| Abstract |
- About 225 lava flows and ash-flow tuffs of the Deschutes Formation (DF) were mapped at Green Ridge. The units fill east-trending paleovalleys which "sky-out" westward and dip east; the eastward dip of the units decreases
as they are traced eastward. The source of the units was west of Green Ridge as evidenced by the increase in the number of ash-flow tuffs and lava flows in the DF from east to west toward Green Ridge, and the presence of viscous silicic lavas and lag deposits in ash-flow tuffs on the west flank of Green Ridge. The exposed DF section
at Green Ridge is 1400 feet thick, and consists mainly of basaltic andesites with subordinate diktytaxitic basalts, andesites, dacites, aphyric basaltic andesites and andesites rich in Fe and Ti, ash-flow tuffs, and sediments. The upper 400 feet of the section is devoid of ash-flow tuffs and dacites. Nine paleomagnetic polarity intervals, five normal and four reversed, are recognized in the section; tentative correlation of these intervals with the
magnetic time scale suggests that the oldest DF unit exposed at Green Ridge is 6.5 Ma, and the youngest 4.4 Ma. The DF units at the crest, and west of Green Ridge are cut by at least five N- to N13W-trending, down-to-the-west normal faults in a zone about five miles wide. These faults were possibly active as much as 5 Ma; the developing faults may have provided structural pathways for the ascent of the mafic lavas in the upper 400 feet of the
section. The faulting which created the Green Ridge escarpment was probably finished by 3 Ma, and perhaps earlier. The presence of dense, Fe- and Ti-rich lavas throughout the DF section implies that magmas had relatively
easy access to the surface during DF time (7.6-4.4 Ma), which suggests that the E-W extensional tectonism that culminated in the formation of the Green Ridge faults was present throughout deposition of the DF. The major element chemistry of DF diktytaxitic
basalts, basaltic andesites, and andesites appears to change regularly with Ti02 content, as shown by decreasing CaO/FeO* (FeO* = total Fe expressed as FeO) with increasing Ti02, and by decreasing A1203 with increasing Ti02 at constant MgO content. The basalts and basaltic andesites with the lowest Ti02 contents also have the lowest alkali contents. These chemical trends appear to be caused by plagioclase, with subordinate olivine, fractionation, and are consistent with the fact that plagioclase and olivine
are virtually the only phenocryst minerals in the mafic DF rocks. It is likely that the diktytaxitic basalt with the lowest Ti02 content is a parental magma. Fractionation may account for the chemical variations
within the mafic rock groups considered separately, but the progression from basalt to andesite appears to be the result of mixing of silicic and mafic magmas. The chemistry of the rocks, especially the CaO/Al203 ratios, is consistent with such mixing, as is the petrography; opaque
minerals are very late crystallizers in the basalts and basaltic andesites, and there is increasing evidence for magma mixing in the series basalt through andesite. Such evidence includes multiple plagioclase populations in the same rock, both of which are resorbed, and resorbed pyroxenes and amphiboles.
Three flows of plagioclase megacryst-bearing basaltic andesite occur near the top of the DF section. The plagioclase megacrysts are up to 5 cm in length, commonly contain apatite inclusions, and resemble the plagioclase
in anorthosite bodies. Ilmenite megacrysts are also present. The megacrysts and the presence of highly plagioclase-fractionated aphyric lavas rich in Fe and Ti in the DF suggest that anorthosite bodies are present beneath the north-central Oregon Cascade Range. Two xenoliths of partly melted cordierite-sillimanite-quartz granulite gneiss were found in a DF ash-flow tuff. The xenoliths demonstrate that granulite-grade
metamorphism and at least local partial melting of the lower crust took place beneath the north-central Oregon Cascades.
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