- The Clarno Formation is a series of volcanic, volcaniclastic, and related intrusive
rocks located in central Oregon. It is the westernmost extent of a broader Eocene
magmatic belt that covers much the western United States. The magmatic belt
stretches eastward from Oregon to western South Dakota, and from the Canadian
Yukon to northern Nevada. While once attributed to subduction of the Farallon Plate
under North America, more recent work suggests that a more complex tectonic regime
involving extension was in place during the early Cenozoic.
In the vicinity of Mitchell, Oregon, the Clarno Formation is well represented
along with Mesozoic metamorphic and sedimentary units, and younger Tertiary volcanic
and volcaniclastic units. In this area, Clarno volcanic activity occurred from
~52-42 Ma, producing mostly andesites and related volcaniclastic rocks. The Mitchell
area is also underlain by related intrusive bodies ranging from basalt to rhyolite in
composition. The Clarno was most active at ~49 Ma, and is dominantly calcalkaline.
In addition, there are several coeval alkaline and peraluminous intrusives
also scattered throughout the Clamo Formation. While these suites are less voluminous
than the calc-alkaline magmatism, they offer insight into the tectonic and magmatic
processes at work in this area during the Eocene.
Whereas silicic intrusions are common in the Clarno, the high-silica rhyolite dike
on the south face of Scott Butte is unusual due to its large garnet phenocrysts. The
existence of primary garnet in rhyolitic magmas precludes middle to upper crustal
genesis, a common source for silicic magmas. ⁴⁰Ar/³⁹Ar age determinations of the
biotite indicate an age of ~51 Ma. This is after andesitic volcanism had commenced,
but prior to the most active period of extrusion. The presence of the almandine garnet
indicates that the dike represents partial melting of lower crustal (18-25 km) material.
The presence of a high field strength element (HFSE) depletion commonly associated
with subduction are magmatism indicates that either the source material had previously
been metasomatised, or that some subduction melts/fluids (heat source) mixed
with the crustal melt.
Two alkaline suites, a high-K calc-alkaline basanite (Marshall and Corporate
Buttes) and alkaline minette/kersantite lamprophyres (near Black Butte and Mud
Creek), were emplaced ~49 Ma, during the height of calc-alkaline activity. The
basanite lacks the HFSE depletion common in the other Clarno rocks. Instead it has a
HIMU-type (eg. St Helena) ocean island basalt affinity, resulting from partial melting
of enriched asthenospheric mantle. In contrast, the lamprophyres represent hydrous
partial melts of metasomatized litho spheric mantle veins and bodies.
Alkaline magmatism was not limited to the most active periods of calc-alkaline
activity. The emplacement of an alkali basalt (Hudspeth Mill intrusion) at ~45 Ma
occurred four million years after the largest pulse of volcanism, but still during calcalkaline
activity. This alkali basalt represents partial melting of metasomatized lithospheric
The occurrence of these alkaline suites coeval with the calc-alkaline activity is
significant in that it disputes prior subduction theories for the broader Eocene magmatism
that are based on spatial and temporal variations from calc-alkaline to alkaline
magmatism. These suites also give further insight into the complex tectonic regime
that existed in Oregon during the Eocene. The occurrence of asthenospheric
melts not caused by fluid fluxing, along with lower lithospheric alkaline melts, are
normally associated with extension. Extension provides these magmas with both the
mechanism for melting, and the ability to reach shallow crust with little or no contamination.
Extension is in agreement with both White and Robinson's (1992) interpretation
that most Clarno Formation deposition occurred in extensional basins, and
with other provinces in the broader Eocene magmatic belt.