I have a pair of SportHill (I think) multisport polypropylene pants that I use for commuting and MTBing. They're close-cut, but not tights, and the fabric feels more durable to me than true lycra. I usually wear them over shorts but, if it's really cold, there's room for long underwear or tights under them.

That said, in my case, there's always the chance of ripping anything I wear on the MTB