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Highly Aluminous Hornblende from Low-Pressure Metacarbonates And

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Highly Aluminous Hornblende from Low-Pressure Metacarbonates And American Mineralogist, Volume 76, pages 1002-1017, 1991 Highly aluminous hornblendefrom low-pressuremetacarbonates and a preliminary thermodynamicmodel for the Al content of calcic amphibole Ar-rnnr Lfcrn, JoHN M. Frnnv Department of Earth and Planetary Sciences,The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A. Ansrucr Calcic amphiboles in carbonate rocks at the same metamorphic grade from the Waits River Formation, northern Vermont, contain 2.29-19.06wto/o AlrO, (0.38-3.30Al atoms per formula unit, pfu). These Al-rich amphibole samplesare among the most aluminous examples of hornblende ever analyzed. The amphibole-bearing metacarbonatesare in- terbedded with andalusite-bearingpelitic schists and therefore crystallized at P < 3800 bars. Theseresults demonstrate that factors in addition to pressuremust control Al content in hornblende. We have identified temperature,mineral assemblage,mineral composition, and rock chemistry [especiallyFe/(Fe + Mg)] as other important factors. To explore semiquantitatively the dependenceof the Al content of calcic amphibole on P, T, and coexisting mineral assemblage,a simple thermodynamic model was developed for mineral equilibria involving tremolite-tschermakite ([CarMgrSi'O'r(OH)r]- [CarMg.AloSi6Orr(OH)r])amphibole solutions. The model uses the thermodynamic data base of Berman (1988), with the addition of new values for standard-stateenthalpy and entropy for pure end-member tschermakite derived from experimental and field data on the Al content in tremolite coexisting with diopside, anorthite, and quartz. Calculated phase equilibria lead to three conclusions:(l) At a specifiedP and T, the Al content of calcic amphibole is strongly dependenton the coexisting mineral assemblage.(2) No uni- versal relationship exists between the Al content of amphibole and P. (3) The Al content of amphibole may changedramatically wilh changesin P and 7. Maximum Al contents calculated by the model are - 1.2 Al atoms pfu. These values are far short of the 3.30 Al atoms pfu measuredin some amphibole samplesfrom Vermont. The principal shortcom- ing of the model is its failure to consider both total Fe in amphibole and the partitioning of Fe and Mg among the Ml, M2, and M3 crystallographicsites. INrnooucrtoN correlation exists between the Al content of hornblende and pressure(cf. Spear, I 98 l; Pluysnina, 1982;,Cao et al., The chemicalcomplexity of hornblendehas long been 1986;Jenkins, 1988, 1989). ofinterest to petrologistsbecause ofits potential as a rec- It came as a surprise, therefore, that during a regional ord of intensive variables during amphibole crystalliza- study of the progressive metamorphism of marls from tion. Recently, studies have focusedon the Al content of the Waits River Formation, northeastVermont, we found hornblendeas a practical geobarometer.Experimental and some hornblende samples with unusually high Al con- empirical studies, for example, show that a positive cor- tent. The most Al-rich amphibole samplesin this study relation exists between total pressureand the Al content contain over 19 wto/oAlrO, (3.30 Al atoms pfu). These of hornblende in certain assemblagesof minerals and sil- are among the most aluminous hornblende samplesever icatemelt (HammarstromandZnn, 1986;Hollister et al., reported (cf. Leake, 1965a, 1965b, l97l1, Doolan et al., 1987; Johnson and Rutherford, 1989a, 1989b).The Al 1978;Selverstone et al.,1984;Sawaki, 1989). Even though content in hornblende is thus regarded as a useful geo- thesehornblende sampleshave high Al content, they are barometer for appropriate plutonic and volcanic rocks. neither from very aluminous rocks (the host marls have Highly aluminous amphibole is likewise often regarded < 15 wto/oAlrOr) nor from rocks metamorphosedat ele- as an indicator of high-pressuremetamorphic rocks be- vated pressure (the marls are interbedded with andalu- causeit is found coexisting withjadeite, kyanite, or glau- site-bearingschists and crystallized at P < 3800 bars). cophane(Leake, 1965a,1965b; Kostyuk and Sobolev, OursecondmajorobservationisthattheAlcontentof 1969; Raase,1974; Selverstoneet al., 1984).Moreover, hornblende varies greatly (from -0.4 to 3.3 Al atoms some experimental studies of mineral assemblagesrele- pfu), even in marls from Vermont at the same metamor- vant to metamorphic rocks have shown that a positive phic grade, depending on the coexisting mineral assem- 0003-004x/9l/0506- I 002$02.00 I 002 LEGER AND FERRY: AI-RICH CALCIC AMPHIBOLE 1003 blage and mineral chemistry. Both observationsindicate that some factor(s) other than pressureand temperature /< a' tr- must exercisea first-order control on the Al content of amphibole.Our study identifiestwo suchcontrols: (l) the assemblageof minerals coexisting with hornblende and (2) the composition of those minerals [particularly Fe/(Fe + Mg) and anorthite content of plagioclasel.We have developeda simple model for the Al content of tremolite- tschermakite solid solutions in the system KrO-CaO- MgO-AIrOr-SiOr-HrO-COr.The model explainsthe de- pendence of the Al content of hornblende both on the coexisting mineral assemblageand the P-7 conditions of crystallization. The model for this simple system, how- Amphibole localities ever, does not predict the high-Al hornblende observed + diopsialezone in some rocks from Vermont. Further researchmust be t tr amphibole zone completed before an adequate quantitative understand- ing of the Al content of calcic amphibole can be attained. ISOGRADS d iops ide amphibole - - - .-biotile Gnor-ocrclr, sET"trNG oF HIGH-AI HoRNBLENDE FROM VERMONT Hornblende-bearing metamorphosed marls were col- lected from the Siluro-Devonian Waits River Formation Fig. l. Geologicsketch map of the areastudied. Thick solid : betweenDevonian Gile Moun- in northeasternVermont (Fig. 1),which has beenmapped lines stratigraphicboundary tain (Dgm)and Waits River (Dwr) Formations.Isograds are based by numerousgeologists (Doll, 1951;Dennis, 1956;Hall, on mineralassemblages in metacarbonaterocks and have ha- Konig and Dennis, 1964;Woodland, 1959;Cady, 1960; chureson their high-gradesides. Filled squares: amphibole 1965).The formation consistsof interbeddedlimestone localitieswith geothemometry;empty squares : otheramph! (2- I 5 cm thick), argillaceousmetacarbonates, and minor bolelocalities; filled circles: other geothermometry localities. LM pelitic schists.The Waits River Formation, togetherwith : I-akeMemphremago$ LW : LakeWilloughby; pre-S : pre- the adjacent and more pelitic Gile Mountain Formation, Silurianrocks; CV-GS : ConnecticutValley-Gaspe Synclinor- are part of the Connecticut Valley-Gaspe synclinorium. ium. Geologicmap basedon that of Doll et al. (1961). The structure and metamorphism now observed in the area developed primarily during the Devonian Acadian II{vESTrGATIoN Orogeny (ca. 370-400 Ma). Two major folds, the Brown- Mnrrroos oF ington syncline (Doll, l95l) on the western side of the Thirty-one samplesof metamorphosedimpure carbon- synclinorium and the Strafford-Willoughby arch (Eric and ate rock containing amphiboles were collected from 27 Dennis, 1958) on the eastern side of the synclinorium field localities (Fig. 1). Mineral assemblageswere identi- control the regional attitude ofbeds. Bedsstrike northeast fied by petrographicobservation of thin sections.Mineral except where locally deflectedaround igreous intrusions. compositions in all sampleswere determined by electron The metasedimentaryrocks were intruded by numer- microprobe analysis using the JEOL JXA-8600 Super- ous late- to posttectonic granitic plutons of the New probe at the Department of Earth and PlanetarySciences, Hampshire Plutonic Series(Doll et al., l96l). From the The Johns Hopkins University, and natural mineral stan- distribution of isogradsand metamorphic zones(Fig. l), dards. Data for both silicates and carbonates were re- it is clear that the plutons were local heat sourcesfor the duced with a ZAF correction scheme.Chemical analysis highest gradesof metamorphism attained in the area. With of 23 rock samples for major elements were performed respectto pelitic schists,metamorphic graderanges from by X-ray fluorescence:13 at X-Ray Assay Laboratories, chlorite zone to the sillimanite zone near the plutons. Ltd., Don Mills, Ontario, and ten at the Geochemical Garnet and amphibole porphyroblasts crosscutthe main Laboratories,Department of GeologicalSciences, McGill schistosity,indicating that the peak of medium- and high- University, Montreal, Quebec. grade metamorphism occurredafter the last period of de- AND cHEMrsrRy AND formation. Four metamorphic zones were also mapped MrNnnc.L CoNTENT basedon minerals in the metamorphosedmarls (Fig. l). PETROI,OGIC FEATURES OF ROCKS WITH The biotite isograd separatesthe ankerite zone at lower TTTCTT-AI HORNBLENDE grade from the biotite zone al higher grade. The amphi- Mineral assemblagesobserved in six representative bole isograd separatesthe biotite and amphibole zones, specimensfrom the amphibole zone are listed in Table and the diopside isograd separatesthe amphibole and l. These specimenswere chosen to show the wide range diopside zones.Amphibole describedin this study is from in amphibole compositions (especiallyin AlrO. content) rocks of the amphibole and diopside zones. even in marls metamorphosedat the same gtade. 1004 LEGER AND FERRY: AI-RICH CALCIC AMPHIBOLE nite, sulfides, chlorite, clinozoisite, potassium feldspar, garnet, muscovite, ankerite, and ilmenite. Compositions Al2O3-CaO- of amphibole, biotite, chlorite, calcite, clinozoisite, gar- net,
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