Metasomatism, Titanian Acmite, End Alkali Amphiboles in Lithic-Wacke
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Amefican Mineralogist, Volwe 70, pages 499-516, 1985 Metasomatism,titanian acmite, end alkali amphibolesin lithic-wackeinclusions within the Coyote Peak diatreme,Humboldt County, California GsRAro K. Cz,c,MANsKEAND SrevEN A. ArrrNr U.S. Geological Suruey 345 Miilillefield Roail, Menlo Park, California 94025 Absfirct Lithic-wacke inclusions within the alkalic ultramafic diatreme at Coyote Peak record a remarkable history of metasomatismand crystal growth. Dominant metasomaticchanges were loss of Si from the inclusionsand massinflux of K, due to an unusuallyhigh K activity in the ultramafic host. Reactionwith K convertedmuch of the clay, quartz, and lithic fraction of the lithic wackeinto microclineand exchangedNa from abundantclastic albite. Sodium releasedfrom albite: (1) combined with Ti and Fe to form myriads of acrnite cystals, many of which are strongly zoned with cores unusually rich in titanium; (2) com- bined with Ti, Fe, Mg, and Ca to form alkali amphiboles,which are zoned from fluor- richterite cores to unusual titanian-arfvedsoniterims; and (3) formed a late-stagezeolite similar to natrolite. The sequenceof ferromagnesianphases is interpreted to have been controlled partly by falling temperature,but dominately by falling /o, as the oxidized sedi- mentaryassemblage equilibrated with the reducedultramafic melt. Preservationof primary sedimentarytextures to within a few millimeters of the lithic- wacke/ultramaficinterface in zoned inclusions and strong zonation betweenthe cores and rims of individual phasessuggest that recrystallizationand chemicalexchange were of short duration becauseof rapid emplacementand cooling of the diatremeat shallowdepth. Introduction reaction process.Rocks characterizedby both mineral as- contain discrete coarse- An alkalic, ultramafic diatreme 260 by 500 m in outcrop semblagesare black; they also grained (to 6 cm) composed of phlogoPite, garnet area penetratesa lithic-wacke sandstonesequence of the clots (zoned TiO2), minor sodalite and pec- Franciscan assemblage,20 km SE of Orick, at Coyote from 12 to 2 wt.% pyrrhotite, least five K-Fe and Na-Fe sulfide Peak,Humboldt County, California. Rocks comprisingthe tolite, and at (Czamanskeet al.,1979,1980, and 1981;Erd and diatreme may be conveniently subdivided into uncon- minerals 1983). mineral assemblages,pris- taminated and contaminatedvariants, based on SiO, con- Czamanske, Considering Peak might bestbe termedmodlibovite tent, color, mineralogy, the character of contained clots, tine rock at Coyote (Scheumann, rocks are greenish in and abundance of inclusions. Poor exposures do not l9l3). Contaminated of the reactedmineral permit these variants to be mapped separately.Uncon- color, but essentiallyare composed rocks. Also pres- taminated rocks comprise two distinct assemblages,each assemblagefound in the uncontaminated pectolite featherygarnet that is containing phenocrystsof subhedralto rounded olivine (to ent are and natrolite, and a (2.6 to 6 2 mm; reversezoned from Foro to Forr; NiO:0.M, less Ti-rich [8 to 1 wt.%) and more aluminous wt.%) than garnetin uncontaminatedrocks. Contaminated MnO:0.4 and CaO:0.5 wt.%) and anhedraltitano- a totally different type ofzeolitic clot, rich in magnetite(zoned from 13 to 9 wt.ohTiO, and from 5 to 4 rocks contain acmite, alkali amphibole, biotite wt.% MgO). Groundmassphases in the most pristine as- natrolite and containing (Xpnro, and barytolamprophyllite (8'E to semblage are phlogopite (xonrog: 0.8 to 0.9), melilite to 0.,14),titanite, nepheline(NetrKs2r), 26wt.%BaO). [(CarFe)rr(CarMg)57(CaNaAl)rr], reacted fragments of magnetite(TiOz < 7 and MgO < 2 wt.o/o),perovskite, apa- The contaminated rocks contain inclusions of lithic wacke tite, sodalite, and poikilitic garnet containing up to 16 mudstone, as well as rounded at the time of wt.%oTiOr. A secondassemblage, characterized by coronas picked up from the Franciscanassemblage lithic-wacke fragments have under- of pyroxeneplus phlogopitearound olivine, containsessen- emplacement.These exchangewith the diatreme,in- tial augite (WoroEnn.Fst) and nepheline(Ne^Ks2r) and gone substantialchemical potassic metasomatism.Small (less than lacks melilite. Study of many thin sectionsshows the as- cluding extreme which are uniformly pale gray semblagesto be relatedto one another by an evolutionary, 3-cm diameter) inclusions, green,initially were thought to representfine-grained po- tassicigneous rocks (Czamanskeet al., 19771.Larger (diam- r Presentaddress: Molycorp, Inc., Questa Division, P. O. Box eter 6-10 on) fragmentscommonly have brown cores and 469,Questa, New Mexico 87556. distinct gray-greenrims, 8 to 12 mm thick, that closely 0003{xxx/8 5/050H499$02.00 499 500 CZAMANSKE AND ATKIN: TITANIAN ACMITE ALKALI AMPHIBOLES IN INcLUsIoNs contactsof the diatreme are not exposed.The mineral as- semblage in the brown cores of the largest inclusion (sampleCYP101) comprises dominant compositionallyin- termedite alkali feldspar, a stilpnomelanelike phase (see below),and minor qtrartz.Relic clasticgrains of quartz and partially exchangedalkali feldspar preserve sedimentary bedding. Interstices between relic clasts are filled with turbid intermediate alkali feldspar, similar in composi- tion to that of the partially exchanged clasts. The stilpnomelaneJikephase (pleochroic in amber hues)occurs as xenomorphicbundles of platy or acicular grains (maxi- mum 0.5 mm) scatteredthroughout the matrix. This phase may also occur, however,as delicateplumes that have nu- cleatedalong the margins of, and appear partly to replace clasticquartz grains(Fig. Fig. 1. Specimen from the Coyote Peak diatreme containing 2A). four lithic-wacke inclusions- Inclusions on left and upper right are Boundaries between the brown cores and gray-green typical of the more uniformly altered, small inclusions (e.g., sample rims of larger inclusions are distinct (Fig. l); this color CYPS); the two larger, rounded inclusions show distinct core and change apparently reflects the presenc€ of the flm components. stilpnomelane-likephase in the core assemblageand of acmite and alkali amphibole in the rim assemblage.The rims of the larger inclusionsare mineralogicallyessentially identical to the more homogeneous,smaller inclusions and resemblein color and texture the smaller,more uniformly consistof titanian-acmite,alkali amphibole,alkali feldspar alteredinclusions (Fig. 1). (which gradesto K-rich microcline as the contact qrith the Due to the extremedisparity in compositionbetween the host diatreme is approached),a natrolite-like zeolite, a fi- ultramafic host and the included lithic-wacke fragments brous unidentified Ca-Na silicate, and rare wollastonite. (Table l), this study provides a specialopportunity to de- The disappearanceof the stilpnomelanelike phase at the scribe pronounced metasomaticexchange. Resulting min- core-rim boundary is abrupt, but there is no microscopic eral assemblagescontain titanian acmite and alkali am- evidencefor any singular reconstitutionreaction. Feldspar phibolesof unusualinterest. compositions do not change abruptly at the core-rim boundary, and remnant clastic alkali-feldspargrains of in- Petrography termediate composition may persist well into the rims. We selectedeight samplesfor study,including: (l) uni- Feldsparsbecome progressively less sodic toward the edge formly altered small inclusions(CYP8 and 103); (2) a of a given inclusion, and individual grains are typically medium-sizeinclusion (CYP52); (3) large inclusions with zoned, with rims that are enriched in K relative to their distinct core and rim alteration facies(Cyp7, 101,102, and cores. Toward the interface with the matrix, feldsparsin 178);and (4) a sampleof lithic-wackecountry rock (Cyp4) some larger zoned inclusions, and in most smaller in- collectedfrom a surfaceoutcrop a few meters beyond the clusions,are thoroughly recrystallizedinto mosaicsof xe- diatreme contact (which is nowhereexposed). In addition, noblastic to idioblastic grains of maximum microcline (as sampleCYP5 was selectedas representativeof the uncon- determined from its powder-diffraction pattern). Alkali taminatedultramafic melt. feldsparmay display turbid cores and clear rims where it The country rock (CYP4) is a light-tan well-induratod contacts the natrolite-like zeolite (Figs. 2C and 2E). This massivefine- to medium-grainedmoderately sorted lithic developmentof a more nearly uniform, granoblastictex- wacke. By volume it is composedof quartz (20%); albite ture obliteratesall relic sedimentaryfeatures. (10%; Ab"" u); lithic fragments,including shale,chert, and Titanian acmite first appearsat the core-rim boundary minor volcanic- and metamorphic-rockfragments (40%); of large inclusions,where chains of acnritegrains, 20 to 60 and a turbid clay-rich matrix (25o/o).Altered biotite, chlor- pm on an edge, commonly surround relic feldspar and ite, and muscovite(total, approximately 5oh)arc also pres- quartz grains (Fig. 2B). As the rim is traversedtoward the (Table ent 6). Franciscangraywacke of the northern Coast inclusion/ultrahafic interfa@,acmite becomeswidely dis- Rangesof California is noted for the absenceof potassium tributed and ever-larger idoblastic grains or clusters of feldspar(Bailey et al.,1964),and no potassiumfeldspar was idioblastic crystalsare found, although fine grains persist. seenin sampleCYP4. Scattereddark-gray shalechips give The domain of alkali amphibolecrystallization appears not the sandstonea salt-and-pepperappearance and, along to extendquite so far inward from the ultramafic/inclusion with other elongateclasts, create