American Mineralogist, Volume 60, pages 785-797, 1975Mineralogical Zoning in a Scapolite-Bearing Skarn Bodyon San Gorgonio Mountain, CaliforniaKrNNnrH SHnylDepartment of Geology, Uniuersity of California,Los Angeles, California 90024AbstractConcentrically 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) Infiltration of fluids, possibly of magmatic origin,formed epidote and garnet from scapolite, and actinolite from diopside.IntroductionThis 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 localequilibrium (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 StudyThe 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 are785786KENNETH SHAYI c.*ro,c ..di."nt"E Mrercic inrrulinsffi eotro:oic rcatn.nrr ond votconic!ljll uctomorotric ond Prccombrlon rocksrO O tO zomilesrO O lO zokrlomc!.rsFrc.l.Index 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 ComplexFrc 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-ol?\-^\ol4\tZONING IN A SCAPOLITE.BEARING SKARN BODYTrslr l. Microprobe Analyses in Weight Percents787Smple No. *Location**si02A1 2O3Ca0Na2oFe as FeOMcoTi02MnOc1HzoTotalSarple No,iLocation* *si02A1203Ca0Na2OFe as FeoMgoTio2Mn0clHzoTotalSarPle No."Location**sio2Ar203Ca0Na20tse as t6uMgoTio2I'hOHzoTotalAC05902 AC12402 ACr2409 4C12410 ACl2503ACT ACT ACT ACT ACTs1.5 52.7 52.4 55.8 51.43.9 2.3 1.5 .8 1.4L2.4 L2.8 !2.7 l3.l L2.60.0 .2 ,3 tr .5t3.2 L2.3 tz,a 6.9 14,713.5 r5.8 16.0 L9.7 rS.5.2 .2 .l 0.0 tr.8 .5 .6 .5 .6na na na na na3.4 3.2 3.6 3,2 3.2100.0 r00.0 100.0 100.0 100.0ACr2504 Drol70l Dr01704 Dr06602A Dr066028ACT SCAP SCAP DIOP DIOP53.6 51,4 52.0 54.2 54.0.9 .7 .4 .4 .3t2.7 24.3 24.4 25.2 25.5r3.3 1L.2 10.8 2.r 2.016.9 11.2 11.4 17.5 L7.60,0 tt tr tr tr.5 .7 .6 .3 .2na na na na nar.7 0 0 0 0100.0 99.7 99.8 99.7 99.6Dr06603A