[SMP38-P07] Crustal fracturing and brecciation processes in middle crust associated with granitoid intrusions in Sør Rondane Mountains, East Antarctica
Keywords:breccia, decarbonation, crust fracturing
The survey area is Balchenfjella, Sør Rondane Mountains, East Antarctica, located within the collision zone during the formation of the Gondwana continent, where the middle and lower crustal materials of the granulite facies are exposed (Ishikawa et al., 2012). The host rock granulite is intruded by breccia dyke with a matrix of carbonate rock (hereinafter, referred to as carbonate breccia dyke) and granitic dyke that are more than 100 m wide, which captures carbonate rock, granite, gneiss as tens of cm to tens of meters of clasts. For two dykes, we estimated the temperature and pressure conditions at the time of intrusion, the origin of the fluid forming two dykes, created a model of the fracturing processes, and finally, estimated the fluid velocity during fracturing.
The results indicate that, the temperature and pressure conditions at the time of intrusion were P = 200 (MPa) and T = 580 to 700 (° C), which is equivalent to pyroxene-hornfels facies, indicating a crustal fracturing in the middle crustal conditions. Based on the carbon-oxygen isotope ratio and trace element composition, the matrix of carbonate breccia dyke has originated from sedimentary carbonate rock (metalimestone) and reacted with the magmatic fluid. Based on the bulk composition, the granitic dyke is syn-collisional granite. The particle size distribution of the captured clasts is 8~102 (m) for carbonate dyke and 17~61 (m) for granitic dyke. By using the Ergun equation, the minimum fluidization rate was calculated to be 190 (km/h) and 40 (km/h), respectively.
This suggests that five stages of magma/fluid activity and two types of brecciation had occurred. In particular, regarding brecciation, as magmatic fluid flows into the metalimestone layer during the intrusion of granite, CO2 is released due to the decarbonation, and the fluid pressure exceeds the lithostatic pressure, disrupts the crust, rises at a flow velocity of over c. 200 km/h, and forms carbonate breccia dyke. Subsequently, the granitic magma intruded at c.40 km/h, forming granitic dyke.
From these observations, it is clarified that the decarbonation of carbonate rock caused by the intrusion of granite causes explosive crustal destruction and can move through the crust at velocities exceeding 200 km/h.