The pilot reported that, during the landing flare the airplane encountered a "sudden massive gust of wind" that pitched the nose up. He pushed the yoke forward, but the wind shifted to a tailwind and the airplane "slammed" onto the runway. Subsequently, the left main landing gear collapsed, the airplane veered right off the runway and came to rest nose down. The airplane sustained substantial damage to the left wing. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The automated weather observation system at the accident airport reported, about the time of the accident, the wind was from 260° at 4 knots. The pilot landed on runway 26. Note: The accident airport is at an elevation of about 6752 ft., with a calculated density altitude about the time of the accident of 9211 feet.