Petrology of the Lower Tertiary Clarno Formation in north central Oregon: The importance of magma mixing

Abstract

Predominantly Eocene rocks of the Clarno Formation in north central Oregon consist of lava flows, volcaniclastic rocks, and tuffaceous sedimentary rocks lying between Cretaceous shallow marine/deltaic sedimentary rocks and younger volcanic rocks. Whole rock Rb and Sr isotopic data yield a poor isochron with an age of 47±5 Ma. Clarno rocks are broadly calc‐alkaline, range from basalt to rhyolite, and show complex textural and compositional properties. Plagioclase and clinopyroxene phenocrysts are present in most lavas, with olivine in mafic rocks and orthopyroxene, quartz, and subordinate biotite and hornblende in intermediate rocks. Spongy (sieve‐textured) plagioclase and glass‐bearing plagioclase are both common. Many quartz grains are resorbed, and some pyroxene grains are spongy. Some samples contain quartz in which plagioclase inclusions have undergone partial melting along the plagioclase‐quartz boundaries; plagioclase inclusions in these rocks are more calcic (An54–66) than the cores of their companion xenocrysts (An41), suggesting digestion of earlier mafic rocks. Mafic cumulates have high MgO, Cr, and Ni values. Some oxide‐ and element‐SiO2 diagrams show linear trends, but scatter from the trends is large. Plots of SiO2 versus Al2O3, Sr, Y, and possibly other components show inflections in trends in the range of 55–60% SiO2. The abundance of disequilibrium textures shows that the dominant process in the evolution of the Clarno suite was mixing of magmas with liquid and (partly) solid sources, probably including earlier crystallized Clarno magmatic rocks. Cumulate textures and compositions, however, indicate that Clarno magmas were modified by crystal fractionation, which also may have contributed to the scatter from linear trends on composition diagrams. Initial 87Sr/86Sr ratios (corrected for a model age of 50 Ma) range from 0.70350 to 0.70397, consistent with mantle source(s) or with contamination by young mafic material but not with interaction with old upper continental crust.