No idea why the Vishnu is a problem.
For one thing, the Vishnu did not start out life as a metamorphic rock. It's protolith are sedimentary and volcanic rocks. In addition, the Vishnu complex itself rests atop even older basement rocks (the Elves Chasm Gniess and the Trinity Gniess), which themselves have their own volcanic and/or sedimentary protoliths AND subsequent metamorphism PRIOR to deposition of the Vishnu protoliths.
For the older Elves Chasm and Trinity gneisses alone, this means that:
1) the protolith sedimentary rocks and volcanic rocks had to have been depositied by non-Flood-related sea water and volcanoes to several kilometers depth,
2) the rocks then had to undergo lithification,
3) the rocks had to be uplifted,
4) then weathered / eroded,
5) then surficially exposed,
6) then covered by more sedimentary and volcanic flows and who knows how much more sedimentary/volcanic material, to some depth,
7) then buried and undergone metamorphism
8) then uplifted,
9) then exposed to weathering and erosion.
Only after all these events can the deposition of the Vishnu protolith sediments finally start, which essentially begins the numbered sequence above, again:
For the Vishnu Metamorphic Complex, this means that:
1) the protolith sedimentary rocks had to have been deposited by non-Flood-related sea water and volcanoes to several kilometers depth,
2) the rocks then had to undergo lithification,
3) the rocks had to be uplifted,
4) then weathered / eroded,
5) then surficially exposed,
6) then covered by more sedimentary and volcanic flows and who knows how much more sedimentary/volcanic material, to some depth,
7) then buried and undergone metamorphism
Also! The entire package of rocks from the gneisses to the Vishnu complex were intruded by numerous igneous plutons, dikes, sills, etc. (the Zoroaster Plutonic Complex) during pre-, syn-, and post metamorphic events (i.e., more than one metamorphic event is suggested by the minerals and textures of the rocks). These igneous rocks take time to cool, too, and the deeper the emplacement of these igneous bodies, the longer they will take to cool.
It is only until after all the above has happened that the entire package of basement rocks can be uplifted, exposed to weathering and erosion, and form the surface upon which more sedimentary rocks (the Grand Canyon Supergroup) can be deposited -- this is, of course, the surface which we today have named the Great Unconformity. The fact is, this is only one of several major unconformities, it just happens this one is the easiest to recognize in the field.
And that, in short, is part of why the Vishnu is a problem for Creationists.
For a succinct summary of Proterozoic-aged events, see
Chapter 8: Early Proterozoic Rocks of Grand Canyon, Arizona
Edited by roxrkool, : No reason given.