MAJOR-ELEMENT COMPOSITIONS AND VARIATIONS

           IN CLARNO ROCKS OF THE MITCHELL AREA

 

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         Geochemical data pertinent to petrology of the Clarno Formation is abundant in published form but has seldom been identified with specific rock units or localities. In support of my interest in field data of the Mitchell area, I have analysed many igneous rocks by combined XRF and AA techniques. The results are summarized here in the form of several classical variation diagrams. Information based upon rare-earth or isotopic analyses of Clarno rocks has not been provided as this material is being offered.

 

         Volcanic and subvolcanic rocks of the Clarno Formation include basalts, basaltic andesites, andesites, dacites, and rhyolites, with their alkaline varieties trachydacite, trachyandesite, trachy basaltic andesite, and trachybasalt as defined by LeBas and others (1986).  

LeBas

Andesite is the most abundant Clarno rock, in contrast to the central High Cascades where basaltic andesite dominates.

Si contents

Rocks of the Clarno series are border-line calcalkaline-calcic  as defined by Peacock (1931), in contrast to the calcalkaline central Cascades, but this may be due to the presence of some secondary calcite in nearly all analyzed Clarno rocks.

alkali-lime

 In addition, it should be noted that the Clarno Formation and its correlatives extend from the Pacific Northwest eastward as far as Idaho and Wyoming in a broad volcanic field, in contrast to the narrow, subduction-driven Cascade arc.

volc fields

 

         Rhyolite and dacite compositions of lava flows are relatively rare in the Mitchell area but both occur as dikes, sills, plugs, and intrusive bodies up to 2 miles in size.  Even the largest of these bodies are aphanitic throughout.  This is probably due to a low content of volatiles in their magmas; the rhyolites and dacites do not contain hydrous minerals, and hydrothermal alteration of country rock extends only a few inches from intrusive margins.  In the transition from dacite to rhyolite, Clarno rocks display increasing K2O and decreasing Na2O.

K2O vs Na2O

         Most intermediate lavas of the Clarno Formation are two-pyroxene andesites that contain phenocrysts of augite, hypersthene, and calcic plagioclase.  It is difficult to distinguish trachyandesites from andesites without accesss to analytical data.  Some of the andesitic lavas display distinctive phenocrysts of quartz which are surrounded by reaction rims of clinopyroxene.  Many andesitic intrusive bodies are characterized by fresh hornblende phenocrysts up to 15 mm in size, while most hornblende in lava flows has been converted to oxy-amphibole.  

         As is true in most high-alumina calc-alkaline rock series, CaO is closely dependent upon SiO2 with a regression coeficient that approaches 0.9 in intermediate Clarno rocks.

CaO vs SiO2

         Olivine-bearing basalt and clinopyroxene-bearing basaltic andesite lavas of the Clarno series are accompanied by compositionally diverse mafic intrusives:

 

(1)Rocks of the Marshall Butte group contain abundant phenocrysts of olivine and clinopyroxene in a matrix of magnetite with nepheline in place of feldspar.  High values of normative neph characterize this group and a relatively large proportion of MgO and Na2O in comparison to other Mitchell-area rocks is illustrated in diagrams MgO vs.SiO2 and Na2O vs. SiO2.

MgOvsSiO2Na2O vs SiO2

(2)Aphyric alkali basalts characterize irregular intrusive bodies near Hudspeth Mill, west of Mitchell.  Their high content of MgO and K2O is also compared to other rocks in diagrams MgO vs. SiO2 and K2O vs. SiO2.  These alkali basalts contain orthoclase microlites in their groundmass and up to 15 percent of orth in their norms.

(3)Many dikes and irregular plug-like bodies of lamprophyric minnet occur in the vicinity of Black Butte.  They are characterized by abundant phenocrysts of phlogopite and ubiquitous carbonate.  Their high content of FeO and TiO2 is revealed in FeO vs. SiO2  and TiO2 vs. SiO2 variation diagrams, and by normative ilmenite.

FeO vs SiO2TiO2 vs SiO2

(4)  The Mafic Dike complex of the John Day Formation is included in these diagrams to provide comparison with Clarno rocks.

 

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