Geological Society of America Bulletin

doi: 10.1130/0016-7606(1971)82[2667:OOPZIP]2.0.CO;2 1971;82, no. 9;2667-2670Geological Society of America Bulletin

 DAVID B JORGENSON Southwestern OregonOrigin of Patchy Zoning in Plagioclase from Gabbroic Rocks of  

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DAVID B. JORGENSON The Anaconda Company, P. 0. Box 638, Grants, New Mexico 87020

Origin of Patchy Zoning in Plagioclase fromGabbroic Rocks of Southwestern Oregon

ABSTRACT

Patchy zoning in plagioclase has been ob-served in gabbroic rocks in southwestern Ore-gon. The plagioclase consists of a partiallyresorbed calcic core, and sodic rims and patchessurrounding and included in the core. Themechanism employed to explain this type ofzoning is an increase in water pressure in themagma during the initial stages of crystalliza-tion, and is discussed in terms of the three-component system diopside (hornblende)-al-bite-anorthite rather than the two-componentsystem albite-anorthite. Textural features seenin thin section support the proposed mech-anism.

INTRODUCTIONPatchy zoning in plagioclase is a common and

widespread phenomenon in gabbroic rocks as-sociated with the Josephine mafic-ultramaficigneous complex of southwestern Oregon andnorthwestern California. The complex lies be-tween lat 4l°30' N. and 42"40' N., and long12330' W. and 124°0' W., and intrudes Up-per Jurassic rocks of the Galice Formation ofthe Klamath Mountain province (Irwin, 1964).

The gabbroic rocks of the Josephine igneouscomplex consist of troctolites, anorthosites,pyroxene gabbros, and hornblende gabbros,the hornblende gabbros being the most wide-spread. The remainder of the complex is madeup almost entirely of high-magnesium perido-tite, which is by far the most abundant rock typeexposed. A few small quartz diorite bodies arealso associated with the complex.

Vance (1965) has presented the most com-plete treatment of patchy zoning in igneousplagioclase. He discussed a number of possiblemechanisms which might produce patchy zon-ing, and concluded that the process dependsupon a release of pressure in an anhydrousmagma. The purpose of this paper is to presentanother mechanism which better explains theorigin of the patchy zoning in plagioclase fromthe gabbroic rocks of the Josephine complex.

DESCRIPTION

Patchy zoning occurs in rocks described ashornblende gabbro (Jorgensen, 1970) andhornblende diorite (Wells and others, 1949).Plagioclase, hornblende, magnetite, and quartzare the principal minerals. Pyroxene, if present,is sparse, and generally occupies the cores oflarge hornblende crystals. Quartz is not alwaysfound in these rocks, but is interstitial whenpresent and constitutes from less than 1 percentto 26 percent by volume.

The plagioclase of the hornblende gabbroranges from Anjo to Angs, but compositionsdown to An44 have been observed in the outerportions of zoned rims. The grains which havepatchy zoning consist of calcic, generally re-

I mmFigure 1. Diagram illustrating patchy zoning and re-

sorption in plagioclase from hornblende gabbro. Stip-pled area is resorbed calcic core; white is more sodic rimand patches.

Geological Society of America Bulletin, v. 82, p. 2667-2670, 5 figs., September 1971

2667

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2668 D. JORGENSON—PATCHY ZONING OF SOUTHWESTERN OREGON

crack

HI mm

Figure 2. Diagram illustrating patchy zoning and re-sorption in plagioclase from hornblende gabbro. Arrowpoints to crack in core plagioclase along which rimmaterial invaded.

sorbed, cores which may display normal zon-ing, and more sodic plagioclase within andsurrounding the cores (Figs. 1 and 2). The rimplagioclase is commonly zoned. The patches areunzoned and have the same composition as therim plagioclase. The compositional differencebetween the cores and the corresponding rimsand patches is sometimes as great as 26 molepercent anorthite, as seen in Table 1. Plagio-clase compositions were determined mainly byuniversal-stage techniques (Slemmons, 1962),but a few were analyzed with an electron micro-probe. One such analysis is shown in Table 2.

Any mechanism that is proposed to explainthe origin of this type of zoning must also ex-plain the obvious resorption of the initiallyeuhedral plagioclase cores. The crystallizationof the rim plagioclase apparently followed a

TABLE 1. COMPOSITIONS OF PATCHILYZONED PLAGIOCLASE SAMPLES

Sample Core An

P68

P75

P77

P20

P85

P98

P6

P3

72

71

78

76

80

84-69

71

75*

60

53

52

53-44

54

53-48

58

49

60

53

52

53

54

53

58*

49

period during which the cores were unstable inthe magma and subjected to partial melting.When conditions were once again suitable forplagioclase crystallization, a more soda-richplagioclase was stable in the melt, and subse-quent crystallization of the rims and internalpatches took place. The patches were probablyformed by the invasion of the liquid alongcracks and cleavages in the solid cores, as evi-denced by the relations shown in Figure 2.

TABLE 2. ANALYSIS OF PLAGIOCLASE SAMPLE P3

Al,0,

core

patch

50

56

.40

.07

33.

27

.83

.66

15.89

9.74

2.56

5.48

0.04

0.07

75

49

Chemical analysis, Table 2.

DISCUSSION

The mechanism involving a release of pres-sure in an anhydrous magma adopted by Vance(1965) cannot explain the zoning observed inthe hornblende gabbro of the Josephine igne-ous complex. Vance's mechanism was discussedin terms of the two-phase plagioclase system.Using this system it is necessary, changing fromhigh to low pressure, to suppress the solidusand steepen both the solidus and liquiduscurves to produce resorption followed by crys-tallization of a more sodic plagioclase (Fig. 3A).The melting relations of plagioclase at highpressure have been determined since Vance'swork (Lindsley, 1967), and are shown in Fig-ure 3B. Although the liquidus and soliduscurves at 1 atm are somewhat steeper than the10 kb curves, the liquidus and solidus are alsoless curved at higher pressures, reducing thesize of the liquid plus plagioclase field. Thisflattening of the curves counteracts the effect ofthe slope differences, and a decrease in pressurefrom 10 kb to 1 atm will produce crystallizationof a more calcic, rather than a more sodic, rim.A decrease from an initial pressure which is lessthan 10 kb, however, may produce the effectsdescribed by Vance, especially in the sodium-rich plagioclase field. The near parallelism ofthe high and low pressure curves in the regionof labradorite and bytownite is another factorwhich makes this mechanism insufficient to ac-count for patchy zoning in plagioclase morecalcic than about An 50.

An investigation of the effects of water onplagioclase crystallization reveals that with in-creasing water pressure the slopes of the liqui-dus and solidus in the albite-anorthite system

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DISCUSSION 2669

lessen slightly (Yoder and others, 1957), butprobably not enough to account for the core-rim compositional gaps observed in the horn-blende gabbro. However, an increase of waterpressure has a profound effect on the crystalliza-tion of plagioclase when the three-componentsystem diopside-albite-anorthite is considered,and can be used to explain the origin of patchyzoning. The equilibrium crystallization trendwithin this system at 1 atm pressure was pre-sented by Bowen (1915). The effects of higher

B

Figure 3A. Hypothetical plagioclase diagram at ele-vated pressure (upper curves) compared to curves at 1atm. Initial crystallization of plagioclase X from liquidJ will occur at higher pressures. With continued crystall-ization the liquid composition moves toward K, and thecrystal composition moves toward Y. A release of pres-sure at this point will cause the plagioclase to begin tomelt, and with falling temperature the liquid composi-tion will move to L and crystallize a plagioclase of com-position Z, producing the compositional gap betweencore (Y) and rim (Z). (Hypothetical crystallization se-quence is from Vance, 1965.)

Figure 3B. Experimentally determined equilibriumcurves for plagioclase at 10 kb (Lindsley, 1967) and 1atm. It is seen that if the crystallization sequence de-scribed above is applied to this diagram, a drop in pres-sure from 10 kb to 1 atm will produce more calcic, ratherthan more sodic, rims (compare compositions Y and Z).

pressures on the system were investigated byLindsley and Emslie (1968), and the diagram at5 kb shown in Figure 4A was interpolated fromtheir work.

The patchy zoning can be explained by fol-lowing a hypothetical crystallization sequencein the ternary system. Under anhydrous condi-tions at 5 kb, a liquid of composition A in Fig.4B will begin crystallizing plagioclase of com-position Xi when the liquidus temperature isreached. With falling temperature, the liquidwill change composition along the curve A-Bwith the crystallization of increasingly moresodic plagioclase. If there is an increase of waterpressure before the liquid composition reachesthe boundary curve (liquid at point B, in equi-

B

Figure 4A. Ternary system diopside-albite-anorthiteat 5 kb (interpolated from Lindsley and others, 1968).The first mineral to crystallize from a melt of composi-tion A will be a plagioclase with composition X i . Withfurther crystallization, the liquid will move along thecurve A-B toward point B, and the plagioclase composi-tion will move toward X2, under equilibrium condi-tions. At B, diopside begins to crystallize with theplagioclase, and the liquid composition will move downthe diopside-plagioclase boundary curve until all the re-maining liquid is used up.

Figure 4B. Diopside-albite-anorthite system at 5 kbwater pressure (adapted from Yoder, 1965). Dashed lineis 5 kb boundary curve. See text for explanation of hypo-thetical crystallization sequence.

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2670 D. JORGENSON—PATCHY ZONING OF SOUTHWESTERN OREGON

librium with plagioclase at point X2), and the?H2O increase is sufficient to push the bound-ary curve below point B, the plagioclase will beunstable and begin to melt, and diopside willbegin to crystallize. This will move the liquidcomposition away from the diopside corner un-til it reaches the new boundary curve at pointC. At this point, plagioclase of composition Ywill begin to crystallize along with the diopside,and will form sodic rims around the partiallyresorbed original calcic cores. The remainingliquid will then change in composition alongthe boundary curve toward D, while plagio-clase and diopside crystallize together.

Diopside is not found in abundance in thehornblende gabbro, but with the increased wa-ter pressure it is easy to imagine how horn-blende could crystallize instead of diopside,incorporating calcium and aluminum and leav-ing the liquid enriched in sodium and silica.There is textural evidence which supports thepatchy zoning mechanism described above. It isquite common to find hornblende crystals indirect contact with the resorbed plagioclasecores, and the rim plagioclase partially enclos-ing the hornblende (Fig. 5). This suggests thatthe hornblende began crystallizing prior to thecrystallization of the rim plagioclase.

The mechanism described here for the devel-opment of patchy zoning is proposed to explainthe plagioclase zoning observed in hornblende

I mm

Figure 5. Diagram illustrating textural relations seenin hornblende gabbro. H is hornblende, P is rim plagio-clase, and the stippled pattern is core plagioclase.

gabbro of the Josephine complex of southwest-ern Oregon. Any attempt to generalize thismechanism to other rock types and other areasmust be done with the realization that differentprocesses can often produce similar features inrocks. The mechanism presented here is basedon hypothetical considerations, and must awaitlaboratory data to prove or disprove it conclu-sively.

ACKNOWLEDGMENTS

This paper resulted from research conductedfor a doctoral dissertation at the University ofCalifornia, Santa Barbara, and was partially sup-ported by a grant from the Society of Sigma Xi,and a National Science Foundation summertraineeship grant. I am indebted to Dr. CliffordA. Hopson for his critical review of the manu-script.

REFERENCES CITED

Bowen, N. L, 1915, The crystallization of haplobas-altic, haplodioritic, and related magmas: Am.Jour. Sci., ser. 4, v. 40, p. 161-185.

Irwin, W. P., 1964, Late Mesozoic orogenies in theultramafic belts of northwestern California andsouthwestern Oregon: U. S. Geol. Survey Prof.Paper 501-C, p. C1-C9.

Jorgenson, D. B., 1970, Petrology and origin of theIllinois River gabbro, a part of the Josephineperidotite-gabbro complex, Klamath Moun-tains, southwestern Oregon [Ph.D. dissert.]:Univ. California, Santa Barbara.

Lindsley, D. H., 1967, Melting relations of plagio-clase at high pressures: Carnegie Inst. Washing-ton Year Book 65, p. 204.

Lindsley, D. H., and Emslie, R. F., 1968, Effect ofpressure on the boundary curve in the systemdiopside-albite-anorthite: Carnegie Inst. Wash-ington Year Book 66, p. 479-480.

Slemmons, D. B., 1962, Determination of volcanicand plutonic plagioclases using a three- or four-axis universal stage: Geol. Soc. America Spec.Paper 69, 64 p.

Vance, J. A., 1965, Zoning in igneous plagioclase:Patchy zoning: Jour. Geol., v. 73, p. 636-651.

Wells, F. G., Hotz, P. E., and Cater, F. W., 1949,Preliminary description of the geology of theKerby quadrangle, Oregon: Oregon Dept.Geology and Mineral Industries Bull., v. 40,23 p.

Yoder, H. S., Stewart, D. B., and SmithJ. R., 1957,Feldspars: Carnegie Inst. Washington YearBook 56, p. 206-214.

MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 10,1971

PRINTED IN U.S.A.

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