CLARNO INTRUSIVE ROCKS
Clarno intrusive bodies of the Mitchell area are of many different compositions and assume different forms representing distinct modes of emplacement. Some were large, nearly solid cylindrical plugs that were forced into overlying rocks, much like an irresistible piston (example: the dacite plug of White Butte, surrounded by uplifted Kgc strata).
Others were fed by more fluid magma rising in central conduits to spread out as sills (example: the dacite sill of Baily Butte and Mitchell Rock just west of Mitchell.
It is characterized by snowflake textures on fresh surfaces and by cooling layers at the base of the intrusion).
Some flat-floored sill-like intrusives bulged upward to form laccoliths (example: the andesite of Black Butte which is characterized by marble-sized spherulites visible on fresh surfaces and abundant hornblende phenocrysts).
Another example might be the mafic intrusion of Marshall Butte just east of Mitchell in which the basal contact is coincident with inclined strata and the central bulge is no longer covered by Clarno mudflow deposits.
The largest sill in the Mitchell area is exposed on the west side of West Branch Valley; it is nearly 2.5 miles long, 600 feet thick, and is basaltic in composition with a diabasic, almost gabbroid texture. Magma ascended from this body to higher levels, forming smaller sills like those exposed in a road cut of highway 26, 1.6 miles north of Ochoco Summit.
Here, a thin sill lifted and fractured thin-bedded lacustrine tuffs (known for enclosed fish scales) and contained sufficient volatiles to produce late-stage segregation veins of alkali-feldspar and zeolite minerals, concentrated in light-colored layers that inexperienced observers might mistake for sedimentary strata.
Dikes were formed where magma ascended through cracks in the shallow crust. Hundreds of small irregular andesite dikes are exposed cutting Hudspeth mudstone in the central part of the Mitchell Anticline but few of them have been traced to a surficial lava.
Examples of more extensive dikes of andesitic composition include a 1.5 mile dike on the southwest flank of Keyes Mountain and a 1.7 mile dike that changes to a sill along the length of Mill Creek, southeast of Mitchell.
Columnar-jointed andesite dikes are common in areas west of Mitchell.
An interesting ring dike of basalt occurs west of Mitchell. It consists of two concentric rings filled in the center with a breccia of angular basalt and Hudspeth mudstone fragments. It may have been an explosive diatreme.
The best example of multiple rhyolite dikes occurs northeast of Stephenson Mountain where more than 9 dikes trend SW-NE. Some of them are over 3 miles in length.
Large intrusive plugs are also common in the Mitchell area and many of them might have been subvolcanic feeders. Some are andesitic (example: Byrds Point near the juncture of Bridge Creek and the John Day river).
Others are more silisic (examples: the remarkable trend of large rhyolite bodies extending from Toney Butte northeast of Mitchell, nearly to Prinevile on the southwest); included here are the rhyolite intrusions of Meyers Canyon, Sargent Butte, Sand Mountain, Sheep Mountain, and Stephenson Mountain.
It might seem anomalous that such large silicic intrusives should have crystallized as aphanitic rhyolites rather than phaneritic granites. The country rocks adjacent to these bodies have not been altered by hydrothermal fluids;
in addition, these intrusive rocks do not contain hydrous minerals such as hornblende or micas; the magmas were probably devoid of volatiles and did not develop coarse-grained textures.
Many of the magmas that were intruded during Clarno times were of unusual composition. Examples include: (1) the melagabbro of Marshall Butte and related plugs which contain crystals of olivine and clinopyroxene and in which plagioclase feldspar is replaced by nepheline.
(2) the group of irregular, seemingly connected, alkali-basalt sills and dikes near the site of the now defunct Hudspeth Mill; these basalts contain orthoclase in their groundmass.
(3) the many small dikes and plugs of minette lamprophyre near Black Butte which contain phenocrysts of phlogopite and clinopyroxene, with abundant small apatite needles.
(4) a rhyolite dike near Scott Butte that contains phenocrysts of quartz, biotite, plagioclase, and distinctive red crystals of almandine garnet.