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Eclogites from Southwestern Oregon

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Eclogites from Southwestern Oregon EDWARD D. GHENT Department of Geology, University of Calgary, Calgary 44, Alberta, Canada ROBERT G. COLEMAN U.S. Geological Survey, Menlo Par\, California 94025 Eclogites from Southwestern Oregon ABSTRACT graphic, and geochronologic data on eclogite blocks from southwestern Oregon are available, Eclogite, high-grade blueschist, and am- but chemical data on bulk rock composition phibolite blocks occur within the Mesozoic and individual minerals are not so extensive as Otter Point Formation of southwestern Oregon those on comparable rocks in California. The and are inferred to have been tectonically purpose of this paper is to (1) report electron emplaced by eastward-directed overthrusting microprobe analyses of minerals and three involving Colebrooke Schist and serpentinite. new whole-rock chemical analyses on this Eclogite from southwestern Oregon is very eclogite, (2) interpret these data in light of similar in bulk chemistry and mineralogy to experimental and computed phase equilibria, the well-studied eclogite of California. and (3) point out certain regularities in the Calculations of phase equilibria at load chemistry of Group C eclogite (Coleman and 1 pressures of 7 and 10 kb and T = 400°C to others, 1965) from Oregon and California and 550°C suggest that many of the hydrates found to contrast this chemistry with that of asso- in eclogite could have been stable at very low ciated blueschist and mafic volcanic rocks. H2O fugacities. The lack of lawsonite and the presence of almandine-grossular garnet set a FIELD RELATIONS AND maximum limit on H»0 fugacity for a given PETROGRAPHY Ps-T. Eclogite, blueschist, and amphibolite blocks Chemically, Group C eclogite from Oregon occur within the Otter Point Formation (Fig. and California characteristically is nepheline 1). Mapping by Lent (1969) and Coleman normative and is enriched in normative (1972) has provided evidence that the Cole- diopside relative to basaltic compositions. The brooke Schist is in tectonic contact with the present chemistry of this eclogite may be the Otter Point Formation and was transposed result of metasomatism in an ultramafic eastward by overthrusting (Fig. 1). The over- environment with low a Si02 and high a Ca, thrusting was facilitated by serpentinite and but outside the stability field of serpentine. produced a tectonic mélange beneath the The generally high jadeite content of thrust consisting of Otter Point Formation, clinopyroxene from Group C eclogite com- high-grade blueschist and eclogite blocks, pared with Group A and Group B eclogite is amphibolite, and low-grade blueschist blocks. largely a function of bulk rock chemistry. Later erosion dissected the Colebrooke thrust Crystallization under low a Si02 conditions sheet, leaving klippen of Colebrooke Schist stabilizes jadeite in clinopyroxene at lower Pe and serpentinite. Landsliding within the for a given T. underlying mélange has produced further mixing of tectonic blocks. INTRODUCTION The locations and mineralogy of eclogite The occurrence of isolated blocks of high- samples from southwestern Oregon are given grade blueschist, amphibolite, and eclogite in Table 1. The mineralogical subdivisions used within the Jurassic and Cretaceous eugeosyn- clinal rocks of southwestern Oregon and the 1 Coast Ranges of California has puzzled geol- Eclogite is divisible into three groups based on mode of occurrence: Group A, inclusions in kimberlite, basalt, ogists for many years. Coleman and Lanphere or layers in ultramafic rocks; Group B, bands or lenses (1971) published new geological and geo- within migmatite gneissic terrains; and Group C, bands chronological data on these rocks and provided or lenses within high-P-low-T metamorphic rocks a review of previous literature. Field, petro- (Coleman and others, 1965). Geological Society of America Bulletin, v. 84, p. 2471-2488, 5 figs., August 1973 2471 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2471/3417835/i0016-7606-84-8-2471.pdf by guest on 24 September 2021 2472 GHENT AND COLEMAN Colîbrooke Schist —I3C million years I I Otter Point Formation Upoer Jurassic grade E; num- shown. Figure 1. Geologic map of part of southwestern high- and low-grade blueschist (after Coleman, 1972; Oregon showing distribution of blocks of eclogite, Lent, 1969). in the table require some comment. It was Minerals which are not clearly of replace- assumed that omphacite, garnet, and rutile ment origin are epidote, glaucophane, phengite' represent the "primary" mineralogy of the some of the sphene, and some Mg-chlorite. eclogite. Other minerals are put into one of Epidote occurs as inclusions within garnet, three categories: (1) minerals showing obvious and sometimes these inclusions are accom- replacement relations; (2) principal minerals panied by chlorite, indicating possible replace- showing no obvious replacement relations; ment relations. In some rocks (56-69; 27-69-1), and (3) accessory and vein minerals. however, epidote is a principal constituent and The following criteria were used to indicate occurs in layers with omphacite and garnet replacement relations. The least ambiguous with no evidence of textural disequilibrium. criterion is that of pseudomorphism, but this Two different compositional ranges and criterion could only be applied to chlorite textural types of chlorite occur in several forming pseudomorphs after garnet. A second, samples (Table 1). Chlorite which shows no less certain criterion, is rimming of a primary evidence of textural disequilibrium is always mineral by other minerals for which a reason- optically positive and shows normal first-order able reaction relation might be inferred, for interference colors, suggesting a Mg-rich example, rutile included within sphene. variety (Albee, 1962). In the same samples, a Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2471/3417835/i0016-7606-84-8-2471.pdf by guest on 24 September 2021 ECLOGITES FROM SOUTHWESTERN OREGON 2473 TABLE 1. MINERALOGY OF ECLOGUE FROM SOUTHWESTERN OREGON Minerals with no Minerals showing Accessory and possible replacement obvious replacement Sample no.* vein minerals relations relations 79-69A FeMg chlorite (garnet) phengite quartz (v) sphene (rutile) quartz albite (v) glaucophane (?)(omphacite) epidote apatite sphene chalcopyrite pyri te 79-69 same same same 95-69-1 MgFe chlorite (garnet) phengite quartz sphene (rutile) epidote chalcopyri te glaucophane (?)(omphacite) sphene glaucophane 82-69-1 FeMg chlorite (garnet) phengite phengite sphene (rutile) sphene FeMg chlorite glaucophane (?)(omphacite) glaucophane sphene(?) MgFe chlorite 56-69 MgFe chlorite (garnet) sphene aragonite MgFe chlorite (omphacite) epidote sphene (v) glaucophane (green amphibole) MgFe chlorite epidote (v) glaucophane (?)(omphacite) glaucophane albite (v) sphene (rutile) phengite 27-69-1 FeMg chlorite (garnet) epidote epidote (v) sphene (rutile) phengite glaucophane (?)(omphacite) glaucophane Sample locations 79-69 White Rock Creek (SW*s, sec. 30, T. 30 S., R. 13 W.) Langlois quadrangle. Banded eclogite block 79-69A (40 ft x 40 ft) in stream bed. Specific gravity = 3.31. 95-69-1 Waterman Ranch (SWig, sec. 7, T. 30 S., R. 14 W.) Langlois quadrangle. Tectonic block (40 ft x 40 ft) on ridge containing layers of eclogite with glaucophane and garnet. Specific gravity = 3.23. 82-69-1 White Creek (SWk, sec. 30, T. 30 S., R. 13 W.) Langlois quadrangle, approximately 1,000 ft downstream from 79-69. Layered eclogite block (10 ft x 20 ft) in stream bed. Edges of mass show strong retrogression. Specific gravity • 3.49. 56-69 South Fork, Floras Creek (SW*s, sec. 1, T. 31 S., R. 13 W.) Langlois quadrangle. Banded eclogite block (50 ft x 50 ft) broken up and associated with extremely coarse-grained blue- schist. 27-69-1 Otter Creek (NWJs, sec. 5, T. 32 S., R. 13 W.) Langlois quadrangle. Small block in stream associated with blueschist. Specific gravity = 3.31 to 3.26. 'All samples contain omphacite, garnet, and rutile. second chlorite forming pseudomorphs after the exception, and if there is a replacement garnet is usually optically negative and shows relation, the evidence is not clearcut. anomalous interference colors, suggesting a more iron-rich chlorite. Electron-microprobe ELECTRON-MICROPROBE ANALYSES analyses confirm the iron-rich character of the latter type of chlorite (Table 6). Introduction Phengite usually occurs in an intersertal Electron-microprobe analyses of garnet, texture with garnet and omphacite. Contacts clinopyroxene, glaucophane, phengite, and between phengite, garnet, and omphacite are chlorite are given in Tables 2 through 6 and smooth and regular, suggesting no replacement. Figure 2. Data were collected on an ARL More rarely, phengite occurs with chlorite in EMX electron microanalyzer, and the data pseudomorphous aggregates after garnet (82- were corrected following methods outlined 69), suggesting that garnet was not stable with by Bence and Albee (1968) and Albee and Ray this phengite. (1970). The analyses quoted represent the Glaucophane-omphacite textural relations averages of 15 to 40 spot analyses per sample. are ambiguous. Smooth, regular contacts be- Garnet. The summary of analytical data in tween the two minerals are the rule rather than Figure 2 indicates that garnet from Oregon Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2471/3417835/i0016-7606-84-8-2471.pdf by guest on 24 September 2021 Figure 2. Triangular diagrams featuring: (a) the associated with kimberlite pipes. Modified
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