or groundwater.The study shows that the Bashuihe rock mass was formed in the low temperature and low pressure environment after the magma emplaced to 3-3.9 km depth,and there was the addition of external fluid in the diagenetic process.During the formation of the rock mass,due to the addition of external fluids,the magma contains a large amount of fluids and volatiles.At the same time,the magma condenses rapidly in the low temperature and low pressure environment,so the large amounts of volatiles and fluids accumulated in the magma cannot overflow,instead,it occupies a certain space in the crystallized rock body,thus forming the large primary cave seen now.In summary,it can be determined that the Laoshan granite large caves are granite primordial caves.At the same time,the diagenetic environment of Bashuihe rock mass meets the requirements for the formation of granite large primordial cave.According to the formation environment of the Bashuihe rock mass and the previous studies,we established the formation pattern of the Laoshan granite large primary caves.The formation process of the cave was divided into four stages according to the geological history of the rock mass,which are:the bubble aggregation stage during magmatic activity,the cave formation stage during granite formation,the cave exposure stage during granite outcropping and the cave transformation stage after granite outcropping.