very new to this

of course i have to chime in...i love lifting and find it (too) challenging to balance lifting and tri's.

of course heavy lifting will be counterproductive to tri training as u will have reduced flexibility, will be sore alot, and be slower WHILE light lifting will be productive by reducing injury through strong joints and tendons.  depends on your goals:

want mass?  want to gain weight?  then u must lift heavy.  u must tax your muscles if u want growth - that means heavy lifting where u go to fatigue be it light or heavy days.  my triathlon goals are NOT to be a front of the pack guy but to offer an alternative to 'cut up' while providing a great calorie burning experience and to also keep my circulation system in tip-top shape.  plus triathlons allows some competition...general weightlifting does not.

if u don't want muscle mass then DONT lift heavy AND don't go to exhaustion.  in my experience, my growth occured ONLY when i had 1) proper intake of protein/carbs 2) mix of heavy and light days 3) no matter what day, every last rep of each set was to exhaustion (need spotter) THAT is growth their.

Now i feel that it will not harm u to lift 'heavy' - just dont' go to fatigue.  make sure u can always handle that last rep.  keep the sets low (3 as opposed to say 5) and the exercises per muscle group few.  having a few heavy days will create stronger joints and build bone density. 

SO THE BIG QUESTION IS: what r your triathlon goals?  do u just want to have fun (myself here)?  OR do u want to place competitively?  being that u love lifting, and assuming u like to lift heavy u must answer that question to tailor your lifting program.  lifting heavy and keeping  or building muscle mass will decrease flexibility and make u slower than someone who is JUST doing light sets (maybe ~70% of max), not going to fatique - they will be fast while maintaining a good degree of joint, bone and tendon strength.

so define your goals, let us know, and i'm sure a lot of us here can help u as most of us lift to some extent depending on our tri goals.

(if u want to be competitive...don't listen to me or follow my weight program! this seems to have gotten very long winded!  not sure if i have really said anything.   )

oh yeah...welcome to the site and, soon, to the awesome world of triathlons!

good luck on your impending marathon!