American Mineralogist, Volume 60, pages 785-797, 1975

Mineralogical Zoning in a Scapolite-Bearing Skarn Bodyon San Gorgonio Mountain, California

KrNNnrH SHny l

Department of Geology, Uniuersity of California,Los Angeles, California 90024

Abstract

Concentrically zoned skarns on Mt. San Gorgonio have been studied in detail from thepoint of view of field and petrographic relations. The skarns are mineralized blocks of marbleof the Precambrian San Gorgonio Igneous-Metamorphic Complex, which are within the con-tact zone between the Complex and the Middle Jurassic Cactus Quartz Monzonite. They con-sist of cores of calcite-forsterite-diopside marble, surrounded by concentric zones of diopside,actinolite * epidote * calcite + quartz, epidote + garnet * calcite * quartz, and scapolite +calcite + quartz. The genesis of the skarns involved three steps. (l) Regional metamorphismformed tremolite and forsterite from calcite, dolomite, and quartz. (2) The intrusion of thequartz monzonite stabilized diopside * forsterite relative to tremolite * calcite. Reactionbetween quartz monzonite and marble resulted in the formation of a zone of diopside * wol-lastonite and a zone of sodic scapolite, by the diffusion of Ca, Mg, and Si across the originalcontact. Components rejected from this zone formed ferroan diopside in the scapolite zoneand in the quartz monzonite, and altered the scapolite to a more calcic form. This process in-volved a net volume loss of 56.6 percent. (3) Inf i l t rat ion of f luids, possibly of magmatic origin,formed epidote and garnet from scapolite, and actinolite from diopside.

Introduction

This paper describes the geology and chronicles thegeological history of a small group of skarns on Mt.San Gorgonio. Combining textural, mineralogic,chemical, and field data from the skarns with fieldand petrographic data from a similar area to thesouth and with published laboratory results, a modelfor the formation of the skarns is proposed. Themodel is consistent with the principle of localequil ibrium (Thompson, 1959), which has beenshown to be a valid assumption in other studies ofmetasomatism (e.9., Vidale, 1969; Joesten, 1974).

The area studied, hereafter called East Ridge, is atan elevation of 10,500 ft. on the eastern ridge of SanGorgonio Mountain (elevation 11,502 ft.), which isthe highest peak in the east-west trending San Bernar-dino Mountains of southern California (Fig. l). TheSan Andreas fault zone defines, and probablyformed, the southern scarp which separates the SanBernardinos from the San Jacinto Mountains to thesouth and the San Gabriel Mountains to the west (Al-len, 1957). San Gorgonio Mountain and the sur-

' Current address: Department of Geological Sciences,University, Cambridge, Massachusetts 02138.

rounding peaks represent a complex of Precambrianmetamorphic rocks and Mesozoic intrusives (Vaughn,1922).

There are relatively few published studies and un-published theses on the geology of the San Bernar-dinos; Vaughn (1922) studied the entire range andBaird et al (1974) have recently discussed its regionalstructure. For additional information on the regionimmediately surrounding East Ridge, the reader isreferred to Allen (1957) and Dibblee (1964).

Method of Study

The geology of East Ridge was mapped betweenMay and November, 1973, using a Brunton compassand tape measure, on a l:240 base map with 5-footcontours mapped in the same fashion (Fig.2). A totalof 120 samples were collected; from these, 70 thin sec-tions, I I polished sections, and 12 thick sections wereprepared, and 60 different whole rock and mineralconcentrate powders were ground for Xnn and Xnostudies.

Seventy different mineral grains were analyzed(Table l) employing an ARL-EMX microprobe. Stan-

Harvard dards comparable in composition to the unknownswere not generally available; the standards used are

785

786 KENNETH SHAY

I c.*ro,c ..di."nt"

E Mrercic inrrulins

ffi eotro:oic rcatn.nrr ond votconic!

ljll uctomorotric ond Prccombrlon rocks

rO O tO zomiles

rO O lO zokrlomc!.rs

Frc . l . I ndex map .

listed in Table 2. Data were reduced using the on a Phillips X-ray fluorescence apparatus, usingmethods of Bence and Albee (1968) and Albee and U.S.G.S. Granite G-2,ZGI Granite GM and BasaltRay (1970).

The Xnn whole-rock analyses (Table 3) were doneBM, and an analyzed limestone as standards.

Geology of East RidgeEast Ridge lies within a zone of intrusion and as-

similation between the Precambrian San GorgonioIgneous-Metamorphic Complex and the MiddleJurassic Cactus Quartz Monzonite (Fig. 2). TheComplex apparently has not been deformed by thisintrusion, as foliation is consistent throughout thecontact zone, and faults, with maximum offset l0 m,postdate the intrusion.

Cactus Quartz MonzoniteThis coarse-grained (l-4 mm) granitic intrusive,

originally called the Cactus Granite by Vaughn(1922) but renamed the Cactus Quartz Monzonite byGuillou (1953), contains about 30 percent each ofquartz, oligoclase (Anru), and K-feldspar, and l0 per-cent biotite with accessory apatite, tourmaline,sphene, zircon, magnetite, and allanite (in biotite).Hornblende is rare and where present, ofglomeroporphyritic habit. Hollenbaugh (1968) con-siders the Cactus to be comagmatic with a pegmatitethat Hewett and Glass (1953) dated as Middle Juras-sic. This is in agreement with more recent K-Ar ages(Armstrong and Suppe, 1973).

San Gorgonio Igneous-Metamorphic Complex

Frc 2. Geologic map of the norrhern part of East Ridge (index at This Precambrian complex, named by Allenupper left).Q shows the sample traverse for Suite l;O for Suite 2. (1957), includes migmatite and gneiss (microcline-

o l? \- ^ \

ol 4

\ t

ZONING IN A SCAPOLITE.BEARING SKARN BODY

Trslr l . Microprobe Analyses in Weight Percents

787

S m p l e N o . *

Locat ion**

s i 0 2A 1 2 O 3Ca0N a 2 oFe as FeO

McoTi02MnOc 1H z o

Total

Sarple No,i

Location* *

s i02A1203Ca0Na2OFe as Feo

MgoTio2Mn0c lHzo

Total

SarP le No. "

Location**

s io2A r 2 0 3Ca0Na20tse as t6u

MgoTio2I'hO

Hzo

Total

AC05902 AC12402 ACr2409 4C12410 ACl2503

ACT ACT ACT ACT ACT

s 1 . 5 5 2 . 7 5 2 . 4 5 5 . 8 5 1 . 43 . 9 2 . 3 1 . 5 . 8 1 . 4

L 2 . 4 L 2 . 8 ! 2 . 7 l 3 . l L 2 . 60 . 0 . 2 , 3 t r . 5

t3 .2 L2 .3 tz ,a 6 .9 14 ,7

1 3 . 5 r 5 . 8 1 6 . 0 L 9 . 7 r S . 5. 2 . 2 . l 0 . 0 t r. 8 . 5 . 6 . 5 . 6na na na na na

3 . 4 3 . 2 3 . 6 3 , 2 3 . 2

1 0 0 . 0 r 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0

ACr2504 Drol70l Dr01704 Dr06602A Dr066028

ACT SCAP SCAP DIOP DIOP

5 3 . 6 5 1 , 4 5 2 . 0 5 4 . 2 5 4 . 0. 9 . 7 . 4 . 4 . 3

t2 .7 24 .3 24 .4 25 .2 25 .5

r 3 . 3 1 L . 2 1 0 . 8 2 . r 2 . 0

1 6 . 9 1 1 . 2 1 1 . 4 1 7 . 5 L 7 . 60 ,0 t t t r t r t r

. 5 . 7 . 6 . 3 . 2na na na na na

r . 7 0 0 0 0

1 0 0 . 0 9 9 . 7 9 9 . 8 9 9 . 7 9 9 . 6

Dr06603A Dr06603c

DIOP DIOP

5 4 . 0 5 4 , 0. 1 , 4

2 5 . 6 2 5 . 5. 1 t r

2 . 7 L 5

t 7 . 4 1 7 . 6

. 2 . 3na na

0 0

9 9 . 5 9 9 . 4

Dr06604 Dr066044

DIOP DIOP

5 3 . 6 5 3 . 5. 4 . 7

25 .0 2s ,7l + r

2 . 9 2 . 5

1 7 , 4 1 7 . 3t ! t r

na na0 0

99.7 100.0

EP12409 EPl2501

EP ACT

3 6 . 9 3 6 . 82 2 , 5 2 3 . 22 3 . 2 2 3 . 3

t ! t rL 2 , 9 L z . T

0 . 0 0 , 0t ?

na na4 . 0 4 . 0

o o o o o a

sc01701 sc0l70rASCAP SCAP

4 6 . 2 5 6 . 32 4 , 8 2 4 . 9L 7 . 3 9 . t3 . 6 6 . 4

. 1 . 1

0 . 0 0 . 00 . 0 0 . 0

tr trtr tr

0 0

92.0** r 96 .1** .

sanp le No.* DI066048 DI06605A DI0790I DI079O2A DrO79O2B 0r07903 DI t23OtE DI12301F DI123O3A DI I2304 DI12305A DI1230SB DI12305C DI12306

LOCAI iON** DIOP DIOP N.V. N.V, N.V. M.V. EP EP EP ACT ACT ACT ACT ACT

s i o ^ 5 3 . 3 s 3 , 7 5 2 . 5 5 2 . 0 5 2 . 8 5 2 . 8 5 2 . 1 S 2 . O 5 1 . 7 5 r . 5 5 1 . 6 5 1 . 3 5 1 . 9 5 1 . 8A l 2 o 3 . 3 . 8 . 5 ' 6 . 6 , 6 . 4 . 5 . 5 . 4 . 6 . 5 . 6 . 6Cad

- 25 .2 25 .2 24 .7 24 .5 24 .9 24 .8 24 .4 24 .4 24 .6 24 .3 24 .5 24 .5 24 .5 25 .1

N a ^ O . l . l . 2 . s . 2 . 2 . 2 . 2 , 2 . 3 . 2 . 2 . 2 , IF e a s F e o 3 . I 2 . 9 9 . 0 1 2 . 4 9 . 0 1 1 . 4 L r . 7 I 0 . 4 1 0 . 6 1 2 ' 0 1 1 . 7 L 2 . 3 1 1 . 3 9 . 8

M g o t 8 . l 1 6 . 8 l 2 . s 1 0 . 8 L 2 . 4 9 . 6 l l , 2 1 0 . 8 1 0 . 9 9 . 9 1 0 . 5 I 0 . l 1 0 . 5 1 1 . 2TiO2 tr tr tr tr tr tr tr tr tr tr tr tr tr trM n o . 3 . 3 . 3 . 2 . 2 , 5 1 . 0 I . 1 L ' 2 1 , 2 L . 2 I . 1 L ' 2 . 8Cl na na na na na na na na Da na na na na naH r o 0 0 0 0 0 0 0 0 0 0 0 0 0 0

T o t a l 1 0 0 . 4 g g . 7 9 9 . 7 1 0 0 . 9 I 0 O . O 9 9 . 8 1 0 1 . 0 9 9 . 5 9 9 . 8 9 9 . 6 l O O . 2 1 0 0 . I 1 0 0 . 2 9 9 . 4

Dr12408 Dr12410 Dr126802 EP01705A EPl2202

ACT ACT EP EP SCAP

5 4 . 5 S 4 . 5 5 2 . 6 3 6 . 9 3 7 . 2. 2 . 5 . 6 2 2 . 9 2 3 . 2

2 5 . 6 2 5 . 4 2 4 , 3 2 3 , r 2 3 . 0t r t r na t r 0 .0

1 . 2 1 . 5 1 1 . 8 r 2 . 8 r 2 , 2

L 7 . ? 1 7 . 8 9 . 6 0 . 0 0 . 0t r t r n a . t 0 . 0t r t r 1 . 3 . 2 . 2na na na na na

0 0 0 3 . 9 4 . 1

9 9 . 3 9 9 . 5 1 0 0 . 2 9 9 . 9 1 0 0 . 0

EP12502 EPl260I EPl26B02 FO0660I F006602

ACT EPe EPe DIOP DIOP

3 6 . 8 3 6 . 5 3 6 . 8 3 9 . 7 4 0 . 02 3 . 4 2 3 , 4 2 3 . 6 . 1 . I2 3 , 3 2 3 . L 2 3 . 0 . I . 1

t r 0 . 0 0 . 0 0 . 0 0 . 0t 2 , 4 t 2 . 8 7 2 . 2 7 . 9 7 . 7

0 . 0 0 . 0 t r 5 1 . 7 5 I . 5. 1 0 . 0 0 , 0 t r t r. 1 . 1 . 2 . 2 . 4na na na na Da

3 . 8 4 . 0 4 . 2 0 0

1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 9 9 , 7 9 9 . 7

EPL22O3 EPL23O3A EPI24OI EPL24O2 EPI24O2A

SCAP EP EP EP EP

3 7 . 2 3 7 . 4 3 6 . 5 3 6 . 6 3 6 . 92 3 . 3 2 2 . 1 2 2 . 2 2 2 , 9 2 2 . 72 3 . 0 2 3 . 7 2 3 . I 2 3 . 2 2 3 . 30 .0 t r t ! t r t r

L 2 . 2 1 3 . 0 1 2 . 2 L 2 . 5 I 2 . 4

0 . 0 0 . 0 0 . 0 . 2 0 . 00 . 0 . 1 0 . 0 0 . 0 . 2

. 2 . 2 . r . l . 2na na na na na

4 . 0 3 . 5 4 . 3 4 . 5 4 . 3

1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0

GA00003 GA00004 GAo7005 GAl230lA GAr2302

EP EP EP EP EP

3 7 . 9 3 7 . 7 ' 3 7 . 7 3 6 . 8 3 7 , 8r 4 . 8 1 4 . 8 t 4 . 2 1 0 . 7 1 1 , 13 4 . 7 3 4 . 7 3 4 . t 3 2 , 3 3 3 . 4

tr tr tr tr t!1 1 . 0 1 1 . 4 1 3 . I 1 6 . 5 1 5 . 7

t r t r 0 .0 t r t !. 3 . 3 t r . 8 . I. 7 . 9 . 5 2 . 0 1 . 3na na na na na

0 0 0 0 0' 9 9 . 4 9 9 . 8 9 9 . 7 9 9 . 2 9 9 . 5

EPr2403 EP12404

EP EP

3 7 . 4 3 7 , r2 3 . 4 2 3 , 02 3 . 5 2 3 . 2

L 2 . 1 t 2 . 4

0 . 0 . 20 . 0 t r

. t . lna na

3 , 4 4 . O

1 0 0 . 0 1 0 0 . 0

GAl2306 SCor502

EP SCAP

3 7 . 9 4 5 . 41 3 . 9 2 6 . 23 4 . 7 t 7 . 3

t r 3 . 91 1 7 t

. l 0 . 0

. 7 0 . 0q t r

na tr0 0

9 9 . 8

Sanple No.* SC017018 SC0l70lE SCol702A SC017028 SC017068 SCol70l SC0780I SC0790l SC07903 g,1220l SCL22O4 SCl2205 w107903 WL07904

Location** SCAP SCAP SCAP SCAP SCAP SCAP n.v. n,v. n.v. SCAP SCAP SCAP DIOP DIOp

s i o 2 5 6 . 5 5 7 . r 4 6 . 1 4 5 . 9 s 7 . 2 4 5 . 0 4 5 . 7 4 6 . 3 4 6 . 3 4 5 . 9 4 5 . 5 4 6 . 7 5 1 . 1 5 r . 1A 1 2 0 3 2 4 . 7 2 4 . 8 2 5 . 5 2 5 . 4 2 4 . 8 2 6 . 0 2 5 . 6 2 6 . 7 2 6 . 3 2 7 . 5 2 6 . 2 2 5 . 5 . l . 1c a o 8 . 7 8 . 5 1 7 . 5 t 7 , 9 8 . 3 L 7 . S 1 6 . 8 1 6 . 7 l 7 . o r S . 4 1 7 . 4 L 7 . r 4 8 . 2 4 8 . 3N a 2 O 6 . 7 6 . 6 3 . 8 3 . 8 6 . 6 3 . 7 3 . 5 3 . 3 3 . 3 3 . 6 3 . 6 3 . 8 t r t !F e a s F e O . 1 . 1 . l . l . 1 . 1 . I . 2 . 2 . l . 1 . I . l . I

M g o 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 t r 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0T i o 2 0 . 0 t r t r t r 0 . 0 0 . 0 t r 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 , 0Mno 0 ,0 t r 0 .0 t r t ! t r t r t r t r t r t r t r . l . lc l t r t r t r t r t r t r .4 .3 .4 t r t r t r na naH.O 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Tot al 9 2 . 6 r * * 9 2 . 9 * * * 9 3 . 2 + * * 9 9 . 6 9 9 . 7

*Fi"st Ato Lettere deeigwtemLwal: lC= act inoLi. tei DI = diopside; EP=epidote; FO= fo"ste?rte; GA= gmet; SC= eqpo| i te; I IL=Dol. laetonite.'aRefers to mLrercLized aone. m.u. = diopeide-ecqol,ite ueias in MR zw.

*4*No mLAsis for COr.

788 KENNETH SHAY

Element Standild Efenent Standard

pyrope garnetvol lastonitehalitemgneti teplEope garnet

spessart lne garnetalbi tepyrope garnetslnthetic ruti.Le

quartz, biotite-microcline-quartz, biotite-amphibole,magnetite-biotite-amphibole), schist (muscovite-bio-tite-quartz, biotite-microcline-quartz), quartzite, andmarble. Allen places the assemblages in the kyanite-staurolite subfacies of Turner's (1948) amphibolitefacies. Silver (1971) reports a zircon age of at least1750 + l5 m.y. for a granite which intrudes a gneissof this complex, thereby confirming the Precarnbrianage suggested by Dibblee (1964).

Marble and Skarn

The marble bodies at East Ridge and the l- to 250-cm wide bands of skarn surrounding them are herediscussed together. A total of eleven individualbodies with nearly complete mineralogic zonation(described below) are present. The maximum exposedthickness is 8.9 m (27 ft), and as that widest part isbounded above by gneiss and below by quartzite andschist, this is taken to be the maximum thickness.Thinner bodies have a thinner marble core or mayeven lack a carbonate core, occurring simply as large

Tesln 2. Standards Used in Microorobe Analvses pods of concentric garnet-, epidote-, actinolite-,diopside-, and scapolite-rich bands. Along strike, thelength of exposure of the skarns is approximately 150m (460 ft).

The bands of skarn surrounding the marble cores(the Men zone) have been designated as the DIot,Act, Ep, and Scn p zones. Two suites of samples fromthe zones were collected, as shown in Figure 2.Calcite is present everywhere except in most of theDtor zone; and with the exception of the Men andDlop zones which contain accessory ilmenite, quartzand sphene are ubiquitous (Fig. 3; Table 4).

MAR Zone. At East Ridge, much of the exposedmarble has been extensively mineralized and altered;consequently many original features, in particular theoriginial composition, are unclear. However, thesouthernmost skarn body at East Ridge contains amarble core which bears an extremely close lithologicand stratigraphic relationship to rock of Area I (Fig.l), an interbedded schist-marble-gneiss sequencewhich has not been intruded so pervasively. Thethickness, grain size, bulk composit ion, andstratigraphic position of the two marbles are virtuallyidentical. It is fair to assume that originally all of theEast Ridge marble was nearly identical to the Area Imarble, and this assumption will be used to augmentand interpret the data from East Ridge. The effects ofintrusion of quartz monzonite at Area I are discussedbelow.

The East Ridge marble, the MeR zone, is 35-40Vo

A]Cac1FeMC

I\4rNab 1

Ti

TesI-r 3. Partial XRF Analyses, Weight Percents

Sanp le No. 73-58 73-85

Locationi MAR MAR

S i O z 1 0 . 8 2 1 1 . 4 5A 1 z 0 s . 2 8 . 1 6Cao 34 .85 34 .02N a 2 0 1 . 3 2 . 4 8F e a s F e O 1 . 1 0 3 7MgO 14.32 13 .75

Sanp]e No. 73-128 73-134

Locat ion t .5 1

s i 0 2 6 9 . 0 5 6 9 - 9 5A 1 z 0 : 1 5 5 4 1 4 . 7 3C a O 3 . 3 2 2 . 7 2Na2O 4 -46 5 . 30F e a s F e o 3 . 2 9 2 . 9 9MgO 1 .04 .93

Sanple No. 73-149 73-150

Locatlon+ 8 8

s i 0 2 7 3 . 2 6 7 6 . 6 2A 1 2 0 3 1 4 . 5 8 1 2 . 7 1C a O . 7 5 1 . 3 5N a 2 0 3 . 1 9 2 . 6 8F e a s F e O 1 . 0 9 8 7MCo 46 .24

7 3 - s 9 7 3 - 1 9

MAR DIOP

t 5 . 6 2 5 t . 2 51 . 6 4 r . 4 2

3 8 . 0 6 2 3 . 8 6. 4 2 . r 2

2 . 4 I 4 . 3 51 3 . 7 1 1 6 8 1

7 3 - 1 3 5 7 3 - 1 3 6

1 1

7 4 . 8 4 7 0 . 6 71 3 . 7 8 1 5 . 3 3

. 8 2 2 . 8 82 . 4 2 4 - 0 7

. 8 6 2 . 3 7

. 2 8 . 6 8

73- ts l 73-152

9 9

7 3 . 6 0 7 5 . 7 11 4 . 0 8 1 3 . 4 0

1 . 4 I . 8 4I . 7 3 3 . 0 2

. 4 7 . 9 s

. 4 7 . 2 7

7 3 - 6 6 7 3 - 7 1 7 3 - ) , 3 I

DIOP DIOP ACT

5 0 . 2 7 5 1 7 7 5 1 . 4 51 . 8 4 r . 7 r 8 . s 3

2 5 . 7 7 2 4 . 8 0 2 2 . 9 r. 1 8 . 1 6 . 0 1

4 4 9 4 . 3 8 9 . 4 51 6 . 9 9 1 6 . 2 6 s . 8 3

73-I38 73-139 73-I40

2 2 3

6 9 . 9 9 7 5 . 9 3 7 3 . 0 71 5 . 7 0 1 3 4 3 1 4 4 4

2 . 6 8 . 9 5 t . O 74 . 0 1 2 . 5 8 3 . 0 72 . 7 2 . 6 9 r . 2 4

. 7 2 . 3 1 - 4 8

73- I53 73-76 73- t54

1 0 1 0 1 5

7 6 . 0 9 7 6 . 0 4 7 6 . 2 31 2 . 7 3 1 2 . 5 7 1 3 . 5 1

. 8 1 I . 2 3 . 8 42 . 9 3 2 . 1 7 3 3 5I . 2 r 1 8 1 . 9 3

. s 6 . 4 3 . 2 8

7 3 - r 2 S 7 3 - t 2 4

ACT EP

5 5 8 5 5 1 . 4 14 8 7 1 3 . 7 3

2 1 8 9 1 9 . 9 60 0 . 0 0

9 . 1 3 6 . 5 76 . 8 8 5 . 2 7

7 3 - t 4 I 7 3 - I 4 2

3 4

7 5 . 7 6 7 5 . 7 01 3 . 6 7 1 3 . 3 9

1 . 0 5 . 9 32 . 8 4 2 . 5 6

. 7 3 . 7 5

. 3 0 . 2 8

7 3 - 1 5 5 7 3 - 7 3

1 6 6 5

7 2 . 8 4 7 4 . 8 51 4 . 7 0 1 3 . 9 1

1 . s ] | . 2 23 . 2 9 2 . 6 4

. 9 1 . 6 75 4 . 3 1

73-132 73-122 73- t29

EP SCAP SCAP

5 3 . 0 2 s 0 . 0 2 5 2 - 3 81 4 5 8 2 5 . 2 s 2 4 . 2 81 2 . 4 0 1 4 . 8 6 l 0 . 6 6

. 0 0 3 . 0 s 5 . 0 86 .48 2 .99 2 -483 5 1 . 9 s . s l

7 3 - 7 2 7 3 - 1 4 4 7 3 - 1 4 5

4 5 6

7 1 . 9 7 7 6 . 6 1 7 6 . L 01 5 . 8 4 ) . 3 - 3 2 1 3 . 3 9

2 . 4 8 | . 2 4 L . L 23 . 7 5 3 . 0 6 2 . 8 7I . 6 1 . 6 9 . 8 2

. 3 6 . 2 4 . 2 6

7 3 - 6 7 3 - 3 2 7 3 - 3 3

> 1 0 0 > 1 0 0 > 1 0 0

7 4 . 0 0 7 3 . 7 8 7 3 . 7 41 4 . 0 5 1 4 . 2 3 1 4 . 4 7

I . 2 7 I . 8 9 1 . 5 33 . 4 2 3 . 2 1 4 . 2 91 . 4 8 1 . 5 3 I . 4 9

- 3 4 3 5 . 4 3

73- -s3q 73-75 73-L4

q . 1 2

8 6 . 7 8 6 8 8 9 6 9 . 9 26 . 0 6 1 4 . 3 9 1 6 . 0 33 6 9 4 . 9 6 1 . 5 5

. 4 0 2 . 0 3 2 . 9 0I . 7 6 2 . 0 6 1 . 4 01 t 5 1 . 3 9 . 6 5

73-146 73-147 73-148

6 7 7

7 2 . 0 0 7 7 - 2 8 7 3 . 8 81 5 . 0 7 1 2 . 7 2 1 3 . 9 9

7 . 7 7 . 7 5 1 . 3 52 . r t 2 . 5 3 2 . 5 6

. 5 . 8 6 1 . 4 s

. 5 3 . 1 9 . 4 3

fRefere to skam zmes, or di,stmce (in feeL) into quutz nanzonite from cont@t. t 'q" is qwrtz-feld.spu uei-ns. Anallst: P.C. stmel

ZONING IN A SCAPOLITE.BEARING SKARN BODY't89

forsterite, 55-60Vo calcite, 5Vo diopside, and lVotremolite; the Area I marble is 45Vo forsterite, 457ocalcite, and llVo tremolite (see Fig. 3). The olivine inthe Mnn zone is f ine- to medium-grained, anhedral,and has the composition Fo"r.u, with trace amounts ofMn, Al, Ca, and Ti. The tremolite is f ine-grained,colorless, elongate, and rimmed by forsterite anddiopside in the Men zone. The diopside grains areuntwinned, equant, f ine- to medium-grained, andpale green. Their composition is DiruHdu; the rims areenriched in Fe.

In all the Area I marble outcrops studied, there is a0.3-l cm wide weathered-out space between marbleand schist (below) or gneiss (above). This may be dueto a metasomatically formed reaction zone which hasbeen weathered out. The outcrops are so extensivelyweathered that no analyses were performed on theArea I samples.

ln several instances, zoned veins are present withinthe M,c.n zone. The centers of such veins are com-posed of qtrartz and medium- to extremely coarse-grained scapolite, in approximately equal proportions.The scapolite is milky and euhedral to subhedral,fluoresces orange, is inclusion free, and has the com-position CarNaAluSirOro.(CaCOr)'.e.(NaCI)0,. Be-tween this assemblage and the host marble is a bandof fine to medium-grained, pale green diopside. Thegrains are equidimensional, generally twinned, andcompositionally homogeneous; the average formulais Di"oHdro with trace Al, Mn, Na, and Ti. Thispyroxene will henceforth be, referred to as "Fe-diop-side," to emphasize the difference between it andthe more magnesian pyroxene, "Mg-diopside," de-scribed two paragraphs above.

DIOP Zone. The Drop zone is defined by the in-nermost appearance of a rock of )50 percent Mg-diopside, and contains diopside, wollastonite, calcite,and forsterite. The center ofthe zone is 90-95 percentdiopside and 5-10 percent wollastonite. Near theboundary with the Mln zone 2-5 percent each of wol-fastonite and calcite, or 2-3 percent each of calciteand forsterite, are present.

The diopside is identical to that present in the Me,nzone. The forsterite has been extensively replaced byserpentine and has the same composition as theforsterite present in the Mln zone. The wollastoniteis fibrous and fluoresces blue-green. Its compositionis very nearly CaSiOr, with I wt percent or less eachof Al and Fe.

ACT Zone. A sharp boundary exists between theDlop and Acr zones, defined by the innermost ap-pearance of green to yellow-green actinolite and

Frc 3. Mineralogic variation at East Ridge, based on the estimatedmodes of Sui te l .

epidote in lesser amounts. Near the Drop/Acrboundary, actinolite is by far the prominent mineralin the Acr zone, but generally decreases in amountwith increasing epidote, toward the Acr/Ep boun-

Tmlr 4. Estimated Modes of Samples From the East Ridge

Zones(ln Volume Percent)

Z o n e :

S u i t e 1 :

aluaf-tzc a l c i t ed o l o n i t eM c - d i o p s i d e

f o r s t e r i t ew o l l a s t o n i t et reno l i ten i z z o n i t ee p i d o t e

garneta c t i n o l i t eI l m e n i t es r'hene

S u i t e 2 |

q u a r l zc a l c r t ed o l o m i t el i i d - d i ^ n e i d a

f o r s t e r i t ew o l l a s t o n r t et r e n o l i t em t z z o n i l , ee p f d o t e

garneta c t i n o l i t ei l n e n i t es phene

JJIOP ACT

6 6 1 9 r 2 51 1

5 - - 1 0

90 90

) - -8 - -

E P S C a PftIAR

5 5

5

40

1

'1,24

1 01 0

122r 5

6 5

5> 2 > 2 > 2

8 5 7 r

L 5 5 5

2 1

C U> 1

> 2

6 0 4

< on

r ) I r ) z \ E p t ) I 2 9? o 1 0 3 a_2 _5_ '_2

3 54

1

> 2

) )

4 08 5

2

S z h n l o . . d a r a I t k o n l ' r n m n o n r F r c . , T r ^ h a < 6 y ^ a r r l

f o r : 66 , taken f rom 1 ' ! i l ! i l /D I0P boundary ; 71 ,L " k e n c l o s e f o L h a L b o u n . l . r \ / r l / < . , ) F a n ^ t rDiiphcr

- ; ; ' ; ; " ; t ; " , ; ; - f

i i , " , , r . " i ' . " , . ' . rcr i" 'boundary .

KENNETH SHAY

dary. Quartz plus calcite constitute 25 percent of thezone.

The actinolite is fine- to medium-grained, exten-sively intergrown with calcite, and is commonlyfound replacing Mg-diopside. The average compo-sition of the analyzed grains is Ca, rMgr.uFet..Alo.usi, uOrr(OH), with trace amounts of Mn, Na,and Ti. The epidote crystals are medium-grained andamoeboid, with inclusions of quartz, calcite, andthe ferroan diopside, Fe-diopside. Compositionwithin a single grain may vary, but no systematiczoning was observed. Throughout East Ridge, theaverage epidote composition is consistently CazAlz.rFeo.'SirO'r(OH).

EP Zone. The outermost occurrence of actinolitemarks the boundary between the Acr and Ep zones.The Ep zone can be broken down into three sub-zones, none of which shows a preference for any par-ticular position within the zone. The least common ofthese, the Eeg, is composed almost entirely of garnet,with 10 percent quartz and calcite. The garnet is ex-tremely coarse-grained, with crystals as large as 2.5cm. Grains are enriched in Mn in the center and Fe atthe rim; the average composition is GruuAndrnu, withminor Mn and Ti. Inclusions of Fe-diopside areabundant, as are inclusions of quartz and thin calcitevelns.

A second, more prevalent subzone, the Ete, iscomposed almost exclusively of extremely coarse-grained, euhedral epidote with the same compositionas the epidote of the Act zone. Inclusions of quartzand Fe-diopside are fairly common.

The most common subzone, considered as general-ly representative ofthe zone and therefore designatedsimply as Ep, consists of 25-70 percent epidote and10-60 percent garnet, with 10-20 percent quartz andl0 percent calcite. The garnet and epidote have com-positions identical to those of the Epe and Erg sub-zones, and seem to have formed simultaneously, asseen by intergrowths between the two minerals. Thegrains tend to be extremely large and anhedral, andcontain inclusions and veins l ike those described. Theepidote:garnet ratio increases slightly from north tosouth.

SCAP Zone. The Scep zone, as mapped, isbounded on the intrusive side by either the qvaftzmonzonite or by aplitic quartz-feldspar veins and onthe Ep zone side by the innermost occurrence, otherthan the Men zone veins, of scapolite. In fact,however, the Sce,r-quartz monzonite contact is notclearly defined, because euhedral, spongiformcrystals of scapolite up to 2 cm long surrounding

quartz, biotite, oligoclase, K-feldspar, and Fe-diopside are commonly observed in the quartz mon-zonite. For [-7 cm beyond this transition zone, theqtrartz monzonite contains up to 5 percent Fe-diopside. Veins of epidote less than two millimetersacross cut across the Sclp zone, running from thequartz monzonite to the Ep zone. Similar veins arepresent at contacts with schist and gneiss all over EastRidge; none are present at Area l.

Three compositional varieties of scapolite are pres-

ent at East Ridge. The predominant scapolites of theSce.p zone, and the only ones in the adjacent quartzmonzonite, are colorless, euhedral, and up to 4.5 cmin length. The largest crystals are spongiforri, con-sisting of 40 percent inclusions of Fe-diopside,qvartz, sphene, and a second scapolite, as well asminerals of the quafi.z monzonite, as noted.Myrmekitic intergrowths of quartz and veins ofcalcite are abundant. The host scapolite grains arehomogeneous, with composition Car.eNar.rAlr.rSir.tOrn.(CaCOr)o.r. The second type of scapolite is foundonly as fine-grained, anhedral, polysynthetically-twinned inclusions within grains of the first type.This scapolite has composition Ca'.,Nat.rAlo.,Sir.,Ozn.(CaCOs)g.'. The third type is the Cl-bearing va-riety in the Mnn zone veins.

ln the East Ridge scapolites, the coefficients of theCaCO, and NaCl ideally should total 1.0, but basedon 34 analyses, Shaw (1960) has found a slightlylower value, 0.855. Analyses in which the sum is lessthan this value could be indicative of variableamounts of K2COg, K2SO1, and KOH, for which noanalyses were performed. Other possible explana-tions for the low sums are vaporization by the analyz-ing beam, natural deviation from the ideal formulae,or poor analyses. The composition of a scapolite canbe approximated as a solid solution betweenmeionite, CagAl.Si.Ozo'CaCOg (Me), and marialite,NarAlrSirOrn.NaCl (Ma). Intermediate species arenamed dipyre (Mero-uo) and mizzonite (Meb'-s'),with appropriate prefixes to designate the "ligandmolecule," e.g., NaCl (Solodovnikova, 1951). To dis-tinguish the different scapolites of East Ridge, thesespecific names will be used: dipyre for the sodic va-riety, mizzonite for the calcic.

Other Features. Mineralized blocks <0.3 macross, of what were originally marble, are presentthroughout the quartz monzonite surrounding themain skarn bodies. These xenoliths lack scapolite,epidote, and garnet, having instead a zonation frominner marble, l ike that of the Mnn zone, to a zone ofMg-diopside. This is in turn surrounded by a thin

ZONING IN A SCAPOLITE-BEARING SKARN BODY 791

zone of dark-green actinolite (too thin to be shown inFig. 2) which lacks calcite intergrowths. The smallestof these xenoliths lack marble cores.

The contact sequence at Area I was studied forcomparison. Here the intrusive is separated from themarble by a band of dipyre (dr zone), followed by azone of Mg-diopside and wollastonite identical to theDtop zone at East Ridge. The diopside and dipyre oc-cur together over a width of 0.6 cm, and dipyre ispresent in the quartz-enriched contact between dr andquaftz monzonite. The widths of the zones vary butare approximately in the ratio dr: Dtop : 2:1 .

Bulk Rock Composition Variations in the

chemical compositions of the rocks of the two EastRidge suites, and quartz monzonite samples up to2.13 m (7 ft) from the contact, are shown in Figure 4.Analyses of quartz monzonite collected further thanthis from the contact are consistent with those from It o 2 . 1 3 m .

Discussion

In order to discuss the processes which are respon-sible for the East Ridge zonation, a rough sequenceof events leading to the present mineralogic con-figuration must first be established. Petrographic and

ao\

v v

a l2o3- O o ON o 2 O - Y V V .o Abol

FQ'' g

3'8u&, ila.:: g:tv

c---Srl(iilo.,'\ --1%

,ft'

si02

V?rfrcol oros or? rcighl 7Js

ot indicoled oride3

\7va

'v

w a vv

$7V

'F?V

d'{/ -wW /

\\

Ftc. 4. Bulk chemical analyses ofSuites I (colored in) and 2, and quartz monzonite up to 7 ft from contact. Dashed lines connect Suite Irocks and do not necessarily represent the character of variation between points. Dots within circles represent analyses of rocks of neitherSui te I nor 2.

C o O - a O OM g O - V V V

792

field evidence show that there were three differentepisodes of metamorphism:

Step l: Regional metamorphism of a calcite-dolo-mite-quartz marble formed tremolite and forsterite.This is suggested by the similar composition, butdifferent mineralogy, of the Area I marble (see Figure6 ) .

Step 2: The next step, which accompanied the in-trusion of the quartz monzonite, was the formationof two mineralogic zones, similar to those seen atArea l, between the intrusive and the marble.Contemporaneous with the formation of these zones,tremolite * calcite in the marble reacted to formdiopside * forsterite + COr. Following this, Fe-diopside and then mizzonite were formed in the outer(dr) contact zone and in veins in the marble. Theseminerals are not present at Area l, indicating thatthis locality may represent a preliminary state of EastRidge.

Step 3: Epidote, garnet, actinolite, calcite, andquartz formed by reactions between late-stagemagmatic fluid and contact minerals.

Step 2

The two initial contact zones of Step 2, Ihe dr andthe Dtor, formed by diffusion and reaction betweenthe intrusive and country rocks. To better understandthe growth of the zones, we must f irst establish areference frame to which we can relate the motions ofcomponents and zone boundaries. The reader shouldnote that only net relative movements can be deducedin this study, because only the final state, andquestionable data on the init ial state, are known forthe system. Determination of detailed mechanisms offormation (e.g., whether the components diffusethrough a crystalline or fluid medium), and values forfluxes and diffusion coefficients of components, re-quire experimental studies.

KENNETH SHAY

Although, in principle, movement relative to onereference frame can be related to movement relative

to any other, certain reference frames may be more i l-

luminating than others for a given situation. Theproblem then, is to choose the one best suited'to theparticular constraints of the system. Brady (1975) has

discussed several possibil i t ies, including (l) meanvolume reference frame, (2) K-h-component reference

frame, and (3) inert marker reference frame.Mouement of Components; Original Contact. The

mean volume reference frame is most useful for con-stant volume processes, and so does not lend itself to

the problem at East Ridge, because of the volumeloss owing to decarbonation reactions. However, theonly requirement for the r(ah-component referenceframe is that the flux of some r(-n-component withrespect to that frame is zero. In other words, using

this frame is the same as considering one componentas motionless, and comparing the net movements ofother components relative to it. In discussing such atransfer of components at East Ridge, it is convenientto express the components as oxides, because only thenet movement can be deduced, and the oxide formsimpl i f ies chemical compar isons between as-semblages.

If a K-h-component which truly did not move with

respect to an inert (non-diffusing) marker frame ispresent, of if some inert marker l ike graphite is pres-

ent (Vidale, 1969), then the location of the originalcontact is easily deduced. Probably all of the compo-nents do move relative to an inert marker frame(Vidale, 1969), so it is conceptually most useful tochoose as the r(ah-component the slowest, or "mostnearly motionless" component. Previous workers(e.g., Carmichael, 1969) have argued that this compo-nent is AlOs/2. Assuming that the AlOs/" referenceframe coincides with an inert marker frame, it followsthat the original contact is within the border zone ly-ing between the dr and Dtop zones, which at East

Ter l r 5. Compbsi t ion of Zones ( in Moles/100 cm3)

IVIAR DI OP ACT -bP S C H P A - v g . Q t z . M o n z .

f ) a n c i i r r ( o n / t o \ *

s i 0a

+At^ o^Ca0

]t la^ o' a

F e 0

Mgo

2 . 6 4

. 7 2 2

. o 6 4

. 2 7 \

.o37

. 0 2 4

2 . 9 4

. 5 4 5

. 0 1 1

1 . 8 0 5

. o B 5

. 0 2 9

1 . O 2 4

3 . 2 42 . 7 1 4

.IT7

r .49r. 0 1 9

.]^96

r , ) 6 7

z . o o

? ? ? l

I . z L A

. 5 9 0< o n

3 . t 02 . 7 3

. 4 0 8r . 2 8 9

. 4 0 1

.489

? q A

, yo:)

L . 2 7 4

. u 1 0

a . > o2 , T 8 2

r . 2 4 4< Q c

. 3 ) 6

. r ) 3" u o a

#Based on da *ua o f Rob ie and Wa ]dbaum (1968 )

ZONING IN A SCAPOLITE.BEARING SKARN BODY 793

Ridge is within the Acr zone. The movements ofother components relative to the AlOa/2 referenceframe have been tabulated (Table 6). These data sug-gest that there was a total volume loss in the contactzone of 56.5 percent. Figures based on the MgOreference frame have been computed for comparison.

The volume loss due to decarbonation may be es-timated by a consideration of the CaO contents of thequartz monzonite and dr zone. Specifically, it may beassumed that in the dr zone, the CaO concentration(increased by 56.5Vo) greater than that of thesimilarly-adjusted quartz monzonite is due to the ad-dition of CaO that was once present as calcite. Usingthe data in Table 5, the volume loss due to decar-bonation in the formation of the dr zone is calculatedas 19.5 percent.

Driuing Forces of Metasomatism. Thompson(1959) and Vidale (1969) discussed movement ofcomponents down a gradient of "non-volati le com-ponent" (e.g., SiOr, MgO) activity. Hewitt (1973) hasshown that mineralogic banding may be due to agradient in "volati le" (e.9., HrO) composition aswell. Both mechanisms are possible for East Ridge:there was most certainly some variation in fluid com-position between the COr-rich fluid of the marble andthe HrO-rich fluid of the magma;and the variation in"non-volati le" composition between the SiOz- andAlOrTr-rich quartz monzonite and the CaO- andMgO-rich marble makes a non-volati le componentactivity gradient possible. The following discussiondemonstrates that both gradients have affected theEast Ridge rocks.

In Figure 5a the bulk compositions of the Step 2zonation, from quartz monzonite to marble, are plot-ted against molecular CaO, MgO, SiOz, and AlOs/2

content. The plot clearly indicates the increase of 4s16,from the marble to the dr zone, and decrease of agusover the same path. It is clear that CaO and SiOz haveindeed diffused "down" the potential gradients (Fig.

5b,c) .Treating each zone as an open system in ther-

modynamic equil ibrium (Thompson, 1959), it hasbeen shown that for an arbitrary temperature andpressure, the minimum number of "inert com-ponents" (those components whose chemical poten-

tials are controlled by the phases-Korzhinsky, 1959)is n for an n-phase zone. Variation in r, and thereforein the number of phases, from one zone to the nextmay be due to an inert component becoming"mobile" for a zone (see Vidale and Hewitt, 1973).

It is worthwhile to discuss (l) why the assemblagesdepicted in Figure 5a only approximate but never at-tain the path along the line calcite-diopside, to thepoint Diopside; and (2) why it is this path that is ap-proached. The second point is fairly obvious: to get

between two divariant, single phase points (one ofwhich is Diopside in this case), the system must atsome moment consist of the two end-point phases. Atthat time the system loses a degree of freedom andbecomes univariant, and at a given P and ? is

therefore constrained to l ie somewhere along a uni-variant path (in this case, a path of decreasing &c'oand increasing ; r r tor) . To understand the f i rs tpoint, we must realize that for an /?-component sys-tem in which m I n - I components are varying inde-pendently, (n-n-1)-phase regions wil l be separatedby (n-m)-phase regions. In Step 2 at East Ridge, thesystem is merely approximated by SiOr-AlO372-MgO-CaO and Figure 5 is therefore merely a projection ofthe (n-m-l)- and (n-ni)-phase regions onto the three-

Tnsln 6 Step 2 Volume Changes and Net Relative Movement of Components

d r z o n e

, Y . o / 7 o o I o r r q l n a lr r n l r r m o a f a l ov r I v !

Addi t ion o f cornponents :s i o 2 - 3 . 1 4 9 m o l e s / 1 c o c m 3

C a O + . 4 8 3

MgO - .040

F e O - . 0 6 2

* N a ^ o + . t 3 6' z

DIOP zong

I I . L L - / o 0 1 o r l g l n a lvo l - ume o f ma rb le

Add i t j . on o f componen ts : ?S i 0 2 - 1 . 6 5 I n o t e s / I o O c m '

Cao - ) ,4 .7 6

lv lgO -7 .85

F e 0 - . 0 6 5

lNar.o -.1b6

MgO f rame;

D I 0 P z o n e

74.90% of o r ig ina lvo lume o f marb le

A d d i t i o n o f c o n p o n e n t s :S iOa +1 .986 no les / fO ' cm;

i - A t ^ o ^ + . t o o

Cao - .9L9

F e 0

*Na-0- a

( + ) = i n t o z o n e( - ) = o u t o f z o n e

(si02)

\:Y-'-,_------+

ouor lz monzoni fe

Frc. 5. a. Hot or oll-n.-Oiopside Step 2 compositional variation.Dash-dot line is possible complete variation. O, analyzed samples;

E, observed at Area l; A, observed at East Ridge. b. Schematicdiagram of variation in &s1q and trrs.e from quartz monzonite tomarble, initiatly; and c. after formation of contact zones.

dimensional diagram; falling directly on any 4-com-ponent, univariant line of Figure 5 would be for-tui tous.

The role that the volatile phase compositiongradient plays concerns only the presence or absenceof wollastonite, and is therefore minor compared tothe non-volatile activity gradients. Inspection ofFigure 6 shows that for the pure system at a fixedXs6", the temperature must be increased from I to atemperature well above the curve Tr * Cc : Fo * Di+ CO, + HrO, if wollastonite is to become stabilizedrelative to calcite and SiOz. At or below thetemperature of the intrusion (for our purposes, atemperature on or just above the curve Tr * Cc : Fo+ Di + CO, + HrO), wollastonite will be stabilizedby a decreased Xgq, in the fluid phase (arrow, Fig. 6),

KENNETH SHAY

(AleOs)

as shown by Greenwood (1969). Therefore it appearsthat a variation in fluid composition between anHrO-rich fluid in the magma and a COr-rich fluid inthe marble, determined the presence or absence ofwollastonite in the contact atea.

Formation of Mizzonite and Fe-Diopside. It is clearthat mizzonite formed after the formation of the drand Dtop contact zones and the Fe-diopside by theinclusions of Fe-diopside and dipyre in the mizzonite.Whether Fe-diopside formed during or after the for-mation of the dr zone is unclear. and therefore twohypotheses for the formation of the two minerals willbe discussed. Both account for the fact that the netresult of this process, which mostly involved chang-ing the dr into a ScrrP zone, is the addition of 0.1moles/100 cms SiO2, 0.03 moles/100 cm8 CaO, 0.13moles/100 cm3 FeO, and 0.06 moles/l@ cms MgO,based on the AlOsrz reference frame.

One possibility is that the CaO, MgO, FeO, andSiO, rejected from the Dtop zone first formed Fe-diopside concurrently with the other Step 2 processes;these components precipitated pyroxene in the adja-cent quartz monzonite as well. CaO reacted with thedipyre to form mizzonite and quartz: (roughly)5CaNarAlnSi'Orn.(CaCO')o.r * 2CaO -t 2.1 CO2 -4CarNaAluSi'Orr.(CaCO')'. ' f 3NarO * l2SiOr.Mizzonite formed in the quartz monzonite at the ex-pense of plagioclase in much the same way, ac-counting for the vein-like character of the Scep zone(Fig. 2). This process followed the formation of Fe-diopside because SiOz must be diluted for (l) toproceed as written. An alternative possibility is thatan Fe-Mg-Si-Ca-bearing f lu id phase , pe rhapsreleased by the magma, reacted with the initial Step 2configuration to form Fe-diopside and mizzonite.

The major drawback of the first possibility is thatthe two minerals are not present at Area 1; unlessconditions there were considerably different fromthose at East Ridge, it is not obvious why Fe-diopsideand mizzonite should not have formed at Area I un-der this model. This drawback does not exist for thesecond possibility, but another does: why is no otherevidence of this fluid, such as veins containing Mg-bearing minerals, seen in the quartz monzonite?

The problem may have a solution in light of thezoned Fe-diopside/mizzonite veins of the M,cn zone.The minerals of these veins are virtually identical tothe Fe-diopside and mizzonite of the Sce.t zone,which would suggest an identical origin. The Menzone veins are merely quafiz monzonite sills whichhave reacted with the marble at constant uolume of theuein, as can be shown by a straightforward com-

t

t

morble

ZONING IN A SCAPOLITE-BEARING SKARN BODY

Ftc. 6. Schematic T-Xssrplot of some assemblages and reactions in siliceous dolomites at -4 kbarBased on data from Greenwood (1967), Robie and Waldbaum (1968), Metz and Trommsdorff(1968), andSkippen (1974). "A" refers to text.

I Y J

parison: the average Alos/z density for the velns ls0.715 moles/100 cm3, and the average SiO, density is3.052 moles/100 cm'. From Table 5, the analogousfigures for quartz monzonite are 0.722 and 3.022,respectively. CaO does not match but could be ac-commodated by the decarbonation of a volume ofMen zone equal to 60 percent of the vein, making atotal volume loss of 38 percent.

This process, operating in the Men zone veins toform Fe-diopside and mizzonite, is probably thesame process responsible for the formation of theseminerals in the Scep zone as well. The figure for totalvolume loss for the Men zone veins does not matchthe figure calculated for the DIor/and dr zones(56.5To), possibly owing to differences in contactgeometry. The absence of Fe-diopside and mizzoniteat Area I remains unexplained.

Step 3

The formation of actinolite, garnet, epidote,calcite, and quartz clearly followed Steps I and 2, asseen by petrographic and field relations. Likewise,these five minerals seem to have all formed at thesame time and by similar processes, such that the out-er part of the Dlop zone became Acr, and the innerpart of the Sc,qp zone became Ep. Based on theAlOsrz reference frame, a 70 percent volume increase,and the addition of 1.93 moles/100 cms SiOr, l.12moles/100 cm3 CaO, and 0.1 moles/100 cm8 FeO and0.243 moles/ 100 cm3 HrO accompanied the transition

from Drop to Acr. A 30 percent volume increase andthe addition of 1.59 moles/100 cms SiOz, 0.82moles/100 cma CaO, 0.32 moles/100 cms FeO372, and0.26'l moles/ 100 cms H2O accompanied the transitionSclp to Ep. These figures, and the presence ofepidoteveins running from the quartz monzonite to the EPzone, suggest that the Step 3 mineralization wascaused by a late stage fluid which may have beenliberated from the cooling qtaftz monzonite. Thatthis fluid appeared well after the initial intrusion is in-dicated by the presence of calcite * quartz, which re-quires low temperatures in the presence of a highXs"6 fluid (Fig. 7; Greenwood, 1967).

fhe chemical potential of SiOz is fixed in the Acrand Ep zones because quartz is present. At con-stant temperature, pressure, and fluid composition(which fixes g.6ue as calcite is also present), the onlycomponents whose chemical potentials might providethe varying conditions necessary for the observedmineral segregations are aluminum, iron (expressedherein as FeOrTr),'and magnesium. Figure 7 is asketch of the stability fields of the East Ridge ac-tinolite, epidote, and garnet with respect Io ltr;o"n,pFeostzs and;ry"e, at fixed P, 7, fluid composition, andCaO content of the phases. This diagram illustrateswhy only epidote and actinolite are present in whatwas originally DIon zone (high &ugo), and why epi-dote predominates toward the Scep zone (higherpoto"n). Note that Lhe f, may have played arole here as well; all of the iron in the epidote and

Xcoz

't96KENNETH SHAY

Ftc. 7. Sketch of the stability fields of the East Ridge actino-lite, garnet, and epidote, as they vary with puso, peros,,, andPreo",,, zl fixed P, l, fluid composition, and CaO content.

garnet is ferrous, whereas only 68 percent of the ironin the actinolite is in the reduced *2 state.

ln the inner Scep zone, p\ags is low, and epidoteand garnet are stable relative to actinolite. Thedifferences in stability conditions between epidoteand garnet are more clearly shown by Gordon andGreenwood's (197 1) work on the pure aluminum endmembers, zoisite and grossularite. This work es-tablished both grossularite's instabil ity relative tozoisite at high Xcs" of the fluid phase, and the widestabil ity range of zoisite. Assuming that these rela-tions hold qualitatively for the iron-bearing system,the Epe subzones may represent pockets of high Xs*fluid and the Eeg, pockets of low Xsq fluid or, fromFigure 6, low Fero"1,. Either condition could resultfrom local concentrations of dipyre inclusions (see(1)). The southward-increasing epidote:garnet ratioobserved at East Ridge may be correlative with thedecreasing degree of pervasive intrusion in the samedirection, for the fluid phase was probably richer inCO, going south, favoring epidote.

A question which remains is: why have epidote andgarnet not replaced all of the ScAp zone, and ac-tinolite. all of the Dtop? Various features indicatethat the cause is not chemical in nature: epidote is ap-parently stable under the conditions of the Sclpzone, judging by the thin epidote veins; the Ep zoneoccurs as small pods in the Scep zone, apart from the

regular, concentric zonation; and veins ofepidote arepresent throughout the surrounding quartzite, schist,and gneiss. These features suggest that infiltrationmetasomatism, and therefore permeability, played akey role in the Step 3 process, as then mineralizationwould only occur where the rock was sufficientlypermeable to allow reactive fluids to flow through it.If this was so, then the permeability was evidentlygreatest on either side of the dr/Dtop contact zone,dropping off rapidly with distance.

Conclusions

Three steps were involved in the formation of theEast Ridge skarns. The first involved the formationof tremolite and forsterite at the expense of dolomite,calcite, and quartz, which occurred during regionalmetamorphism.

The second and third steps can be chronicled usinga fixed-component reference frame. The second stepbegan with the intrusion of the Cactus Quartz Mon-zonite, which raised the temperature of the marble,stabilizing the assemblage diopside * forsterite. Atthe quartz monzonite-marble contact, a zone of diop-side and a zone of dipyre (sodic scapolite) began toform about the contact, due to the diffusion of Ca,Mg, and Si. Wollastonite became stable relative tocalcite + SiO, near the marble, due to a high Xs,ofluid phase. The components expelled from the con-tact zone formed Fe-diopside and reacted with thedipyre to form mizzonite (calcic scapolite) andquartz. The Step 2 process involved a 56.5 percentvolume loss.

In the third step, mizzonite of the inner Sclp zoneand Mg-diopside of the outer Dtop zone reacted witha late-stage fluid liberated from the quartz mon-zonite, forming epidote, garnet, and actinolite whereinfiltration of the fluid was possible.

Acknowledgments

This paper is a summary and extension of an undergraduate

thesis submit ted to U.C.L.A. in June, 1974. I am grateful to W. G.Ernst and Douglas Rumble III for their assistance and direction,

and I am particularly indebted to John B. Brady for the ex-

ceedingly generous amount of time he spent on my behalf. I thank

W. G. Ernst , D. Rumble I I I , T. M. Gordon, D. A. Hewit t , and

J. B. Brady for their comments on preliminary drafts, and I owe

especial thanks to an isolated system whose components' non-

ideal behavior increased my activity. This work was supported by

a U.C.L.A. Undergraduate Fel lowship, U.C.L.A. departmentalfunds, and an NSF Graduate Fellowship.

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ZONING IN A SCAPOLITE.BEARING SKARN BODY'197

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Manuscript receit:ed, May jl,l974; accepted

for publicalion, December 3, 1974

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