| Abstrakt: |
Phase relationships for four granitoids from the central Sierra Nevada batholith in California have been determined in the presence of excess water to 10 kb water pressure. Water pressure-temperature curves were constructed to locate the solidus and the disappearance of potassium feldspar, quartz, plagioclase, biotite, and hornblende in a tonalite, two granodiorites, and a granite. Complete fusion patterns were ascertained for the tonalite, one of the granodiorites, and the granite at 1, 2 and 10 kb water pressure. Electron microprobe analyses of liquids in equilibrium with crystals and vapor, obtained at selected temperatures throughout the melting interval of one of the granodiorites at 2 kb water pressure, reveal that the normative anorthite content, MgO, and FeO of liquids increase with increasing temperature. Concordance between the trends of liquid compositions and those illustrated by batholithic crystalline rocks suggests, but does not prove, that liquidcrystal equilibria was the operative petrogenetic process that produced the granitoids of the central Sierra Nevada batholith. Experimental conditions prohibit detailed comparison with natural occurrences, but the new results are compatible with a crustal anatectic model provided that residue refractory material is carried upward with the liquid formed by differential fusion, provided that high temperatures are reached in the upper crust, or provided that high water pressure existed during the partial fusion process. However, consideration of all experimental results; of recent information on the abundance of lead, strontium, and rubidium isotopes in Sierran rocks; and of rare earth element distribution and potassiumargon ages of minerals suggests that mantle source material is involved in the petrogenesis of the Sierra Nevada batholith. |
