Thingberry tree

I am wondering if anyone can identify this tree. It grew up as a weed next to another long-established and virtually unkillable tree. It's only been in the yard about five years or so, but is already well over twenty feet high. It has a rough, golden bark, and, perhaps for the first time ever, it is bearing small red raspberry-like berries that taste pretty good.

The berries. Slightly sweet, slightly tart, not very juicy, 1-2 cm long. Maybe I need to let them ripen a bit.

Update,6/29/2018: Don Williams, the Susquehanna River Sentinel, has identified this as a mulberry tree. I just have to let the fruits fully ripen and they'll be delicious!