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Significance of Hornblende in Calc-Alkaline Andesites and Basalts

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Significance of Hornblende in Calc-Alkaline Andesites and Basalts American Mineralogist, Volume 65, pages 837-851, 1980 Significance of hornblende in calc-alkaline andesites and basalts A. T. ANDERSoN. JR. Department af the Geophysical Sciences, The University of Chicago 5734 South Ellis Avenue Chicago, Illinois 60637 Abstract Hornblende occursin someandesitic and basalticrocks of calc-alkalineaffini1y, where it is commonly associatedwith olivine. The texture,eruption history, and compositionare inter- pretedto indicate that hornblendecommonly forms as a product of reactionbetween olivine and basalticand andesiticliquids. The natural hornblendesprobably formed within the crust at temperaturesbetween 960o and 1080'C from liquids with lessthan about 6 weight percent HrO. The formation of hornblendefrom basalticliquid within the crust has implications for evolution of continentalcrust, as well as for the origin of andesiteand thermal conditions in subductionzones. Introduction Definitions Boettcher (1973) and others (Mueller, 1969;Hol- In using the word liquid, I mean to refer to a single loway and Burnham, 1972;Green, 1972;Helz, 1976; state of matter (a homogeneous liquid phase). It is vi- Cawthorn and O'Hara, 1976;Allen and Boettcher, tal to distinguish between the composition of a liquid 1978)argue that amphibole plays an important role (generally a residual liquid) and the composition of in governing the compositionsof calc-alkalineande- the bulk material. Petrographically a certain horn- sites. The mechanismswhereby amphibole suppos- blende may be associated with a particular residual edly exertsits role are not clear. Is it left behind in a liquid. Experimentally a certain bulk composition crystallins residue-where is it: in the crust or in the may crystallize hornblende at or below the liquidus. mantle? Does amphibole crystallize and separate The residual liquid associated with hornblende will from a parental liquid-what are the composition generally differ in composition from the bulk. Ther- and temperatureof the parental liquid? Does amphi- modynamically the composition and temperature of bole crystallizefrom a derivative liquid which mixes a certain liquid can remain constant as the propor- with a parental liquid to yield andesite?Amphibole tions of crystals and liquid vary. To interpret a given occurs in some calc-alkaline volcanic and plutonic association of hornblende and liquid, the petrogra- rocks with basaltic and andesitic bulk compositions, pher must seek experimental conditions which yield but the significance of such occurrencesis uncertain hornblende and liquid with compositions similar to without textural evidenceof how it got there: is the those observed. If analogy is drawn with a particular amphibolea primary igneousmineral or a product of bulk composition (and liquidus hornblende) the im- alteration?If igneous,over what interval of crystalli- plied pressure (and concentration of HrO in the liq- zation did it form, and was the composition of the uid) is larger than if analogy is made with a residual liquid in that interval andesitic,dacitic, or basaltic?If liquid. The implications for temperature are complex amphibole can be shown to have actually formed and depend on the concentration of HrO in the liquid from a fiquid with a certain composition, what does as well as on compositional features such as Fe/Mg that portend for the concentrationof HrO in and the and concentrations of alkalis. temperatureof the liquid? My aim is to help point the way to answersto someof the abovequestions by Hornblende in experimental products critically sxxmining evidenceon the growth of am- Experimental studies yielding hornblende in prod- phibole from basaltic and andesiticliquids, both ex- ucts with either calc-alkaline bulk compositions or perimental and natural. with residual liquids having analogous compositions 0003-o04x/80/09I 0-0837$02.00 83'l ANDERSON:HORNBLEN DE IN CALC-A LKA LI N E A N DESITES reveal:(l) Hornblende is a liquidus or near-liquidus mineral in some basaltic bulk compositionsif P",o exceedsabout l0 kbar (Yoder and Tilley, 1962; Green and Ringwood, 1968;Allen et al., 19751'Stern et al., 1975)and if H,O dissolvedin liquid exceeds about 12 weight percent. (2) In some andesiticbulk compositionshornblende is a liquidus mineral rf Pn,o exceedsabout 2 kbar (Piwinskii, 1973).(3) As P",o 1 decreasesbelow P,o,u,,the hornblende-outcurve lies progressivelyfarther below the liquidus (Robertson T and Wyllie, l97l; Holloway and Burnham, 1972; Green, 1972;Eggler and Burnham, 1973;Allen and Boettcher, 1978).(4) The proportion and composi- tion of residual liquid change dramatically over an interval of temperatureless than 50"C after the be- ginning of crystallization of hornblende (Holloway and Burnham, 1972).(5) Near the liquidus low oxy- gen fugacity favors the crystallization of hornblende f-t--ii:) comparedto pyroxene(Helz, 1973,1976). (6) The re- actionsbetween hornblende, liquid and other crystals 1:1a(ry22 (Bowen, 1928,p. 61, 85, lll; Helz, are complex a\*>,,/ ./'// 1976).(7) Hornblendeis stableto the highesttemper- atures and lowest concentrationsof HrO in liquid in systems(a) where olivine is present(Helz,1976), (b) where P,,o ( P,",",(Holloway, 1973),and (c) where Fig. l. Simplified hypothetical P-T-XH2o equilibrium diagram for a high-alumina basalt. The diagram is drawn in perspective. significant F is present(Holloway and Ford, 1975). The origin is not shown. The concentration of H2O refers to the The above relations are summarizedon Figure l. bulk material. The stippled and dashed regions are saturated with The coordinatesof Figure I are qualitative. Quan- vapor rich in H2O. Below the liquidus surface the pyroxene (Px)- titative values will vary with bulk composition. out and olivine (Ol)-in surfaces are assumed to coincide for Lesser MgO and Mg/(Fe+Mg) will diminish the simplicity. Fields for spinel and magnetite are omitted for simplicity. Hornblende (Hb) is a liquidus mineral in both vapor- temperature of olivine, pyroxene, and hornblende absent and vapor-present regions. The liquidus hornblende is liquidus surfaces.The field for liquidus hornblende accompanied either by liquidus Ol or by both plagioclase (Pl) and will then extendfarther into the vapor-absentportion Ol. Only one symbol is shown for pyroxene (Px). Liquidus Px is of the diagram. Lesserconcentrations of AlrO. will calcium-rich. Low-calcium pyroxen€ probably predominates if Hb shift more of the liquidus surfaceinto the olivine and is abundant. pyroxenefields and diminish the area of the liquidus field for plagioclase.Consequently, the combination simplifications,the diagram is consistentwith exist- of low Al,O, and high MgO may obliterate the liq- ing experimentaland natural data. uidus surfacehaving hornblende,olivine, and plagio- It would be helpful to know the positions of sur- clase(see Bowen, 1928, p. 1l l). For a naturalmagma facesof equal SiO, in the liquid (H,O-free basis).At with CO, as well as HrO, there will be a rangein X",o presentthe coordinatesof such surfacesin Figure I in addition to a boundary marking vapor-absentand are uncertain. Qualitatively, it is possible to give vapor-present regions. Reactions between olivine, somelimits. I considera surfaceof SiO, : 60 weight pyroxene,hornblende, and liquid add complications percent on an anhydrous basis, corresponding not shown on the diagram. Spinel and magnetiteare roughly to andesite.Because the compositionof the probable additional liquidus minerals but are not residual liquid changessteeply with temperatureaf- shown.For simplicity only one pyroxeneis depicted. ter the beginning of crystallization of hornblende Calcium-rich clinopyroxene is the liquidus pyroxene (Holloway and Burnhan, 1972),it is reasonableto but is joined atd/or replacedby orthopyroxenebe- expect the 60 percent SiO, surface to lie close to the low the liquidus at temperaturesbelow or near that hornblende-outsurface. In the region ofthe diagram of the appearanceof hornblende. Except for such where hornblendeis a liquidus mineral (hornblende- ANDERSON: HORNBLENDE IN CALC-ALKALINE ANDESITES out corresponds to liquidus), the 60 perc€nt SiO, sur- by either contrastinginitial concentrationsof HrO in face must lie below the hornblende-out surface. In parental high-alumina basaltic magma, variable the region ofthe diagram where the hornblende-out depth of crystallization,or extent of reaction during surface is far below the liquidus (for example, sub- crystallization. aerial basalts with about 0.1 weight percent HrO), re- Some natural calc-alkahneliquids associatedwith sidual liquids with more than 60 percent SiO, may hornblende are less siliceous and hotter than most develop in the absence ofhornblende. Therefore, the experimental analogs.The natural occurreneessug- 60 percent SiO, surface may intqisect the horn- gest that the field of liquidus hornblendeextends to blende-out surface, unless crystallization intervenes. lower concentrationsand partial pressuresof HrO It is probable, but uncertain, that residual liquid in than is indicated by most existing experimentalre- anhydrous high-alumina basalt attains 60 percent sults. SiO, before complete crystallization if equilibrium is maintained. Residual glassesin natural high-alumina Hornblendeassociated with natural andesiticliquid basalts have 62 (Fuego volcano, Guatemala-Rose e/ Many andesiteshave phenocrystsof hornblende al., 1978) and 67 (Hat Creek flow, California-An- (see for example,Moorhouse, 1959, p. 189-190; derson and Gottfried, 19'71)percent SiOr. However, Turner and Verhoogen,1960, p.272-282) but most equilibrium
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