AN EXPERIMENTAL STUDY OF THE ALKALINE INTERMEDIATE RATTLESNAKE MOUNTAIN SILL

I have run piston-cylinder melting experiments on a sample of Rattlesnake Mountain sill (RMS), from Big Bend National Park in the Trans-Pecos Magmatic Province. The sill has an outer analcite-syenodiorite border zone enclosing a slightly more mafic interior which contains segregations of analcite-syenite. Carman et al. (1975) and Carman (1994) provide comprehensive studies of the intrusion and similar “RMS-type” bodies in the park and elsewhere. I selected a sample of the RMS border syenodiorite to serve as an approximation of the integrated composition of the intrusion, and thus of the intruding magma. The composition is benmoreitic and is similar to Carmen et al.'s (1975) calculated integrated compostion.

I have completed runs at a range of pressures between 8 and 20 kb, with each run containing capsules of sample plus 0, 2, 4, and 8% H₂O added as AgOH (see Mirante, 2003). The experiments have yielded super-liquidus returns at 2% H₂O, 12.5 kb, 1110° C and at 8% H₂O, 8 kb, 1085° C. Ilmenite is a liquidus phase at 4% H₂O, 15 kb, 1125°. Garnet is common at 15 kb and above with 4% and less added H₂O. Apatite and ilmenite are nearly ubiquitous, and apatite appears accompanied only by traces of clinopyroxene near the liquidus at 15 kb. Other common phases are edenite or ferroedenite, biotite, and alkali and plagioclase feldspar. None have contained olivine or an identified silica-undersaturated phase. All of the returns contain some melt and most show good equilibrium textures.

These piston-cylinder experiments may provide information as to the possible line of descent from more mafic undersaturated alkaline rocks in the Trans-Pecos region through identification of liquidus phases. Additionally, they allow the investigation of intermediate alkaline melts at high pressure and of the partitioning of elements between minerals and melt in a benmoreitic magma at a range of conditions.