League of Legends (LoL) Question: Mage Supports building Tank Items?

To answer your question directly: yes.

The reason is that most supportive items (which give utility and aura's) are essentially defensive items (e.g. Locket of the Iron Solari or Mikael's Blessing). Rules to live by as a support are that you'll (usually) get more damage as a team if you just keep building utility over selfish damage items.

Examples to this rule are supports that have good AP scalings like Karma or Vel'Koz.

Tbh I don't think Karma is working like that. The other two ones probably work better than Karma when building more defensive items. Based on great team-oriented play especially in a Ranked Team opting for rather defensive - supportive items is the best option in my opinion (also depending on the game).

I get Randuin's and Locket on Lulu (whoever made that guide, I love you forever) and it works out pretty well. I'm also happy to see that other people buy Mikael's.

It's pretty "logical" if that's your question. It's unwise to build a Warmog's on Soraka but with some utility-like tank items it's absoloutly helpful. Thornmail's good if you find you don't need the other two; while Sunfire seems to be reccomended on a couple of tanky support (Leona, Thresh, etc.) I feel it's not as useful.

High utility supports like Sona? Yeah, going tanky Utility with CDR or just raw tank is a legit option and build path. Crucible, Locket, even some of the outlyer niche items like Zekes all have tanky stats. Someone like Sona can really benifit from lots of CDR, and being tanky lets her surrvive longer in fights to get into the thick of things and constantly pump out her utility.

I've seen quite a lot of aggressive mage supports build Randuin's Omen due to the passive lowering attack speed (and really hurting the damage output of ADC's, especially on-hit counters like Vayne's Silver Bolts), and anyone with a gap closer like Thresh or Leona can really take advantage of the active in chases. Many times, especially as a second/split item, most of the damage being thrown around bot lane is physical, and the 70 armor given by the item is very helpful in surviving trades, and quite possibly saving tower or setting up your ADC. Anything that gives you Mana Regen is also a big buy on a lot of supports, especially the spammy type like Sona, so Athene's Unholy Grail and Mikael's Blessing are good picks based on the AP scalings of your support. Exceptions to these kinds of rules are off-pick supports like Shen, who is best when built as a full-on tank, Sunfire Aegis is a great item along with Randuin's Omen, and anything that helps you with massively tanking anyone who gets within taunt range.

One good thing about tanking out a mage support is that the other team will think they can delete you, and they keep focusing your champ (while your teammates jump in, and take some heads). If I use Morgana, I often go with almost all tank items, and it works great. For squishier folks like Sona, you would simply have to change your play style. Instead of poking and retreating, you can just hang out in the middle of the action while constantly buffing and healing. Still, when you're used to a high damage output with a certain champ, using a tanky build can throw you off your game.

you can get some tank items like locket of iron solari or frozen heart (depending on the enemy and your comp) but I wouldn't recommend going full tank on them. Consider either support items with tanky stats or ap items that give hp-armor-mr.

Why not just pick a tanky support, and build tank items on him instead? Lol