What melted my tweeter?

Jeff,
Two possibilities, but they are simply guesses. It is possible that the amplifier (or even the pre-amp upstream) has been damaged by the power fluctuation. If the component has a high frequency oscillation or instability, then exposure to the high frequency energy for an extended period (relative to heat dissipation capability of the tweeter) can overheat and damage/destroy the tweeter.
The other is that the power spike resulted in a high frequency surge, that destroyed the tweeter in one brief event. The amp/pre-amp may or may not be damaged.
Good luck.

The amp seems to be performing OK.

Whatever it is, you need to file a claim with your local power company right away. Then take the affected components to a reputable shop / dealer and have them inspected. If the tweeter physically melted / is deformed, it's quite possible to have other internal damage to the speaker too. As far as the amp working "ok", it might be. Then again, unless you can have it thoroughly checked out, you'll never know whether it is really up to snuff or still just "working". Just because it turns on and sound comes out of it doesn't mean that it is still working as it was before the snafu. Sean
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Thanks Sean,
I live in Bangkok and I doubt the power company will be very sympathetic.
I have an MIT Power Conditioner and thought it would control power surges.

Upon close inspection with a flashlite, I see some burned up parts on a circuit board in my amp.