Again, from all sources I've ever seen, you can't ignite aluminium blocks and have a sustained fire, whereas you can with magnesium. If the aluminium is liquid then perhaps by spraying it around because of the spark it would give the effect you describe, but it sounds dubious that it melted so quickly in a large enough amount, and it seems somewhat unlikely to me that once it started the sprayed liquid aluminium would stay close enough to melt and burn the remainder, too. When you see pictures of large arc flashes, a lot of metal is sprayed from the location of the arc and then burns, but it burns flying away so it cannot sustain the reaction; the cause of the metal burning and flying away is the arc. In your case you didn't have enough electric power to get such a strong arc (the 10-15 A you get from house hold power before tripping the fuse is not that much), so the energy to sustain burning up the remainder of the metal must have come from the metal fire itself. That is definitely possible with magnesium, but from all I've seen it is not with aluminium.
Magnesium can be set alight even when multiple cm in thickness [1], and lighting up thin strips is much more spectacular[2]. You can hold aluminium foil to a blow torch, what happens is it will melt and oxidize, but not produce any flame or light whatsoever[2]. If aluminium foil doesn't do it, I don't see how a thicker sheet would do it. And all I've found confirms this. (BTW steel wool however does burn continuously. But I agree that a steel lamp base burning this way doesn't look feasible, either.)
Since you say that it was thin, it must have been an alloy in either case. It seems Magnesium alloys need to be cast. I've got a laptop with a magnesium base which looks like sheet metal from the outside but when you open it up, on the inside it has all sorts of structures coming out of the "sheet", clearly confirming it was cast. I've never seen aluminium parts made that way, aluminium cast parts seem to always be relatively thick, not like sheet metal. Aluminium can be pressed, however (think beer cans), but then it doesn't have a fancy structure. I'm not a specialist in these matters in any way, but maybe this might give some further ideas.
Magnesium can be set alight even when multiple cm in thickness [1], and lighting up thin strips is much more spectacular[2]. You can hold aluminium foil to a blow torch, what happens is it will melt and oxidize, but not produce any flame or light whatsoever[2]. If aluminium foil doesn't do it, I don't see how a thicker sheet would do it. And all I've found confirms this. (BTW steel wool however does burn continuously. But I agree that a steel lamp base burning this way doesn't look feasible, either.)
[1] e.g. video on https://en.wikipedia.org/wiki/Magnesium [2] source: own experiments, and there will be videos from others
Since you say that it was thin, it must have been an alloy in either case. It seems Magnesium alloys need to be cast. I've got a laptop with a magnesium base which looks like sheet metal from the outside but when you open it up, on the inside it has all sorts of structures coming out of the "sheet", clearly confirming it was cast. I've never seen aluminium parts made that way, aluminium cast parts seem to always be relatively thick, not like sheet metal. Aluminium can be pressed, however (think beer cans), but then it doesn't have a fancy structure. I'm not a specialist in these matters in any way, but maybe this might give some further ideas.