Variations in the ratio of K 2 O to SiO 2 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

western United States↗

USGS1970

Lead and strontium isotopes in rocks of the Absaroka volcanic field, Wyoming

The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr 87 /Sr 86 , 0.7042 to 0.7090; Pb 206 /Pb 204 , 16.31 to 17.30; Pb 208 /Pb 204 , 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.

Wyoming↗

USGS1962

Unit-cell dimensions of natural and synthetic scapolites

In natural scapolites the cell dimension a shows a regular increase from marialite to meionite composition, while c remains constant. Both a and c of synthetic meionite are larger than the corresponding dimensions of synthetic marialite. The cell volume of both natural and synthetic scapolites is a nearly linear function of composition. Variations in cell dimensions of scapolites may be caused by differences in structural state similar to those in plagioclase feldspars.

Science↗