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Amphibole Pairs, Epidote Minerals, Chlorite, and Plagioclase In

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Amphibole Pairs, Epidote Minerals, Chlorite, and Plagioclase In American Mineralogist, Volume 59, pages 2240, 1974 AmphibolePairs, Epidote Minerals, Chlorite, and Plagioclasein MetamorphicRocks, Northern Sierra Nevada,California' ANNI HtereNBN U. S. Geological Suruey, Menlo Park, Calilornia Electron microprobe analysesby G. K. CzannNsxB Abstract Two kinds of amphiboles-actinolite and aluminous Ca-amphibole-and two to three epidote minerals coexist with chlorite and plagioclasein the meta-volcanicand meta-igneousrocks in the contact aureoles of the Cretaceous plutons in the Feather River area, northern Sierra Nevada. Small prisms and rims of actinolite occur with green hornblende outside the l-mile- wide contact aureoles of the plutons. In the zone next to the plutons, blue-greenhornblende forms rims around large grains of actinolite and also forms small prisms that are included in plagioclase.Electron-microprobe analyses show that the alkali and Al content of the amphi- boles increaseswith an increasein the grade of metamorphism, the ratio of NafK to tetra- hedral Al remaining close to 1:3. The ratio between two coupled substitutions, edenitic (NaAAlt' for nASi'") and tschermakitic(Al"t Al"'for Mg'r5;t"r, remains3:7 and leadsto the compositionNaooCa.Mg,"Fe2*ro(Al,Fe3*),.Sio"Al'")O*(OH), at the highest grade of meta- morphism in the Feather River area and further to NanuCarP*suR3*rnSiuAlrOrr(OH)athe ex- treme member of this actinolite-hornblendeseries. The Mg/Fe ratio changeswith the bulk com- position and with the degreeof oxidation of the host rocks. Epidote and clinozoisite coexist in all zonesas separategrains. In addition, many grainsof clinozoisiteinclude high index domains, lamellae, and stringersof epidote.In the highest grade zone next to the plutons, zoisite occurs with epidote,clinozoisite, and anorthitic plagioclase.Farther from the plutons, the plagioclaseis albite. There is no regular changein the distribution coefficientKn(Mg/Fe) for chlorite/amphi- bole although it is highest in the zone next to the plutons. Introduction zone around the plutons. In the pelitic layers, the common occurrence of andalusite, cordierite, and The minerals studied occur as major constituents staurolite in the outer contact aureole and andalusite in Paleozoic(?) metavolcanic and related meta- and cordierite in the inner one indicates PZ condi- morphosed intrusive rocks around Cretaceous plu- tions of the epidote amphibolite facies (Hietanen, tons in the southwestern part of the Bucks Lake 1967). Eilsewhere, mineral assemblages such as quadrangle, northern Sierra Nevada. The general muscovite-biotite-chlorite in pelitic layers and albite- geology of this quadrangle and the petrography and epidote-actinolite-hornblendeindicate the upper limit chemical composition of the host rocks have been of the greenschistfacies. describedin anotherreport (Hietanen, 1973);there- In the metavolcanic rocks and related intrusive fore only a short summary of the geologic setting rocks, the increase in the grade of metamorphism is and some additional features are given in the follow- indicated by the change of the color of hornblende. ing description. In the low-grade zone, the pleochroism parallel to y Two metamorphic events can be recognizedin the is light green to green but becomes bluish green host rocks near the plutons. The earlier regional towards the plutons. The most interesting feature metamorphism was to the greenschistfacies and was is that two amphiboles-actinolite and aluminous modified by a later higher-temperaturecontact meta- hornblende-and two to three epidote minerals co- morphism that afiected only the rocks in a narrow exist in all zones, as shown by indices of refraction measured in immersion liquids. It was therefore 'Publication authorized by the Director, U. S. Geological highly desirable to determine the composition of Survey. these coexisting minerals. The rock samples chosen 22 AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS for this study had already been chemically analyzed Description of the Rocks (Hietanen,l9l3,Table 1). They includeintermedi- Metagabbrosample 796, O.3 mile south of the ate and basic metavolcanicrocks and related intru- Granite Basin pluton, representsthe highest meta- sive rocks, metadiorites,and metagabbros. morphic grade. It is a coarse-graineddark rock in Samplesof meta-andesiteand metadacite (sam- which black hornblende contrasts with white an- ples 463 and464) are from successivelayers in the orthite (Ansr). Thin sectionsshow that most of the Franklin CanyonFormation, 3.2 miles east-southeast large amphibole grains are pale-greento colorless of the GraniteBasin pluton (Fig. 1). Sample465 is actinolite (yAc - 20o) mottled with blue-green from a small body of metadiorite a few meterswide amphibole (Fig. 2). All large grains are rimmed by just south of the locality of metadacitesample 464. blue-greenamphibole that is strongly pleochroic in Becauseof their proximity, thesethree rocks most blue green (y), green (F), and light green (o) and likely recrystallizedat the samePT conditionsat the has yAc = 22o. In addition, small euhedralprisms border of the greenschistand epidote amphibolite of blue-greenamphibole are included in anorthitic facies. The four other samples (Sp. 796, 546, 551, plagioclase.Most plagioclasegrains are granulated, 532) were collectedfrom 0.3 to 0.8 mile outside individual grains averaging0.01 mm in size, but the contact of the Granite Basin pluton. The rocks some are large and have mottled extinction and ir- in this contact zorre are coarsergrained and show a regular twinning lamellae. Some of these lamellae higher grade of metamorphism than those farther consistof rows of tiny small grains;others have wavy from the contact. borderssimilar to thosedescribed in an earlier report XPLANATIOI{ Ftc. L Maior geologic units and locations of the specimens studied (796, 546, 463, 532 arrd 464) in the Feather River area. ANNA HIETANEN included in albite. Albite is in granulated grains among the hornblende prisms and includes small Ft grains and clusters of epidote and some clinozoisite. \ Numerous scatteredanhedral to subhedralgrains of ilmenite (about0.02 mm long) are includedin horn- blende; some larger ore grains are clusteredparallel to the foliation.Some leucoxene is includedin horn- blende and epidote minerals. Meta-andesitesample 463, 3.2 miles east-south- east of the Granite Basin pluton, is a fine-grained gray-greendense rock that is either massive or -j-.r,aar.........- weakly foliated. Major constituents are light-green to pale-green hornblende, colorless actinolite, epi- Frc. 2. Amphibole grains consisting of hornblende dote, clinozoisite, and albite. Small amounts of (shaded) and actinolite (unshaded). Black is magnetite.fn chlorite, leucoxene, magnetite, and pyrrhotite are the high grade zone (796) actinolite has rims and patches of blue green hornblende, and small prisms of hornblende ubiquitous.The minerals are clustered;aggregates are included in anorthitic plagioclase. In the low grade consisting of amphibolesand some chlorite and zone (465) large grains consist either of actinolite with epidote minerals probably were originally pyroxene patchesof green hornblende,or of hornblende that includes phenocrysts.In the massiverock, green hornblende and is rimmed by actinolite. Small prisms of actinolite are occursin anhedralto subhedralgrains, 0.01 mm to included in albite. 1 mm long.Many grainshave pale-green to colorless actinolitic ends and rims; all contain inclusions of (Hietanen,1951, Fig. 2I) and are thoughtto have leucoxene.Actinolite alsooccurs as needles in albite. crystallized from zoisite at elevated temperatures Albite is in small (0.1 to 0.3 mm) grainsernbedded near the plutons. All three epidoteminerals, epidote, in a mixture of small anhedralgrains of epidote and clinozoisite, and iron-poor zoisite, are present. clinozoisite that is clouded by leucoxene. In some Epidote and clinozoisiteare in individual grains or in small patches,however, euhedralcrystals of epidote clusters,which are surroundedby a mass of small minerals free of inclusions are included in albite or grainsof zoisitedusted with tiny inclusions(Fig. 3). in a mixture of albite, fine-grained epidote, and Someclinozoisite grains contain patches of epidote; leucoxene.Chlorite is in clustersof small flakes next some others have twinning lamellae. All epidote to the amphiboles in the massive rock and with minerals contain inclusions of leucoxene, and are epidote minerals along the foliated shearzones. The accompaniedby somechlorite and quartz. A few tiny opaque minerals,pyrrhotite and magnetite,are in grains of ilmenite-magnetiteoccur as an accessory. subhedralgrains, 0.01 to 0.5 mm in size. Metagabbro sample 546, O.4 mile east of the Metadiorite sample 465, which is consideredto Granite Basin pluton, is mineralogicallyand textur- be an intrusiveequivalent of the meta-andesiteand ally similar to the metagabbroof locality 796, except metadacite,is an equigranularrock in which dark- for the absenceof zoisite. a lower anorthite content green hornblendecrystals contrast with the white to (Anto) in plagioclase,and less actinolite. A few blue-greenhornblende prisms have actinolite ends. 6n Metabasaltsample 551, 0.8 mile east-northeastof L'l @ lcLt ,b the Granite Basin pluton, is a dark-greento black, t/ weakly foliated rock that consists mainly of small prisms of hornblende,opaque minerals, and albitic 464 .a plagioclase.In thin sections,hornblende prisms (0.2 ep.t-f-A'Y'/ to 1 mm long) are pleochroicin light blue-greenand OS mm 7i; light greenand havecolorless
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