pegmatite. The complex crops out in the adjacentnortherly-trending West Humboldt, Stillwater, andClan Alpine ranges in northwestern Nevada and hasa known extent of at least 35 miles in maximumdimension. The gabbroic rocks were studied chieflyin the West Humboldt Range (Fig. 1) where theyform a quasi-tabular body which dips east off theeast flank of the range under the Quarternary de-posits of the adjacent basin. Stratigraphic evidenceindicates that the age of the complex is betweenLower Jurassic and Upper Tertiary. Emplacement ofthe complex was at a shallow depth, probably lessthan 2500 feet, and was apparently synchronouswith regional deformation of the intruded Mesozoicsediments.

Hornblende picrite and strongly foliate horn-blende leucogabbro which compose the West Hum-boldt layered body crop out chiefly along the westernperiphery of the Humboldt gabbroic complex in theWest Humboldt Range. These units were earliest inthe sequence of rock types forming the complex.Some of the relatively large bodies of these two faciesare shown in Fig. 1. The consistent areal associationof picrite and foliate leucogabbro indicates that thesefacies are cogenetic. Structural evidence suggeststhat all of these bodies are contemporaneous andmay once have been contiguous. Deformation, wide-spread deuteric alteration, and poor exposures pre-clude more definite correlation of these bodies.

MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963INTERNATIONALMINERALOGICAL ASSOCIATION,PAPERS, THIRD GENERAL MEETING

LAYERED PICRITE-ANORTHOSITIC GABBRO SHEET, WEST HUMBOLDT RANGE, NEVADA

ROBERT C. SPEEDl

Department of Geology, Stanford University, Stanford, California

ABSTRACT

The West Humboldt layered intrusion is a two-layer sheet exposed over a half square mile in the West HumboldtRange, Nevada. The body is composed of 235+ feet of hornblende picrite overlain by at least 100 feet of anorthositichornblende gabbro. The strong segregation of cumulus minerals into an ultramafic basal layer and anorthositic upper layeris remarkable in view of the absence of variations in the composition of the cumulus phases throughout the body and thelack of sorting or rhythms within each layer.

The layering apparently resulted from the separation of plagioclase from olivine and clinopyroxene during sedimenta-tion following emplacement of a highly crystalline basaltic magma. The picritic layer contains all the mafic cumulates andmuch plagioclase that was dragged down by the sinking mafic grains. The anorthositic layer is composed chiefly of feldsparthat was not carried down by the cloud of mafic crystals.

Tabular plagioclase in both layers has a strong preferred orientation. Fabric attitudes are similar in both layers andlie at high angles to the picrite-leucogabbro contact. The planar fabric appears to have formed in response to folding of theigneous body prior to complete consolidation.

INTRODUCTION

A two-layer stratiform intrusion composed of basalhornblende picrite and overlying anorthositic horn-blende gabbro is exposed over a half square mile inthe West Humboldt Range, northwestern Nevada.The strong bistratal segregation of early crystallizedminerals in this body is quite remarkable in view ofthe absence of cryptic layering (as defined by Hess,1960), grain size sorting, or mineral rhythms withineach layer. A second striking feature of this intrusionis the pervasive planar fabric defined by tabularplagioclase. The attitude of this foliation is similarin both layers and lies at high angles to the interfaceof the two layers and the base of the sheet.

The layering and fabric of this body are clearlyunlike those of other layered gabbros reported inthe literature and in this symposium. The object ofthis paper is to suggest a possible origin of the lay-ered body which accounts for these unusual features.Detailed study of the intrusion is yet to be com-pleted, but the problems mentioned above are suffi-ciently well established and of such an unusual naturethat presentation at this symposium seems war-ranted.

GEOLOGIC SETTING

The West Humboldt layered intrusion is part ofthe Humboldt gabbroic complex (Speed, 1962), alarge mass of hornblende gabbro with minor picrite,anorthosite, dolerite, keratophyre, and gabbroic

1 Present address: California Institute of Technology, Jet Pro-pulsion Laboratory, Pasadena, California.

WEST HUMBOLDT LAYERED SHEET

Structure. The West Humboldt layered intrusion,shown in Fig. 2, is tabular and consists simply of

69

70 ROBERT C. SPEED

235+feet of hornblende picrite overlain by at least100 feet of anorthositic hornblende gabbro. The con-tact of the two layers is not exposed, and an approxi-mate contact was mapped by tracing the sharp

mineralogical change in the surficial debris. Fieldexamination ill nearly vertical sections through thebody indicated that the contact is either sharp orgradational through no more than 15 feet.

EXPLANATION_ QUARTERNARY DEPOSITS

I:0.,° I CENOZOIC VOLCANICS

16 6 61 QUARTZ MONZONITE

HUMBOLDT GABBROIC COMPLE

IX X X 1 UNDIFFERENTIATED

STRONGLY FOLIATEHORNBLENDE LEUCOGABB

HORNBLENDE PICRITE

MESOZOIC SEDIMENTSMETASEDIMENTS

.I,~:'

(,~j~,I

'---

WEST HUMBOLDTLAYERED INTRUSI(FIG.2l X

:x

lI:l01&,1CDI=1----,T----~o~~~--4WEST HUMBOLDT

I RANGEI

II

CONTOURINTERVAL400 FEET

BASE MAP FROMLOVELOCK (15')QUADRANGLE

x "(, XX ,--, x',,. _ ... "" ...... _... j

J X <:» X

X

2 MI

FIG. 1. Generalized geologic map of West Humboldt Range, Nevada, showing distribution of gabbroic rocks andlocation of West Humboldt layered intrusion.

LA YERED GABBRO SHEET

ooIi

,/ IGNEOUS FOLIATION

»«: ATTITUDE OF SEDIMENTARY BEDS

TOPOGRAPHIC CONTOUR

/- CONTACT DASHED WHERE POORLY( EXPOSED

r" FAULT

LATER ROCKS OF THE HUMBOLDTGABBROIC COMPLEX

I-o..Js:::E

~O~t;;~ HORNBLENDE PICRITEWW3:5

~ UPPER TRIASSIC METASEDIMEI\iTS~ m:MARBLE

HORNBLENDE LEUCOGABBRO

<D ANALYZED SPECIMEN

CONTOUR INTERVAL 200 FEET

o 500 1000 2000 FEET

6000 A

5800

5600

~ ~b """$M~ I5400 ,-~5200 .".,.,,,.............. . -_.\ 1'"01lATION

FIG. 2. Geologic map of the West Humboldt layered sheet.

The roof of the intrusion has been completelyeroded, and the maximum thickness of the leuco-gabbro above exposed picrite is about 100 feet. Thislayer may thicken in the eastern part of the body ifthe picrite-leucogabbro contact maintains a constantattitude east of its easternmost exposure. The picriteis estimated to be around 235 feet thick in the west-

ern part of the body. The thickness of other parts ofthe picrite is poorly known because of the lack ofvertical exposures through this layer and uncer-tain ty in the a tti tude of the base of the sheet. Thepicrite, however, appears to thicken in the easternpart of the body.

The base of the picrite lies partly on deformed

71

\N

\

B

72 ROBERT C. SPEED

marble and hornfels and partly on younger horn-blende gabbro. The concordance between the igneoussheet and the marble suggests that the body is a sill.Marble, however, is also widespread in thrust zonesin the West Humboldt Range; therefore, the struc-tural setting of the intrusion is uncertain. Attitudesof the marble segments form an anticlinal pattern,the axis of which roughly parallels axes of folds inthe metasedimentary terrane which trend aboutN.1O°E. and plunge shallowly. Most of these folds areoverturned to the west. The degree of folding of thelayered intrusion, however, cannot be determinedas most of the western limb of the body is faulted.The internal fabric of the layered sheet suggests thatthe intrusion was deformed prior to final consolida-tion.

Hornblende picrite. Hornblende pieri te forms arather homogeneous petrographic unit in which in-ternal layering is absent. Modal variation in thepicrite consists chiefly of a gradual increase in colorindex vertically in the layer. Modal and chemicalanalyses of specimens taken near the bottom, middle,and top of the picrite are reported in Table 1. An-alyses 1 and 3 represent the approximate limits ofmineralogical and chemical variation in the picrite.

Textures in both the picrite and the leucogabbroclearly indicate a sequence of crystallization of min-eral assemblages. The early crystallizing assemblagein both facies is believed to be composed of accumula-tive minerals. This is indicated by the strong segrega-tion of these minerals into two layers on the basis oftheir densities. The later crystallizing assemblagesformed by reaction of the cumulus minerals with theintercumulus liquid and by in situ precipitation ofthis liquid. Terminology is taken from Wager et al.(1960) .

The early crystallizing assemblage in the picrite iscomposed of olivine, labradorite, and clinopyroxene,That crystallization of olivine started first is indi-cated by the inclusion of small olivine grains in clino-pyroxene and labradorite. Olivine has a wide rangeof grain size and idiomorphism. Maximum dimensionof olivine grains in thin section ranges from 0.5 to 5.0mm. This range of grain intercepts is believed to indi-cate variation in actual size of olivine grains. In somesections olivine grains with small intercepts are clus-tered in a manner which indicates that all the grainsin the cluster could not have been considerablylarger at levels above or below the plane of the thinsection. Furthermore, euhedral olivine grains whoseintercepts differ by an order of magnitude are in near-contact; it is difficult to envision how the two grains

TABLE1. CHEMICALANDMODALANALYSESOFROCKSFROMTHEWESTHUMBOLDTLAYEREDINTRUSION

2 3 4---------~----------------Chemical Analyses

Si02 46.56 45.25 43.85 51.30TiO, 0.54 0.44 0.49 0.49AJ,03 9.91 9.74 7.85 20.01Fe203 3.16 2.46 3.27 1. 29FeO 5.34 6.17 6.10 2.93MnO 0.14 0.14 0.17 0.05MgO 18.69 22.78 23.40 6.11CaO 7.91 6.70 6.33 10.75Na,O 1.47 1.48 1.15 3.63K20 0.48 0.36 0.30 0.60H2O+ 3.90 3.02 5.12 1.86H2O- 0.91 1.04 1.14 0.35P205 0.11 0.10 0.14 0.14CO2 0.18 0.17 0.14 0.15Cl 0.06 0.05 0.05 0.05F 0.02 0.01 0.02 0.01

---------------------

Total 99.38 99.91 99.52 99.72

NormsOr 3.06 2.26 1.93 3.74Ab 13.23 13.15 10.43 31.60An 20.20 19.80 16.60 37.50Di 16.29 11.32 12.90 12.75Hy 22.98 14.48 19.93 6.1701 18.09 34.51 32.25 4.89Ap 0.26 0.23 0.33 0.33II 1.08 0.88 1.02 0.96Mt 4.86 3.75 5.10 1.92

MgO%----- 90 90 90 85MgO+FeO

An% 59 59 60 53----------------------------Modes

Olivine 25.4 34.0 38.5Fo% (85-87) (87-92) (85-90)

Bronzite 3.5 2.5 3.3En% (80-82) (82-84) (81-83)

Clinopyroxene 13.1 14.7 13.2 5.0Kaersutite 11.4 10.5 8.5 18.0Plagiocalse 27.8 28.1 21.6 73.7

An% (55-60) (55-60) (50-58) (49-57)Magnetite 2.3 2.3 3.8 0.3Titanobiotite 2.2 2.0 3.2Chlorite 1 14.2 6.1 6.7 2.4Sphene - - - 0.5

1 Deuteric chlorite replacing both plagioclase and olivine.l-Hornblende picrite-1O feet above base.2-Hornblende picrite-170 feet below leucogabbro contact.3-Hornblende picrite-15 feet below leucogabbro contact.4-Hornblende lexcogabbro-30 feet above picrite contact.

Specimen locations shown on Fig. 2.Analyst: Y. Chiba.

LA YERED GABBRO SHEET 73

could have similar actual dimensions. Perfectlyeuhedral and highly irregular anhedral olivines areboth abundant. Optical measurements indicate acomposition range of FO~5-92,but it is probable thatthe spread is due to errors in measurement as the ma-jority of measurements indicates Fo87. Zoning is ab-sent in olivine. Layering with respect to composition,size, or shape of olivine was not observed.

Labradorite in the picrite occurs chiefly in tabletswhich are as long as 12 mm. The tablets contain asmany as 50 oscillatory zones. The most calcic zone isAn67, but most zone compositions are in the rangeAn55-65.Some labradorite tablets have a thin rim ofnormally zoned plagioclase, the periphery of whichis as sodic as An25. The rims probably crystallizedafter accumulation. Labradorite forms irregularmonomineralic clusters at a few places. Long dimen-sions of the clusters generally parallel the planarfeldspar fabric.

Clinopyroxene occurs both as discrete subhedraland euhedral grains and in monomineralic glomero-porphyritic clusters in which component grains arehighly intergrown. Many clinopyroxenes have oscilla-tory zoning, but others are apparently of uniformcomposition. Optical variations in zoning are chieflyin optic angle (52-54° and 58-60°) whereas differ-ences in refractive index ({3 = 1.682 -1.686) andZl\c (40-42°) between zones are small. Unzonedclinopyroxene optic angles are 53-57°. Accordingly,clinopyroxene compositions fall in the intervalCa43-50Mg39-48Fe7-1o.Maximum grain intercepts ofclinopyroxene clusters and single grains are 4.0 mm.

Two intercumulus assemblages occur in the picrite.The earlier consists of orthopyroxene and sodic rimson cumulus labradorite. The later assemblage is com-posed of kaersutite, titanobiotite, and magnetite.Orthopyroxene is virtually restricted to sinuouspoikilitic nets which envelop and replace olivinegrains. In other picrite bodies in the West HumboldtRange, however, orthopyroxene occurs in large (12mm) poikilitic nets enclosing clinopyroxene andplagioclase as well as olivine. This texture indicatesthat orthopyroxene was a product both of reactionof olivine with the intercumulus liquid and continueddirect precipitation from the liquid. Orthopyroxenecompositions range from En80 to En87.

The foregoing minerals are enveloped by largepoikilitic nets of kaersutite (4.9% Ti02) and titano-biotite. Amoeboid grains of magnetite occur only inkaersutite and titanobiotite and are apparently con-temporaneous with the latter minerals. These threeminerals compose the late assemblage which crystal-lized from the intercumulus liquid. Clinopyroxene

and, to a less extent, orthopyroxene are highly re-placed by kaersutite and titanobiotite, but replace-ment of olivine and plagioclase by these minerals didnot occur.

Most of the rocks of the layered sheet werestrongly hydrated in the deuteric stage. Much of theolivine and orthopyroxene was al tered to talc, trerno-lite, bowlingite, magnetite, and serpentine minerals.Labradorite was partly replaced by combinations ofprehnite, chlorite, pumpellyite, grossular, analcite,albite and white mica. Kaersutite was partly alteredto actinolite, chlorite and sphene. Reciprocal move-ment of Ca and Mg is indicated by the replacementof labradorite by chlorite and olivine by tremolite atplaces where the two primary minerals were in con-tact. The original grain contacts are obscured by thealteration, and the amount of chlorite representingformer olivine or plagioclase cannot be ascertained.This chlorite is reported separately in the modes inTable 1, but textures indicate that the chlorite chieflyreplaces plagioclase.

Hornblende leucogabbro. Modal variations in horn-blende leucogabbro are largely in the relative amountof plagioclase (70-95%) and late amphibole. Clino-pyroxene rarely forms more than 5% of the leuco-gabbro, and in many specimens, it is absent.Systematic vertical or lateral trends in these varia-tions in the leucogabbro were not found. Chemicaland modal analyses of a representative leucogabbroare given in Table 1.

Labradorite and clinopyroxene form the earlycrystallizing assemblage in the hornblende leuco-gabbro facies. Relics or alteration products of olivineor orthopyroxene were not observed in the leuco-gabbro. Plagioclase tablets are generally larger andthe peripheral jackets are thicker in the leucogabbrothan the picrite. The contrast in zoning and composi-tion of the jackets and the core suggests that the twocrystallized under considerably different conditions.The cores almost certainly are part of the cumulusassemblage; it is suggested, therefore, that the rimsgrew from the intercumulus liquid. The averagecomposition of the cumulus parts of the leucogabbroplagioclase appears to be uniform throughout thelayer and to differ little from that of the picrite plagi-oclase. Clinopyroxene in the leucogabbro occurs bothin equant and highly elongate subhedra. Clinopyrox-ene compositional limits in the leucogabbro are sim-ilar to those in the picrite.

The inter cumulus assemblage in the leucogabbrois composed chiefly of kaersutite with a little magne-tite and apatite. Titanobiotite does not occur in the

74 ROBERT C. SPEED

FIG. 3. Planar fabric of tabular plagioclase in hornblende pic-rite, West Humboldt layered intrusion. Trace of fabric (strikeN. 10° W., dip 80° E.) about parallel to hammer handle.

leucogabbro. Modal apatite appears to be far moreabundant in the leucogabbro than the picrite,though normative apatite is similar in the two facies.The abundance of the intercumulus minerals in theleucogabbro is between 1.5 and 2 times that in thepicrite.

Fabric. One of the most striking features of the WestHumboldt layered sheet is the strong planar fabricformed by the alignment of plagioclase tablets inboth layers of the body (Fig. 3). Foliation symbolsin Fig. 2 indicate the similarity of attitude of thefabric in the two layers and the fact that the planarfabric is at a high angle to the interface of the twolayers and the projected base of the sheet. Figure 4,an equal area plot of poles to the feldspar fabric,shows that the preferred orientation is generally sim-ilar throughout the body.

Bent twins, shattered grains, and other evidenceof protoclasis are absent. Discordant plagioclasetablets contact one another without deformation.Clearly, the fabric must have been formed in a be-nign manner. Sawed slabs of leucogabbro were ex-amined for lineation, and it is clear that prominentlineation is absent. It is possible, however, that if alarge number of careful measurements were made, aslight lineation might be discovered.

Vertical variations. Vertical variations in the WestHumboldt layered intrusion are summarized as fol-lows:A. Mineral Layering

1. Restriction of olivine, orthopyroxene, and titanobiotite tothe picrite.

2. Greater content of clinopyroxene in the picrite than theleucogabbro.

3. Greater amount of plagioclase, kaersutite, and apatite inthe leucogabbro than the picrite.

4. Ratio of olivine and clinopyroxene to plagioclase increasingstratigraphically upward in the picrite.

B. Textural Variations.

1. Average plagioclase grain size larger in the leucogabbrothan the picrite.

2. Clinopyroxene single grains and clusters a little larger in thepicrite than the leucogabbro; clinopyroxene in the Ieuco-gabbro more elongate.

The mineral variations indicate the strong separa-tion of the cumulus minerals into two layers on thebasis of density. The greater content of the inter-cumulus minerals in the leucogabbro than the picriteindicates the higher intercumulus porosity (lowercrystal/liquid ratio) in the upper layer prior tocrystallization of the intercumulus liquid.

Cryptic layering of the cumulus phases is appar-ently absent within the intrusion. Olivine composi-tion is constant throughout the picrite, and therange of 2V and (3 of clinopyroxene at differen t levelsin the sheet is similar. The consistent values ofnormative MgO/MgO+ FeO on the analyzed picritesin Table 1 supports the modal evidence that themafic minerals do not become more iron-rich athigher stratigraphic levels. Magnetite is a product ofdeuteric alteration of olivine in the picrite. Conse-quently, normative magnetite content of the picrites

o" 0

,, ,o ~ 0

o 0 J 01 I x

0: ~ : X;1 XX, 0

"o80 0

, 0

o 0

s

FIG. 4. Equal area projection of poles to planar fabric (circles)in West Humboldt layered intrusion and to bedding (X) in meta-sedimentary rocks within one mile of the layered intrusion. Plottedon lower hemisphere.

LAYERED GABBRO SHEET 75

is higher than that of modal magnetite, and the ratioof normative MgO/MgO+FeO is higher than it wasprior to alteration. Bulk modal plagioclase composi-tion, though difficult to estimate because of complexzoning, appears not to differ at succeeding strati-graphic levels in what is taken to the cumulus partof the mineral. This is supported by the similarvalues for normative plagioclase in the analyzedpicrites of Table 1. The larger average volume of theintercumulus plagioclase rims in the leucogabbro,however, makes the normative plagioclase in theleucogabbro more albitic than that of the picrite.

Glomeroporphyritic clots of clinopyroxene andplagioclase are the only monomineralic aggregates inthe intrusion. The clinopyroxene clots are generallyequant; the plagioclase clots are both equant andlensoid, the latter type lying parallel to the planarfabric of the rock. Rhythmic mineral layering is ab-sent. Sorting within each of the facies as a functionof grain size or shape was not observed.

PETROLOGY OF THE LAYERED INTRUSION

The rocks along the base of the sheet are so alteredthat the existence of a chilled zone could not be deter-mined. Consequently, the composition of the parentmagma or the degree of crystallinity of the magmaon emplacement are unknown. In a qualitative way,however, the two facies appear to be complementaryfractions of an olivine gabbro. This suggests that thetwo layers are differentiates of a single injection ofbasaltic magma. The continuity of the feldspar fabricbetween the two layers strongly supports the viewthat the layers were not separate injections. The factthat rocks throughout the complex have a late-crystallizing alkaline assemblage indicates an alkalimagma type.

Differentiation of the magma consisted of twoconcomitant processes, vertical segregation of theearly formed minerals into upper feldspathic andbasal mafic layers and strong changes in the composi-tion of the liquid phase with progressive crystalliza-tion. Following accumulation of olivine, plagioclaseand clinopyroxene, succeeding events were: 1)crystallization of orthopyroxene and sodic plagio-clase rims and development of the planar fabric, and2) crystallization of kaersutite, titanobiotite, mag-netite and apatite. The contemporaneity of thefeldspar rims with orthopyroxene rather than theother intercumulus minerals is suggested by the co-existence of kaersutite with magmatic analcite inother rocks of the complex. The contrast of ortho-pyroxene textures in the foliate picrite of the layeredintrusion to those in nonfoliate picrite to the south

in the West Humboldt Range suggests that crystal-liza tion of large continuous orthopyroxene webs inthe former picrite may have been mechanically pre-vented by concomitant reorientation of platy plagio-clase.

The second group of intercumulus minerals filledthe remaining pore space as well as partly replacingthe earlier pyroxenes. The composition of this latecrystallizing assemblage must be close to the non-volatile composition of the intercumulus liquid nearits final consolidation. The late magma containedonly 31-40% Si02 but was enriched in iron (15-30%total iron as FeO), soda, and probably in volatiles.This liquid was probably highly fluid. The liquid lineof descent shows a remarkable retention of iron inthe liquid phase. An account of the differentiationof the entire complex will be presented elsewhere.

ORIGIN OF THE LAYERING

The two-layer distribution of early crystallizingminerals strongly indicates the influence of gravityduring differentiation. The absence of rhythms orsorting in either layer, however, indicates that deposi-tion of the cumulates in this body did not involvethe current mechanisms that were apparently opera-tive in other layered gabbros (Wager and Deer, 1939;Hess, 1960). The original thickness of this body isunknown, but the known or inferred thickness ofother parts of the gabbroic complex in the WestHumboldt Range suggest that it did not greatlyexceed 500 feet. The rate of cooling, therefore, wasprobably similar along all surfaces of the magmachamber, and the change of melting point of crystal-lizing phases with depth in the chamber was prob-ably not a significant factor in their distribution.

The uniformity of composition of the cumulusminerals, each of which forms a solid solution series,indicates that they crystallized from magma of con-stant composition with respect to the ratios of hightemperature to low temperature components of theseries. Fresh magma, therefore, must have contin-ually been brought to the cooling surfaces wherecrystallization occurred, and magma movement mustbe invoked to account for the absence of crypticlayering. Yet, convective overturn or other flowmechanisms cannot be called upon to circulate freshmagma considering the lack of rhythms or sorting ofthe cumulates which would have been depositedconcomitantly with the rise of fresh magma. Anotherfeature to be accounted for is the upward increase incolor index in the picrite.

The layering can be explained in part by the sug-gestion that the cumulus minerals had largely crystal-

76 ROBERT C. SPEED

lized before the magma was emplaced. Olivine, clino-pyroxene, and plagioclase could have crystallizedwith little change of composition during rise of themagma through the conduit if the upward flow wasturbulent enough to bring fresh magma to the wallsand sweep the crystals away from the peripheralparts of the conduit. The complex zoning of thefeldspar and some clinopyroxene suggests rapidcrystallization under varying conditions. The rela-tive movement of crystals and crystal aggregates ofolivine, clinopyroxene, arid plagioclase during sedi-mentation that followed magma emplacement mayaccount for the layering and petrographic variations.The mafic phases would clearly have settled throughthe liquid, and it is suggested that plagioclase mayhave had a slight tendency to rise. The ratio ofplagioclase to interstitial amphibole is lowest at thebase of tabular bodies of feldspathic gabbro at otherplaces in the complex. This suggests that plagioclasewas less dense than the late liquid.

The settling of olivine and clinopyroxene simul-taneously throughout the magma chamber probablyinvolved increasing amounts of aggregation of thesinking grains with time. The sedimentation processmight be viewed as the compaction of a cloud ofcrystal aggregates rather than as the settling of dis-crete particles. Sorting during deposition of thistype should have been poor. Much plagioclase wasprobably dragged down by the descending mafics.Feldspar crystals near the chamber roof had fewerwaves of mafic grains and aggregates sinking pastthan did feldspars at lower levels, and the opportu-nities for feldspar to have been swept down were lessat higher levels in the chamber. On the other hand,feldspar at the base of the magma chamber was prob-ably covered by cumulates immediately after em-placement and had little chance to move away frommafic crystals at this horizon. Consequently, it issuggested that the degree of separation of cumulusmafic grains and feldspar during sedimentation wasa function of their height in the magma chamber justafter emplacement (excepting, of course, the pos-sibility of a thin chilled or rapidly crystallized layeralong the roof of the chamber in which all the earlyphases were trapped as phenocrysts.

If this mechanism is correct, the top of the picritecontains mafic cumulates which were near the roofof the chamber just after emplacement. Olivine,clinopyroxene, and labradorite at the bottom of thepicrite, however, probably did not move after em-placement, and their modal ratio should representthe ratio of these minerals in the magma on emplace-

men t. Good modal data on basal specimens are lack-ing, but, roughly, labradorite appears to have beena little more abundant than olivine and olivine abouttwice the amount of clinopyroxene. The leucogabbrolayer consists of plagioclase which successfully sepa-rated from the compacting cloud of mafic material.However, a major problem not covered by this ex-planation is the origin of the clinopyroxene in theleucogabbro. It seems unlikely that clinopyroxene inthe leucogabbro was a member of the pre-emplace-ment assemblage that failed to accumulate com-pletely. Olivine occurs only in the basal layer, andthe difference in density of olivine and clinopyroxeneis not enough to suggest that the former mineralshould have settled more than the latter. The ap-parent difference in shape of clinopyroxenes in theleucogabbro and the picrite supports the view thatthey did not crystallize together. Yet, it is not clearwhy the composition of clinopyroxene from the twolayers is apparently similar if clinopyroxene in theleucogabbro crystallized during or following accumu-lation.

Considering that the sheet was probably not muchgreater than 500 feet thick, the completeness ofaccumulation of the mafic grains is remarkable. Therate of settling of the cumulates must have vastlyexceeded the rate of crystallization of the liquid infrom the walls. In fact, the crystallization of the lateintercumulus assembalge apparently did not occuruntil sedimentation was complete. The probablehigh fluidity of the liquid phase during sedimenta-tion greatly assisted the rapid settling.

Some aspects of the Shonkin Sag laccolith (Hurl-but and Griggs, 1939) are similar to this intrusion.The laccolith is also highly differentiated for a thinbody, and the color index of the lower shonkinitelayer increases upward. The mode of the chilled basallayer of the Shonkin Sag body (47% phenocrysts)indicates that the magma was rather crystalline onemplacement, Unfortunately, petrographic coverageof that body does not show whether or not crypticlayering exists.

ORIGIN OF THE FOLIATION

The feldspar fabric appears not to be primary butto have formed by reorientation of feldspars in place.Absence of prominent lineation of the feldspar andlack of feldspar crystals stemming from a planarsurface indicate that this is not a growth fabric. Thefact that the foliation lies at high angles to the baseof the intrusion and the interface of the two layersindicates that laminar flow of plagioclase tablets past

LAYERED GABBRO SHEET 77

a planar surface did not cause alignment. It could besuggested that feldspar tablets were oriented duringaccumulation so that the tablets lay vertically. Bythis mechanism, however, the normals to the tabletsshould be randomly distributed in the horizontalplane rather than strongly aligned as shown in Fig. 4.Thus, the fabric could not have developed by sedi-mentation alone. As noted previously, the contrastbetween orthopyroxene textures in the foliate picriteof the layered body and in a non-foliate picrite a fewmiles south suggests that plagioclase alignment andorthopyroxene crystallization were contemporaneous.Finally, it is clear from the undeformed poikilitictexture of the late intercumulus kaersutite andtitanobiotite that these minerals crystallized afterthe fabric had formed. Together, these points indi-cate that reorientation occurred after accumulationbut before final consolidation.

Formation of a strong planar fabric by reorienta-tion of crystals in a liquid requires that the abun-dance of crystals be high enough that differentialmovement of crystals and liquid can occur. A changeof shape of the chamber is necessary for this differ-ential movement. Figure 4 shows a definite relationbetween the preferred orientation in the igneous bodyand the attitudes of the bedding of the intrudedmetasediments. The bedding orientation is largelycontrolled by near-isoclinal overturned folds. Thecomparable orientation of igneous and country rockfabrics suggest that the foliation is an axial plane

HESS, H. H. (1960) Stillwater igneous complex. Montana, Geol.Soc. Am. Mem. 80,

HURLBUT, C. S., JR. AND D. GRIGGS (1939) Igneous rocks of theHighwood Mountains, Montana: Pt. I, The laccoliths, Bull.Geoi. Soc. Am. 50, 1043-1112.

SPEED, R. C. (1962) Humboldt gabbroic complex (abs.), Geol.

fabric and that the foliation resulted from deforma-tion of the chamber in response to external stress.

The a tti tude of the fold axis defined by the baseof the picrite roughly corresponds to attitudes ofaxes of major and minor folds in the surroundingrocks. This relation and the similar orientation of theigneous foliation and axial planes of folds in thecoun try rocks suggest that intrusion occurred beforeregional deformation but that folding occurred beforefinal consolidation of the igneous body.

The absence of protoclastic textures in these rocksindica tes that reorien ta tion occurred in a benignmanner. It is suggested that the tabular grains werealigned by the flow of intercumulus liquid duringdeformation. The grains were reoriented by the mov-ing liquid so that their largest surface lay in the planeof maximum liquid flow. The cumulus porosity ofthe rocks (12-25%) seems low compared to estimatesfrom other cumulus rocks and to results of packingexperiments (Hess, 1960; Wager et al. 1960). Thissupports the view that some of the intercumulusliquid was pressed out, but the poor sorting of thesecumulates may also have yielded low porosities.

ACKNOWLEDGMENTS

I am grateful to C. O. Hutton and R. R. Comptonof Stanford University for helpful criticism of thismanuscript and to my colleague, A. A. Loomis, forhis valuable discussion and advice on the problem aswell as for manuscript comments.

REFERENCES

Soc. Am., Spec. Paper 73.WAGER, L. R., G. M. BROWN AND W. J WADSWORTH (1960)

Types of igneous cumulates, Jour. Petrol. 1, 73-85.-- AND W. A. DEER (1939) The petrology of the Skaergaard

Intrusion, Kangerdlugssuaq, East Greenland, Medd. Gr enland,105,314.