Long term effect on rocks of high pressure and temperature? The mean surface temperature of Venus is about 460 C and the pressure is about 92 atmospheres. Assuming this situation (or similar) has prevailed for millions of years what changes (if any) would there have been in the structure and chemistry of the rocks on the surface of Venus? 

Wikipedia(Venus) http://en.wikipedia.org/wiki/Venus
Consider not only the physical attributes of the Venusian atmosphere, but its composition, from the NASA Venus fact sheet:

Major:       96.5% Carbon Dioxide (CO2), 3.5% Nitrogen (N2) 
Minor (ppm): Sulfur Dioxide (SO2) - 150; Argon (Ar) - 70; Water (H2O) - 20;
             Carbon Monoxide (CO) - 17; Helium (He) - 12; Neon (Ne) - 7


The 1st and 3rd most abundant components - carbon dioxide and and sulphur dioxide, at the pressures and temperatures of the surface would result in:


  The reactions probably include the deterioration of silicates by carbon dioxide to produce carbonates and quartz, as well as the deterioration of silicates by sulfur dioxide to produce anhydrate calcium sulfate and carbon dioxide.


From the Venus Wikipedia page

A study reported in the paper Venus Geochemistry: Progress, Prospects, and New Missions (2009), focusing on the basalt-atmosphere interaction, determined that the conditions on Venus is roughly the equivalent of:


  a metamorphic environment of ‘amphibolite
  grade,’ ‘hornfels facies;’ the paucity of H2O
  in Venus’ atmosphere (30±15 ppm) prevents formation
  of amphibole


The metamorphic facies for Earth that are used in the above quote are shown below:



Image source

The implications of the surface-atmosphere chemistry, pressure and temperature would, according to the Venus geochemistry article (linked above), result in the formation of:


  anhydrite from essentially
  all the Ca in basaltic minerals and rocks. Iron in the
  rocks and minerals will likely react to either oxides or
  sulfides, depending on the oxidation state of the atmosphere.
  The fate of alkalis is not clear – our calculations
  yield only alkali feldspar (albite), but alkalis
  could be partitioned into ionic (carbonate-sulfate)
  melts. Some Venus basalts are so rich in potassium
  that phases besides feldspar may be present (e.g.,
  leucite, nepheline-group, or sodalite-group).