American Mineralogist, Volume 61, pages 732-750, 1976
Limitsto theassemblage forsterite#:ill$*illll;X. fromperidotite hornfelses,Icicle
B. RoNelo Fnosr Department of Geological Sciences, Uniuersity of Washingtonl Seattle, Washington 98I 95.
Abstract
Hornblende-plagioclaseperidotite has formed in aluminous reaction zones around mafic lensesin chromite-bearing forsterite-enstatite-tremolite metaperidotite close to the contact of a gabbroic phaseof the Mount Stuart Batholith. Assemblagesencountered in thesealuminous rocks are consistent with a theoretically-derived topology for the reactions governing the occurrenceof plagioclasein peridotite, which indicates that the low-temperaturelimit of pfagioclaseperidotite in the pure systemis the reaction 5 anorthite * 8 forsterite+ 2HzO : 2 tremolite * diopside * 5 spinel. Associatedpelitic hornfelsescontain assemblageswhich indicate that the metamorphism occurred at pressuresbetween 2 and,4 kilobars. The reaction chlorite = forsterite * enstatite * spinel + HrO, which occurs in the metaperidotitenear the contact with the batholith, '715 establishesthat the temperatureat the time of metamorphismreached + 25"C. Only in a small pendant enclosedwithin the gabbro do mafic hornfelsescontain the assemblagehorn- blende-clinopyroxene-orthopyroxene-plagioclase,indicative of transition to pyroxene- hornfels facies. The reaction-zonerocks contain assemblageswhich are indicative of both sides of the reaction 2 forsterite * anorthite : enstatite * diopside * spinel, with the enstatiteand diopside-bearingassemblages containing spinel richer in chromite than the forsterite * anorthite-bearingassemblages. Thermodynamic calculationsusing observedspinel composi- tions suggest pressures at least 2 kilobars below the pressure inferred from the pelitic assemblages.lt is arguedthat the main causefor this discrepancyis error in the assumptionof ideality in the spinel solid solution. Hornblende in the hornblende-plagioclaseperidotite is colorlessand shows a systematic variation in alkali content and tetrahedralaluminum along a trend rich in the tschermakite endmember.By expressingthis compositional changemathematically, diagrams are derived which show that for the hornblendecompositions used, the low-pressureperidotites can be divided into three chlorite-free mineral facies. They are, in increasing grade, horn- blende-spinelperidotite, hornblende-plagioclase peridotite, and finally plagioclaselherzolite.
Introduction 1964; Onuki, 1965; and Irvine and Smith, 1967), plagioclaseperidotite from low-pressureregionally- Models for the mineralogyof aluminouslherzolite, metamorphic or contact-metamorphicenvironments basedupon experimentalwork (Kushiro and Yoder, has not been reported. Plagioclaseand olivine-bear- 1966;Emslie, 19701'Green and Hibberson, 1970;and ing assemblageshave been describedfrom basaltic Herzberg, 1972) indicate a fairly wide stability field rocks which have beensubjected to metamorphismin for plagioclasein peridotite at high temperaturesand pyroxene hornfels facies(Richey and Thomas, 1930; relatively low pressures ( cf. O'Hara, 1967a). AI- MacGregor, l93l; Phillips,1959, l96l; and Almond' though plagioclase* olivine is a common magmatic 1964),but ultramafic rocks from high-gradecontact- assemblageand plagioclaseperidotite of clearlymag- metamorphic environments contain spinel rather matic origin does occur (cf Brown, 1956; Leake, than plagioclase(Matthes, l97l). This report describes the occurrence of horn- I Present address: Department of Geology, University of in the contact Minnesota-Duluth. Duluth, Minnesota 55812 blende-plagioclaseperidotite hornfels 732 THE ASSEMBLAGEFORSTERITE-ANORTHITE 733
zone of a gabbro, where it has formed in aluminous zones in an enstatiteand chromite-bearingmetape- ridotite. The relationships displayed permit refine- ment of the low-temperatureand high-pressurelimits to the stability of plagioclasein peridotite, and pro- vide an explanation for the rarity of plagioclasein metamorphosedultram afi cs.
Location and regional geology The hornblende-plagioclaseperidotite occurs in a metamorphosed ultramafic complex on the north slope of the lcicle Creek valley in the central Cas- cades of Washington, about I l5 km east of Seattle (Fig. l). It is a part of the Mount Stuart block, a fault-bound structural unit which comprises the Mount Stuart Batholith and the rocks it has intruded and metamorphosed.These country rocks includethe Chiwaukum Schist,a pelitic schistthat is the south- ern continuation of the Skagit Metamorphic Suite Ftc. l. Map showinglocation of areasin the centralCascades of (Misch, 1966), and the Ingalls Complex, a tecton- Washington where contact metamorphism of peridotite and re- ically emplacedultramafic-mafic complex. lated rocks has been studied; Icicle Creek, this paper, Paddy-Go- Easy Pass,Frost (1975). Dashed symbols : Chiwaukum Schist, The Chiwaukum Schistunderwent an early Barro- blank = Ingalls Mafic-Ultramafic Complex, v-symbols : Mount vian-type regional metamorphism of unknown age, Stuart Batholith, dotted symbols = Tertiary sedimentaryrocks. which reachedthe staurolitegrade in the lcicle Creek area. The Ingalls Complex was emplaced after this regionalmetamorphism, and the associateddeforma- Petrographic description tion jumbled the units so that inclusionsof diabasic In the Icicle Creek area the Ingalls Complex sur- to gabbroic rock, and less commonly, Chiwaukum vives as two bodieswhich cover a total area of about Schist, chert, and felsic plutonic rock occur locally 8 km'zadjacent to and within the mafic phaseof the within the ultramafic units. Concurrent with the em- Mount Stuart Batholith (Fig. 2). Throughout these placement,or somewhat afterward, the complex un- bodies the metaperidotite contains the assemblage derwent a low-grade hydrothermal event which ser- forsterite-enstatite-tremolite-chlorite,even in local- pentinizedlarge portions of the peridotite. ities more than a kilometer from the nearestcontact The Mount Stuart Batholith is of Late Cretaceous (Pongsapich,1974). ln the larger body the associated age and is composedof two major phases,an earlier mafic inclusionswithin the metaperidotiteare typical mafic phasewhich rangesfrom gabbro to diorite and hornblende hornfelses,but in a small pendant in the a later silicic phasewhich rangesfrom quartz-diorite batholith (Fig. 2), the mafic hornfelses locally de- to granodiorite (Pongsapich,1974). Intrusion of the velop the assemblagehornblende-orthopyroxene- batholith resultedin contact metamorphismof both clinopyroxene-plagioclase(An60). A pelitic hornfels the Chiwaukum Schist and the Ingalls Complex. In with the assemblage biotite-plagioclase (An40)- the aureole of the silicic phase of the batholith, cordierite-hypersthenealso occurs within the pendant. staurolite, cordierite, and andalusite crystallizedor Wherever the mafic hornfels and metaperidotite recrystallizedin the schist,while in the aureoleof the are in contact, a mineralogicallyand chemicallycom- mafic phase, sillimanite grew as well (Plummer, plex metasomatic reaction zone has developedbe- 1969).Contact metamorphism of the Ingalls Com- tween them. There seemto be two different types of plex by the silicicphase of the batholith at Paddy-Go- reactionzones (Fig. 3), a two-part reactionzone, and Easy Pass,about 23 km southeastof the Icicle Creek an amphibole-richone. The more common two-part area, caused the formation of the forster- reactionzone is similar to that describedfrom Paddy- ite-enstatite-tremolite assemblagein the metape- Go-Easy Pass (Frost, 1975), where an aluminous ridotite and the forsterite-enstatite-spinelassem- "blackwall" zone and a calcic "metarodingite" zone blage in associated aluminous metamorphosed surround the mafic inclusion.The amphibole-richre- blackwallrocks (Frost. 1975). action zone, in which hornblende or tremolite make B, RONALD FROST
embayedby the other phasesand containsnumerous inclusionsof greenspinel. Anhedral, rounded olivine makes up 30 to 40 percentof the hornblende-plagio- clase peridotites, and irregularly shaped, spongy enstatite occurs in amounts between 0 and 20 per- cent. Anorthitic plagioclase(An 98-99) makesup 20 to 25 percent of the rock, forming large, irregular grains, commonly rimming spinel. Brown to green spinel is usually quite abundant, forming up to 20 percent of some rocks. Most of the rocks have a granoblastictexture, and many of them are banded with layers richer in olivine and plagioclasealter- nating with those containing more hornblende and spinel.
Mineral comPositions In order to understandhow the mineral composi- tions affectthe phaseequilibria, constituentminerals from five samples of hornblende-plagioclase peridotite, two samples of enstatite- and diopside- bearing tremolite hornfels, and one sample each of forsterite-enstatite-tremolite metaperidotite, horn- ROCK UNITS SYMBOLS blende-bearing pyroxene hornfels, and cordierite- - CONTACT,rrposad, were analyzed on an Applied Mount Stuorl Bolholilh :.:.: opprorimola,inltrr€d hypersthenehornfels uotr.Pt''o"" Research Laboratories S-channelEMX-SM micro- ffi .-I-13 SAMPLE LOCATION orcoi {har€ plqoioclo$ probe (Table l). The sample locations are shown siti.i.enor. '//gqrldolilcoccurs// / Sl on Figure 2. Ingolls Compler oreos shlra pyroxons *QX lndividual grains of olivine, enstatite,and diopside Metoleridotite \a {/ qssem- ! homtels focies blo90$ occu. were of uniform composition for all samplesstudied. podi [T] Mofic Homfels, includrs SCALE l;4J ol molop?,idolile However, there was grain-to-grain variation ranging I O.5 O lkm from moderate to extreme,especially in rocks from fl..-TnlChowoukum Schlst the reactionzones. In order to minimize the effectof FIc. 2. Geologic map of the Icicle Creek area, Washington. this variation, analysesreported from a particular Geologic mapping by Pongsapich(1974) with revisionsby Frost. sample were taken from neighboring grains within a Topographic base from U S. Geological Survey l5 minute circle 0.5 cm in diameter. Spinel, plagioclase,and Chiwaukum Mountains quadrangle. hornblende, although uniform in composition in some samples,showed within-grain zoning as well as up more than 75 percent of the rock and spinel is grain-to-grain variation. Unless otherwise indicated, absent, seemsto show a transition from tremolite- analysesare averagevalues. rich metaperidotite,through a rock rich in an amphi- and orthopyroxene bole which is colorlessin thin section,through horn- Olivine blendite, where the amphibole is brown or olive- The olivine and orthopyroxene from the Icicle colored, to mafic hornfels. Creek rocks show similar composition ranges to The hornblende-plagioclaseperidotites occur as those from Paddy-Go-Easy Pass (Frost, 1975), part of the aluminous reaction zones within several namely Foeo,Ene,in the metaperidotiteand Forr-rr, hundred metersof the contact with the mafic phaseof Enro-roin the aluminous reaction zone rocks (Ta- the batholith. They are orange or green on a weath- bles 2, 3). Orthopyroxene richer in iron (En6r,Enrr) ered surface and cinnamon-brown to dark-green occurs in the hornblende-pyroxene and biotite- when fresh. Modal proportions of the constituent cordierite-hypersthenehornfelses, respectively.As phasesare quite variable. For example,colorless or at Paddy-Go-Easy Pass, clinoenstatite lamellae in very pale brown or green hornblende makes up be- orthoenstatiteoccur locally throughout the area.Un- tween 5 and 40 percent of a rock. Generally it is dulose extinction along with minor bending and kink- TH E A SSEM BLA GE FORSTERITE_AN ORTH ITE 735
TYPE 1 - Twcparr R€actron Zone TYPE 2 - AmphiboleRich ReactronZone
Amphibole.Rich Reaction Zone wilh Mafic aflinity
I n r!'ri " \ni lr r v v ,'lll,!' l' Amphibole-Rich I tlit# Reetion Zone y' '' ",tlt,I i / r,tr,t , I r'.v r':tt),tl,ti./ Ultramafic altinitv \\) ::-.-,'.:ii4it
Malic Hornfeli ''.-.-,/
ASSEMBLAGESENCOUNTERED Amphibole-rich Amphibole-rich Metaporidotite AluminousR.Z. Metarodingite Ultramaticaffinity Mafic affinity Mafic Hornlels
= OI.Opx-Tr-CteChromire (Plaq- An ) lPlag- - An ll (Amphrbole=colorless {Amphrbole=brown- {Amphibole=brown 98.99 90-96 colorlesshornblendel olive hornblende) hornblende) (Plaaioclase=An Hb.ot sp Cpx-Plag Hb (Plag ioclase=An ) - ) 65,70 Hb Ol Opx-Sp Cpx-Plag'Garn Hb-Opx 70_80 Ol-PlagHb-Sp Cpx'Plag'Sp Hb Opx'Ol Hb Plag Hb Plag Ol Opx-Hb-Plag Cpx-Sp Hb-Cpx'OpxChromite' Hb-Di Plag Hb.Plag-Di Ol Opx-Hb-Plag-Sp Hb-OlPlag Hb'Pla9-Opx ' Ol'Opx-Plag-Sp Hb-EnDi Plag- Hb'Plag-Opx-Cpx Ol-Hb-DiSp
' Assemblagestransitional to pyroxene,hornfelsfacies
Cpx=clinopyroxene,Cte=chlorite, Garn=garnet, Hb=hornbtende, Ol=olivine, Opx=orthopyroxene, Plag=plagi@lase,Sp=spinet, Tr= tremotire
Ftc. 3. Diagramatic display of the two typesof reactionzones found around mafic inclusionsin the metaperido- tite. The mineral assemblageslisted are those encountered in the highest-gradeareas, indicated by ruled lines in Fig 2. Width of the reactionzones is variable,ranging from l0 to 50 centimeters. ing of the orthoenstatiteindicate that the clinoensta- not containinghighly aluminousphases, to 3.7'per- tite is strain-induced. cent forrthe hypersthenecoexisting with cordierite. AlrO, contentof the orthopyroxeneranges from The orthopyroxene occurring with green spinel and lessthan 1.0percent for thosepyroxenes from rocks olivine contains only about 3.2 percentAlrOr, some- what lessthan one would expectfrom the experimen- Tnsls l. List of analyzedrock samplesfrom Icicle Creek tal work of MacGregor (1974). Distribution of Fe and Mg between olivine and : l-13 Hornblende'PlagioclasePeridotile enstatite(Fig. a) shows a Kd 0.9, consistentwith l-15 H0rnblende-PlagroclasePeridotite other pairs from ultramafic rocks (Ross e/ al., 1954; l-21 Hornblende-PlagioclasePeridotite Green, 1964;Challis , 1965:'White, 1966;Peters, 1968; l-26 Hornblende-PlagioclasePeridotite Loney et. al., 197l; Medaris, 1972; Onyeagocha, l-36 Metapendotite,Forsterite-Enstatite-Tremolite Assemblage 1973; Evans and Trommsdorff, 1974; and Frost, l-52a Reaclion.zonerock with maficaflinities, Forsterite-Anorthite-SDinel Assemblage te1s). l-52b Reaction-zonerock with maficaffinitres, Hornblende-ortho-pyroxene- Clinopyroxene-LabradoriteAssemblage Clinopyroxene l-59 Hornblende-PyroxeneHornfels Clinopyroxene from all of the rocks is strongly l-72 Tremolite-Enstatite-Diooside-ChromrteHornfels magnesian, and even that from the horn- l-82 Tremolite'Enstatite-Diopside-ChromiteHorntels l-89 Cordierite-HyperstheneH0rnlels blende-pyroxenehornfels has an Xy" greater than 0.80.AlrO3 and NarO contentsare low, lessthan 2.00 736 B RONALD FROST
Tnsls 2. Microprobe analysesof olivine definesa Ka :0.60 (Fig' 4). This is lower than the Ka of between 1.00 and 0.7 found in other or- f-36 l-15 121 l-13 l-26 l-52a tho-clinopyroxene pairs (Ross et. al., 1954; Kretz, si02 40 56 39 57 40 27 38 45 40 08 38 80 1961;White,1966; Peters,1968; Medaris,1972 and Fe0' 917 1430 1024 1820 1130 1852 Onyeagocha,1973). Apart from the obvious temper- l\,190 49 45 45 29 48 32 41 81 47.51 41 55 ature effect, this difference is also related to the fact [In0 016 028 007 039 016 027 lower-temperature diopside from Icicle Ni0 0,25 004 008 032 027 024 that the Ca0 002 003 004 006 004 ND Creek is more calcic than diopside from the higher- T0lal 99 62 99 50 99.02 99 22 99 36 99 38 temperaturerocks. As pointed out by Blander(1972), CationProportion ona basisol 4 oxygens iron content of clinopyroxeneis dependentupon the Si 0 996 0 996 0 999 0 992 0 997 0 998 extent to which the M2 site is occupied by calcium. 0188 0301 0212 0392 0235 0398 Fe pairs from Icicle Creek lvg 1 810 1 699 1 786 1 607 1 762 1 595 The ortho-clinopyroxene Mn 0 003 0 006 0 002 0 008 0 003 0 006 definea solvuswhich is considerablywider than that Ni 0 005 0 001 0 002 0,007 0 006 0 005 determinedat 810"C by Lindsleyet al. (1974)(Fig. 0 001 0 001 0 001 0 002 0 001 ND 5). Other pyroxene geothermometersalso yield un- Mg/Fe+ Mg 0 906 0 850 0 894 0 804 0 882 0 800 reasonably high temperatures for these pairs, ca. 900oC for the equationsof Wood and Banno (1973) ' Totaliron as Feo I'lD: notdetermined and ca. 1000"C for those of Saxena and Nehru (1975).This discrepancyis undoubtedlydue to the and 0.30 percent, respectively,values which are un- fact that their equations were derived by extrapola- doubtedly an indication of the low-pressureand rela- tion from high-temperatureexperiments. As a result, tively low-temperatureorigin of theserocks. they are probably not valid for theserelatively low- Distribution of Fe and Mg betweenthe pyroxenes temperaturerocks. Furthermore, Saxenaand Nehru
Tlnlr 3. Microprobe analysesof pyroxenes
l-89 r-36 r-26 t-21 t-52b' 1.52b" t-ss' l-59" l-72' t-72" si02 5709 56 06 5551 55 53 53 07 5397 529s 5743 5477 49 23 Ti02 000 0,06 013 007 014 011 0 23 000 0 01 012 nt203 121 187 316 168 190 095 128 046 051 349 Cr203 013 007 000 0 02 0 11 003 006 007 015 006 Fe0"' 574 716 670 1262 473 1889 654 936 279 3001 Mgo 3542 33 69 33 s8 2896 1648 2410 1525 3213 1781 1572 c) Ca0 0 18 032 022 050 2286 061 2275 059 2361 24 Mn0 016 015 008 029 016 099 045 020 011 046 Ni0 006 008 003 004 003 002 002 006 005 000 Na20 000 000 000 000 023 000 029 001 019 000 Tot.l 99 99 99 46 9942 99 72 99 70 9967 9981 10022 99 97 99 33 Cationproportions ona basisot 6 oxygens 1 Si 1 964 1 952 1 927 1 975 1 951 1 986 1 963 2 000 1 990 917 AIIV 0 036 0 048 0 073 0 025 0 049 0 014 0 037 0 000 0 010 0 083 AIVI 0 012 0 028 0 057 0 045 0 033 0 027 0 019 0 019 0 012 0077 Ti 0 000 0 002 0 003 0 002 0 004 0 003 0 006 0 000 0 000 0 004 Cr 0,004 0 002 0 000 0 000 0,003 0 001 0 002 0 002 0 004 0 002 Fe 0 165 0 209 0 195 0 375 0 14s 0 581 0 203 0 273 0 085 0 977 Mg 1 816 1 749 1 738 1 535 0 903 1 322 0 843 1 668 0 965 0 913 0 007 0 012 0 008 0 019 0 900 0 024 0 904 0 018 0 919 0 010 Mn 0 005 0 004 0 002 0 009 0 005 0 030 0 014 0 006 0 003 0 015 Ni 0 002 0 002 0 000 0 001 0 001 0 001 0 000 0 002 0 001 0 000 Na 0 000 0 000 0 000 0 000 0 017 0 000 0 021 0 000 0 012 0 000 SumVl ions 2 011 2 008 2 003 1986 2011 1 998 2 012 1 988 2 001 1 998
WO 034 060 043 099 4620 125 4636 095 4669 053 En 9136 8880 89 55 79 56 46 34 68 s8 43 23 8513 4900 4803 Fs 830 1059 1003 1945 746 3017 1041 1392 430 5144
' 0rthopyroxenelrom orthopyroxene-clinopyroxene pair " Clinopyroxenefrom orthopyroxene-clinopyroxene pair '.' Totaliron as Feo THE ASSEMBLAGE FORSTERITE-ANORTHITE 737
. OPX- OLIVINE PAIRS
0.8 r OPX- CPX PAIRS
OPX-CORDIERITE PAIRS FROMTHE LITERATURE o.6 OPX-CORDIERITEPAIRSI THISSTUDY = = cD o.4 =
q, L o.2
0.0 o.o o,2 0,4 0.6 0.8 l.o t.2 Fe/Mg OPX
FIc. 4 Distribution of iron and magnesiumbetween orthopyroxene and coexistingolivine, clinopyro- xene, and cordierite. Analyses quoted from the literature are those of Barker (1964), Chinner and Sweatman(1968), Reinhardt (1968),and Knorring et al. (1969)
(1975) have derived their geothermometer using a Cordierite seriesof complex expressionsindicating the amount Cordierite from the pelitic hornfelsesis fairly iron- of the various ions in the Ml and M2 sites.Since the rich and may contain about 3 weight percentHzO, as site preferencesare very poorly known, particularly determined by totals of 97 percent from microprobe in the clinopyroxenes,and since they are surely a analysis(Table 5). This value is within the range of function of temperature,if not of composition, this 0.32-4.8 percent found in wet-chemicalanalyses of geothermometer can hardly be applied rigorously to cordieritetabulated by Leake (1960).The Ko for iron low-temperaturepyroxene pairs. and magnesium distribution betweencordierite and hypersthenefrom Icicle Creek is consistentwith that Spinel from other cordierite-hypersthenepairs (Barker, Spinel ranges in composition from green, alu- 1964;Chinner and Sweatman,1968; Reinhardt, 1968; minous spinel in the hornblende-plagioclaseper- and Knorring et al., 1969)(Fig. 4). These mineral idotite to chromite with more than 60 percent pairs came from rocks which formed under a wide (Fe,Mg)CrrO, in the tremolite-enstatite-diopside variety of P-T environments, and the cordierite in hornfels(Table 5). Three of the samples(l-15, I-21, them contains differing amounts of HrO (ranging and I-52) contain green spinelwhich is unzoned and from 0.3 to 3.0 percent), indicating that the water shows only minor grain-to-grain variation. The re- content of cordieritemay not have as strong an affect maining samplescontain weakly-zonedspinels with a on the 1(6 between cordierite and other ferromagne- rather sizablegrain-to-grain variation. The iron and sian phasesas has been postulated(Wood, 1973). magnesiumdistribution betweenolivine and spinel is Calcic dependantupon the chrome content of the spineland amphiboles shows a similar trend to that describedby Evans and The calcic amphibolesfrom the Icicle Creek rocks Frost (1975)(Fig. 6). range in composition from tremolite to hornblende I B RONALD FROST
/ sotvus ar Stooc !t NDStEyer.ol. (1974)
EN FS
Ftc. 5 Solvus defined by coexisting orthopyroxene and clinopyroxene as compared with that determined experimentally by Lindsley et al. (1974)
(Table 6), with two different trends in amphibole where it occurs with clinopyroxene,orthopyroxene, composition (Fig. 7). Hornblende from the horn- and labradorite, and the tremolite from the tremo- blende-plagioclaseperidotite, which is colorless in lite-enstatite-diopside-chromiterocks are relatively thin section and orcurs with olivine, enstatite,anor- more sodic, showing a (Na * K)/A,ltu trend more in thite, and spinel,shows a trend toward a hornblende the direction of pargasite.The colored hornblendeof rich in the tschermakiteend member, with a (Na * the mafic rocks further differs from the colorless K)/Al'u ratio of 0.15. This trend is different from hornblendeof the ultramafic rocks in that it contains that described by Frost (1975) from forste- more iron, apparentlyas Fe'+, more titanium relative rite-enstatite-spinelrocks, but is similar to the (Na * to aluminum, and accordingto calculationsof Papike K)/eltu values from highly tschermakitic amphi- et al. (1974), has minor amounts of sodium in the boles reported by Leake (1971). M4 site. The brown hornblende from the mafic hornfels. The solid line in Figure 7 reflectsthe manner in which the amphibole composition is controlled by
T,c.st-r4. Microprobe analysesof plagioclase that of the coexistingphases. From the slope of this line the hornblende composition in the horn- blende-plagioclaseperidotites can be approximated t21 t-26 r-59 si02 43 16 4345 5285 by the formula Ar203 36 32 3650 3004 Nao,u,CarMgu-oru, (Sir-,AlrorrXOH), Fe203' 006 0 13 023 Ca0 1987 20,15 1247 This hornblende coexistswith olivine, enstatite,and Mgo 000 001 003 spinel;therefore, its composition is fixed by the reac- Na20 017 009 443 Kzo 000 000 012 tion: Sum 99 58 10032 10017 tremolite* 0.925xspinel * 0.075xNazO Cationproportion on a basisof 8 oxygens : hornblende(Al'u : x) + 0.775xforsterite
JI 2 007 2 007 2 392 + 0.225xenstatite (l ) AI 1 991 1 987 1 603 - r'l Fe'" 0 002 0 004 0 008 Assuming that the NarO is in the fluid phase, this Ca 0 990 0 997 0.605 reaction involves six components and five phases, Mg 0 000 0 000 0 002 and, therefore, is isothermally and isobaricly Na 0 016 0,008 0 389 K 0 000 0 000 0 007 univariant. The univariant nature of this reaction is Total 8 000 8 000 I 000 expressedas the linear relationshipin Figure 7. Why An 994 992 OUJ the hornblende from olivine, enstatite, and spinel- bearing assemblagesfrom Paddy-Go-EasyPass has a Totaliron as Fe203 different value of (Na * K)/AIIV from that at Icicle THE ASSEMBLAGE FORSTERITE_ANORTHITE 739
Tlnln 5. Microprobeanalyses of spineland cordierite
l-15 t-21 l-26. l-27' r-36 l-72" l-89*r r Si02 ND ND ND ND ND ND 48.81 Ti02 0.02 0.03 0.08 0.00 0.05 0.02 0.00 A1203 66.1 67.0 35.7 27.4 21.1 15.3 31.52 Cr203 0.04 0.05 27.3 32.2 43.0 50.0 0.00 Fe203 1.55 0.85 5.64 7.42 4.60 3.33 Vzog 0.16 0.07 0.09 0.06 0.35 0.05 ND Mgo 19.7 21.2 12.4 6.88 9.24 7.13 9.07 Fe0 12.1 9.62 18.2 29.2 20.2 22.7 7.22* Mn0 0.12 0.03 0.11 0.21 0.45 0.17 0.12 Ni0 0.02 0.12 0.21 0.07 0.04 0.03 0.00 Zn0 0.11 0.12 0.17 0.21 0.37 0.40 ND Total 99.92 99.09 99.90 99.65 99.40 99.13 96.88
Cationproportions ona basisof 32 oxygens Si ND ND ND ND ND MD 5.074** Ti 0.003 0.00s 0.014 0.000 0,010 0.004 0.000 AI 15.726 '15.852 9.891 8.158 6.348 4.788 3.863 Cr 0.006 0.008 5.066 6.421 8.678 10.527 0.000 ps+3** * 0.236 0.129 0.998 1.408 0.882 0.666 V 0.026 0.011 0.017 0.012 0.072 0.011 ND
Mg 5.927 6.343 4.341 2.587 3.516 2.828 1.405 Fe 2.036 1.614 3.581 5.315 4.319 5,053 0.628 Mn 0.021 0.005 0.022 0.045 0.097 0.038 0.010 Ni 0.003 0.019 0.040 0.014 0.008 0.006 0.000 Zn 0.016 0.018 0.030 0.039 0.070 0.079 0.000 YRt 0.985 0.992 0.620 0.510 0.399 0.300 Ycr 0.000 0.000 0.318 0.402 0.546 0.659 Mg/Fe+ Mg 0.744 0.797 0.548 0.327 0.449 0.359 0.691
* Valuerepresents anaverage ol the six most chromiferous points. ** Valuerepresents anaverage ofthe six most aluminous points. *** Analysisofcordierite, includes 0.05% CaO and 0.08% Na20. * Totaliron as FeO. * * Cationproportions forcordierite calculated ona basisof 18oxygens. * * * Fe203for spinel calculated assuming stoichiometry. ND: Notdetermined.
Creek is not understood, but it could be related to removal of silica from the amphibole. As the horn- different external parameters between the two au- blende-plagioclaseperidotite contains olivine and reoles. spinel and is relatively saturated in aluminum and The hornblendein the hornblende-plagioclaseper- undersaturatedin silica,the admissionof sodium into idotite generallycontains more sodium than coexist- the amphibole is obviously favored. ing plagioclase.For reasonsof charge balance,the substitution of sodium into plagioclase is accom- Estimate of pressureand temperature panied by addition of silica and expulsion of alumi- at the time of metamorphism num, whereasthe substitutionof sodium into horn- The pelitic mineralsin the contact aureoleindicate blende requires the addition of aluminum and the a relativelylow pressureof metamorphism.Plummer 740 B, RONALD FROST
3.O
2.O *-o )< c
o.o o.o o.l o.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 l.o Ysp'Cr
Frc. 6. Logarithm of the partition coefficient for the distribution of iron and magnesium between olivine and spinel as a function of Yc,"p (where Yc."p : Cr/Al * Cr * Fe'+), calculatedassuming that YF."p3+ : 0.05 (ln Ko = ln Kd + 4(0.05 - YF.3+)"p. The dashedline is the ca.700oC "isotherm" of Evans and Frost (1975).I : composition range of spinel from the hornblende-plagioclaseperidotite, solid circles,B = composition range of spinelfrom metaperidotite,open circles,C: composition range of spinel from tremolite-enstatite-diopside-chromite assemblage.
(1969) noted that stauroliteseemed to be stablewith until the breakdown of hornblende. Lower Pg,e in both cordierite and andalusite within the aureole. the mafic inclusionsrelative to the metaperidotiteis The presenceof staurolite would place the low-pres- indicated by the fact that the only rocks containing sure limit at about 2 kilobars (Richardson,1968) and assemblagesin the pyroxene-hornfels facies (i.e. or- the occurrenceof the other minerals would suggest thopyroxene * clinopyroxenein either the mafic or pressuresof between2.5 and 4 kilobars (Hess, 1969). ultramafic hornfelses)are found associatedwith the The lack of cordierite as a breakdown product of larger mafic inclusionsin the roof pendant. This lo- chlorite is not inconsistentwith theseestimated pres- calization does not seem to be due to variations in sures,for as noted by Frost (1975) the presenceof rock composition, as in one locality an ultramafic, minor elements,most notably iron and chromium, tremolite-rich rock included in mafic hornfels con- and the occurrenceof unknown amounts of water in tains the assemblageenstatite-diopside-tremolite, in- the cordierite, lower the upper pressurelimit for the dicating that the reaction forsterite * tremolite : reaction forsterite * cordierite : enstatite t spinel to enstatite * diopside + HrO has gone to completion pressuresmuch lower than the 3.3 kilobars (730"C) with the elimination of olivine from the rock. The found in the pure system(Fawcett and Yoder, 1966). metaperidotite elsewherein the roof pendant still Although P11r6is unknown, its maximum valuecan contains the assemblageforsterite * tremolite. This be safely set at P1o5"1.This is particularly true in the suggests that the rocks containing the pyrox- metaperidotite where progressive metamorphism ene-hornfels facies assemblagesformed in areas of would have continually enriched the fluid phase in relatively low P11p where hornblende had become HrO. The same may not be true for the mafic horn- unstable, while throughout the metaperidotite Ps,6 felses,because when epidote (which is only a minor was great enough to maintain amphibole as a stable phaseto begin with) reactsout at relativelylow tem- phase. peratures,there are no major dehydration reactions The breakdown of chlorite occurs in the metape- THE A SSEM BLA GE FO RSTERITE_A N ORTHITE 741 ridotite near the contact with the batholith, allowing Topologies for phaseequilibria a temperatureestimate to be obtained from the ex- governing the occurrenceof perimentallydetermined curve for the reaction chlo- plagioclasein metaperidotite : rite forsterite * enstatite * spinel t HrO. By Phaserelations in aluminous peridotite can be ap- setting the pressurelimits at 2 and 4 kilobars and proximated by the CaO-MgO-AlrOa-SiOr-H2O sys- taking into account iron substitution (Frost, 1975), tem. As no plagioclasehas beenfound in metaperido- then the temperature limits can be calculated from tite occurring below the anthophyllite-out isograd the equation: (Frost, 1975), low-temperature Mg-silicates antigo- rite, talc, and anthophyllite can be disregarded.The lnf'"o: -'7620/T + 7'59 participating phases are therefore limited to anor- + 0.18s6(P- 3000)/T(Frost, 1975) thite, chlorite, diopside, enstatite, forsterite, spinel, calcic amphibole, and water. The composition of The resulting temperatures are 690' and 740" for chlorite is taken to be MguAlrSi'O'o(OH)r,as that is pressuresof 2 and 4 kilobars, respectively.Although close to the composition of chlorite found at its this result (715 + 25"C) is subjectto error inherentin breakdown in high-grademetamorphosed blackwall the determination of the equilibrium expression rocks (Frost,1975). (Frost, 1975),this error will haveonly a minor effect A Schreinemakers'analysis of this systemusing the on the conclusions. phaseslisted, after the method of Zen (1966),yields 8
Tlst-s 6. Microprobe analysesof calcic amphiboles
t-15 t-21 l-26 l-13 l-26* l-26'* t-72 l-59 Si02 46.08 46.46 50.46 46.30 42.63 42.50 55.74 47.88 Ti02 1.59 1.73 0.28 1.06 0.97 3.02 0.01 1.66 A1203 13.23 12.70 9.20 11.65 14.59 12.24 3.08 8.39 Cr203 0.02 0.00 0.39 0.59 0.07 0.38 0.33 0.16 Fe0*** 4.86 J.U I 4.03 5.99 7.75 8.30 3.57 10.0.1 Mgo 17.24 18.13 19.74 17.41 15.76 15.59 22.32 15.80 Mn0 0.12 0.04 0.05 0.12 0.10 0.10 0.05 0.27 Ni0 0.00 0.03 0.08 0.07 0.06 0.07 0.11 0.02 Ca0 12.24 12.32 12.45 11.91 11.73 11.63 11.72 1116 Na20 0.36 0.22 0.48 0.10 2.47 2.68 0.51 1.28 Kzo 0.81 1.01 0.09 1.40 0.39 0.64 0.15 0.43 Total 96.54 96.55 97.26 q6 5q 96.51 97.14 97.59 97.07 Cationproportion on a basisof 23oxygens Si 6.550 6.581 7.041 6.640 6.209 6.210 7.682 6.928 Altv 1.450 1.419 0.959 1.360 1.791 1.790 0.318 1.072 Alvt 0.723 0.702 0.554 0.611 0.715 0.318 0.182 0.360 Ti 0.167 0.184 0.029 0.115 0.106 0.332 0.001 0.181 0.002 0.000 0.043 0.067 0.088 0.044 0.036 0.018 Fe 0.578 0.463 0.470 0.718 0.944 1.014 0.412 1 212 Mg 3.653 3.827 4.106 3.771 3.420 3.394 4.586 3.408 Mn 0.014 0.004 0.005 0.014 0.012 0.012 0.006 0.331 Ni 0.000 0.003 0.010 0.009 0.007 0.008 0.012 0.003 Ca 1.865 1.869 1.862 1.830 1.830 1.820 1.731 1.730 Na 0.100 0.060 0.130 0.027 0.699 0.758 0.137 0.360 K 0.146 0.183 0.015 0.256 0.072 0.120 0.026 0.078 Na+ K/AllV 0.170 0.174 0.151 0.208 0.430 0.490 0.513 0,408
* Marginof zoned hornblende grain, colorless in PPL ** Coreol zoneshornblende grain, brown in PPL *** Totaliron as FeO. B. RONALD FROST
o-Amphiboletrom hornblende-plogioclose peridolife t.0 e-Amphibolecoexisling wilh opx, cpx, ond chromife r-Amphibolecoexisling with opx, cpx,ond plogioclose | -l- 0.8
- 0.6 + =oAC'
0.2
0. 0.5 r.0 t.5 29 Atlv Ftc.7. Variationsof thealkalicontentof amphibolesasafunctionoftetrahedral aluminum,calculated on an anhydrous basisof 23 oxygens. invariant points which are connected by 32 semblagesencountered in the Icicle Creek rocks are univariant curves. Fortunately the relative positions consistentwith the theoretically derived phase dia- of three of the univariant curvesare known. Experi- gram (Fig. 8). Two rock samples,however, contain mental work cited above (e.g. Kushiro and Yoder, the assemblage tremolite-diopside-enstatite-chromite, 1966) places the reaction forsterite * anorthite : which is diagnostic of the "higher pressure" divariant enstatite* diopside f spinel at relativelyhigh pres- field bounded by reactions (An,Sp) and (Fo). The sure (6-10 kbar) with a relatively small value of Icicle Creek hornfelses,though, may be assumedto dP/dT. Field evidence indicates that the reaction have formed under isobaric conditions. In the next chlorite : forsterite * enstatite * spinel + HrO sectionit is shown that the reaction(V, Tr) and hence occurs at lower temperaturesthan the reaction forste- the whole diagram (Fig. 8) is translated to lower rite * tremolite : enstatite * diopside * HrO for pressuresby impurities such as Fd+, Fd+, and Cr in most metamorphic pressureconditions (Trommsdorff the participating phases. and Evans, 1974; Frost, 1975).This enablesone to construct a unique topologic net for equilibria in- Effect of impurities on the reaction forsterite volving these phases (Frost, 1973, Fig. 48). Com- I anorthite : enstatite * diopside t spinel parison of the assemblagesfound in the metarodingite The effect of other components (e.9. chromium, and metamorphosedblackwall rocks at Paddy-Go- iron), particularly in spinel, on the HrO-absentreac- Easy Passwith those predicted by the topologic net tion can be estimated by simple thermodynamics. indicates that the plagioclase-producingreactions and this resultcan be comparedwith the estimatesfor occur at temperatures and pressuresabove those temperatureand pressurederived independentlyfor of the invariant point produced by the intersection the Icicle Creek rocks. The equilibrium is: of the reactions chlorite = forsterite + enstatite + : spinel * HzO and tremolite * diopside * spinel 2Mg, SiO, * CaAlzSi,O8 : MgzSirOo anorthite * forsterite + HrO. This allows one to forsterite anorthite enstatite further reduce the significant reactions to those shown on Figure 8 (Frost, 1973).The stoichiometryof these f CaMgSirO6 { MgAlrOn reactionsis given in Table 7. diopside spinel Chemographic diagrams projected from forsterite and, (ali'Xaji.Xaii"',"o.) to the CaO-AlrOr-SiO, plane of the MgO-CaO- ^ _ AlrOs-SiO, tetrahedron show that most of the as- (a?l)'("1i,..) TH E A SSEM B LAG E FORSTERITE_A N ORT H ITE t15
For minoramounts of dilution,Raoult's law canbe used. Activity of the pyroxene end members in the assumedto hold approximately,and thereforefor pyroxene solid-solutioncan be approximatedon the spineland olivine: basisof a two-site model as first advocatedby Muel- ler (1961, 1962)and discussedin depth by Blander : (Xii.XYii)= ai.l'*,''"o. (1972\ and Wood and Banno (1973),such that when molecule where (Yili) : Al/Al + Cr * Fe"n enstatite is considered as being the MgrSirO.: : (x#"f ";: (a3H.): (XJn,) As plagioclasein theserocks is nearly pure anorthite, tx,lli,,l the activity coefficient can be set equal to unity; for and for diopside: plagioclaseof more sodic compositions,the activity- = compositiondata from Saxena(1973, Fig. 56) can be (a€1.) ({:;,) (x;";')
I ,g* I
EN+DI+SP (v)- FO+AN
/l I 1 L
(Dr)
T- Frc. 8. P-7 plot for the reactions governing the occurrence of plagioclase in the : CaO-MgO-AIrOB-SiO,-H,O system An : anorthite, Di = diopside, En : enstatite, Fo forsterite, Sp : spinel,Tr : tremolite,V : vapor, in this diagram HrO. Chemographicdiagrams show phaserelations as projected from forsterite to the CaO-ALOs-SiO, plane of the CaO-MgO-AlrOs-SiO, tetrahedron and = are valid for olivine-bearingassemblages formed in an HrO-saturatedenvironment. .r assemblage found at Icicle Creek. NOIE: Reactions labeled (V) and (Fo) (dashed lines) are metastable in rocks of peridotitic composition in the presenceof an HrO-saturated vapor phase. 744 B RONALDFROST
Tnnls 7. Reactions governing the occur- pressure when log K : 0, and P2 : equilibrium rence of anorthite in peridotite, Part I pressurewhen log K I 0. Amphibole consideredas pure tremolite Extrapolation of unpublished work of Windom (personal communication) shows that the reaction P.rlicipatingPhasos forsterite* anorthite = enstatitet diopside* spinel An = Anorthite,CaAt2 Si206 occursat about 6.4 kilobarsat 715'C, a valuewhich Di : Diopside,CaMgSi206 is within 0.2 kilobars of that extrapolatedfrom the En= Enstatite,MgSiO3 work of Kushiro and Yoder (1966). This can be Fo: Forsterite,Mg25i04 consideredas P2,with a hypotheticalP' existingat a Sp : SpinelMgAl204 pressurewhere K : 1.0. In the pure systemthe only Tr = TremoliteCa2Mg5(Sig022) (0H)2 substitutions which can affect the equilibrium con- V : Vapor,H20 stant are those by which aluminum dissolvesin the pyroxenes and enstatite dissolvesin diopside. At Reactions theselow temperaturesthe substitution of calcium in (V,Tr) An : 2Fo+ 2En+ Di + Sp enstatitecan be consideredto be negligible.Assum- (Sp,An) Fo Tr = 2Di H20 + 5En+ + ing that the iron content of the pyroxene does not (Di) Tr 2Sp= + 2An+ 3Fo+ En + H20 affect the aluminum content and that, as there is no (Fo) 2Tr : + Sp An + 8En+ 3Di+ 2V jadeite component in the clinopyroxene, aluminum (En) 2Tr+ Di + 5Sp= 5An 8Fo 2V + + will dissolve equally into orthopyroxene and cli- nopyroxene,the aluminum content of the pyroxenes coexistingwith olivine, spinel,and plagioclasecan be estimatedfrom analysesin Table 2. Orthopyroxenes Thusthe equilibrium constant becomes: from rocks containing green spinel have an Xlpr}n, : 0.06 (e.5. # 3, Table 2).X|o"irrcan be estimated - _ ( Xii?,,, X Xii;., XXii;,n, X X:'.;,)( X;'-Xyii)" from Figure 5 to be about 0.98 in the pure systemat (xi;,)-(x:i"-) 715"C. By combiningthese values with equations'(2) (2) and (3) and solvingfor P' at a temperatureof 7l5oC, we get P : 6.8 kilobarswhen K : 1.0. The next problem is in determining how to apply Equation (2) indicatesthat the largesteffect on the this value for the equilibrium constantto calculations equilibrium pressurewill be causedby the dilution of pertaining to the pressure of equilibrium. The ex- aluminum in spinel.Thus in a P-Yil diagram the pression for log K as a function of P and Z from pressureof equilibrium for the reaction forsterite * Bacon and Carmichael (1973) yields unrealistically anorthite : enstatite f diopside * spinel will fall low pressuresfor the lcicle Creek rocks (e.g:.- 3.9 with decreasingaluminum in the spinel. This curve kilobars for rock I-26). This error is probably due to can be calculatedwith the information now available, the fact that, as vapor-freereactions such as this have assumingfor this model that the ion substitutingfor small valuesof AS, they are sensitiveto small errors aluminum in spinel is chromium, as chromium is far in the thermochemicaldata. The error in determina- more abundant in spinel from high-gradeultramafics tions of entropy and enthalpy for the participating than ferric iron (Evans and Frost, 1975). phasescan be eliminated if the equilibrium pressure First the activity of anorthite in plagioclasecan be can be estimatedat 715'C. assumed to be unity, as plagioclase from all oli- lf the equilibrium pressureis known at a given vine-plagioclaserocks is nearly pure anorthite. Sec- temperature,the dependanceof the equilibrium con- ond, by specifyingthe iron content of the olivine, the stant on pressurecan be derived from the following iron content of the other participating phasescan be relation: calculated from using the previously determinedK6 values.For example:let X$, : 0.S5.about mid-way d lnK/ dP : - AV/RT in the composition rangeof olivine from IcicleCreek. The resulting expressionfor the forsterite-anorthite From K6 betweenolivine and enstatite,Xii; can be reaction is: determinedas 0.862.Ko betweenMl and M2 sitesin orthopyroxene can be calculated from the data of - log K : 0.1025(Pt - P,)/T (3) Saxenaand Ghose (1971)to be 7.0 (Stroh,personal where 0.1025 : - AV/2.303R, P, : equilibrium communication) at 715'C. From this. the site occu- THE ASSEMBLAGE FORSTERITE_ANORTHITE pancy can be calculated as: XilJ', 0.958, thermodynamic calculations could be due to three Xi,?;, : 0.765. From Figure 4 it can be deter- causes,namely, error in the location of the reaction mined that at thesetemperatures, diopside coexisting in the pure system,error in estimating the location with enstatite with an XiaoJ: 0.862 should have of the box bracketing the pressureand spinel com- Xii; : 0.925 and Xtp^.', -- 0.94. Assuming that position, and error in the model of thermodynamic that portion of the M2 site not occupiedby calcium activities. contains iron, then Xi,?;, : 0.98. When one varies It is conceivablethat there is some error in the the aluminum content of spinel in this system,the location of P"ou,riu,r'-at Yi? : l.0, but it probably Fe2+content of it will also vary. Assuming that Y$&+ is not much greater than * 0.5 kilobar. Although : 0.05, the expressionfor the dependenceof Ka be- there seemsto be conflicting resultsin experimental tween olivine and spinel on Yi! can be expressedas determinations, they agree more than is generally Ko* : exp (0.85 - 2.94 Yif) (Evans and Frost, recognized. Discounting those experiments which t97s). usednatural olivine-plagioclasepairs, the only exper- With thesedata, usingequations (2) and (3) and P' imental work which doesnot agreewith the resultsof : 6.8 kilobars and calculating P, for various values Kushiro and Yoder (1966) and Windom (personal of YiY, the solid line in Figure 9 can be generated. communication)is that of Herzberg (1972). How- As noted on Figure 6, spinels from the horn- ever, data from that study are internally conflicting' blende-plagioclaseperidotite have Yip' of 0.58 or and thereforethe resultsare not reliable' greater, whereas those in the tremolite-enstatite- The bracketing spinel compositions on Figure 9 diopside-spinelhornfelses have YiI of 0.39 or less. seem to be fairly well substantiated.They come from The range of these values and the pressurebracket, spinelswhich clearly occur in assemblagesformed on 2-4 kilobars, are shown as the cross-hatched rec- either side of the reaction forsterite I anorthite = tangle in Figure 9. This indicatesthe area in which enstatite * diopside * spinel. Some question could the reaction should occur accordingto the field data. be made about the presenceof spinels of varying The large discrepancy,a minimum of 2 kilobars, be- composition within individual samples of horn- tween the observedrelations and those predicted by blende-plagioclaseperidotite and whether they imply
1.0 0.9 0.8 0,7 ^-0.6 0.5 0.4 0.3 vsp'Al
Frc.9 Theeffectofspinelcompositiononthereactionforsterite*anorthite:enstatite*diopside* spinel in an iron-bearing system An = anorthite, Di = diopside, En = enstatite, Fo = forsterite, Sp : spinel. Cross-hatched box indicates the range of spinel compositions and pressuresat which the reaction seemsto take place at Icicle Creek. Z : 715"C. B. RONALD FROST
T,tsI-B8 Reactionsgoverning the occurrenceof anorthite in per- the activity of various end members in the spinel idotite, Part Il Amphibole with variable composition solid-solution is valid. First described by Irvine (1965),it assumesthat Fe2+,Fe3+, Mg, Al, and Cr ParticipalingPhasos each substituteindependently in the spinel structure. An = Anorthite,CaAt2Si208 In the more recentmodel of Gooley et al. (1974)it is Di : Diopside,CaMgSi206 assumedthat FeCr, substitutesideally for MgAlr. As En = Enslatile,MgSi03 Fo : Forsterite,Mg25i04 there is no chargeconstraint on the substitutionpat- Hb = Hornblende, and Na15x Ca2Mgs _ 85y(Si6 _ ,Atr022)(0H)2 tern, this is an unreasonableassumption, there- Sp : Spinel,MgAl204 fore the model of Irvine (1965) is preferred. V = Vapor,H20 + ( 075x)Na20 Stroh (personal communication) found that the
= two-site model works reasonablywell in calculations Reactions,where x theamount of AllVinHb : (V,Hb") 2Fo+ An : 2En+ Di + Sp on the reaction MgAlrSirO. * MgrSiOo MgrSi2Ou (An-) Hb+ (1 + 775x)Fo=(5- 225x)En+ 2Di+ 925xsp+V * MgAlrOa for rocks which formed at high temper- (Sp-) Hb+ (1- 1 075x)Fo= (5- 2075x)En+ (2- 925x)Di+ atures, but rather poorly for those from Paddy-Go- ,925xAn + V Easy Passand lcicle Creek, which formed at rather (Di') Hb + (2 - 925x)Sp = (3 - (1 - ,77sx)F0 + 225x)En + low temperatures.As there is clearly immiscibility 2An+V 1ro-) 2Hb+ (1- 1 075x)Sp= (8-200x)En + (3- 775x)Di+ within the spinel tetrahedron at metamorphic tem- (1 + 775x)An + 2V peratures(Turnock and Eugster,1962), it would be (En') 2Hb+ (1- 225x\Di+ (5-207x)Sp: (8-200x)Fo+ reasonableto assumethat, although the substitution - (5 225x)An + 2V of chromium for aluminum in the spinel structure may be nearly ideal at magmatic temperatures,there should be increasingdeviation from ideality with de- disequilibrium.Similar rangesin spinelcomposition creasingtemperature. It appears that it is this non- from chlorite-forsterite-enstatite-spinelrocks were ideality which is producing the discrepancyseen in interpretedby Evansand Frost (1975)as beingdue to Figure 9. localequilibrium caused by limitedmobility of Alros. The same explanation is valid for the rocks from Efect of changes in Icicle Creek, but as the reaction responsiblefor this hornblende composition on the topology plagioclase-forming variation, tremolite * spinel : forsterite * anorthite of t he reactions *enstatite*HrO, will proceedwith a continualdeple- A source of possibleerror inherent in interpreting tion of aluminum in the spinel,it is the more alu- Figure 8 is that changesin topology could be asso- minous spinelswhich areout of equilibriumwith the ciated with the fact that the amphibole involved in rock assemblage,not the more chromiferousones. the reactionsis not tremolite. In order to estimatethe The third sourceof error is in the assumptionsused effect of amphibole composition on the topology, a in the thermodynamiccalculations. As the reaction Schreinemakers'analysis was madg on the invariant has a very smalldP / dT, evenif the temperaturevalue point in Figure 8 using a variablehornblende compo- of 715"C was off 6y 20", it would affect the results sition. only by a factor of 2 percent.The assumption that By coupling the reaction controlling the horn- LVr"",, : LV""".p also adds an error of about 2 per- blende composition (reaction (1)) with reaction (Sp, cent. The other major error in the thermodynamicsis An) in Table 7, and solving so that tremolite is elimi- involved in modeling the activity of various com- nated,reaction (An*) (Table 8) is produced.Then by ponents in the participating phases,particularly the coupling reaction(An*) with (Hb, V*), the restof the pyroxenesand spinel.The model usedfor the pyrox- reactionson Table 8 can be generated.The addition enes does not take into account the minor sub- of NarO to the systemdoes not add another phase. stitutionsof Al, Cr, Fe3+,or Na, but thesewould Sincethe alkali is dissolvedin hornblendeon one side only tend to decreasethe activity of the magnesium ofthe reactionand in the fluid phaseon the other, the end membersin the pyroxenesand henceincrease the reactionson Table 8 are divariant. However, if AlIv differenceseen in Figure 9. As all the aboveerrors are in hornblende is specified, the reactions become relatively small compared to the observed dis- univariant and their intersectionbecomes invariant in crepancy,the expressionfor the activity of MgAlrOn a P-T plane. in spinel seemsto be the major culprit. If AIIV in hornblendeis allowed to vary from 0.0 to There seemslittle doubt that the two-sitemodel for 2.4, with 2.4 being nearly the limit for natural horn- THE A SSEMB LA GE FO RSTERITE-A N ORTH ITE
T- T4
T5 T-
Frc. 10. Aseriesof P-TsectionsthroughP-I-compositionspace.Thecompositionalvariable,x,isthe value of Alrv in the hornblende.The reactionsshown are those listedin Table 8. The diagramsshow the changes in topology associated with changes in hornblende composition assuming that the hornblende composition changesaccording to the solid line in Figure 7.
blendes,then some of the coefficientson Table 8 will ature for the amphibole-breakdownreactions would changefrom a positive value through 0.0 to a nega- increasewith increasingAlIv in the hornblende. In tive value. In other words, the phasewill become a other words, the hornblende composition will be- product instead of a reactant. For example, in the come bufferedby the system,and the hornblendewill reaction labeled(Fo*), the coefficientsfor spinel be- becomeprogressively more aluminous as the reaction come 0 at x : 0.93.At that value the reactionwould proceeds. The majority of the samples of horn- be labeled(Fo, Sp*). Clearly, at this samevalue of -x, blende-plagioclaseperidotite from Icicle Creek seem the coefficients for forsterite in reaction (Sp*) will to represent rocks which were buffered along the also equal 0.0, and the stoichiometry for the two divariant reaction surface(Di*), as they contain the reactionswill be the same.At valuesof x greaterthan assemblage forsterite-hornblende-anorthite-ensta- 0.93, spinel will be on the opposite side of the reac- tite-spinel, which would be divariant in the tion to which it is written on Table 8. Topologically, NazO-CaO-MgO-AlzO'-SiOr-HrO system. Similar for values of Alrv in hornblende of less than 0.93, buffering along (En*) was not observedas it would stable(Fo* ) is to the right of the m'etastableextention require a composition more calcic than that found in of (Sp*) (Fig. l0A). When x : 0.93,(Fo*) is colinear most metamorphosedblackwall rocks. with (Sp*) (Fig. l0B), whereas for x greater than It should be noted, however,that the reactionson 0.93,(Fo*) is to the left of the metastableextention of Table 8 and Figure l0 are not valid for all natural (Sp*) (Fig. l0C). The point in Figure l0 where the systems,as hornblende compositions could be buf- sequenceof stable reactions changesis that of dia- fered along different trends depending upon differ- gram D, where the AlIv content of hornblende is encesin coexistingphases or intensivevariables. For about 2.15. This value is well above that found in example, hornblende buffered by reactions(Fo*) or hornblende from Icicle Creek (Table 6), indicating (Sp*) might have entirely different compositions that the topology for the naturally-occurringphases from thoseoccurring along (En*) evenif they formed is essentiallythe same as that for the pure Na-free at similar temperaturesand pressures,as forsterite system(Fig. 8). and spinel are essentialfor fixing a low activity of As the presenceof sodium and aluminum in the silica or a high activity of aluminum, respectively,in amphibole would tend to stabilizeit relative to other a coexistinghornblende. The reaction(Sp*) would be phases,it seemslikely that the equilibrium temper- particularly complex,for with the constraintof spinel 748 B. RONALD FROST saturation removed from the assemblage,albite tremolite = enstatite* diopside + H2O, as is sugges- might form in the plagioclase,adding an extra degree ted by forsterite-enstatite-diopside-chloriterocks of freedom to the assemblage forsterite-enstatite- from the Alps (Evansand Trommsdorff, 1974)is not plagioclase-hornblende-diopside. However, these clear as yet. As noted above,the occurrenceofa field added complexitiesdo not changethe basictopology of hornblende-plagioclaseperidotite between those shown in Figure 8, which indicates that there is a of hornblende-spineI peridotite and plagioclase divariant region in which olivine, anorthite, lherzolite seemsto be valid even in fairly aluminous enstatite,and hornblende can coexist. systems. Figure I I can also be usedto explain somecoronas Conclusions commonly found betweenolivine and plagioclasein The major conclusionthat can be drawn from this gabbro and anorthosite. The sequences oli- work is that the transition from the plagio- vine-clinopyroxene-hornblende* spinel-plagioclase clase-peridotitefield to that of spinel-peridotite is and olivine-orthopyroxene-hornblende + spinel- strongly compositionally dependent.The transition, plagioclase have both been described in the lit- which is univariant in the CaO-MgO-AlrOs-SiO, erature (Shand, 1945; Murthy, 1958). Recently, system, becomesdivariant or multivariant with the similar coronas have been described from gabbroic addition of other components, particularly NaO, rocks which have undergone no major metamor- CrrOr, FeO, or Fe2O3.Some of the apparent dis- phism(Irvine,19741' Nishimori, 1974).These coronas crepanciesin experimentalresults can be accounted seem to represent reactions similar to (En*) and for on this basis.The experimentsof Green and Hib- (Di*) in Table 8 and probably reflect local reequili- berson (1970) and Emslie (1970) were done with bration of the assemblageforsterite * anorthite un- other componentspresent, particularly sodium, and der retrograde,hydrous conditions. the differencesbetween their results and those of Conclusions bearing on the validity of thermody- Kushiro and Yoder (1966) and Windom (personal namic calculations involving spinel are also signifi- communication)should not be surprising.Further- cant. It has been shown here that the use of the more, as most natural peridotites are fairly poor in reactionforsterite * anorthite : enstatite* diopside sodium and contain minor-to-significantamounts of * spinel as a geobarometer is fraught with diffi- chromium and iron (both of which are concentrated culties,not the leastof which is the problem of defin- in spinel), the transition from plagioclase- to ing the activity of end members in the spinel solid- spinel-peridotitecan take placein natural peridotites solution series. Expressions using thermochemical at pressureslower than those indicated experimen- constantsfor the participating phases(cl Bacon and tally. Thus, the occurrenceof spinel, evenone rich in aluminum, is not necessarilyprima facie evidencefor high-pressure,mantle origin for that rock. Figure I I gives a rough revision of O'Hara's SPINEI. (1967b) facies diagram for the CaO-MgO-AlrO.- tH€RZOtIIE SiOr-HrO system at low pressuresand high tem- CHIORIIE-BEARING peratures.Before discussingthis figure it is best to M ETAPER I OOTIT E stressagain that the bulk rock composition will have a large effect, both on the relative location of the curves in the P-T plane, and on the variance of the transition boundaries, when applied to natural 1 rocks. o- 'o PtAG IOCTA5E The low-temperature limit for the horn- o THERZOTIIE blende-spinelperidotite is shown here as being the reaction chlorite : forsterite * enstatite * spinel /a q +HrO. Below this reaction the rocks are referredto as /*?'=
et -o metaperidotites,for if a rock has an equilibrium chlo- q(J, o< | rite-bearingassemblage its origin as a result of crustal zi"/ metamorphism is usually obvious. Whether the field of hornblende-spinelperidotite is truncated at high T- : pressuresby the crossingof the reactionschlorite Frc. I l Mineral faciesin aluminousperidotite at pressuresof less forsterite* enstatite* spinel+ HrO and forsterite* than about l0 kilobars and temperaturesabove 700"C. THE ASSEMBLAGE FORSTERITE_ANORTHITE 749
'76535. Carmichael, 1973)contain far too much inherent er- lunar rock of deep crustal origin: sample Geochim. Cos- ror to make them applicable to geologic problems mochim Acta, 38, 1329-1140. Gneen, D. H. (1964) The petrogenesisof the high-temperature dealing with vapor-free reactions.Thus, with even peridotite jntrusion in the Lizard area, Cornwall. J. Petrol.5, adequate expressionsfor the activity of the com- I 34- I 88. ponents in the participating phases,the reaction can - ANDW. Hrsnpsol.l (1970)The instability of plagioclasein only be usedas a geobarometerthrough careful ap- peridotite at high pressures.Lithos, 3, 209-221. plications of thermodynamics to an experimentally Henznrnc, C. T. (1972) Stability fields of plagioclase- and spinel-lherzolite. Prog. Experimenlal Petrol 2, 145-148. determinedcurve. Hess, P. C. (1969)The metamorphic paragenesisof cordierite in pefitic rocks. Contib. Mineral. Petrol. 24, 191-207 Acknowledgments lRvtNE,T. N. (1965)Chromian spinelas a petrogeneticindicator. The author would like to thank the following individuals for Part l-Theory. Can J. Earth Sci. 2, 648-674. their assistancein this project: - (1974) Petrologyof the Duke Island Ultramafic Complex, Wasant Pongsapich who first alerted me to the occurrence of SoutheasternAlaska. Geol. Soc. Am Mem. 138. high-grade ultramafic hornfelses in the lcicle Creek valley. - AND G. H. SvrrrH(1967) The ultramafic rocks of the Mus- Robert Vocke and John Kramer who assistedin the field work kox intrusion, Northwest Territories,Canada. In, P. J Wyllie, and carried rocks. Ed. Ultramafic and Related Rocl
Ntsnlvronr, R. K. (1974) Cumulate anorthositegabbros and per- gneissesfrom the Gananoquearea, Ontarjo. Can J. Earth Sci.5, idotitesand their relation to the origin of the calc-alkalinetrend 455-482. of the Peninsular Ranges Batholith (abstr.) Geol Soc Am RrcH.uosoN, S W. (1968) Staurolite stability in a part of the Abstr Progr. 6,229. system Fe-Al-Si-O-H. J Petrol 9, 467-488. O'Henn, M. J. (1967a)Mineral faciesin ultrabasicrocks. In, P. J. RrcHEy, J. E. eNr H. H. Tnotrr,ls(1930) The geology of Ardna- wyllie Ed. Ultramafic and Related Rocks John Wiley and Sons, murchan, northwest Mull and Coll. Geol. Suru U K. Mem , New York. p.7-18. 393p - (1967b) Mineral petrogenesisin ultrabasicrocks. In, P. J. Ross, C. S., D. M. Fosrsn rNo A T. MvEns (1954) Origin of Wyllie Ed., Ultramafcand Related Rocks.John Wiley and Sons, dunites and of olivine-rich inclusions in basaltic rocks. Am. New York, p. 393-403. Mineral. 39, 693-737. ONurr, H. (1965) Petrochemical research on the Horoman and SnxrN,l, S. K (1973) Thermodynamicsof Rock-Forming Crystal- Miyamori ultramafic intrusions,northern Japan. Sci Rep To- line Solutions.Springer Verlag, New York. hoku Uniu Ser. III,9,217-216. - AND S Guose (1971) Mg+'z-Fe+2order-disorder and the ONYtecocnr, A. C. (1973) Petology and Mineralogy of the Twin thermodynamics of the orthopyroxene solid crystalline solution Sislersdunite Ph.D. Thesis,University of Washington,Seattle, Am. Mineral.56, 532-559. Washington. - AND C. E. NBnnu (1975) Enstatite-diopsidesolvus and PAPIKE,J. J., K L CluenoN nNr K. BllowrN (1974)Amphiboles geothermometry.Contrib Mineral. Petrol. 49,259-267. and pyroxenes:characterizationof other than quadrilateralcom- SHeNo,S. S. (1945) Coronas and coronites.Geol. Soc.Am. Bull. ponents and estimatesof ferric iron from microprobe data. Geol 56, 247-266. Soc. Am. Abstr Progr.6, 1053-1054. Tnolrl,rsoonrr, V. nNo B. W. EvnNs (1974)Alpine metamorphism PETERS,T. (1968) Distribution of Mg, Fe, Al, Ca, and Na in of peridotitic rocks Schweizer. Mineral Petrcgr. Mkt. 54, coexistingolivine, orthopyroxene,and clinopyroxenein the To- 333-353 talp Serpentinite(Davos, Switzerland)and in Alpine metamor- TunNocr, A. C. ,qNo H. Eucsrrn (1962) Fe-Al oxides: phase phosed Malenco Serpentinite(N. Italy). Contrib. Mineral. Pet- relationsbelow 1,000"C.J. Petol 3,533-565. rol. tE,65-15. Wnrrr, R. W. (1966) Ultramafic inclusionsin basalticrocks from PnIrrrps, E. R. (1959) An olivine-bearinghornfels from south- Hawaii. Conlrib Mineral Petrol. 12,245-314. easternQueensland. Geol Mag.196,37'7-384. Wooo, B. J. (1973)Fe*'-Mg*'partition betweencoexisting cord- - (1961) An olivine-bearing hornfels from southeastern ierite and garnet-a discussionof the experimentaldata. Con- Queensland-a correction. Geol Mag.98, 431-433. trib Mineral. Petrol. 40, 253-258. Pt-uuunn,C.(1969)GeologyoftheCrystallineRocks,Chiwaukum - AND S BINNo (1973) Garnet-orthopyroxene and or- Mountains and Vicinity, WashingtonCascades Ph.D. Thesis, thopyroxene-clinopyroxenerelationshipsinsimpleandcomplex University of Washington, Seattle, Washington. systems Contrib. Mineral Petrol 42, 109-124 PoNcsnrrcH, W. (1974) The Geology of the Eastern Part of the Zru, E-nN (1966) Construction of pressure-temperaturediagrams Mounl Stuart Batholith, Cental Cascades, Washington. Ph.D. for multicomponent systems after the method of Schreinema- Thesis, University of Washington, Seattle, Washington kers-a geometric approach. U. S. Geol. Suru. Bull 1225. RstNHlnnr, E. W. (1968) Phase relations in cordierite-bearing