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Home , Gneiss, San Gorgonio Mountain, Scapolite

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

Mineralogical Zoning in a -BearingSkarn Body on San Gorgonio Mountain,

KrNNnrH SHnyl Departmentof Geology, Uniuersityof California, ,California 90024

Abstract

Concentrically zoned skarns on Mt. San Gorgonio have been studied in detail from the point of view of field and petrographic relations. The skarns are mineralized blocks of of the Precambrian San Gorgonio Igneous-Metamorphic Complex, which are within the con- tact zone betweenthe Complex and the Middle JurassicCactus Monzonite. They con- sist of cores of -forsterite-diopsidemarble, surrounded by concentric zones of , actinolite * * calcite + quartz, epidote + * calcite * quartz, and scapolite + calcite + quartz. The genesisof the skarns involved three steps.(l) Regional formed tremolite and forsterite from calcite, dolomite, and quartz. (2) The intrusion of the quartz monzonite stabilized diopside * forsterite relative to tremolite * calcite. Reaction between 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 original contact. Components rejected from this zone formed ferroan diopside in the scapolite zone and in the quartz monzonite, and altered the scapolite to a more calcic form. This processin- volved a net volume lossof 56.6percent. (3) Infiltration of fluids,possibly of magmaticorigin, formed epidote and garnet from scapolite, and actinolite from diopside.

Introduction rounding peaksrepresent a complex of Precambrian This paper describesthe geology and chroniclesthe metamorphicrocks and Mesozoicintrusives (Vaughn, geologicalhistory of a small group of skarns on Mt. 1922). San Gorgonio. Combining textural, mineralogic, Thereare relatively few published studies and un- published theses geology chemical, and field data from the skarns with field on the of the San Bernar- dinos; Vaughn (1922) and petrographic data from a similar area to the studied the entire range and Baird et al (1974) have south and with publishedlaboratory results,a model recently discussedits regional for the formation of the skarns is proposed. The structure. For additional information on the region model is consistent with the principle of local immediately surrounding East Ridge, the reader is referred (1957) equilibrium (Thompson, 1959), which has been to Allen and Dibblee (1964). shown to be a valid assumptionin other studiesof (e.9., Vidale, 1969;Joesten, 1974). Method of Study The area studied, hereafter called East Ridge, is at The geology of East Ridge was mapped between an elevation of 10,500ft. on the eastern ridge of San May and November, 1973,using a Brunton compass Gorgonio Mountain (elevation 11,502ft.), which is and tape measure,on a l:240 basemap with 5-foot the highest peak in the east-westtrending San Bernar- contoursmapped in the samefashion (Fig.2). A total dino Mountains of southernCalifornia (Fig. l). The of 120 sampleswere collected;from these,70 thin sec- San Andreas fault zone defines, and probably tions, I I polished sections, and 12 thick sections were formed, the southern scarp which separatesthe San prepared, and 60 different whole and Bernardinosfrom the to the concentrate powders were ground for Xnn and Xno south and the SanGabriel Mountains to the west(Al- studies. len, 1957). San Gorgonio Mountain and the sur- Seventy different mineral grains were analyzed (Table l) employing an ARL-EMXmicroprobe. Stan- ' Current address:Department of Geological Sciences,Harvard dards comparable in composition to the unknowns University, Cambridge, Massachusetts02138. were not generallyavailable; the standardsused are 785 786 KENNETH SHAY

ol ?\ -^\ c.*ro,c ..di."nt" o I l4 Mrercic inrrulins \t E eotro:oicrcatn.nrr ondvotconic! ffi ljll uctomorotricond Prccombrlonrocks

rO O tO zomiles

rO O lO zokrlomc!.rs

Frc.l.Index map. listed in Table 2. Data were reduced using the on a Phillips X-ray fluorescenceapparatus, using methodsof Benceand Albee(1968) and Albeeand U.S.G.S. G-2,ZGI GraniteGM and Basalt Ray (1970). BM, and an analyzedlimestone as standards. The Xnn whole-rockanalyses (Table 3) weredone Geologyof East Ridge East Ridge lies within a zoneof intrusionand as- similation betweenthe PrecambrianSan Gorgonio Igneous-MetamorphicComplex and the Middle JurassicCactus Quartz Monzonite (Fig. 2). The Complex apparentlyhas not beendeformed by this intrusion, as is consistentthroughout the contactzone, and faults,with maximumoffset l0 m, postdatethe intrusion.

CactusQuartz Monzonite This coarse-grained(l-4 mm) granitic intrusive, originally called the Cactus Granite by Vaughn (1922)but renamedthe CactusQuartz Monzonite by Guillou (1953),contains about 30 percenteach of quartz, (Anru), and K-feldspar,and l0 per- cent with accessoryapatite, , sphene,zircon, magnetite,and allanite (in biotite). is rare and where present, of glomeroporphyritichabit. Hollenbaugh(1968) con- sidersthe Cactusto be comagmaticwith a that Hewettand Glass (1953) dated as Middle Juras- sic.This is in agreementwith more recentK-Ar ages (Armstrongand Suppe,1973).

San GorgonioIgneous-Metamorphic Complex

Frc 2. Geologicmap of the norrhernpart of EastRidge (index at This Precambrian complex, named by Allen upperleft).Q showsthe sampletraverse for Suitel;O for Suite2. (1957), includes and (microcline- ZONING IN A SCAPOLITE.BEARINGSKARN BODY 787

Trslr l. MicroprobeAnalyses in WeightPercents

Smple No. * AC05902 AC12402 ACr2409 4C12410 ACl2503 ACr2504 Drol70l Dr01704 Dr06602A Dr066028 Dr06603A Dr06603c Dr06604 Dr066044 Location** ACT ACT ACT ACT ACT ACT SCAP SCAP DIOP DIOP DIOP DIOP DIOP DIOP si02 s1.5 52.7 52.4 55.8 51.4 53.6 51,4 52.0 54.2 54.0 54.0 54,0 53.6 53.5 .3 .1 ,4 .4 .7 A12O3 3.9 2.3 1.5 .8 1.4 .9 .7 .4 .4 Ca0 L2.4 L2.8 !2.7 l3.l L2.6 t2.7 24.3 24.4 25.2 25.5 25.6 25.5 25.0 2s,7 l+r Na2o 0.0 .2 ,3 tr .5 .1 tr Fe as FeO t3.2 L2.3 tz,a 6.9 14,7 r3.3 1L.2 10.8 2.r 2.0 2.7 L5 2.9 2.5 Mco 13.5 r5.8 16.0 L9.7 rS.5 16.9 11.2 11.4 17.5 L7.6 t7.4 17.6 17,4 17.3 Ti02 .2 .2 .l 0.0 tr 0,0 tt tr tr tr t! tr MnO .8 .5 .6 .5 .6 .5 .7 .6 .3 .2 .2 .3 c1 na na na na na na na na na na na na na na Hzo 3.4 3.2 3.6 3,2 3.2 r.7 0 0 0 0 00 00 Total 100.0 r00.0 100.0 100.0 100.0 100.0 99.7 99.8 99.7 99.6 99.5 99.4 99.7 100.0 sanple No.* DI066048DI06605A DI0790I DI079O2ADrO79O2B 0r07903 DIt23OtEDI12301F DI123O3A DII2304 DI12305A DI1230SB DI12305CDI12306 LOCAIiON** DIOP DIOP N.V. N.V, N.V. M.V. EP EP EP ACT ACT ACT ACT ACT sio^ 53.3 s3,7 52.5 52.0 52.8 52.8 52.1 S2.O 51.7 5r.5 51.6 51.3 51.9 51.8 .5 .6 .6 Al2o3- .3 .8 .5 '6 .6 ,6 .4 .5 .5 .4 .6 Cad 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 Na^O .l .l .2 .s .2 .2 .2 .2 ,2 .3 .2 .2 .2 ,I Fe as Feo 3.I 2.9 9.0 12.4 9.0 11.4 Lr.7 I0.4 10.6 12'0 11.7 L2.3 11.3 9.8 Mgo t8.l 16.8 l2.s 10.8 L2.4 9.6 ll,2 10.8 10.9 9.9 10.5 I0.l 10.5 11.2 TiO2 tr tr tr tr tr tr tr tr tr tr tr tr tr tr Mno .3 .3 .3 .2 .2 ,5 1.0 I.1 L'2 1,2 L.2 I.1 L'2 .8 Cl na na na na na na na na Da na na na na na Hro00000000000000 Total 100.4 gg.7 99.7 100.9 I0O.O 99.8 101.0 99.5 99.8 99.6 lOO.2 100.I 100.2 99.4

Sarple No,i Dr12408 Dr12410Dr126802 EP01705A EPl2202 EPL22O3 EPL23O3A EPI24OI EPL24O2 EPI24O2A EPr2403 EP12404 EP12409EPl2501 Location* * ACT ACT EP EP SCAP SCAP EP EP EP EP EP EP EP ACT si02 54.5 S4.5 52.6 36.9 37.2 37.2 37.4 36.5 36.6 36.9 37.4 37,r 36.9 36.8 A1203 .2 .5 .6 22.9 23.2 23.3 22.1 22.2 22,9 22.7 23.4 23,0 22,5 23.2 Ca0 25.6 25.4 24,3 23,r 23.0 23.0 23.7 23.I 23.2 23.3 23.5 23.2 23.2 23.3 Na2O tr tr na tr 0.0 0.0 tr t! tr tr t! tr Fe as Feo 1.2 1.5 11.8 r2.8 r2,2 L2.2 13.0 12.2 L2.5 I2.4 L2.1 t2.4 L2,9 Lz.T Mgo L7.? 17.8 9.6 0.0 0.0 0.0 0.0 0.0 .2 0.0 0.0 .2 0.0 0,0 Tio2 tr tr na .t 0.0 0.0 .1 0.0 0.0 .2 0.0 tr t? Mn0 tr tr 1.3 .2 .2 .2 .2 .r .l .2 .t .l cl na na na na na na na na na na na na na na Hzo 0 0 0 3.9 4.1 4.0 3.5 4.3 4.5 4.3 3,4 4.O 4.0 4.0 Total 99.3 99.5 100.2 99.9 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 ooo ooa

SarPle No." EP12502 EPl260I EPl26B02 FO0660IF006602 GA00003 GA00004 GAo7005 GAl230lA GAr2302 GAl2306 SCor502 sc01701sc0l70rA Location** ACT EPe EPe DIOP DIOP EP EP EP EP EP EP SCAP SCAP SCAP sio2 36.8 36.5 36.8 39.7 40.0 37.9 37.7' 37.7 36.8 37,8 37.9 45.4 46.2 56.3 Ar203 23.4 23,4 23.6 .1 .I r4.8 14.8 t4.2 10.7 11,1 13.9 26.2 24,8 24.9 Ca0 23,3 23.L 23.0 .I .1 34.7 34.7 34.t 32,3 33.4 34.7 t7 .3 L7.3 9.t Na20 tr 0.0 0.0 0.0 0.0 tr tr tr tr t! tr 3.9 3.6 6.4 tse as t6u t2,4 t2.8 72.2 7.9 7.7 11.0 11.4 13.I 16.5 15.7 11 7 t .1 .1 Mgo 0.0 0.0 tr 51.7 5I.5 tr tr 0.0 tr t! .l 0.0 0.0 0.0 Tio2 .1 0.0 0,0 tr tr .3 .3 tr .8 .I .7 0.0 0.0 0.0 I'hO .1 .1 .2 .2 .4 .7 .9 .5 2.0 1.3 q tr tr tr na na na na Da na na na na na na tr tr tr Hzo 3.8 4.0 4.2 0 0 00000 00 00 '99.4 Total 100.0 100.0 100.0 99,7 99.7 99.8 99.7 99.2 99.5 99.8 92.0**r 96.1**.

Sanple No.* SC017018SC0l70lE SCol702ASC017028 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 sio2 56.5 57.r 46.1 45.9 s7.2 45.0 45.7 46.3 46.3 45.9 45.5 46.7 51.1 5r.1 A1203 24.7 24.8 25.5 25.4 24.8 26.0 25.6 26.7 26.3 27.5 26.2 25.5 .l .1 cao 8.7 8.5 17.5 t7,9 8.3 L7.S 16.8 16.7 l7.o rS.4 17.4 L7.r 48.2 48.3 Na2O 6.7 6.6 3.8 3.8 6.6 3.7 3.5 3.3 3.3 3.6 3.6 3.8 tr t! Fe as FeO .1 .1 .l .l .1 .1 .I .2 .2 .l .1 .I .l .I Mgo 0.0 0.0 0.0 0.0 0.0 0.0 0.0 tr 0.0 0.0 0.0 0.0 0.0 0.0 Tio2 0.0 tr tr tr 0.0 0.0 tr 0.0 0.0 0.0 0.0 0.0 0.0 0,0 Mno 0,0 tr 0.0 tr t! tr tr tr tr tr tr tr .l .l cl tr tr tr tr tr tr .4 .3 .4 tr tr tr na na H.O 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Tot al 92.6r** 92.9*** 93.2+** 99.6 99.7

*Fi"st Ato Lettere deeigwtemLwal: lC= actinoLi.tei DI = diopside; EP=epidote; FO= fo"ste?rte; GA= gmet; SC= eqpo|ite; IIL=Dol.laetonite. 'aRefers to mLrercLized aone. m.u. = diopeide-ecqol,ite ueias in MR zw. *4*No mLAsis for COr. 788 KENNETH SHAY

Tesln 2. StandardsUsed in MicroorobeAnalvses pods of concentric garnet-, epidote-, actinolite-, diopside-,and scapolite-rich bands. Along strike,the Element Standild Efenent Standard lengthof exposureof the skarnsis approximately150 A] pyrope garnet I\4r spessartlne garnet Ca vollastonite Na albite m (460 ft). c1 halite b1 pyrope garnet The bandsof skarnsurrounding the marblecores Fe mgnetite Ti slnthetic ruti.Le MC plEope garnet (the Men zone) have beendesignated as the DIot, Act, Ep,and Scn p zones.Two suitesof samplesfrom the zones were collected,as shown in Figure 2. Calciteis presenteverywhere except in most of the quartz, biotite-microcline-quartz, biotite-, Dtor zone;and with the exceptionof the Men and magnetite-biotite-amphibole), (-bio- Dlop zoneswhich containaccessory ilmenite, quartz tite-quartz, biotite-microcline-quartz), , and and spheneare ubiquitous(Fig. 3; Table4). marble. Allen places the assemblagesin the - MAR Zone. At EastRidge, much of the exposed subfaciesof Turner's (1948) marblehas beenextensively mineralized and altered; facies.Silver (1971) reports a zircon age of at least consequentlymany original features, in particularthe 1750+ l5 m.y. for a granite which intrudes a gneiss originial composition,are unclear.However, the of this complex, thereby confirming the Precarnbrian southernmostskarn body at East Ridge containsa age suggestedby Dibblee (1964). marblecore which bearsan extremelyclose lithologic andstratigraphic relationship to rock of Area I (Fig. Marble and Skarn l), an interbedded schist-marble-gneisssequence The marble bodiesat East Ridge and the l- to 250- which has not been intruded so pervasively.The cm wide bands of skarn surrounding them are here thickness,grain size, bulk composition, and discussed together. A total of eleven individual stratigraphicposition of thetwo marblesare virtually bodies with nearly complete mineralogic zonation identical.It is fair to assumethat originallyall of the (describedbelow) are present.The maximum exposed EastRidge marble was nearly identical to the Area I thicknessis 8.9 m (27 ft), and as that widest part is marble,and this assumptionwill be usedto augment bounded above by gneissand below by quartziteand and interpretthe datafrom EastRidge. The effectsof schist, this is taken to be the maximum thickness. intrusionof quartzmonzonite at Area I arediscussed Thinner bodies have a thinner marble core or may below. even lack a carbonate core, occurring simply as large The EastRidge marble, the MeR zone,is 35-40Vo

TesI-r 3. Partial XRF Analyses, Weight Percents

Sanple No. 73-58 73-85 73-s9 73-19 73-66 73-71 73-),3I 73-r2S 73-t24 73-132 73-122 73-t29 73--s3q 73-75 73-L4 Locationi MAR MAR MAR DIOP DIOP DIOP ACT ACT EP EP SCAP SCAP q .1 2 SiOz 10.82 11.45 t5.62 5t .25 50.27 51 77 51.45 55 85 51.41 53.02 s0.02 52-38 86.78 68 89 69.92 A1z0s .28 . 16 1.64 r .42 1.84 r.7r 8.s3 4 87 13.73 14 58 25.2s 24.28 6.06 14.39 16.03 Cao 34.85 34.02 38.06 23.86 25.77 24.80 22.9r 21 89 19.96 12.40 14. 86 l0 .66 3 69 4.96 1.55 Na20 1.32 .48 .42 .r2 .18 . 16 .01 00 .00 .00 3.0s 5.08 .40 2.03 2.90 Fe as FeO 1.10 37 2.4I 4. 35 4 49 4.38 9.45 9.13 6.57 6 .48 2.99 2 -48 I .76 2.06 1.40 MgO 14.32 13.75 13.71 16 81 16.99 16.26 s.83 6.88 5.27 3 51 .9s .sl 1 t5 1.39 .65

Sanp]e No. 73-128 73-134 73-135 73-136 73-I38 73-139 73-I40 73-t4I 73-I42 73-72 73-144 73-145 73-146 73-147 73-148 Locationt .5 1 11 223 34 456 677 si02 69.05 69-95 74.84 70.67 69.99 75. 93 73.07 75.76 75.70 71.97 76.61 76.L0 72.00 77-28 73.88 A1z0: 15 54 14.73 13.78 15.33 15.70 13 43 14 44 13.67 13.39 15.84 ).3-32 13.39 15.07 12.72 13.99 CaO 3.32 2.72 .82 2.88 2.68 .95 t.O7 1.05 .93 2.48 | .24 L.L2 7.77 .75 1.35 Na2O 4 -46 5. 30 2.42 4 -07 4.01 2.58 3.07 2.84 2.56 3.75 3.06 2.87 2.rt 2.53 2.56 Fe as Feo 3.29 2.99 .86 2.37 2.72 .69 r.24 .73 .75 I .61 . 69 .82 .5 .86 1.4s MgO 1 .04 .93 .28 .68 .72 .31 -48 .30 .28 .36 .24 .26 .53 .19 .43

Sanple No. 73-149 73-150 73-tsl 73-152 73-I53 73-76 73-t54 73-155 73-73 73-6 73-32 73-33 Locatlon+ 8 8 99 10 10 15 16 65 >100 >100 >100 si02 73.26 76.62 73.60 75.71 76.09 76.04 76.23 72.84 74.85 74.00 73.78 73.74 A1203 14.58 12.71 14.08 13.40 12.73 12.57 13.51 14.70 13.91 14.05 14.23 14.47 CaO .75 1 .35 1.4I .84 .81 I.23 .84 1.s] | .22 I .27 I .89 1.53 Na20 3. 19 2.68 I.73 3.02 2.93 2.17 3 35 3.29 2.64 3.42 3.21 4.29 Fe as FeO 1.09 87 .47 .9s I.2r 1 81 .93 .91 .67 1 .48 1.53 I .49 MCo 46 .24 .47 .27 .s6 .43 .28 54 .31 -34 35 .43

t'q" fRefere to skam zmes, or di,stmce (in feeL) into quutz nanzonite from cont@t. is qwrtz-feld.spu uei-ns. Anallst: P.C. stmel 't89 ZONING IN A SCAPOLITE.BEARINGSKARN BODY forsterite, 55-60Vo calcite, 5Vo diopside, and lVo tremolite; the Area I marble is 45Voforsterite, 457o calcite, and llVo tremolite (see Fig. 3). The olivine in the Mnn zone is fine- to medium-grained,anhedral, and has the composition Fo"r.u,with trace amounts of Mn, Al, Ca, and Ti. The tremolite is fine-grained, colorless, elongate, and rimmed by forsterite and diopside in the Men zone. The diopside grains are untwinned, equant, fine- to medium-grained, and palegreen. Their compositionis DiruHdu;the rims are enriched in Fe. In all the Area I marble outcropsstudied, there is a 0.3-l cm wide weathered-outspace between marble and schist(below) or gneiss(above). This may be due to a metasomatically formed reaction zone which has been weathered out. The outcrops are so extensively Frc 3. Mineralogic variation at East Ridge, basedon the estimated weathered that no analyses were performed on the modes of Suite l. Area I samples. ln severalinstances, zoned veins are present within epidote in lesser amounts. Near the Drop/Acr the M,c.n zone. The centers of such veins are com- boundary,actinolite is by far the prominentmineral posed of qtrartz and medium- to extremely coarse- in the Acr zone, but generallydecreases in amount grained scapolite,in approximatelyequal proportions. with increasingepidote, toward the Acr/Ep boun- The scapolite is milky and euhedral to subhedral, fluorescesorange, is inclusion free, and has the com- 4. Estimated Modes of Samples From the East Ridge position CarNaAluSirOro.(CaCOr)'.e.(NaCI)0,. Tmlr Be- Zones tween this assemblageand the host marble is a band (ln Volume Percent) of fine to medium-grained, pale green diopside. The grains are equidimensional,generally twinned, and Zone: ftIAR JJIOP ACT EP SCaP '1,24 compositionally homogeneous;the averageformula Suite 1 : 66 19 r25 122 is Di"oHdro with trace Al, Mn, Na, and Ti. This aluaf-tz 11 10 r5 calcite 55 5--10 10 will henceforth be,referred to as "Fe-diop- dolonite side," to emphasize the difference between it and Mc-diopside 5 90 90 the more magnesian pyroxene, "Mg-diopside," de- forsterite 40 )-- wol lastonite 8-- scribed two paragraphsabove. t renol i te 1 ni zz onite 65 DIOP Zone. The Drop zone is defined by the in- epidote L5 55 nermost appearance of a rock of )50 percent Mg- garnet 21 diopside,and containsdiopside, , calcite, actinolite CU I lmenite >2 >2 >2 >1 and forsterite. The center ofthe zone is 90-95 percent s r'hene >2 5 diopside and 5-10 percent wollastonite. Near the Suite 2 | 85 7r r)I r)z\Ept) I29 boundary with the Mln zone 2-5 percent each of wol- quarl z ?o 10 3a calcrte 604 '_2 fastonite and calcite, or 2-3 percent each of calcite d olomite _2 _5_ and forsterite, are present. liid-di^neida < on The diopside is identical to that presentin the Me,n forsterite 35 wollastonrte 4 zone. The forsterite has been extensivelyreplaced by trenolite 1 serpentine and has the same composition as the mtzzonil,e e pf dote )) forsterite present in the Mln zone. The wollastonite garnet 85 is fibrous and fluorescesblue-green. Its composition actinolite 40 ilnenite >2 is very nearly CaSiOr, with I wt percent or less each s phene 2 of Al and Fe. Szhnlo..dara Itkon l'rnm nonrFrc.,T r^ha< 6y^arrl ACT Zone. A sharp boundary exists between the f or: 66, taken from 1'!il!il/DI0P boundary; 71, L"ken close-;;';;";t fo Lha L",;;-f boun.l. r\/ r l / <. , ) Fa n^tr Dlop and Acr zones, definedby the innermost ap- Diiphcr ; ii,",, r. .rcri"' "i'.",.' pearance of green to yellow-green actinolite and boundary. KENNETH SHAY dary. Quartz plus calcite constitute 25 percent of the quartz, biotite, oligoclase, K-feldspar, and Fe- zone. diopside are commonly observed in the quartz mon- The actinolite is fine- to medium-grained, exten- zonite. For [-7 cm beyond this transition zone, the sively intergrown with calcite, and is commonly qtrartz monzonite contains up to 5 percent Fe- found replacing Mg-diopside. The average compo- diopside. Veins of epidote less than two millimeters sition of the analyzed grains is Ca, rMgr.uFet.. across cut across the Sclp zone, running from the Alo.usi,uOrr(OH), with trace amounts of Mn, Na, quartz monzonite to the Ep zone. Similar veins are and Ti. The epidotecrystals are medium-grainedand presentat contacts with schist and gneissall over East amoeboid, with inclusions of quartz, calcite, and Ridge; none are present at Area l. the ferroan diopside, Fe-diopside. Composition Three compositional varieties of scapolite are pres- within a single grain may vary, but no systematic ent at East Ridge. The predominant scapolitesof the zoning was observed.Throughout East Ridge, the Sce.pzone, and the only ones in the adjacent quartz average epidote composition is consistently CazAlz.r monzonite, are colorless,euhedral, and up to 4.5 cm Feo.'SirO'r(OH). in length. The largest are spongiforri, con- EP Zone. The outermost occurrenceof actinolite sisting of 40 percent inclusions of Fe-diopside, marks the boundary betweenthe Acr and Ep zones. qvartz, sphene, and a second scapolite, as well as The Ep zone can be broken down into three sub- of the quafi.z monzonite, as noted. zones, none of which shows a preferencefor any par- Myrmekitic intergrowths of quartz and veins of ticular position within the zone.The leastcommon of calcite are abundant. The host scapolite grains are these,the Eeg, is composed almost entirely of garnet, homogeneous,with composition Car.eNar.rAlr.rSir.t with 10 percent quartz and calcite. The garnet is ex- Orn.(CaCOr)o.r.The secondtype of scapoliteis found tremely coarse-grained,with crystalsas large as 2.5 only as fine-grained, anhedral, polysynthetically- cm. Grains are enrichedin Mn in the centerand Fe at twinned inclusions within grains of the first type. the rim; the averagecomposition is GruuAndrnu,with This scapolite has composition Ca'.,Nat.rAlo.,Sir., minor Mn and Ti. Inclusions of Fe-diopside are Ozn.(CaCOs)g.'.The third type is the Cl-bearing va- abundant,as are inclusionsof quartz and thin calcite riety in the Mnn zone veins. velns. ln the East Ridge scapolites,the coefficientsof the A second, more prevalent subzone, the Ete, is CaCO, and NaCl ideally should total 1.0,but based composed almost exclusively of extremely coarse- on 34 analyses,Shaw (1960) has found a slightly grained, euhedral epidote with the same composition lower value, 0.855.Analyses in which the sum is less as the epidote of the Act zone. Inclusions of quartz than this value could be indicative of variable and Fe-diopsideare fairly common. amounts of K2COg,K2SO1, and KOH, for which no The most common subzone,considered as general- analyses were performed. Other possible explana- ly representativeofthe zone and therefore designated tions for the low sums are vaporization by the analyz- simply as Ep, consists of 25-70 percent epidote and ing beam, natural deviation from the ideal formulae, 10-60 percent garnet, with 10-20 percent quartz and or poor analyses.The composition of a scapolitecan l0 percent calcite. The garnet and epidote have com- be approximated as a solid solution between positions identical to those of the Epe and Erg sub- , CagAl.Si.Ozo'CaCOg(Me), and , zones,and seem to have formed simultaneously,as NarAlrSirOrn.NaCl (Ma). Intermediate speciesare seen by intergrowths between the two minerals. The named dipyre (Mero-uo) and mizzonite (Meb'-s'), grains tend to be extremely large and anhedral, and with appropriate prefixes to designate the "ligand contain inclusionsand veinslike thosedescribed. The molecule," e.g.,NaCl (Solodovnikova,1951). To dis- epidote:garnet ratio increasesslightly from north to tinguish the different scapolites of East Ridge, these south. specific names will be used: dipyre for the sodic va- SCAP Zone. The Scep zone, as mapped, is riety, mizzonite for the calcic. bounded on the intrusive side by either the qvaftz Other Features. Mineralized blocks <0.3 m monzonite or by aplitic quartz-feldspar veins and on across, of what were originally marble, are present the Ep zone side by the innermost occurrence, other throughout the quartz monzonite surrounding the than the Men zone veins, of scapolite. In fact, main skarn bodies. These xenoliths lack scapolite, however, the Sce,r-quartz monzonite contact is not epidote, and garnet, having insteada zonation from clearly defined, because euhedral, spongiform inner marble, like that of the Mnn zone, to a zone of crystals of scapolite up to 2 cm long surrounding Mg-diopside. This is in turn surrounded by a thin ZONING IN A SCAPOLITE-BEARINGSKARN BODY 791 zone of dark-greenactinolite (too thin to be shown in chemicalcompositions of the rocksof the two East Fig. 2) which lacks calciteintergrowths. The smallest Ridge suites,and quartz monzonitesamples up to of thesexenoliths lack marble cores. 2.13m (7 ft) from thecontact, are shown in Figure4. The contact sequenceat Area I was studied for Analysesof quartz monzonitecollected further than comparison.Here the intrusive is separatedfrom the this from the contactare consistent with thosefrom I marble by a band of dipyre (dr zone), followed by a to 2.13m. zone of Mg-diopsideand wollastoniteidentical to the Dtop zone at East Ridge.The diopsideand dipyre oc- Discussion cur together over a width of 0.6 cm, and dipyre is presentin the quartz-enrichedcontact betweendr and In orderto discussthe processeswhich are respon- quaftz monzonite. The widths of the zones vary but sible for the East Ridge zonation,a rough sequence are approximately in the ratio dr: Dtop : 2:1. of events leading to the presentmineralogic con- Bulk Rock Composition Variations in the figurationmust first be established.Petrographic and

CoO-aOO al2o3- O o O MgO-VVV a Abol o\ No2O-YV V .o

vv

3'8u&,ila.:: g: FQ'' g tv si02 lo. ,'\ --1% V?rfrcol oros or? rcighl 7Js c---Srl(ii ot indicoled oride3 ,ft'

\ 7va \ \ 'v wav v $7'F?V V d'{/ -wW /

Ftc. 4. Bulk chemical analysesofSuites I (colored in) and 2, and quartz monzonite up to 7 ft from contact. Dashed lines connect Suite I rocks and do not necessarilyrepresent the character of variation between points. Dots within circlesrepresent analyses of rocks of neither Suite I nor 2. 792 KENNETH SHAY

field evidence show that there were three different Although, in principle, movement relative to one episodesof metamorphism: reference frame can be related to movement relative Step l: Regional metamorphismof a calcite-dolo- to any other, certainreference frames may be more il- mite-quartz marble formed tremolite and forsterite. luminating than others for a given situation. The This is suggestedby the similar composition, but problem then, is to choosethe one best suited'to the differentmineralogy, of the Area I marble (seeFigure particular constraintsof the system.Brady (1975)has 6). discussedseveral possibilities, including (l) mean Step 2: The next step, which accompanied the in- volume referenceframe, (2) K-h-component reference trusion of the quartz monzonite, was the formation frame, and (3) inert marker referenceframe. of two mineralogic zones, similar to those seen at Mouement of Components; Original Contact. The Area l, between the intrusive and the marble. mean volume referenceframe is most useful for con- Contemporaneouswith the formation of thesezones, stant volume processes,and so doesnot lend itself to tremolite * calcite in the marble reacted to form the problem at East Ridge, becauseof the volume diopside * forsterite + COr. Following this, Fe- loss owing to decarbonationreactions. However, the diopside and then mizzonite were formed in the outer only requirement for the r(ah-component reference (dr) contact zone and in veins in the marble. These frame is that the flux of some r(-n-component with minerals are not present at Area l, indicating that respectto that frame is zero. In other words, using this locality may representa preliminary state of East this frame is the sameas consideringone component Ridge. as motionless,and comparing the net movementsof Step 3: Epidote, garnet, actinolite, calcite, and other componentsrelative to it. In discussingsuch a quartz formed by reactions between late-stage transferof componentsat EastRidge, it is convenient magmatic fluid and contact minerals. to expressthe componentsas oxides,because only the net movement can be deduced,and the oxide form between as- Step 2 simplifies chemical comparisons semblages. The two initial contact zones of Step 2, Ihe dr and If a K-h-componentwhich truly did not move with the Dtor, formed by diffusion and reaction between respect to an inert (non-diffusing) marker frame is the intrusiveand country rocks.To betterunderstand present,of if some inert marker like is pres- the growth of the zones, we must first establish a ent (Vidale, 1969),then the location of the original referenceframe to which we can relate the motions of contact is easilydeduced. Probably all of the compo- componentsand zone boundaries.The readershould nents do move relative to an inert marker frame note that only net relative movements can be deduced (Vidale, 1969),so it is conceptually most useful to in this study, because only the final state, and choose as the r(ah-componentthe slowest,or "most questionabledata on the initial state, are known for nearly motionless" component. Previous workers the system.Determination of detailedmechanisms of (e.g., Carmichael, 1969)have argued that this compo- formation (e.g., whether the components diffuse nent is AlOs/2. Assuming that the AlOs/" reference through a crystalline or fluid medium), and values for frame coincideswith an inert marker frame, it follows fluxes and diffusion coefficients of components, re- that the original contact is within the border zone ly- quire experimentalstudies. ing between the dr and Dtop zones, which at East

Terlr 5. Compbsition of Zones (in Moles/100 cm3)

IVIAR DI OP ACT -bP SCHP A-vg. Qtz. Monz.

qA f)anciirr (on/to\* 2.94 3.24 z.oo 3.t0 ? a. >o 2 .64 si0^ .545 2.714 ? ? ?l 2.73 2,T82 a +At^o^ . 011 .IT7 I.zLA .408 , yo:) r.244 .722 Ca0 1.805 r.49r .590 r.289 L.274

Mgo 1.O24 r,)67 .489 "uoa .024 #Based on da*ua of Robie and Wa]dbaum (1968) ZONING IN A SCAPOLITE.BEARINGSKARN BODY 793

Ridge is within the Acr zone. The movements of content. The plot clearly indicates the increase of 4s16, other components relative to the AlOa/2 reference from the marble to the dr zone, and decreaseof agus frame have been tabulated (Table 6). These data sug- over the samepath. It is clear that CaO and SiOzhave gest that there was a total volume loss in the contact indeed diffused "down" the potential gradients (Fig. zone of 56.5 percent. Figures based on the MgO 5b,c). referenceframe have been computed for comparison. Treating each zone as an open system in ther- The volume loss due to decarbonationmay be es- modynamic equilibrium (Thompson, 1959), it has timated by a considerationof the CaO contentsof the been shown that for an arbitrary temperature and quartz monzonite and dr zone.Specifically, it may be pressure, the minimum number of "inert com- assumedthat in the dr zone, the CaO concentration ponents" (those componentswhose chemical poten- (increased by 56.5Vo) greater than that of the tials are controlledby the phases-Korzhinsky, 1959) similarly-adjustedquartz monzonite is due to the ad- is n for an n-phasezone. Variation in r, and therefore dition of CaO that was once presentas calcite.Using in the number of phases,from one zone to the next the data in Table 5, the volume loss due to decar- may be due to an inert component becoming bonation in the formation of the dr zone is calculated "mobile" for a zone (seeVidale and Hewitt, 1973). as 19.5 percent. It is worthwhile to discuss(l) why the assemblages Driuing Forces of Metasomatism. Thompson depicted in Figure 5a only approximate but never at- (1959) and Vidale (1969) discussedmovement of tain the path along the line calcite-diopside, to the componentsdown a gradient of "non-volatile com- point Diopside; and (2) why it is this path that is ap- ponent" (e.g.,SiOr, MgO) activity. Hewitt (1973)has proached. The second point is fairly obvious: to get shown that mineralogic banding may be due to a between two divariant, single phase points (one of gradient in "volatile" (e.9., HrO) composition as which is Diopside in this case), the system must at well. Both mechanismsare possiblefor East Ridge: somemoment consistof the two end-pointphases. At there was most certainly somevariation in fluid com- that time the system loses a degree of freedom and position between the COr-rich fluid of the marble and becomes univariant, and at a given P and ? is the HrO-rich fluid of the ;and the variation in therefore constrainedto lie somewherealong a uni- "non-volatile" composition between the SiOz- and variant path (in this case,a path of decreasing&c'o AlOrTr-rich quartz monzonite and the CaO- and and increasing ;rrtor). To understand the first MgO-rich marble makes a non-volatile component point, we must realize that for an /?-componentsys- activity gradient possible.The following discussion tem in which m I n - I components are varying inde- demonstrates that both gradients have affected the pendently, (n-n-1)-phaseregions will be separated East Ridge rocks. by (n-m)-phaseregions. In Step 2 at East Ridge, the In Figure 5a the bulk compositionsof the Step 2 systemis merely approximatedby SiOr-AlO372-MgO- zonation, from quartz monzonite to marble,are plot- CaO and Figure 5 is therefore merely a projection of ted against molecular CaO, MgO, SiOz, and AlOs/2 the (n-m-l)- and (n-ni)-phase regions onto the three-

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

MgO frame;

dr zone DIOP zong DI0P zone

,Y.o/7o oI orrqlnal II. LL-/o01 orlglnal 74.90% of original rrnlrrmo af alo v r I v! vol-ume of marble volume of marble

Addition of cornponents: Additj.on of components: ? Addition of conponents: sio2 -3.149 moles/1co cm3 Si02 -1.65I notes/IoO cm' SiOa +1 .986 noles/fO' cm; CaO + .483 Cao -),4 .7 6 i-At^o^+.too MgO - .040 lvlgO -7 .85 Cao -.9L9 FeO - .062 Fe0 -.065 Fe0

*Na^o'z + .t36 lNar.o -.1b6 *Na-0-a (+) = into zone (-) = out of zone KENNETH SHAY

(si02) asshown by Greenwood(1969). Therefore it appears that a variation in fluid composition betweenan HrO-rich fluid in the magmaand a COr-richfluid in the marble, determinedthe presenceor absenceof wollastonitein the contactatea. Formationof Mizzoniteand Fe-Diopside. It is clear (AleOs) that mizzoniteformed after the formation of the dr and Dtop contactzones and the Fe-diopsideby the \:Y-'-, inclusionsof Fe-diopsideand dipyre in themizzonite. WhetherFe-diopside formed during or after the for- mation of the dr zone is unclear.and thereforetwo _------+ hypothesesfor the formationof the two mineralswill be discussed.Both accountfor the fact that the net resultof this process,which mostly involvedchang- ing the dr into a ScrrPzone, is the addition of 0.1 moles/100cms SiO2, 0.03 moles/100 cm8 CaO, 0.13 moles/100cm3 FeO, and 0.06moles/l@ cmsMgO, basedon the AlOsrzreference frame. t One possibilityis that the CaO, MgO, FeO, and SiO, rejectedfrom the Dtop zone first formed Fe- diopsideconcurrently with the otherStep 2 processes; ouorlz monzonife morble thesecomponents precipitated pyroxene in the adja- cent quartz monzoniteas well. CaO reactedwith the dipyre to form mizzonite and quartz: (roughly) t 5CaNarAlnSi'Orn.(CaCO')o.r* 2CaO -t 2.1 CO2- 4CarNaAluSi'Orr.(CaCO')'.'f 3NarO * l2SiOr. Mizzoniteformed in the quartz monzoniteat the ex- pense of plagioclasein much the same way, ac- countingfor the vein-likecharacter of the Scepzone

Frc. 5. a. Hot or oll-n.-Oiopside Step 2 compositional variation. (Fig. 2). This processfollowed the formation of Fe- Dash-dot line is possible complete variation. O, analyzed samples; diopsidebecause SiOz must be diluted for (l) to E, observed at Area l; A, observed at East Ridge. b. Schematic proceedas written.An alternativepossibility is that diagram of variation in &s1q and trrs.efrom quartz monzonite to an Fe-Mg-Si-Ca-bearingfluid phase, perhaps marble, initiatly; and c. after formation of contact zones. releasedby the magma,reacted with the initial Step2 configurationto form Fe-diopsideand mizzonite. The major drawbackof the first possibilityis that dimensionaldiagram; falling directly on any 4-com- the two mineralsare not presentat Area 1; unless ponent,univariant line of Figure 5 would be for- conditions there were considerablydifferent from tuitous. thoseat EastRidge, it is not obviouswhy Fe-diopside The role that the volatile phase composition and mizzoniteshould not haveformed at Area I un- gradientplays concerns only the presenceor absence der this model.This drawbackdoes not existfor the of wollastonite,and is thereforeminor comparedto secondpossibility, but anotherdoes: why is no other the non-volatileactivity gradients.Inspection of evidenceof this fluid, such as veinscontaining Mg- Figure 6 showsthat for the pure systemat a fixed bearingminerals, seen in the quartz monzonite? Xs6",the temperaturemust be increasedfrom I to a The problemmay havea solutionin light of the temperaturewell abovethe curveTr * Cc : Fo * Di zonedFe-diopside/mizzonite veins of the M,cnzone. + CO, + HrO, if wollastoniteis to becomestabilized The mineralsof theseveins are virtually identicalto relative to calcite and SiOz. At or below the the Fe-diopsideand mizzonite of the Sce.t zone, temperatureof the intrusion (for our purposes,a which would suggestan identicalorigin. The Men temperatureon or just abovethe curve Tr * Cc : Fo zone veinsare merely quafiz monzonitesills which + Di + CO, + HrO), wollastonitewill be stabilized havereacted with the marbleat constantuolume of the by a decreasedXgq, in the fluid phase(arrow, Fig. 6), uein, as can be shown by a straightforwardcom- ZONING IN A SCAPOLITE-BEARINGSKARN BODY IYJ

Xcoz

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

parison: the averageAlos/z densityfor the velns ls from Drop to Acr. A 30 percentvolume increase and 0.715moles/100 cm3, and the averageSiO, density is the addition of 1.59 moles/100cms SiOz, 0.82 3.052moles/100 cm'. From Table5, the analogous moles/100cma CaO, 0.32 moles/100 cms FeO372, and figures for quartz monzonite are 0.722 and 3.022, 0.26'lmoles/ 100 cms H2O accompanied the transition respectively.CaO doesnot matchbut could be ac- Sclp to Ep.These figures, and the presenceofepidote commodatedby the decarbonationof a volumeof veinsrunning from the quartz monzoniteto the EP Men zoneequal to 60 percentof the vein, making a zone, suggestthat the Step 3 mineralizationwas total volumeloss of 38 percent. causedby a late stagefluid which may have been This process,operating in the Men zoneveins to liberated from the cooling qtaftz monzonite.That form Fe-diopsideand mizzonite,is probably the this fluid appearedwell afterthe initial intrusionis in- sameprocess responsible for the formation of these dicatedby the presenceof calcite* quartz,which re- mineralsin theScep zone as well. The figure for total quires low temperaturesin the presenceof a high volumeloss for the Men zoneveins does not match Xs"6fluid (Fig. 7; Greenwood,1967). the figure calculated for the DIor/and dr zones fhe chemicalpotential of SiOzis fixed in the Acr (56.5To),possibly owing to differencesin contact and Ep zones becausequartz is present.At con- geometry.The absenceof Fe-diopsideand mizzonite stant temperature,pressure, and fluid composition at Area I remainsunexplained. (which fixesg.6ue as calciteis also present),the only componentswhose chemical potentials might provide Step 3 the varying conditions necessaryfor the observed The formation of actinolite, garnet, epidote, mineral segregationsare aluminum, (expressed calcite,and quartzclearly followed Steps I and2, as herein as FeOrTr),'andmagnesium. Figure 7 is a seenby petrographicand field relations.Likewise, sketchof the stability fields of the East Ridge ac- thesefive mineralsseem to have all formed at the tinolite, epidote,and garnet with respectIo ltr;o"n, sametime and by similarprocesses, such that the out- pFeostzsand;ry"e, at fixedP, 7, fluid composition,and er part of the Dlop zonebecame Acr, and the inner CaO content of the phases.This diagramillustrates part of the Sc,qpzone becameEp. Basedon the why only epidoteand actinoliteare presentin what AlOsrzreference frame, a 70 percentvolume increase, was originallyDIon zone(high &ugo), and why epi- and the additionof 1.93moles/100 cms SiOr, l.12 dote predominatestoward the Scep zone (higher moles/100cm3 CaO, and 0.1moles/100 cm8 FeO and poto"n).Note that Lhe f, may have played a 0.243moles/ 100 cm3 HrO accompaniedthe transition role here as well; all of the iron in the epidoteand 't96 KENNETH SHAY

regular,concentric zonation; and veins ofepidote are presentthroughout the surroundingquartzite, schist, and gneiss.These featuressuggest that infiltration metasomatism,and thereforepermeability, played a key role in the Step3 process,as then mineralization would only occur where the rock was sufficiently permeableto allow reactivefluids to flow throughit. If this was so, then the permeabilitywas evidently greateston either sideof the dr/Dtop contactzone, droppingoff rapidly with distance. Conclusions Three stepswere involvedin the formation of the East Ridgeskarns. The first involvedthe formation of tremoliteand forsterite at theexpense of dolomite, calcite,and quartz,which occurredduring regional metamorphism. The secondand third stepscan be chronicledusing a fixed-componentreference frame. The secondstep beganwith the intrusionof the CactusQuartz Mon- zonite,which raisedthe temperatureof the marble, Ftc. 7. Sketch of the stability fields of the East Ridge actino- lite, garnet, and epidote, as they vary with puso, peros,,, and stabilizingthe assemblagediopside * forsterite.At Preo",,,zl fixed P, l, fluid composition, and CaO content. thequartz monzonite-marble contact, a zoneof diop- side and a zoneof dipyre (sodicscapolite) began to form aboutthe contact,due to the diffusionof Ca, garnet is ferrous, whereas only 68 percent of the iron Mg, and Si. Wollastonitebecame stable relative to in the actinolite is in the reduced *2 state. calcite + SiO, near the marble, due to a high Xs,o ln the inner Scep zone,p\ags is low, and epidote fluid phase.The componentsexpelled from the con- and garnet are stable relative to actinolite. The tact zone formed Fe-diopsideand reactedwith the differences in stability conditions between epidote dipyre to form mizzonite (calcic scapolite) and and garnet are more clearly shown by Gordon and quartz.The Step2 processinvolved a 56.5percent Greenwood's(197 1) work on the pure aluminum end volumeloss. members, zoisite and grossularite. This work es- In the third step,mizzonite of the inner Sclp zone tablished both grossularite'sinstability relative to and Mg-diopsideof the outerDtop zonereacted with zoisite at high Xcs" of the fluid phase, and the wide a late-stagefluid liberated from the quartz mon- stability range of zoisite. Assuming that these rela- zonite,forming epidote,garnet, and actinolitewhere tions hold qualitatively for the iron-bearing system, infiltrationof the fluid was possible. the Epe subzonesmay representpockets of high Xs* fluid and the Eeg, pockets of low Xsq fluid or, from Acknowledgments Figure 6, low Fero"1,.Either condition could result This paper is a summary and extension of an undergraduate from local concentrationsof dipyre inclusions (see thesissubmitted to U.C.L.A. in June, 1974.I am gratefulto W. G. (1)). The southward-increasingepidote:garnet ratio Ernst and Douglas Rumble III for their assistanceand direction, observed at East Ridge may be correlative with the and I am particularly indebted to John B. Brady for the ex- ceedingly generous amount of time he spent on my behalf. I thank decreasingdegree pervasive of intrusion in the same W. G. Ernst, D. Rumble III, T. M. Gordon, D. A. Hewitt, and direction, for the fluid phase was probably richer in J. B. Brady for their comments on preliminary drafts, and I owe CO, going south, favoring epidote. especial thanks to an isolated system whose components' non- A questionwhich remainsis: why haveepidote and ideal behavior increasedmy activity. This work was supported by garnet not replaced all of the ScAp zone, and ac- a U.C.L.A. Undergraduate Fellowship, U.C.L.A. departmental funds, and an NSF Graduate Fellowship. tinolite. all of the Dtop? Various features indicate that the causeis not chemicalin nature:epidote is ap- References parently stable under the conditions of the Sclp AllrN, C. R. ( 1957)San Andreasfault zonein , judging zone, by the thin epidote veins; the Ep zone . Geol. Soc. Am. Bull.68, 315-350. occursas small pods in the Scep zone,apart from the ALBEE.A. L.. ,.,No L. Rlv (1970) Correction factors for electron '197 ZONING IN A SCAPOLITE.BEARING SKARN BODY

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3L a