A STRONTIUM ISOTOPIC STUDY OF NEWBERRY VOLCANO, CENTRAL OREGON - STRUCTURAL AND THERMAL IMPLICATIONS

Ratios of 87Sr/86Sr were determined in twenty-seven basaltic rocks from Newberry. These ratios range from 0.70330 to 0.70414; most fall between 0.7035 and 0.7038. Covariance of Al2O3/FeO* ratio, TiO2 content, and Th content with 87Sr/86Sr suggests that some isotopic ratios reflect shallow crustal contamination, but the data also suggest that two mantle sources, a prominent one at 0.7036 and a less-prominent one at 0.7033 or less, are represented. The hypothesis of crustal contamination is supported by variations in 87Sr/86Sr ratios with locations of the vents from which these rocks erupted. Sr tends to be more radiogenic in rocks from vents either close to a silicic complex such as the caldera of the volcano or the Amota Butte-Sugar Pine Ridge complex, or close to axes of the fault zones that underlie the edifice. Presumably, local heating of the upper crust facilitates contamination of basaltic magma rising through it. © 1990.