Electron probe microanalyses of minerals in the nine intrusive rock types of the Diablo Plateau document compositional changes accompanying magmatic differentiation. Plagioclase occurs only in the least differentiated rock type. Alkali feldspar phenocrysts show little compositional variation. Groundmass feldspar compositions were modified by exsolution and deuteric alteration, particularly during growth of natrolite. Nepheline compositions in these hypabyssal rocks are intermediate between those of volcanic and plutonic nephelines elsewhere.
Mafic minerals show wider compositional variations. Clinopyroxene, the most abundant of these, changed from sodian augite toward hedenbergite, and then toward acmite. The more sodic pyroxenes crystallized later than feldspars and nepheline from residual liquids after emplacement at the now-exposed level. Amphiboles and biotites show progressive decreases in Ti, Mg, and tetrahedral Al/Si. In the least differentiated rock type and in some autoliths, the amphibole is kaersutite; in later formed rocks, hornblendes and sodic amphiboles are widespread. Other prominent minerals showing compositional variation are manganoan fayalite, aenigmatite, and eudialyte.
For the nepheline syenites, crystallization temperatures are estimated at 700 to 725 °C, with log (fO2) = −20 and log (fH2O) = + 1.6 to 2.1.
↵*Present address: Department of Geology, Furman University, Greenville, South Carolina 29613.
- Geological Society of America