2000 Chevy Blazer Thermostat?

driving and gauge shot up to 270 and then back. No coolant leakage. Car idols fine until gauge goes up, then we shut it down. Noticed a small steam leak at the top hose point of connection to the radiator. No white color in the oil. Any idea what is causing the guage to rise immediately? Is the thermostat needing to be changed? Headgasket?

Thanks in advance.