THE MAFIC DIKE SYSTEM OF THE MITCHELL AREA
A set of basaltic dikes extends west-by-northwest across the Mitchell area and displays such uniformity of orientation, composition, and age that it is referred to as The Mafic Dike System (Taylor, 1981). The aggregate length of the system is 17.8 km but the dikes crop out in an en-enchelon pattern of many separate segments (see map).
Individual segments range in length from 2 km to a few meters. Most dike segments are 4 to 10 m in width, but they diminish in thickness to thin edges instead of blunt terminations at their extremities. In some locations, the segments are joined nearly at right angles by short connective dikes; elsewhere, bulbous protrusions may extend into incompetent mudstones. In contrast, some dikes apparently were unable to invade competent rocks; they terminate just below thick resistant conglomerates or preexistent sills. The dikes are exposed best in the central part of the Mitchell Anticline where they have intruded Cretaceous marine sedimentary rocks of the Gable Creek and Hudspeth Formations. The dikes also have penetrated the overlying volcanic rocks of the Clarno Formation on both limbs of the anticline and were probably associated with basaltic lava flows of similar composition in the lower John Day Formation. The dike segments might coalesce with a continuous dike at depth and they might have bifurcated into more segments at higher levels, now removed by erosion.
The best age data for time of emplacement of the dike system is 34.1 Ma, based upon Ar-Ar determinations of a fresh marginal sample by Appel (2001) using U.S.G.S. facilities at the Denver Laboratory. Several other questionable determinations of varying ages have been offered; but they were performed on extremely altered samples. See detailed observations of “Radiological Ages” available from the MENU.
Margins of the dikes consist of very fine-grained, black, devitrified glass in a chilled zone up to 1 m wide. Dike interiors are gray where fresh, uniformly coarse grained, and commonly display crude columnar jointing. Weathering of dike rocks is always more advanced in the interior than at the margins and produces a dark reddish-brown, ferruginous soil. Elongate depressions are produced by erosion of dikes that cut resistant strata, such as Gable Creek conglomerate.
Elsewhere, as in soft mudstones of the Hudspeth Formation, low ridges mark the position of the dikes. Volcanic mudflow deposits of the Clarno Formation have been so indurated in close proximity to the dikes that double walls of erosion-resistant mudflow can be found standing in relief along dike margins.
Rocks of the mafic dike system in the Mitchell area are nonporphyritic low-alumina tholeiites enriched in Fe and Ti. They are distinct from Mitchell-area basaltic flows and dikes of the Clarno Formation in which the Al, Ca, and Mg contents are much smaller. Major-element composition of the dikes is also distinct from basalts of the Picture Gorge Formation in the Mitchell area because the dikes contain much more Fe and Ti. The closest compositional match to the mafic dike system is with basaltic lavas in the lower part of the John Day Formation northwest of Mitchell. These John Day lavas lack phenocrysts and contain high Fe and Ti. It is not suggested that the mafic dike system was the direct source of these particular John Day lavas, because the dikes are high-Si, low-K tholeiites while the lavas are low in Si and high in K, transitional to alkali basalts.
Wherever the width of dikes exceeds 3 m, the interior is texturally hypidiomorphic-granular, consisting of plagioclase, monoclinic pyroxene, and opaque oxides up to 1.5 mm in length. Plagioclase of composition An50-60 makes up 75 percent of the dike rocks. The pyroxene is a dusky augite with a 2V of 45-48 degrees and commonly constitutes 18-20 percent of the rock. The opaque oxide, which is titanomagnetite in the fine-grained margin, becomes a coarse-grained skeletal and dendritic admixture of ilmenite and magnetite in the interior. Alteration of dike rocks by deuteric processes and weathering has converted all olivine to green chlorite minerals.
The dike segments are not randomly located; in both position and orientation their emplacement appears to have been systematically controlled. It is especially significant that the dikes are parallel to and confined to the vicinity of the Mitchell Fault (see map again).
However, the dikes seem to have avoided the trace of the fault itself. What could have localized these dikes? This question can be readily answered if it is assumed that the dikes were emplaced after most of the right-lateral movement had occurred on the Mitchell Fault but before the responsible conditions of stress had died out. If a deep-seated body of mafic, nonporphyritic, fluid magma, elongate in a west-northwest direction, was intersected by a zone of crustal disruption along a still-active Mitchell Fault, the magma could have found paths of least resistance along gash fractures at acute angles to the fault trace, while passage of magma into the fault zone itself would have been hindered. Indeed, without this combination of tectonic pressures and avenues of escape, the dense Fe-rich magma might never have reached shallow crustal levels.