Phase relationships of Stype granite with H2O to 35 kbar Muscovite granite from Harney Peak, South Dakota

Muscovite granite (13.8% muscovite, 4.8% normafive corundum) was reacted, with varying percentages of H20, in cold-seal vessels at 2 kbar and in piston-cylinder apparatus between 10 and 35 kbar. The diagrams illustrating melting/crystallization relationships are: P-T sections with both excess H20 and with no H:O added (0.66% H20 in rock); T-X•2o sections at 15 kbar and 25 kbar showing H:O-undersaturated conditions; the H20-undersaturated surface for the crystallization of quartz/coesite (small amounts of aluminosilicate minerals persist to higher temperatures). Glass compositions measured by electron microprObe from samples with 5% H:O at 15 kbar confirm that liquids are syenitic through at least 100øC above the solidus, as predicted from the effect of pressure on the Residua System. Results are explained successfully by phase relationships involving muscovite, quartz, and orthoclase in K20-A1203SiO:-H:O, with reactions depicted ina Ptotal-PeH_o-T model, with special reference tothe divariant surfaces in the region Peri o< PtotaV With reduced Pe• o (or a•2o), produced ither by small amounts of H20 (and H:O-undersaturat•on) or by CO: + H:O m•xtures, subsohdus dehydraUon reaction temperatures decrease, vapor-present solidus temperatures increase, and muscovite stability in presence of liquid increases. In general, muscovite, biotite, and amphibole can be precipitated from magmas containing only a few tenths per cent H:O (although the H:O-undersaturated liquids coexisting with crystals may contain 3% or more dissolved H:O). This particular granite cannot be a primary magma from mantle or subducted oceanic rust. It is a possible product of partial fusion of pelitic rocks between about 20 km and 40 km depth given sufficient H20, and xenocrystal muscovite or sillimanite from the source rocks. The phase relationships are consistent with the idea of S-type granites, but not sufficient o prove the origin of this rock. Additional tests require phase relationships of other associated granitic rocks, and details of geochemistry, geophysics, and field relationships.