Laccolith-like emplacement model for the Papoose Flat pluton based on porphyroblast-matrix analysis

ABSTRACT

Detailed porphyroblast-matrix analysis with-in the concordant metasedimentary aureolerocks surrounding the Papoose Flat pluton ofeastern California indicates that inclusiontrails within porphyroblasts can be used asstrain markers to restore the aureole rocks totheir prepluton emplacement position. Usingporphyroblast-matrix relationships in combi-nation with measurement of stratigraphicsections and whole-rock geochemical analy-ses, we have determined the kinematics of ro-tation, the change in thickness and volume,and the amount of translation of the metased-imentary formations within the aureole.These data are consistent with initial em-placement of the magma as an inclined silland subsequent inflation into a pluton or lac-colith. The combination of structural andporphyroblast-matrix analysis leads to athree-dimensional kinematic history of thewall rocks wherein vertical upward transla-tion represents a significant part of the plutonemplacement-related strain history.

INTRODUCTION

In recent years there has been a resurgence ofinterest among structural geologists and petrolo-gists over the potential mechanisms by whichspace may be made in the Earth’s crust to ac-commodate emplacement of plutons. Emplace-ment mechanisms frequently cited in the litera-ture are either driven by tectonic processes thatcreate space for magma intrusion (shear zones,strike-slip bends, p-shears), or depend on themagma to create its own space (diapirism, stop-ing, ballooning, sheeting, in situ melting) (seereviews by Pitcher, 1979; Hutton, 1988; Clarke,

1992; Paterson and Fowler, 1993). In spite ofnew research initiatives, however, the emplace-ment of granite into the Earth’s crust remains acontroversial and poorly understood process, be-cause the space needed to accommodate the vol-ume of magma can rarely be totally accountedfor by the structures surrounding granite plutons.This space problem can be particularly acutewhere the aureole rocks surrounding a plutonlack strain markers. Similarly, the deflection ofrock units around plutons, as shown on geologicmaps, can also be misleading because the three-dimensional aspect of the kinematics involved inproducing such deflections are rarely known.

Recent advancements in our understanding ofmagmatic and solid-state flow fabrics in graniteshave led to a better understanding of flow duringfinal emplacement (see de Saint Blanquat andTikoff, 1997, for details), but how the wall rockswere displaced in order to make sufficient spacefor magma emplacement often remains unknown.Detailed structural analyses of aureole rocks arefew (e.g.,Akaad, 1956; Pitcher and Berger, 1972;Sylvester et al., 1978; Fyson, 1980; Sandersonand Meneilly, 1981; Davis, 1993), partly becauseof a lack of sufficient strain markers, and partlybecause it is seemingly difficult to separate theeffects of pluton-related strains from regionallyconcentrated deformation within the narrowaureoles of many plutons (Vernon et al., 1993a;Guglielmo, 1994). This is unfortunate, becauseas observed by Read (1957, p. 332), “The mech-anism of emplacement of these circumscribedbodies is more often to be determined from thestructures of their country-rock walls than fromtheir own (structure). Evidence of drag andmovement of the wall-rocks, combined withthat presented by the metamorphic history of theaureole-rocks, may be decisive.”

This paper describes the deformation featureswithin the highly strained aureole rocks sur-rounding the Papoose Flat pluton of eastern Cali-

fornia (Figs. 1 and 2). The Papoose Flat plutonhas been considered to be a classic example of aballooning pluton since Nelson et al. (1972) andSylvester et al. (1978) documented the intenseattenuation and concordancy of the aureole rockssurrounding the pluton. Doubt has been cast onthe applicability of the ballooning model by thediscovery of simple shear indicators at the marginof the pluton (Law et al., 1990, 1992, 1993;Paterson et al., 1991), and by volume balance cal-culations in which the calculated pluton “vol-ume” was compared with the observed wall-rockstrains (Paterson and Fowler, 1993).

The aim of this paper is to describe a detailedporphyroblast-matrix analysis from samples takenwithin the metamorphic aureole surrounding thePapoose Flat pluton. The results of this analysissupport the use of porphyroblasts as strain markersand also demonstrate how porphyroblast-matrixrelationships may aid in determining the sequenceof metamorphism versus deformation. Combiningthe porphyroblast-matrix results with more tradi-tional structural analysis on the aureole and sur-rounding wall rocks has led to: (1) a full strainanalysis of the aureole rocks taking into accountshape change, rigid-body rotation, volume change,and rigid-body translation and (2) a two-stageintrusion model of forcible emplacement for thePapoose Flat pluton whereby magma is initiallyintruded as an inclined sill, which then inflates intoa laccolith-like pluton.

GEOLOGIC BACKGROUND AND FIELDRELATIONSHIPS

The Papoose Flat pluton is one of severalMesozoic granitic plutons that intrude the cen-tral White-Inyo Range of eastern California andare associated with emplacement of the SierraNevada batholith (Bateman et al., 1963; Ross,1965; Bateman, 1992). The Papoose Flat plutonis among the youngest dated plutons in the

96

Laccolith-like emplacement model for the Papoose Flat pluton based onporphyroblast-matrix analysis

Sven S. MorganRichard D. Law* } Virginia Polytechnic Institute and State University, Department of Geological Sciences,

Blacksburg, Virginia 24061–0420

Matthew W. Nyman University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1063

GSA Bulletin;January 1998; v. 110; no. 1; p. 96–110; 12 figures; 1 table.

*e-mail: [email protected]

Geological Society of America Bulletin, January 1998 97

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Figure 1. Simplified geologic map of Papoose Flat pluton and southwestern end of the Inyo anticline (after Nelson et al.,1991). Line of section (A–A′) for Figure 2 is indicated.

Figure 2. West to east crosssection parallel to long axis (seeFig. 1) of the Papoose Flat plu-ton. Surface geology is based onthe geologic map of Nelson et al.(1978, scale 1:62 500) (V = H;no vertical exaggeration). Wavyornament within pluton indi-cates extent of pluton’s gneissicborder facies; note that forclarity, the thickness of thegneissic border facies is highlyexaggerated.

range; it has a U-Pb monazite age of 83.1 ± 0.4Ma (Miller, 1996) and K-Ar biotite ages of75–81 Ma (Kistler et al., 1965). The pluton wasoriginally described by Sylvester et al. (1978) asa biotite quartz monzonite containing K-feldsparmegacrysts, although normative compositions ofmegacryst-poor samples collected from the plu-ton by Brigham (1984, p. 55) plot within thegranite and granodiorite fields as defined byLeMaitre (1989).

The White-Inyo Range is an elongate horstblock that trends north-northwest and parallelsthe trend of the eastern Sierra Nevada Range15–20 km to the west. The sedimentary rocks inthe central White-Inyo Range consist of a latePrecambrian through Paleozoic platform marginsequence that exhibits open to tight folds fromregional to outcrop scale. The central part of therange is structurally dominated by the Inyo anti-cline (Fig. 1), which is a north-northwest–trend-ing upright fold that can be traced for more than40 km along its hinge. The Papoose Flat plutonintruded the southwestern limb of the anticline(Nelson et al., 1972; Sylvester et al., 1978), andlocally overturned and deflected strata aroundthe pluton. The sedimentary rocks in the rangeare weakly metamorphosed (lower greenschistfacies, see Ernst et al., 1993; Ernst, 1996), exceptwhere they are in contact with the Mesozoic plu-tons. Within the Inyo anticline north of the Pa-poose Flat pluton, a well-developed regionalslaty cleavage strikes north-northwest and dips5° to 20° more steeply to the southwest than bed-ding (Fig. 3).

The Papoose Flat pluton is roughly elliptical inshape; the long axis of the pluton trends east-west(Fig. 1). A narrow apophysis of granite protrudesat the eastern end. Around the western half of itsperimeter the pluton is surrounded by a concor-dant aureole of metasedimentary rocks. Withinthe aureole these plastically deformed LowerCambrian metasedimentary units have been at-tenuated to 10% of their regional stratigraphicthickness and parallel the contact with the pluton

for approximately 20 km (Sylvester et al., 1978).The contact with the metasedimentary forma-tions is characteristically very sharp, and withinthe pluton a solid-state foliation (referred to as the“gneissic border facies” by Sylvester et al., 1978)is strongly developed at distances of up to 10–20m from the margin and decreases in intensity to-ward the interior of the pluton. The gneissic foli-ation is best developed around the northwestern,western, and southern margins (Sylvester et al.,1978) as well as along the northeastern margin(de Saint Blanquat et al., 1994) of the pluton andis parallel to the pluton–wall-rock contact. On thenorthwestern, western, and southern marginsboth the compositional layering within the aure-ole and the foliation within the granite dips gently(25°–40°) away from the pluton, lending the plu-ton and surrounding metasedimentary units a do-mal shape at the present level of exposure, whichis probably close to the pluton roof (Sylvester etal., 1978). On the eastern margin the pluton–wall-rock contact dips steeply toward the pluton,indicating that the current exposure level is muchcloser to the base of the pluton.

An east to west cross section (Fig. 2) illustratesthe sill-like geometry of the pluton. On the west-ern margin, the Poleta Formation is structurallyabove the pluton. Stratigraphically, the next low-est formation is the Campito Formation, which isfound on the eastern margin structurally belowthe pluton. If the pluton is removed from thecross section, there is no missing stratigraphytraced from east to west.

The approximate extent of the contact aureolebased on the appearance of porphyroblasts inslate, phyllite, and schist was mapped bySylvester (1966). An internal, higher temperaturecomponent of the aureole was determined by thepresence of biotite and andalusite in pelitic schist,and a lower temperature component was definedby the presence of spots of muscovite and biotitein slate. Sylvester et al. (1978) estimated that theaureole is approximately 100–600 m wide alongthe western margin and as much as 1000 m in

width around the eastern part of the pluton. Cor-recting for contact dip indicates that the actualthickness of the aureole is on the order of100–200 m, measured normal to the contact.Petrologic study of highly strained metacarbon-ate and metapelitic rocks within the contact aure-ole surrounding the western part of the pluton in-dicates that temperature gradients were relativelyflat and narrow (<100 m); there was only a slightdecrease in temperature, from 500–550 °C at thepluton–wall-rock contact, to 450–500 °C at theaureole’s outer margin (Nyman et al., 1995). Onthe basis of stratigraphy and structure, Sylvesteret al. (1978) estimated that the pluton was in-truded at a depth between 6.4 and 9.2 km.

A well-developed stretching lineation ob-served in the western part of the aureole and plu-ton margin lies within the plane of the foliation,plunging to the north-northwest on the north sideof the pluton and to the south-southeast on thesouth side (Fig. 1). Crystallographic fabrics mea-sured in both the plastically deformed aureolequartzite and in quartz veins within the pluton’sgneissic border facies indicate that this stretchinglineation is associated with plane strain (k = 1)deformation (Law et al., 1992, 1993). The lin-eation is oriented perpendicular to the long axesof triaxial (approximate chocolate tablet) boudinsin schist and calc-silicate units within the aureolealong the south and western margin of the pluton.

Anisotropy of magnetic susceptibility (AMS)data from the Papoose Flat pluton (to be presentedin a separate paper), combined with micro-structural examination of the same samples fromwhich AMS analyses were obtained, indicate thatthe solid-state foliation and lineation within boththe pluton’s aureole and gneissic border facies areparallel to magmatic foliation and lineation withinthe interior of the pluton (de Saint Blanquat et al.,1994). The parallelism between the solid-stateand magmatic fabrics indicates that the two fab-rics developed synchronously and are related tothe same event (de Saint Blanquat et al., 1994).

PORPHYROBLAST-MATRIXRELATIONSHIPS

Porphyroblast-matrix relationships within theaureole of the Papoose Flat pluton impose im-portant constraints for modeling rigid body rota-tion and translation of the aureole rocks associ-ated with pluton emplacement, and also allowthe determination of the relative sequence ofmetamorphism, or porphyroblast growth, versusdeformation. The reference frame chosen for pre-senting our porphyroblast data is the foliation-parallel compositional layering (which we be-lieve represents bedding) within the aureolemetasedimentary rocks. In all figures, composi-tional layering has been rotated into a horizontal

MORGAN ET AL.

98 Geological Society of America Bulletin, January 1998

Figure 3. Contoured stereograms of poles to bedding and slaty cleavage within the westernlimb of the Inyo anticline north of Papoose Flat pluton.

orientation in order to view the angular relation-ships between porphyroblast inclusion trails andcompositional layering in the same referenceframe, regardless of the dip of the compositionallayering at an individual sampling site. Unlessotherwise stated, all porphyroblast data are pre-sented on section planes oriented at right anglesto the compositional layering and parallel to thestretching lineation; all these diagrams areviewed toward the northeast.

All samples analyzed for porphyroblast-matrix relationships are Harkless schist. In theWhite-Inyo Range the Harkless Formation con-sists of a series of shale and quartz sandstoneunits that can be traced into concordancy with thePapoose Flat pluton (Fig. 1), where it has beenmetamorphosed into a series of quartzite unitsinterlayered with andalusite schist. In the westernlimb of the Inyo anticline, north of the pluton andits aureole, the average dip of the bedding is 32°to the southwest, and the shale has a well-definedregionally developed slaty cleavage that dipsapproximately 20° more steeply to the southwest(Fig. 3). As the pluton is approached, the slatycleavage fabric, consisting of fine-grained chlo-rite, muscovite, and quartz, is gradually over-printed by a younger foliation associated withcontact metamorphism. The younger foliationis oriented parallel to the compositional layer-ing and defined by coarse muscovite porphyro-blasts. Within the Harkless schist adjacent to thePapoose Flat pluton, the contact-metamorphicassemblage is quartz + muscovite + andalusite +biotite + plagioclase.

On the basis of differences in inclusion traildensity and orientation (Morgan, 1992), anda-lusite porphyroblasts within the Harkless schistexhibit two distinct internal domains: (1) a coreregion with a high density of planar inclusiontrails (Sic) that are usually oriented at a low angleto compositional layering (Lc) and (2) a rim re-gion with a low density of inclusions (Sir) that arecoarser grained than in the core and that are con-tinuous in orientation from Sic and curve sharplyinto parallelism with the external foliation (Se)(Fig. 4, a, b, and f). Seis defined by elongate mus-covite grains that anastomose around andalusiteporphyroblasts; in hand sample Se generally de-fines a well-developed foliation that is parallel tocompositional layering (Lc).

The orientation of Sic relative to Lc was mea-sured in thin sections cut both parallel and per-pendicular to the lineation, and always perpen-dicular to the compositional layering and/orfoliation. In most samples andalusite is partiallyor completely replaced by sericite, thereby lim-iting the number of samples suitable for porphyro-blast-matrix analysis. Porphyroblast inclusiontrails from 12 samples taken from around thepluton were measured in thin sections cut paral-

lel to lineation (Fig. 5a). When these data arecombined onto one plot and viewed toward thenortheast, the two-dimensional mean orientationof Sic is inclined at 8° to Lc, measured in a clock-wise direction from Lc (i.e., in the thin-sectionplane, the mean angle between compositionallayering Lc and the internal inclusion trailswithin porphyroblasts is 8° measured down tothe southeast relative to a horizontal composi-tional layering, Lc). Note that Sic has the sameangular relationship to Lc regardless whether anindividual sample is taken from the northern partof the aureole, where Lc dips to the north, orfrom the southern part of the aureole, where Lcdips to the south.

Porphyroblast inclusion trails from six sam-ples taken from around the pluton were measuredin thin sections cut perpendicular to lineation(Fig. 5b). When these data are combined ontoone plot and viewed toward the north-northwest,the two-dimensional mean orientation of Sic isinclined at 8° to Lc, measured in a counterclock-wise direction from Lc.

In order to obtain the average three-dimen-sional orientation of Sic (relative to a horizontalLc), the data from thin sections cut parallel andperpendicular to lineation were combined on asingle stereogram (Fig. 5c). Figure 5c portraysthe three-dimensional orientation of Sic in sam-ple coordinates when the aureole metasedimen-tary formations are horizontal, or as if they wereundomed from concordancy with the pluton androtated into a horizontal orientation.

Porphyroblast-Matrix Interpretations

There is a spread of porphyroblast inclusiontrail (Sic) orientations within each thin section.However, regardless of geographic positionaround the pluton, and regardless of the directionand angle of dip of compositional layering (Lc),the majority of inclusion trails within individualthin sections are inclined at a very similar averageangles to compositional layering (Figs. 4d and 5).Four interrelated interpretations based on thisconsistent angular relationship between Sic andLc are proposed and help to clarify structuralrelationships.

(1) Deformation and metamorphism subse-quent to andalusite core growth have not causedporphyroblasts to rotate enough (relative to com-positional layering) to destroy the orientation ofan earlier fabric now preserved as Sic within theporphyroblasts. The spread in orientation of in-clusion trails indicates that most porphyroblastshave rotated by a maximum angle of between 10°and 30° with respect to compositional layering,even though these layers have been attenuated toas little as 10% of their original thickness(Sylvester, 1966; Sylvester et al., 1978). Very few

porphyroblasts have rotated more than 60°. Ingeneral, the most convincing argument for non-rotation, or little rotation, of porphyroblasts is theobservation of planar to sigmoidal-shaped inclu-sion trails that are found in the same orientationover large areas (Fig. 4d), even though significantdeformation postdated porphyroblast growth(Ramsay, 1962; Fyson, 1980; Vernon, 1988a,1988b, 1989; Jamieson and Vernon, 1987; John-son, 1990; Morgan et al., 1992).

(2) Porphyroblast growth has preserved a pen-etrative and planar fabric (Sic) in the aureolerocks that was at a low angle to compositionallayering (i.e., bedding) and that predated anda-lusite porphyroblast core growth. This older pen-etrative fabric (Sic) represents a slaty cleavage inthe original shale that cut bedding at a low angle,as discussed below.

Thin sections from all samples, regardless oflocation, exhibit a strong alignment of andalusiteporphyroblast inclusion trails that are inclined at asmall average angle (5°–25°) to compositionallayering. The slaty cleavage fabric, which is foundthroughout the western limb of the Inyo anticlineto the north and outside of the Papoose Flat plu-ton’s aureole, also consistently dips between 5°and 25° more steeply than bedding (Fig. 3).

The progressive overprinting of this regionalslaty cleavage has been studied in a transect sev-eral hundred meters south of the central section ofthe pluton’s southern margin; here attenuation andrecrystallization associated with contact meta-morphism decrease with distance from the plutonmargin. At 200 m from the pluton margin, bed-ding is concordant to the pluton margin, but theregional slaty cleavage has not been transposedinto the aureole fabric. Both bedding and cleavagedip toward the southwest; cleavage dips slightlymore steeply than bedding. As the pluton isapproached and metamorphism and deformationincrease, the cleavage is progressively overprintedby a new metamorphic foliation. At 50 m from thecontact, andalusite porphyroblasts contain inclu-sion trails that dip to the southwest slightly moresteeply than compositional layering, similar to thebedding-cleavage relationship in the outer aureolea few hundred meters to the south.

(3) Andalusite porphyroblast cores grew whenthe compositional layering (i.e., bedding) withinthe aureole metasedimentary rocks was originallyplanar, prior to being domed into its current struc-tural position during pluton emplacement. Whenthe aureole rocks are undomed (i.e., the dippingcompositional layering surrounding the pluton isunfolded back into a planar, although not neces-sarily horizontal, configuration) the core inclusiontrails (Sic) in andalusite porphyroblasts from thenorthwestern part of the pluton’s aureole becomealigned parallel to Sic in porphyroblasts from thesouthwestern part of the aureole (Fig. 6). This sug-

LACCOLITH-LIKE EMPLACEMENT MODEL



Figure 4. Photomicrographs and photomicrograph tracings of andalusite porphyroblast-matrix relationships. Thin sections are viewed towardthe northeast and are cut parallel to lineation and perpendicular to foliation. (a–c) Sketch and micrographs of andalusite porphyroblast in sam-ple PF 47 (see Fig. 5a for field location); sketch (a) is based on photomicrograph (b). Sic = alignment of inclusion trails observed within the coreof porphyroblast. Sir = alignment of inclusion trails observed within the rim of porphyroblast. Se = schistosity observed in the (exterior) matrix;Seis parallel to compositional layering. Photomicrograph (c) shows the same porphyroblast as in (a-b), but viewed at a smaller scale to include ma-trix foliation. (d–e) Sketch and micrograph of andalusite porphyroblast-matrix relationships taken from sample PF 281 (see Fig. 5a). Note con-stant angular relationship between inclusion trails in porphyroblasts and exterior foliation. (f) Euhedral andalusite porphyroblasts with distinctcores and rims defined by relative abundance of inclusions.


Figure 5. Location map of samples and rose-like diagrams of two-dimensional angular relationship between inclusion trails within cores of an-dalusite porphyroblasts (Sic) and compositional layering (Lc). All samples are rotated so that compositional layering, Lc (thin horizontal line), ishorizontal. (a) Angular relationship measured from thin sections cut parallel to lineation and perpendicular to compositional layering. View is tothe east-northeast. The individual black bars originating from the center of the circle that reach the outer circle indicate that 30% of the porphy-roblasts measured in that thin section contain orientations of Sic which vary within the 10° interval that the black bar covers. (b) Angular rela-tionship measured from thin sections cut perpendicular to lineation and perpendicular to foliation. View is to the north-northwest. Black bars thatreach the outer circle indicate that 20% of the porphyroblasts measured in that thin section contain orientations of Sic that vary within the 10° in-terval that the black bar covers. (c) Stereogram in sample coordinates that combines the two-dimensional data from the thin sections cut perpen-dicular and parallel to lineation to determine the three-dimensional orientation of Sic relative to a horizontal compositional layering Lc.

gests that the andalusite porphyroblasts grew andincorporated the planar fabric as inclusion trailsbefore the aureole rocks were domed, or deformedaround the evolving pluton (Fig. 6). This, in turn,suggests that most porphyroblasts have not rotatedsignificantly (less than 10°–30°) with respect tocompositional layering, but have rotated with re-spect to the geographic horizontal as the composi-tional layering was deformed. In addition, the ob-servation that Sic is planar and is not folded orsigmoidal in cross section indicates that theandalusite cores grew during a period of “static”contact metamorphism, and that core growth wasnot associated with penetrative deformation.

(4) The intense attenuation of the aureolerocks around the margins of the evolving plutoncaused the original regional slaty cleavage in theaureole rocks to be rotated with respect to com-positional layering (Lc) and become aligned par-allel to Lc. The orientation of the original slatycleavage within the aureole is only preserved asinclusion trails (Sic) within cores of andalusiteporphyroblasts and is oriented at a low angle toLc. Sic curves from the porphyroblast cores intothe rims to become aligned parallel to the presentmatrix foliation, Se. This curvature tracks the

rotation of the slaty cleavage, from its originalorientation at a low angle to Lc, to its present ori-entation parallel to Lc (Fig. 7).

Two additional interpretations based onporphyroblast-matrix relationships help deter-mine the timing of andalusite porphyroblastgrowth in relation to the intense attenuation of themetasedimentary formations.

(5) Andalusite core growth predates the de-velopment of the present exterior foliation, Se.Vernon et al. (1993a, 1993b) suggested that oneline of evidence for growth of porphyroblastsprior to “ductile foliation development” (i.e., de-velopment of a contact-metamorphic foliationparallel to pluton margins) is the fine grain sizeof inclusions within porphyroblasts when com-pared with the coarser grained matrix outside theporphyroblasts. The fine-grained size of inclu-sions indicates that when porphyroblasts initiallystarted growing and incorporating their matrix,the matrix assemblage had not yet begun react-ing to the changing physical environment in theaureole. Within the Harkless schist adjacent tothe Papoose Flat pluton, the older planar fabric,Sic, is preserved only within the cores of anda-lusite porphyroblasts, and is much finer grained

than the exterior foliation, Se (Fig. 4c).(6) Andalusite rim growth is synchronous

with the contact metamorphism and intensestructural attenuation that produced the exteriorfoliation, Se. Inclusion trails in the rims of anda-lusite porphyroblasts (Sir) curve from the planarcore regions (Sic) into parallelism with the ma-trix foliation (Se) tracking the attenuation event.Inclusions in the rims are also coarser grainedthan in the core region, indicating that the matrixwas finally reacting to the changing physical en-vironment as andalusite porphyroblasts contin-ued to grow.

AUREOLE STRAIN ANALYSIS

Complete specification of the strain history arock body has undergone is an inherently difficultand often impossible task. A complete analysis ofthe strain path requires information on distortion(shape change), rigid-body rotation, volumechange, and rigid-body translation (Ramsay,1969). Measurement of all these displacementcomponents is typically impossible, and we sug-gest that one of the prime reasons for the currentcontroversy over how plutons are emplaced maybe the difficulty in completely specifying the dis-placements associated with pluton emplacement.The concordant metasedimentary aureole rockssurrounding the Papoose Flat pluton present arare opportunity for a complete determination of

MORGAN ET AL.


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Figure 6. Undoming the aureole rocks (b) from concordancy with the pluton (a) rotates an-dalusite porphyroblasts so that inclusion trails are parallel in all porphyroblasts regardless ofposition around the pluton. This relationship indicates that porphyroblasts initially grew whenaureole rocks were originally planar, prior to doming into concordancy with the pluton. AfterMorgan (1992).

Figure 7. Sequential growth of andalusiteporphyroblast and incorporation of matrix asinclusion trails. Note that rotation of exteriormatrix foliation into parallelism with compo-sitional layering (Lc) begins late in growth his-tory of porphyroblasts.

the strain history. With the aid of porphyroblast-matrix relationships as strain markers, togetherwith measurement of stratigraphic sections andwhole-rock geochemical analyses, we attempt todemonstrate that: (1) the kinematics of rotation,(2) the change in thickness, (3) the change in vol-ume, and (4) the amount of translation of themetasedimentary formations within the aureolerocks, may all be quantified.

Rigid-Body Rotation

Assuming that the geometric relationshipbetween inclusion trails within andalusite porphyro-blasts and compositional layering represents thesame relationship between bedding and slaty cleav-age observed outside the pluton aureole, the inter-section angle between these two planes can be usedas a passive marker to restore the concordant aure-ole formations back to their prepluton emplacementpositions. Our preferred model restores the western

aureole rocks back into the Inyo anticline so that theinclusion trails become realigned parallel to the re-gionally developed slaty cleavage fabric. Thismodel is based on vertical motion of the pluton andassociated translation and rotation of the surround-ing wall rocks. The motion of the wall rocks is di-vided into two separate steps that allow modeling ofthe motion with a stereonet, although the motionwas probably one continuous event.

The first restoration step is to undome thenorthwestern and southwestern aureole composi-tional layering by rotating around horizontalpoles orientated east-northeast (Fig. 8a). Thesepoles of rotation are perpendicular to the lin-eation and parallel to the long axis of the pluton.This rotation brings the northwestern and south-western “limbs,” which dip away from the plu-ton, down into a horizontal plane (Fig. 8b). Thesecond restoration step involves rotation of thecompositional layering around a horizontal poleoriented north-northwest, parallel to the strike of

the southwest limb of the Inyo anticline (Fig. 8b).In this motion the southwest side is rotated down,simultaneously bringing the compositional layer-ing into parallelism with bedding in the south-west limb of the anticline (Fig. 8c), and the inclu-sion trails into parallelism with the slaty cleavagefound throughout the central White-Inyo Range(cf. Figs. 3 and 8d).

A model based on horizontal motion, in whichthe aureole rocks were bulged horizontally out ofthe Inyo anticline as the pluton inflated outwardto the west during emplacement, was also testedby porphyroblast-matrix analysis. This is theoriginal translation and rotation model assumedby Sylvester et al. (1978), and is consistent withthe map pattern of the aureole rocks as they aredeflected westward out of the anticline to sur-round the pluton. This alternative model is testedby independently rotating the northwestern andsouthwestern sides of the aureole around verticalpoles (Fig. 9a) until they strike parallel to thesouthwest limb of the Inyo anticline (Fig. 9b).The vertical poles of rotation are located where,in map view, the beds in the western limb of theInyo anticline begin their deflection into concor-dancy with the pluton. When the aureole rocksare restored back into the western limb of theanticline with this rotation, inclusion trails are notaligned parallel to one another with respect tocompositional layering from the northern limb tothe southern limb (Fig. 9b), and they are also notparallel to the orientation of the preexisting re-gionally developed slaty cleavage. Therefore thisemplacement model, which incorporates hori-zontal bulging and rotation of the western aureolearound vertical poles, is rejected. However, thevertical and horizontal pluton-emplacementmodels investigated here only represent endmembers of a spectrum of possible solutions.

Distortion (Shape Change)

Sylvester (1966) recorded the pinch and swellof the Lower Cambrian metasedimentary forma-tions around the western margin of the pluton bymeasuring 21 stratigraphic sections through theaureole (see also Sylvester et al., 1978, Fig. 11).The regional stratigraphic thickness of 570 to 700m for the Harkless Formation is reduced toroughly 45 m around the western margin of thepluton. This indicates that the Harkless Forma-tion has been thinned to less than 10% of its orig-inal thickness. The greatest degree of attenuationis observed at the westernmost end of the pluton,and decreases in intensity eastward along thenorthern and southern margins (cf. Fig. 1). TheSaline Valley Formation decreases in thicknessfrom 62 m to 27 m to 16 m traced from east towest in Sylvester’s three easternmost traversesalong the southern border. Along the same inter-



Figure 8. Diagram illustrating the palinspastic restoration of aureole rocks into their pre-pluton position in the western limb of the Inyo anticline using vertical translation and rotation.(a) Present structural position of aureole metasedimentary layering surrounding the PapooseFlat pluton. Open arrows depict the undoming and rotation of the concordant aureole rocks inthe first step of restoration. (b) Aureole rocks are undomed. Open arrows depict the second-stage restoration of the aureole rocks to become aligned into parallelism with the bedding inthe southwestern limb of anticline. (c) Aureole metasedimentary rocks repositioned into par-allelism with bedding in the Inyo anticline. Note how restoration places inclusion trails fromandalusite porphyroblasts (stereograms at bottom of diagram) into parallelism with slatycleavage observed in pelitic rocks from western limb of Inyo anticline indicated by stereogram(d) at top right of diagram. Arrows defined by narrow lines indicate stratigraphic way up.

val, the upper Harkless quartzite and quartz-micaschist decrease in thickness from 66 m to 35 m to31m traced from east to west.

Volume Change

Seven samples from the Harkless Formationwere analyzed for major elements by direct-current plasma spectrometry. The samples weretaken at various distances from the pluton, rangingbetween 5 km north of the pluton and 20 m fromthe pluton–wall-rock contact in the concordantsection around the western margin (Fig. 1).Whole-rock geochemical analyses indicate thatthere is no significant change in major elementconcentrations in the Harkless Formation shale asit is metamorphosed and attenuated into aureoleschists surrounding the Papoose Flat pluton (Table1). Therefore, the change in thickness of the Hark-less Formation—from a regional thickness of 570to 700 m down to 45 m around the pluton—is con-sidered to be due to shape change associated withpenetrative strain, and not volume loss.

Rigid-Body Translation

Vertical Translation. The amount of rigid-body translation of the aureole rocks is controlledby the kinematics of their rotation, the shapechange (thinning) of the metasedimentary forma-tions, and the amount of deflection of the aureolerocks out of the Inyo anticline. By limiting thepluton motion to a vertical upward translationand assuming that beds in the southwest-dippinglimb of the Inyo anticline were consistently ori-ented across the area prior to intrusion of thePapoose Flat pluton, the amount of vertical trans-lation can be quantified by simple trigonometry(Fig. 10). Area balancing the attenuated stratigra-phy also helps to control the amount of transla-tion and helps in understanding the geometry ofthe pluton at depth (Fig. 11).

The westward deflection of the Lower Cam-brian section out of the Inyo anticline when incontact with the Papoose Flat pluton is clearlyrevealed in the geologic map of the region (Fig.1). At the present topographic level, the Poleta-

Harkless contact within the aureole on the north-western margin of the pluton is deflected approx-imately 4.2 km west-southwest of its regional po-sition in the Inyo anticline, assuming that theregional structures were consistently orientedthrough the area before intrusion. The Poleta-Harkless contact within the aureole northwest ofthe pluton is at an elevation of 2680 m, as is thePoleta-Harkless contact 3 km northeast of the plu-ton, where it is found in the undisturbed south-west limb of the Inyo anticline. Simple trig-onometry indicates that the structural depth to theoriginal Poleta-Harkless contact, prior to upwarddeflection by the pluton, was approximately 2 kmbelow its current outcrop position northwest ofthe pluton (Fig. 10). These calculations assume(1) that the pluton rose vertically out of the south-west-dipping limb of the anticline, (2) a homo-clinal structure for the southwestern limb, and (3)a dip of 25° for the beds within the homocline. Adip of 25° is a minimum dip for the southwesternlimb of the Inyo anticline north of the pluton. Us-ing a dip of 30° for the southwestern limb of theanticline, a vertical translation of 2.4 km is neces-sary to account for the outcrop position of thePoleta-Harkless contact. Similarly, a dip of 40°would indicate a vertical translation of 3.5 km tobring the Poleta-Harkless contact up to its presentoutcrop position northwest of the pluton.

The possible subsurface geometry of the Pa-poose Flat pluton and its aureole has been inves-tigated by constructing a series of cross sectionsdrawn at right angles to the pluton long axis.These cross sections take into account differentpossible dip values for the western limb of theInyo anticline, and therefore different possibleamounts of vertical upward translation of the plu-ton and its aureole. The cross sections are drawnassuming that the thinning of the metasedimen-tary layers is a result of stretching as the aureolelayers were domed over an inflating pluton. In or-der to test the cross sections, the aureole forma-tions have been area balanced (aureole layerscontain the same area before and after deforma-tion), given that they must be 10% of their origi-nal thickness by the time they reach the currentlevel of exposure (Fig. 11). Cross sections are

MORGAN ET AL.


Figure 9. Diagram illustrating incorrect palinspastic restoration of aureole rocks into theirprepluton position in the western limb of Inyo anticline using horizontal translation and rotation.(a) Present structural position of aureole metasedimentary layering surrounding Papoose Flatpluton. Open arrows indicate rotation necessary to restore compositional layering in aureolerocks into parallelism with bedding in western limb of anticline. (b) Aureole metasedimentaryrocks restored into parallelism with bedding in Inyo anticline. Note how inclusion trails from north-ern aureole margin do not restore into parallelism with inclusion trails from southern aureolemargin, and how inclusion trails are not parallel to the regionally developed slaty cleavage.

TABLE 1. HARKLESS FORMATION CHEMCAL ANALYSES

Sample SIO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O Ba Sr Total

Gc-1 60.75 0.82 22.01 7.57 0.08 2.17 0.47 0.40 4.02 579.6 99.3 98.3Gc-2 59.16 0.99 25.75 7.86 0.07 1.87 0.09 1.08 3.62 523.1 146.5 100.49Gc-3 60.40 0.88 23.27 8.31 0.11 2.42 0.26 0.73 3.70 623.9 141.9 100.06Gc-4 67.20 0.64 16.64 9.38 0.10 2.60 0.45 0.30 2.00 344.9 88.6 99.31Gc-5 60.95 0.86 21.86 9.07 0.13 2.68 0.56 0.58 3.32 484.8 119.6 1.00Gc-6 61.28 0.91 25.13 6.65 0.07 2.31 0.29 0.11 4.80 613.3 134.3 101.54Gc-7 58.53 0.88 24.91 8.16 0.12 2.39 0.17 0.36 4.45 763.8 121.4 99.98

Note. Data are in weight percent except for Ba and Sr, which are in parts per million. Sample locations are shown in Figure 1.

drawn parallel to the north-northwest–orientedstretching lineation, assuming plane strain withno volume loss. The cross sections illustrate howthe aureole formations need to be overturned atdepth if the western limb of the Inyo anticline isgently dipping (Fig. 11b). In order for the crosssection to be area balanced (with shallower dipsfor the western limb), there is less distance for theaureole layers to be stretched and thinned. Morearea is created by overturning the layers. If thewestern limb of the anticline has a dip closer to40°, then minimal overturning is necessary toarea balance the cross section (Fig. 11c).

The assumption of plane strain inherent in thetwo-dimensional area balancing is locally sup-ported by the fact that both the plastically de-formed Harkless quartzite of the aureole and thesimilarly deformed foliation-parallel quartzveins in the gneissic border facies of the plutonare characterized by a well-developed north-northeast–south-southwest–trending stretchinglineation within the mylonitic foliation. Quartzcrystallographic fabrics measured by both opti-cal and X-ray texture goniometry methods inmore than 150 samples of these plastically de-formed rocks consistently indicate that strainsymmetry was very close to plane strain (k = 1)conditions (Law et al., 1992, 1993). However,triaxial boudinage structures, indicating defor-mation within the flattening (1 > k > 0) strainfield, have been recorded in skarn deposits from

the Poleta Formation (Sylvester and Christie,1968) located between the pluton margin and theoverlying Harkless quartzite, and in limestone ofthe Mule Springs and Saline Valley formations,above the Harkless Formation. These plasticallydeformed rocks also contain a north-north-west–south-southeast–trending stretching lin-eation; the greatest separation between the tri-axial boudins is observed in sections cut parallelto lineation and perpendicular to foliation. Thelocal occurrence of flattening (1 > k > 0) strainsindicates that, with respect to area and volumebalancing, less vertical translation and/or lessoverturning of stratigraphic units at depth (Fig.11b) is necessary to account for attenuation ofthe stratigraphy. We reiterate, however, that theminimum possible dip angle for the southwest-ern limb of the anticline is 25°, indicating a min-imum possible vertical translation for the plutonand its aureole rocks of 2.0 km.

Horizontal Translation. The PrecambrianWyman Formation, which is the lowest strati-graphic formation exposed in the White-InyoRange, constitutes the core of the Inyo anticline(Fig. 1). The Inyo anticline is a continuous north-northwest–trending upright fold that can betraced for more than 40 km along its hinge, andonly where the Wyman Formation comes intocontact with the eastern margin of the PapooseFlat pluton are the beds overturned, suggestingthat emplacement of the pluton is responsible for

the overturned section (Sylvester et al., 1978;Nelson, 1987). The Reed dolomite, which strati-graphically is above the Wyman Formation, isalso overturned and the entire succession, fromthe Wyman Formation through the Reeddolomite and overlying Deep Spring andCampito Formations, is more tightly folded onthe eastern end of the pluton (Fig. 1) than any-where else in the Inyo anticline, indicating thatthe fold was tightened by lateral translation as thepluton locally expanded outward toward the east.

EMPLACEMENT MODEL

Porphyroblasts and Two Stages of Emplacement

Porphyroblast matrix relationships observedwithin the Harkless Formation surrounding thewestern and southern margin of the Papoose Flatpluton can be explained by invoking a model in-volving two stages of magma injection, the firstpassive and the second forcible (Fig. 12). Thefirst magma injection produces a sill and intro-duces a heat source that decreases the viscosity ofthe overlying sedimentary succession and resultsin static contact metamorphism and growth of an-dalusite porphyroblast cores (Fig. 12a). The firstinjection of magma predates attenuation of thesurrounding aureole rocks. A second pulse ofmagma, under pressure high enough to lift theoverlying roof rocks, translates but also plasti-cally attenuates the overlying sedimentary suc-cession (Fig. 12b) now that it is thermally weak-ened and causes the continued growth ofandalusite (rims). The magma may have initiallyrisen along a preexisting fault (Fig. 12) in a man-ner similar to that proposed for the Birch Creekpluton (Nelson and Sylvester, 1971), an almostidentical age pluton, located 34 km to the north ofthe Papoose Flat pluton.

Our two-stage model for evolution of the Pa-poose Flat pluton is based on the relative timingof porphyroblast growth versus the developmentof structures within the matrix of the Harklessschist. Within the porphyroblasts evidence existsfor a significant period of andalusite growth be-fore ductile deformation (attenuation and dom-ing) but postdating the regionally developed slatycleavage. Porphyroblast core growth is related toinitial intrusion of magma as a sill primarilywithin or below the Poleta Formation. The dura-tion of time taken for andalusite porphyroblastswithin the Harkless shales to nucleate and growto the size of their core region is believed to bethe duration of time over which the magmaponded as a sill (Fig. 12a). Porphyroblasts over-grew the slaty cleavage fabric, which was ori-ented at a low angle to bedding, and incorporatedthe fabric as inclusion trails.



Figure 10. Block diagram illustrating that approximately 2 km of vertical translation isneeded to bring the Harkless and Poleta Formations to their present exposure level northeast ofthe pluton. Dip of beds in the southwest limb of the Inyo anticline is assumed to be a constant25°, and deflection of the aureole rocks is limited to vertical translation. Dashed lines on sidesand top of pluton represent orientation of stretching lineation.

The fine grain size of inclusions relative to ma-trix grains and planar alignment of inclusiontrails in the core region of andalusite porphyro-blasts (Fig. 4) indicates that the cores grew priorto ductile deformation (Vernon et al., 1993a,1993b) that was associated with final emplace-ment of the Papoose Flat pluton. In contrast, in-clusion trails within the rim regions of porphyro-blasts, Sir, curve into parallelism with the matrixfoliation (Fig. 4), a feature commonly used as ev-idence for syndeformational growth of porphyro-blasts (Bell and Rubenach, 1983; Bell et al.,1986; Vernon, 1989). We therefore interpret theporphyroblast core to rim boundary as represent-ing a change in metamorphic and kinematic con-ditions as inclusions become coarser grained andcurved into parallelism with the matrix foliation.This curvature records the onset of ductile defor-

mation within the aureole rocks.The second pulse of magma is associated with

three phenomena: (1) the dominantly verticaltranslation of the overlying sedimentary succes-sion; (2) the extreme attenuation of the immedi-ately overlying metasedimentary aureole rocks;and (3) the growth of andalusite porphyroblastrims (Fig. 12b). Although our model for evolu-tion of the Papoose Flat pluton calls for the em-placement of at least two separate pulses ofmagma, no documented examples of mineralog-ical zonation or internal contacts within the plu-ton have been published. However, recently com-pleted work on the anisotropy of magneticsusceptibility, mineralogy, and microstructures ofthe pluton (the subject of a separate paper) indi-cates that the pluton is zoned mineralogically(M. de Saint Blanquat, 1997, personal commun.)

and exhibits at least rare examples of internalcontacts (M. Barton, 1992, personal commun.;M. de Saint Blanquat, 1996, personal commun.).The previous difficulty in recognizing distinct in-trusions within the pluton that may be associatedwith these separate magma pulses may, at least inpart, be explained by the fact that the western halfof the pluton is only exposed close to its roof(Sylvester et al., 1978).

We suggest that upward dilation of the magmachamber produced the doming of the sedimentaryunits around the pluton now seen at the current ex-posure level. The sedimentary succession fartherabove the pluton, unaffected by the thermal pulse,was passively translated upward in a style similarto that of the overburden above a laccolith. In thisinterpretation the north-northwest–south-south-east orientation of the stretching lineation ob-served in the pluton’s gneissic border facies andaureole rocks is a result of the east-west orienta-tion of the elongate shape of the inflating magmachamber. The lineation is oriented subperpendic-ular to the elongate shape of the pluton and devel-oped as the solid-state carapace (pluton marginand aureole) was stretched around an upward-inflating tube-like magma chamber (Fig. 10). At-tenuation of the aureole rocks was accompaniedby strain-path partitioning, i.e., plastic deforma-tion of the quartz-rich units occurring under planestrain (k = 1) conditions (Law et al., 1992, 1993),whereas the limestone and dolomite units appearto have been simultaneously deforming underflattening (1 > k > 0) conditions (Morgan, 1992).

Within the pluton’s aureole, extreme attenua-tion of the metasedimentary layers resulted in ro-tation of the regional slaty cleavage into paral-lelism with compositional layering. Porphyro-blasts continued to grow during the attenuationevent, which is recorded in the rims of andalusiteporphyroblasts where inclusion trails curve intoparallelism with the present foliation. Porphyro-blast growth outlasted the deformation, becausethe outermost rims of andalusite porphyroblastsovergrow the youngest foliation. The metasedi-mentary units maintained their coherent stratigra-phy and were thinned uniformly due to thedecreased viscosity of the rocks at elevated tem-peratures and the relatively homogeneous up-ward expansion of the pluton.

The sharp curvature of inclusion trails as theyturn into parallelism with the matrix foliation at-tests to the rapid time scale of the deformationrelative to the time taken for porphyroblastgrowth. The sharp curvature occurs over a smallvolume within the porphyroblasts, and there is nodiscontinuity or break in inclusion trails fromcore region through rim region and into the ma-trix, indicating that porphyroblast growth kept upwith the imposed strain rate. On the basis of ther-mal modeling studies of the Papoose Flat pluton

MORGAN ET AL.


Figure 11. Three cross sections extrapolating the half-pluton shape and geometry of the con-cordant aureole rocks to depth. Cross sections are drawn parallel to the axis of the Inyo anti-cline (and parallel to the stretching lineation) and perpendicular to the long axis of the pluton.Vertically lined area (deformed wall rocks) equals horizontally lined area (undeformed wallrocks). In cross section (a), deformed wall rocks (unlined) equal 50% of undeformed wall rocks.In (b), area is conserved due to overturning of wall rocks. In cross sections (a) and (b), dip ofbedding in southwest limb of Inyo anticline before pluton emplacement is assumed to be a con-stant 30°, and therefore bedding is transported 2.4 km vertically to be exposed at presentground surface (see Fig. 10). In cross section (c), dip of bedding in southwest limb of the Inyoanticline before pluton emplacement is assumed to be a constant 40°, which produces 3.5 km ofvertical translation.

and its aureole rocks, Nyman et al. (1995) esti-mated that the observed 90% attenuation of theaureole rocks may have taken place in fewer than80 k.y. at an average strain rate on the order of10–12s–1 assuming that the pluton was intruded asa single magma pulse. They pointed out, how-ever, that if high aureole temperatures were main-tained over a longer time period, by injection ofseveral magma pulses (as inferred from the an-dalusite porphyroblasts), then strain rates mayhave been slower (Nyman et al., 1995, p. 640). Inour two-stage emplacement model, penetrativedeformation is only associated with the secondpulse of magma injection (during laccolith for-mation) and therefore the average strain rate of10–12 s–1 calculated by Nyman et al. (1995)would remain valid.

Direct observation of the growth of laccolithsis rare; however, examples of the rapid progres-sive growth of near-surface laccoliths have beendocumented (see review by Correy 1988, p.45–46). One of the most spectacular near-surfaceexamples was recorded by Minakimi et al. (1951)in Japan, where growth of a near-surface lacco-

lith, 1 km long by 0.8 km wide, raised groundlevel by 170 to 200 m over a 9 month time period,and involved emplacement of approximately 0.1km3 of magma.

Sill-like Geometry

The Papoose Flat pluton is in concordant con-tact with the Lower Cambrian Poleta Formationon its western and southern margins and with thestratigraphically underlying Campito Formationon its eastern margin (Fig. 1). The Poleta Forma-tion dips 30° to 40° away from the pluton on thewest and south sides of the pluton and theCampito Formation locally dips steeply towardthe pluton on the eastern margin. Only along thecentral section of the pluton’s northern margin,and around the apophysis at the southeast end ofthe pluton, is the pluton discordant to the sur-rounding metasedimentary formations (Nelsonet al., 1978). The generally concordant shape ofthe pluton and the observation that there is nomissing stratigraphy suggest that the magma ini-tially intruded along bedding between the under-

lying Campito Formation and the overlying Po-leta Formation in the west-dipping limb of theInyo anticline. An east to west cross sectionillustrates the concordancy and sill-like geome-try of the pluton (Fig. 2).

Further support for an initial sill-like intrusioncomes from porphyroblast-matrix relationships,which indicate that the first metamorphic event re-lated to intrusion (growth of porphyroblast cores)took place when the overlying Harkless Forma-tion was planar in geometry and situated withinthe western limb of the Inyo anticline, prior to be-ing domed and intensely strained (attenuated)during vertical inflation of the western part of themagma chamber. Porphyroblast-matrix analysisalso indicates that the first metamorphic event wasstatic and occurred prior to ductile attenuation ofthe aureole rocks.

Speculation on the Nature of the Pathwayfor Transport of Magma into a Sill-like orLaccolith-like Intrusion

In their original model for emplacement of thePapoose Flat pluton, Sylvester et al. (1978, Fig.9) proposed that the pluton was intruded as asteeply dipping dike or wall diapir that truncatedthe Wyman, Reed, Deep Spring, and CampitoFormations before reaching the level of the Po-leta Formation, where it then inflated outward tothe west, deflecting the aureole rocks out of theiroriginal position in the Inyo anticline. Sylvesteret al. (1978, p. 1217) suggested that the apophysis,which is discordantly intruded into the surround-ing country rocks at the eastern end of the pluton(Fig. 1), might “be a vestige of the initial stage ofintrusion, judging from the vertical lineations inthe granite of the apophysis and the apparentlyhigher temperature metamorphic mineral phasesin the adjacent wall rocks.” Our palinspasticrestoration of the aureole rocks, based on an-dalusite inclusion trails, has demonstrated that fi-nal emplacement of the pluton could not have in-volved westward-directed horizontal translationof the wall rocks (Fig. 9), but the suggestion thatthe apophysis might be a feeder dike to the mainpluton remains appealing.

The apophysis is situated at a deeper structurallevel than the main pluton. Inspection of the geo-logic map by Nelson et al. (1978) revealed that theapophysis–wall-rock contacts are currently ex-posed at an elevation of 1700–2070 m above sealevel, and the eastern margin of the main pluton isexposed at elevations ranging between 2070 m(mapped junction between apophysis and mainpluton) and 2987 m above sea level. Inspection ofthe geologic map of the pluton (Fig. 1) also indi-cates that if the west-northwest–east-southeast–trending apophysis were to continue westwardbeneath the main pluton, its map position would



Figure 12. Schematic diagram illustrating two stages of magma emplacement and porphyro-blast-matrix development. (a) Magma is emplaced initially as a sill within southwest limb of theInyo anticline. Andalusite porphyroblasts overgrow regionally developed slaty cleavage fabricin surrounding shale during contact metamorphism. (b) Subsequent pulses of magma into sillcause sill to inflate vertically, but also outward toward the east. Andalusite porphyroblasts con-tinue to grow as the aureole rocks are thinned. Matrix foliation recrystallizes in response to pen-etrative deformation during continued contact metamorphism, and inclusion grains becomecoarser and curve as matrix foliation wraps around andalusite porphyroblasts during shorten-ing. Note how angular relationship between regional slaty cleavage and bedding is preserved asangular relationship between inclusion trails and matrix foliation.

coincide with the crest of the pluton roof, whichhas been domed upward at the western end of thepluton. The projected extension of the apophysisbeneath the main pluton also coincides with themapped position of several other west-north-west–east-southeast–trending geologic featuresin the pluton: (1) the hinge line of an elongatemagnetic foliation dome recently identified byde Saint Blanquat et al. (1994) using AMS analy-sis; (2) a central domain of higher calculatedquartz-biotite δ18O temperatures (Brigham, 1984,p. 142); (3) an elongate central core domaincharacterized by magmatic microstructures (deSaint Blanquat et al., 1994); and (4) the zone ispoor in both myrmekite and microaplite within,and along the axis of, the magmatic domain (M. deSaint Blanquat, 1997, personal commun.). On thebasis of these observed spatial relationships, wetentatively suggest that the apophysis may be partof a steeply dipping west-northwest–east-south-east–striking intrusion that extends beneath thePapoose Flat pluton and acted as a feeder dike tothe pluton during its progressive developmentfrom an inclined sill to a laccolith.

DISCUSSION

Three-Dimensional Control and the Role ofTranslation

Structural aspects of pluton emplacement arecommonly determined by the kinematics inferredfrom two-dimensional map and outcrop patterns.The map pattern of the deflected wall rocks sur-rounding the Papoose Flat pluton was used bySylvester et al. (1978) to infer a model of em-placement for the pluton involving westward(horizontal) translation and expansion. Formationof the gently plunging, north-northwest–south-southeast–trending stretching lineation observedacross the western part of the pluton and its aure-ole is not explicitly considered in this model in-volving westward horizontal expansion. The pres-ence of the stretching lineation was interpreted byLaw et al. (1990, 1992) and Paterson et al. (1991)as indicating that the high strain zone surroundingthe western part of the pluton could be associatedwith regional deformation. In our model, how-ever, lineation developed as the solid-state cara-pace (pluton margins and aureole) was stretchedabove an upward-inflating magma chamber (cf.Fig. 10 and Law et al. 1993, Fig. 15).

Many detailed analyses of the structures andstrain associated with pluton emplacement haveconcluded that the space created by aureolestrains is insufficient to accommodate the vol-ume of magma intruded (Holder, 1979; Johnand Blundy, 1993; Paterson and Fowler, 1993;McNulty et al., 1996). Paterson and Fowler(1993) calculated that a maximum of 23% of the

volume of the Papoose Flat pluton could beaccommodated by ductile flow within the aure-ole and therefore argued that mechanisms otherthan forcible expansion must be operating tocreate space for the Papoose Flat pluton. Manypossible deformation paths can explain thestrain surrounding plutons, and inferring the de-formation path on the basis of the deflection oftwo-dimensional map patterns is at best an ap-proximation (see Schwerdtner, 1995) and maybe why the space problem still exists. Further-more, forcible emplacement has typically beenassociated with structures indicative of flatten-ing (1 > k > 0) strains, and models of forcibleemplacement have been based on radially ex-panding plutons (Holder, 1979; Ramsay, 1989;Paterson and Fowler, 1993; Guglielmo, 1994).Structures indicative of simple shear surround-ing plutons are often attributed to synchronouspluton emplacement and regional deformation.If translation is a large component of forcefulpluton emplacement, which has often beenoverlooked (Guglielmo, 1994), then simpleshear strains should be expected and the radiallyexpanding pluton model for forceful emplace-ment can only be expected to account for part ofthe volume of such plutons.

In this paper, wall-rock flow is separated intoshortening (thinning), and rigid-body translation,because all the sedimentary rocks within the con-cordant aureole surrounding the Papoose Flatpluton have been translated upward, but not all ofthe same sedimentary rocks have undergone pen-etrative shortening (attenuation). Calculating thevolume produced in the aureole by measuringshortening strains alone will never give a valueequal to the volume of the pluton if translation isignored. The space problem is not as acute whenthe wall and roof rocks move upward, becausethere is an unlimited amount of space that can becreated by vertical translation.

Stoping Versus Diapirism Versus LaccolithFormation

Stoping is not a viable mechanism for creatingspace for the Papoose Flat pluton because all ormost of the wall rocks are accounted for. Whenthe pluton is removed from the surrounding rocksand the void closed up, the section is con-formable from the western contact to the easterncontact, i.e., the sedimentary section is mostlycomplete and there is little missing stratigraphy(Figs. 1 and 2).

If the Papoose Flat pluton rose as a diapir, it isdifficult to explain the overturned Precambrianformations along the eastern margin. Instead of arim syncline, there is a rim anticline. Away fromthe pluton to the northwest, the Precambrian for-mations dip to the southwest as do all the forma-

tions in the southwest limb of the Inyo anticline,but their dips increase and overturn abruptly nearthe pluton contact, rotating counterclockwisewhen viewed to the northwest. This geometrysuggests that bedding initially rotated downwardin an overall tightening of the anticline aroundthe eastern end of the pluton. A rising diapirshould rotate all bedding upward, in a clockwisemotion when viewed to the northwest, as thebeds are uplifted and pushed aside by a risingmagma body, even during return flow. This ac-tion would produce a diapir-associated rim syn-cline. The interpretation that the dipping beds ofthe Precambrian formations rotated downward tobecome overturned indicates that in situ outwardhorizontal expansion was at least locally impor-tant as an emplacement mechanism along thepluton’s northeastern margin, and suggests thatthe Precambrian formations were initially belowthe magma chamber (as the floor beneath an in-clined sill) (Fig. 12).

The observation that regionally developedslaty cleavage is preserved within the outer aure-ole south of the pluton, even though bedding wasdeflected out of its regional position and orientedparallel to the pluton margin, indicates that somewall rocks were translated and at the same timenot affected by significant penetrative deforma-tion. This observation has general implications interms of a mechanism for providing space foremplacement and helps to alleviate the spaceproblem above the Papoose Flat pluton. If therocks above plutons can be translated upward byfolding and faulting, similar to the roof rocksabove a laccolith, then the space problem is sig-nificantly alleviated, if not removed.

Jackson and Pollard (1988) demonstrated thatone of the diorite laccoliths in the Henry Moun-tains of Utah uplifted its sedimentary overburdena minimum of 2.5 km and was originally intrudedat a depth of 3–4 km. The laccolith is concordantto the overlying sedimentary section and proba-bly initially intruded as a sill; there are numeroussills in the region surrounding the main intrusion.The sedimentary section does not exhibit signifi-cant penetrative deformation. Paleomagnetic dataon the surrounding sills, which dip away from thepluton parallel to the contact, indicate that theyinitially crystallized in a horizontal position, par-allel to bedding, and have since been rotated 75°to 80° from horizontal as the laccolith inflatedand domed the overlying sedimentary layers(Jackson and Pollard, 1988). The sills and lacco-liths in the Henry Mountains, which were in-truded at a shallow level, clearly illustrate thatwall rocks can be translated vertically withoutsignificant penetrative deformation, and thattranslation can provide room for intrusions. Inthis paper we have attempted to demonstrate thatintrusion of the deeper level Papoose Flat pluton,

MORGAN ET AL.


which on the basis of structural and stratigraphicgrounds is calculated to have been intruded at adepth of between 6.4 and 9.2 km (Sylvester et al.,1978, p. 1213), may also have involved kilometer-scale translation of its wall and roof rocks. Min-eral paragenesis of the Papoose Flat pluton aure-ole rocks indicate a pressure of contact meta-morphism of between 3 and 4 kbar (Nyman et al.1995, p. 635).

If magma flow is responsible for the kilometer-scale translation of wall rocks in the upper crust,which seems to be well documented for lacco-liths, then how do magmas and plutons expressthis extreme pressure at deeper levels in thecrust? Buoyancy seems to be a controlling factorof where magma originally ponds in the crust(Corry, 1988; Ryan, 1993), but the driving forcethat produces the characteristic shapes of intru-sions such as laccoliths and plutons is not simplybuoyancy, and locally magmas seem to be able toexert extreme pressures, in a dominantly verticaldirection, on their surrounding wall rocks.

CONCLUSIONS

The deformation features surrounding the Pa-poose Flat pluton have been interpreted by mod-eling the intrusion as a mid- to upper-crustal lac-colith. The concordant metasedimentary aureolesurrounding the Papoose Flat pluton is unusual inthat a complete strain analysis is possible. Withthe aid of porphyroblast-matrix relationships, thekinematics of rotation, the change in volume, thechange in thickness, and the amount of translationof the metasedimentary formations within the au-reole have been quantified and are consistent withinitial magma emplacement as an inclined sill andsubsequent inflation into a laccolith and/or pluton.

Detailed porphyroblast-matrix analysis indi-cates that inclusion trails within the andalusiteporphyroblasts record the orientation of a region-ally developed slaty cleavage within the originalshale and that these inclusion trails may be usedas strain markers to restore the aureole metasedi-mentary layers back to their prepluton emplace-ment position. Inclusion-trail analysis indicatesthat wall-rock translation is compatible with avertical motion for final pluton emplacement.

The combination of structural and porphyro-blast-matrix analyses leads to a three-dimensionalkinematic history of the wall rocks whereby spaceis produced by a combination of penetrative strain(shortening), vertical and horizontal translation,and rotation (cf. Figs. 10 and 11). Space prob-lems, encountered in pluton emplacement modelsthat cannot account for all of the volume of a par-ticular pluton by penetrative strain in the sur-rounding wall rocks, may be an indication thattranslation plays a significant role in the emplace-ment history.

ACKNOWLEDGMENTS

Field and laboratory work was funded throughgrants from the Geological Society of America,Sigma Xi, and University of California WhiteMountain Research Station to Morgan, and Na-tional Science Foundation grants EAR-9018929and EAR-9506525 to Law. We thank particularlyAllen Glazner for carrying out the chemicalanalyses in Table 1, Basil Tickoff for suggestingimportant organizational changes to the manu-script, and Clem Nelson for support and encour-agement of our work in the White-Inyo Range.We also thank Barbara John, Cees Passchier,Stefan Schmid, Carol Simpson, Art Sylvester,Basil Tickoff, and Associate Editor Mike Coscafor critical reviews.

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Laccolith-like emplacement model for the Papoose Flat pluton based on porphyroblast-matrix analysis