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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. Bll, PAGES 10242-10256, NOVEMBER 10, 1981

The Mineralogyand Chemistryof the AnorogenicTertiary SilicicVolcanics of S.E. Queenslandand N.E. New South Wales, Australia

A. EWART

Departmentof & ,University of ,St. Lucia,Brisbane, Queensland 4067

The Late Oligocene-EarlyMiocene volcanismof this regionis chemicallystrongly bimodal; the (volmetrically dominant) comprise , hawaiites, and tholeiiticandesites, while the eruptivesare mainly ,potassic , and potassic,high-silica .The comendites and rhyoliteshave distinctivetrace element abundancepatterns, notably the extreme depletionsof Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable em'ichraentof suchelements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytesexhibit thesecharacteristics to lesserdegrees. The comenditesare distinguished from the rhyolitesby their overall relative enrichmentof the more highly chargedcations (e.g., LREE, Nb, Y, and especiallyZr) and Zn. The phenocrystmineralogy of the trachytesand rhyolitescomprises various combinationsof the following phases:sodic (-), calcic anorthoclase, , , ferroaugite-ferrohedenbergite,ferrohypersthene, fayalitic , ilmenite, titano- ,and rarely (near annite) and Fe-hastingsiticamphibole. Accessories include , , chevkinite (ferrohedenbergite-bearingrhyolites only), and allanite ( and botite rhyo- lites only). The comenditesgenerally contain Ca-poor anorthoclase-sanidine,quartz, fluorarfvedsonite, and aegirine-(Zr-bearing), aenigmatite,and + ilmenite. CoexistingFe-Ti oxidesare ab- sent in the comenditesand relatively uncommonin the rhyolitesand trachytes.Where present,they in- dicate equilibration temperaturesof 885ø-980øC and fo2 betweenQFM and WM buffers.The arethus interpreted to havebeen strongly water undersaturated during equilibration, which is also consistentwith the generalpaucity of pyroclastics,the rarity of hydrousmineral phases,and the ex- tremeFe-enriched ferromagnesian phenocryst compositions. The chemicaland mineralogicaldata are in- terpretedto indicatethe operationof extremefractionation processes controlling the developmentof the silicicmagmas, and the comendites,trachytes, and certaintrachyte- series are consideredto have evolvedfrom a mafic parentage.The availableoxygen, St, and Pb isotopicdata, however,point to some modificationof the magnasthrough crustalequilibration processes. The remaining high-silicarhyolites are consideredto mostlikely representcrustal partial meltsbut are again modifiedby extensivefraction- ation.

INTRODUCTION quence followed by several distinct rhyolite units (two of The region in questionforms part of the broad belt of Cai- which are consideredin this paper, namely the Binna Burra nozoic volcanism,some 3000 km long, that extendsalong the and Springbrookrhyolites) that are overlain by a younger easternand southeasterncoastal region of Australia. Wellman mafic sequence;a separatecomenditic dyke phasealso occurs. and McDougall [1974] have shownthat volcanic activity com- Volcanism is dated from 20.5-22.3 Ma [Ewart et al., 1977]. menced about 70 Ma ago and has continuedthrough the Cai- Focal Peak ShieM . This again constitutes the eroded remnants of a situated within a com- nozoic at a nearly constantrate. More than 50 volcanic prov- inces are recognized throughout the whole coastal region. plex ring structuresome 14 km in diameter,the periphery of Although basalts, hawaiites and tholeiitic andesitesare the which is marked by rhyolite intrusionsand extrusions(the Mt. dominant lavas, some of the provincescontain minor associ- Gillies Rhyolites).The massiveMt. Barney granophyreplug ated silicic volcanism, namely trachytes, rhyolites, and/or also outcropswithin this ring structure.The generalizedvol- comendites.In certain areas,especially southern Queensland, canic successioncomprises an older mafic sequencefollowed isolatedoutcrops of trachytesand comenditesare found with by the Mt. Gillies Rhyolites.Reported ages range between 23 apparently no associatedmafic extrusives. and 25.6 Ma [Ross, 1977]. Considerationof the overall distributionof chemistryof the Glass Houses and Maleny Centers. The Glass House erupted magmas in S.E. Queensland reveals that the vol- Mountains constitutea spectaculargroup of eroded domes canism is very strongly bimodal [Ewart et al., 1980]. Where and plugscomposed mainly of comenditesbut with subordi- both mafic and silicic eruptivesoccur within a .given center, nate trachytes.A few kilometersto the north of theseplugs is estimatessuggest that the relative volumes are at least 7:1 in the isolatedmafic Maleny eruptivecenter, which may be re- favor of the mafic eruptives. lated to the GlassHouse extrusions. This is supportedby K- The following is a brief outline of the volcanism found Ar datesof 25.2 and 25.4 Ma for a youngMaleny flow and a within someof the more prominent volcanic centersto which , respectively.Extending NNE along the S.E. particularreferences are madein thispaper and whichin- Queenslandcoast are severalquite isolated silicic extrusive dicate the diversityfound among the variousprovinces. A lo- centers,namely, the Mt. Coolurn comenditeand the Fraser Is- cality map, showing the extent of the S.E. Queensland-N.E. land trachytes. New South Wales volcanic regions,is provided in Figure 1. Mt. AlfordRing Complex. This representsan erodedplug Tweed $hieM Volcano. This constitutes the eroded rem- of microdioriteand subordinategranophyre intruded by a nants of a former shield volcano some 100 km in diameter. The coreof tholeiiticandesite; the complexis cut by a semicircular generalized½olcanic succession comprises a lower mafic se- zoneof rhyolitic and trachytic ring dykes (25.2 Ma) and a sec- ondnonarcuate set of cross-cuttingrhyolite, comendite, and Copyright¸ 1981by the AmericanGeophysical Union. dykes [Stevens,1962].

Paper number 1B0531. 10242 0148-0227/81/001B-0531 $01.00 EWART:QUEENSLAND TERTIARY ANOROGENIC SILIClC VOLCANICS 10243

0 100 200 km I I I •.•. '• Minerva Springsure••_•

• WaddyPoint Middle Rocks

Indian Head

FRASER ISLAND

Coolum

Bunya Maleny Mountains Mountains

RISBANE

Blaine Mt. Fraser Pea k

•ar Mt. Maroon Edwards Mt. Barney Alford Mt. Ernest Burra Springbrook Din sey Rock Mr. Gillies Warning

Tweed Shield Volcano Nandewar Mountains

Mt. Kaputar Warrumbungle Mauntains••l•• • A • t.Oliver Fig. 1. Map of S.E. Queenslandand N.E. New SouthWales showing the distributionof the Tertiaryvolcanic centers.

Minerva Hills. The volcanic sequencefrom this rather subordinaterhyolite. These are the Minerva Hills volcanics complexcenter has been broadly divided into a basalseries of [ Veeverset al., 1964]. marielavas (some 70 m thick) that are overlainby a younger seriesof intcrcalatcdmarie lavas and trachyticpyroelastics. K- CHEMISTRY Ar datesof approximately33-34 Ma and 24-28.5 Ma are ob- It is emphasizedthat within the volcanicprovinces being tained from the two series,respectively. The pyroelasticsare consideredthere is a strongbimodality of magmacomposi- closelyassociated with a concentrationof numerousplugs, tionswith respectto their SiO2 distributions[Ewart et al., domes,thick flows,sills, and dykesof mainly trachytcwith 1980].This paperis concernedwith the siliciccompositions, 10244 EWART: QUEENSLANDTERTIARY ANOROGENIC$1LICIC VOLCANICS i.e., thosethat are greaterthan 59% SiO2. In addition, some Trace Elements mineralogicalcomparisons are made with associated,al- thoughrare, microsyenitesthat occurin certainof the vol- Three distinctpatterns of traceelement behavior axe recog- canic centers.Averaged major and trace element data and nizable within the silicic eruptivesof this province: mineralogicaldata, are summarizedin Table 1. The 1. Those behaving as incompatible elements,which thus data are basedon sequentialaverages with increasingSiOn_ show a progressiveand regular enrichment through the tra- contentsof the respectivesamples. chyte to rhyolite compositionalspectrum: Rb, Th, and to a For purposesof thispaper the distinctionbetween trachytes lesserextent Pb are the most notable examplesin this prov- and rhyolitesis taken as 73% SiOn_.The peralkalinetypes are ince. For example,there is an almostlogarithmic relationship readily distinguishedby their distinctivesodic mineralogy. between Rb versusK/Rb [Ewart et al., 1976], which suggests Very rare lavasoccur that are describedas ;these are that Rb is effectivelyexcluded from fractionatingphenocryst re•.dilydistinguished from trachytesby the absenceof both phases(except biotite, which is, however,a very rare phasein phenocrystalalkali feldsparand quartz.In this paper,four theextrusives of theregion). This is confirmedby analysesof distincttypes of rhyolitesare recognizedon the basisof both phenocrystphases from the silicic eruptives.Thus, measured occurrenceand mineralogica!criteria (Table 1). Rb partition coefficientsfor alkali feldsparsrange between 0.31 and 0.65 (averaging 0.46) and are <0.1 for all other Major Elements phases(except biotite). Th partition coefficientsare also low, A. Metaluminous and œeraluminousrhyolites and tra- except for certain accessories(zircon, allanRe, and chevki- chytes. The majorityof the trachytesand rhyolitesanalyzed nite), while Pb partition coefficientsare uniformly higher for are chemicallyvery closeto the metaluminous/peraluminousthe (0.5 to 1.7). boundary,and it is believedthat the magmaswere metal- 2. Those elements exhibiting very strong depletion uminousprior to the processesof coolingand devitrification. throughthe compositionalspectrum from trachyteto rhyolite, No evidence,however, of the occurrenceof systematicselec- most notably Ba, St, V, Ni, and Cr, and also including Mg, tive alkali leaching within the various extrusiveshas been Mn, Ca, and P. Extreme Eu depletion alsooccurs JEwart et al., found. The silicic extrusivesof Minerva Hills and the hyper- 1976].Such depletionsare illustratedby the plot of Rb versus sthene-bearingrhyolites are noteworthyin thatthey consis- Ba (Figure 3). The behavior of these elementscan, in prin- tently showperaluminous chemistry. ciple, be explainedby fractionationof feldsparsand Fe-Mg The trachytesand rhyolitesare all relativelypotassic and silicates. exhibit progressivelydepleted Ti, Fe, Mg, Mn, Ca, and P 3. The highly chargedcations, notably Zr, Ce, Y, and Nb. abundanceswith increasingSiOn_, culminating in very strong Although theseelements normally exhibit enrichmentduring depletionsof theseelements in the rhyolites(the fractionationwithin the S.E. Queenslandsilicic mag- rhyolitesexhibiting least depletion).Figure 2 indicatesthat mas, they also exhibit a mutally 'decoupled'behavior, result- the nonnative compositionsof the rhyolites(excepting the ing in poor correlations(Figure 4). This behavior can prob- Minervatype) closely approach the quartz-saturatedtwo-feld- ably be attributed mainly to the crystallization and sparboundary curve (Figure 2a) and the 'minimum'(piercing fractionation of minor microphenocrystphases such as zir- point) in the projectionof the [Ab-Or-Q]97An3system (Figure con, chevki•.itc, allanitc, and possibly apatite, in the non- 2b) [Jamesand Hamilton,1969], although plotting within the peralkalinelavas, and to sodicpyroxenes agd sodicamphi- sanidine fields. These can be classedas high-silica, potassic boles in the peralkaline types. rhyolites.The plotsof the trachyteswithin the quartz-feldspax The averageddata for the rhyolites (presentedin Table l) system(Figure 2b) showthat they lie in a zoneclose to, and illustrates the above patterns of behavior, and their overall essentiallyparallel to, the projectionof the two-feldsparelement abundancepatterns would seem to be readily inter- boundarycurve as definedby Jamesand Hamilton[1969] for preted as indicatingstrong crystal fractionation control (see, thosecompositions low in normativeAn (seealso Figure 2a). however,Hildreth [1979] and later discussion).Nevertheless, it The Minerva Hills extrusivesseem to be unique within the re- is apparentthat each rhyolit e typehas its own distinctive trace gionin that theyshow a completecontinuity of majorelement element patterns,indicating an independentline of evolution. chemistryfrom trachyteto rhyolitein the onevolcanic center; The Minerva Hills trachytesand rhyolitesexhibit a complete it is thus noteworthythat the Minerva rhyolitesare lower in continuity of trace element abundances,in conformity with normativeQ (Figure 2b) and generallyexhibit a more tra- their major element chemistry, and the rhyolites are notably chyticchemistry compared to the otherrhyolites of the region enriched in Zr and Nb comparedto the other rhyolite types (Table 1). within the region. B. Peralkaline extrusives. These are characterized by The comendites have certain trace element characteristics normativeac, and lesscommonly normative ns. The majority (Table l) that tend to distinguish them from the non- of the extrusivesare highly silicicand can be classifiedas peralkalinerhyolites, most notably the strongenrichments of comendites(Table 1), althoughperalkaline trachytes sporadi- the highly chargedcations (e.g., Zr, Nb, LREE, Y) and Zn. cally occur.The comenditesexhibit highly depletedMg, Mn, Nevertheless,Table 1 and Figure 4 show that the non- Ca, and P abundances,with Na and Fe3+ significantlyen- peralkalinetrachytes and rhyolites may also exhibit enriched riched comparedto the nonperalkalinerhyolites. Their nor- concentrationsof certain of theseelements, overlapping the mative compositions,when plottedin the quartz-feldspaxsys- abundancesfound in the comendites.The factorthat seemsto tern (Figure2b), projectin a zonelying betweenthe thermal mostcharacterize the comendit&is the relativeenrichment of valley in the ternary system[Tuttle and Bowen,1958] and the the wholespectrum of the highly chargedcations, which is not projectionof the two-feldspaxboundary curve in the [Ab-Or- found in the nonperalkalinelavas. The enrichmentof Zr in Q]9?An3system; a systematicshift of Ab/Or ratios therefore the comenditesis particularlycharacteristic [see also Nicholls occurswith increasingnormative Q. and Carmichael,1969] and is readily explicableby the experi- EWART.'QUEENSLAND TERTIARY ANOROGENIC SILIClC VOLCANICS 10245

v 10246 EWART:QUEENSLAND TERTIARY ANOROGENIC SILIClC VOLCANICS

[] []

AID 50 Or

/

** J • Average Binna Burr a, chI Mt. Gillies,& Hypersthene-• '•/ bearingrhyolites []oø •r MinervaHills rhyolite o MinervaHills trachytes [] [] Trachytes(excluding - a'mervaH'dls) [] . Peralkaline lavas / [] / ß I 'alkalin/e/ lavas/ / Ab 50 Or Fig.2. Normatire(CIPW) quartz and compositions oftrachytes, rhyolites, and peralkaline lavas plotted in thequartz-feldspar andfeldspar ternary systems. Heavy solid curve in plot(a) representsquartz-saturated two-feldspar boundarycurve, and in plot (b) (curve 2) represents thequartz-feldspar andtwo-feldspar boundary curves for composition (Ab-Or-Q)o7An3(atl-kbar water vapor pressure), both after James and Hamilton [1969]. Curve labeled (1) in plot (b) is after Tuttleand Bowen[1958] (also for I kbar). mentalwork of Watson[1979], which showsthat zirconcrys- MINERALOGY tallizationis inhibitedin peralkalineliquids, supporting the Introduction petrographicobservations of the absence of zirconin thecom- endites.This may also explain the enrichment of Y andTh in The following characteristicphenocryst assemblages are theseperalkaline magmas. observed: The comenditesalso exhibit enrichmentsof Rb and Pb (and Rhyolites. (a) Sodicplagioclase + sanMine+ quartz+ Fe- probablyalso such elements as Ga, Be,and U, forwhich only biotite + allanite +_ilmenite +_titanomagnetite. (b) Sanidine limited data are available; seeEwart et al. [1976]). In contrast, + quartz+ + ferrohedenbergite+ chevkinite + ilme- extremedepletions are observed for Ba, St, Eu, Ni, Cr, V, Mg, nite + titanomagnetite.(c) Sodicplagioclase + quartz + sani- Mn, Ca, and P. dine + ferrohypersthene+ ilmenite.

ß ß

ß A ß • ß ß • .o• z,.,.. ß ½.o [] oo ß amOi•io i 0 0 [] [] [] [] [] ø o [] o

[]

ß KEY "'' ß ß ße

ß Binna Burra Rhyolites ß e "'•"e •e ee' ß MinervaHills Rhyolites . ' ;;:,•'/ ' ß Other Rhyolites ß '. :

A Cornendites ß ee ' ß ' o Trachytes ' ß ' ß Mafic lavas ß

ß I I I I I IIII I I I I I ]lml I I I I I I Iml IO.O 100.O I000 Ba (p.p.m.) Fig. 3. Rb versusBa plot of thesilicic and mafic eruptives of theregion. EWART:QUEENSLAND TERTIARY ANOROGENIC SILICIC VOLCANICS 10247

350

300

250

W•7v

E 200 ,4

150

ß • KEY

lOO v ß Binna Burra Rhyollte v . Mt.Gillles Rhyolite ß ß O Springbrookand Mt.Alford ß •, •',•' 0 0 Rhyolites

ß • . ß. O ß •, v MinervaandHills RhyolitesTrachytes 5o ß" '•"•; ' * • • ß • • • Trachytes(excluding

•.'..' .•;.. :t.•.?..• • ß ß.• O ß ß ComenditesMinerva Hills) ::•A•.O,;f...... ,;•.• ß 0 . ßMaflc lavas i i i i i i i i i i i i i i i i, i i i i i i soo ,ooo ,soo ooo Zr (p.p.m.) Fig.4. Zrversus Nb plot of the silicic and marie eruptives ofthe region.

Nonperalkalinetrachytes. (a) Sanidine+ sodicplagioclase trachytes, rhyolites, and comendites in their volume percent + Fe-olivine+ ferroaugite+ titanomagnetite + ilmenite. (b) . Sanidine-anorthoclase+ ferroaugite + Fe-olivine+ titano- magnetite+ ilmenite. (c) Sanidine+ hastingsiticamphibole Feldspar Phenocrysts + allanite+ ilmenite+ titanomagnetitie+ fayalite + ferrohe- denbergite. A. Nonperalkalineeruptives. Individual specimens may Dacites. Plagioclase+ augite + ferropigeonite+ ilmenite. containcoexisting plagioclase + sanidine,calcic anorthoclase Comendites.(a) Phenocrysts:Ca-poor anorthoclase-sani- + sanidine,sanidine, calcic anorthoclase zoned to sanidine,or dine+ quartz+ ferrohedenbergite(zoned to aegirine-augite)rarely, plagioclase + anorthoclase + sanidine. Compositional + ilmenite.(b) Groundmass:Ca-poor anorthoclase-sanidine relations are thus complex. Extensive chemical zoning occurs + quartz+ aegirine+ fluorarfvedsonite+ aenigmatite + il- in bothplagioclase and the alkali feldspars, usually amount- menite. ing to at least20 tool.%; the plagioclasesand sanidinesare Thesilicic magmas from this province were erupted with zonedto moresodic rims, while the calcic anorthoclase relativelylow phenocryst contents (<10%), although some phenocrysts arecommonly zoned toward sanidine rims (vary- variationisfound, asillustrated inFigure 5(see also Table 1). ingfrom Or,_•_so). Incertain ofthe lavas, cores ofsodic plagio- No systematicoverall differences are apparentbetween the claseare enclosedby sharplydefined zones of sanidine. Compositeplots of thefeldspar compositions (microprobe analyses)occurring within the microsyenites,trachytes, and rhyolitesare presented in Figure6. Thetrachytic feldspar z phenocrystsexhibit extensive ternary solid solution, which de- tu 20 creasessomewhat in the mostsiliceous trachytes. Calcic anorthoclaseis characteristic of the less siliceous trachytes, whereasthe more siliceoustrachytes contain a muchwider o spectrum of compositions, which extend from andesine o throughto sanidine(Ores). The rhyolitic feldspars are equally >. lO variable.The MinervaHill typeis dominatedby anortho- z clase-sanidine,with rare occurrences of a primarybut unusu-

o ally sodicplagioclase (averaging Ane.sOrs.0. The Mt. Gillies rhyolitescontain almost exclusively sanidine, which occasion- ally containscores of corrodedplagioclase. The BinnaBurra •'•'•' •,/'/,•/////////••//•///////•////•, , •///////////• andthe hypersthene rhyolites contain coexisting plagioclase 0 10 20 30 andsanidine, the latter rhyolite type having generally more Total Phenocrysts (vol.%) calcicplagioclases. Fig. 5. Histogramof thedistribution of percenttotal phenocrysts B. Peralkalineeruptives. Composite plots of the feldspars in the siliciceruptives (including peralkaline and nonperalkaline(Figure 7) in thesemagnaas reveal that Ca-poor anorthoclase- types). sanidineisthe only type found, although considerable compo- 10248 EWART: QUEENSLANDTERTIARY ANOROGENICSILICIC VOLCANICS

) Microsyenites (b) Trachytes: (c) Trachytes: / . _ / ,/ /

Ab 50 50 50 Or

/ &. (d)T rachytes: (e)Trachytes' f) Minerva Hills t t Rhyolites: >73% SiO 2 ee i I ß •e e ß / / , . , /'/ Ab 50 5o 5o Or ;' An

ß

) Springbrook a (g) Mt. Gillies (h)Binna Burra L / j•. AlfordHypersthene Rhyolites' Rhyolites: >73% SiO 2 >73% S

Ab 50 50 50 Or

Fig. 6. Compositeplots of phenocrystfeldspar compositions (mol. %), basedon microprobedata, occurring within the microsyenites,trachytes (sequentially subdivided according to total rocksilica contents), and rhyolitetypes.

sitionalvariation is apparent,that can be broadlycorrelated ment in the pyroxenesfrom the peralkalinemagmas, non- with the wholerock normativeAb/Or ratios(Figure 2b). peralkalinemagmas, and intrusive microsyenites is compared Thusthe feldspars in thesiliceous comendites tend to be more in Figure9. It is clearthat Na-enrichmentis relativelyminor potassic(Or37_•) than thoseoccurring in the lesssiliceous per- in thenonperalkaline magmas, whereas the peralkaline mag- alkaline magmas•,•Ji""'- 22-45)- ' mas are characte6•edby completesolid solutionsbetween pureaegirine and ferrohedenbergite, nearly always as ground- Pyroxenesand mass phases. It is therefore evident that Na-enrichment in Four distinctpyroxene occurrences are recognized,as fol- thesepyroxenes occurs only after extremeFe-enrichment has lows: (a) phenocrystsof ferroaugite-ferrohedenbergite(+ beenattained. A notablefeature of the chemistryof the sodic fayalitic olivine);(b) phenocrystferrohypersthene; (c) pheno- pyroxenesis their relativelyhigh ZrO2 (Table 4), which is, crystferropigeonite (+ ferrohypersthene);and (d) ferroheden- however,very variable even within the onesample. bergite-aegirinesolid solution series occurring as groundmass Possiblefactors leading to Na-enrichmentin phaseswithin the peralkalineextrusives. duringfractional crystallization can be approachedby consid- In addition,magnesian orthopyroxene, augitc, and olivine eringthe followingtwo exchangereactions: phasesare found in a flow (hybrid), and very rarely in the hypersthenerhyolites, and are clearly xenocrystal. Figure 8 illustratesthe pyroxenecompositions found within thetrachytes, thepyroxene-bearing rhyolites, and a hybrid / / / e•B•IB / / / 63-66%/ / Si0/ 2 -•n dacite. It is clear that these anorogenicsilicic magmas are 50 Or characterizedby the developmentof extremeFe-enrichment intheft phenocryst pyroxenes andolivines, with ferroaugite- __A __ ß ß / / .... ,. / / / / ferrohedenbergitebeing the dominantpyroxene in the non- _ _ _ 66-69%SiO2-• peralkalinemagnaas (Table 2). Ferrohypersthene(extending 50 to Fs70 occursas the characteristicferromagnesian pheno- 69-73% Si 0 2 cryst phase in the Springbrookrhyolite and also within iso- / I / / lated rhyolitesfrom the Mt. Afford and Mt. Gillies groups 50 (Table 3). Ferropigeoniteoccurs as a distinctivephenocryst phaseina unique hybrid dacite flow and very rarely insam- plesof the Mt. Gilliesferrohedenbergite-fayalite rhyolites. / / aSl• _--'•_L__--• -• -•"- / / >73%! Si0! 2 '-•n The conditions of precipitation of ferropigeonite are un- 50 Or known, as it doesnot occur with coexistingFe-Ti oxidesin Fig. 7. Compositeplots of phenocrystfeldspar compositions these Queensland lavas. (mol. %), basedon microprobedata, occurring within the peralkaline Na-enrichmentin the pyroxenes. The extentof Na-enrich- extrusives(subdivided according tototal rock silica contents). EWART:QUEENSLAND TERTIARY ANOROGENIC SILICIC VOLCANICS 10249

(a) Microsyenites (b)Trachytes: • (c)TrachYtes: •

• • / / •. / ß ,aam• x .... a& \ Mg 50 Fe ß Mn

(d) Trachytes: • (e)Trachy (f) Rhyolites:•

Mg 5o Fe + Mn ': . Ca ee el X

/ (g)Binna Bur,•a h) Hypersthene Rhyolites •

"Dacit;,m ß ß / , ",. ,. >73•', SiO• 2:•,t,,•,.. • \ Mg 5o 50 Fe + Mn Fig. 8. Compositeplots of phcnocrystpyroxenes (solid dots) and olivines (solid triangles), based on microprobedata, occurringwithin the microsyenites, trachytes, and various rhyolite types. Trachyte data subdivided according to totalrock silicacontents. Atomic percent.

NaAISi308 + •AO2+ •FeeSiOn-- •AleO 3 ,and also to the syenitesof the Mt. Warning com- albite gas fayalite liquid plex [Ewartet al., 1976];these microsyenites and syenites show late-stageNa-enrichment of the constituentcalcic py- + NaFeSieO6+ 1•SiOe roxenesin the presenceof quartz,alkali feldspar,ilmenite, acmite quartz and fayalite. ( The firstreaction suggests that -'•NaFeSi206 py...... is likely to be s.s.) , • liquid• sensitiveto changesof UA1203'- liquid and foe (and also •Si02 NaAISi308+ •FeeO3 -- •/2AleO3+ NaFeSieO6+ SiOe This is illustrated in Figure 10a (modified from Ewart et albite hematite liquid acmite quartz al.[ 1976],) assumingasio2 quartz ----aFe2SiO nolivine --__ 1, and (ilmenite (pyroxene a•aAlSi308feldspar ----0.75, with the calculationsrepeated for the s.s.) s.s.) three stated oxygen buffers. The results suggestthat for a given aA•203uquid and T, increasingfo2 favorsincreasing These reactions can be applied in principle to the non- aNaFeSi206 • py ...... , whereasif crystallizationfollows a constant peralkaline rhyolitesof the region (containingalkali feldspar, foe buffer, then decreasinga AI203 uquiafavors increasing a pyroxene calcicpyroxenes, ilmenite, quartz, and fayalite), to the micro- NaFeSi206, . ! ß TABLE2. SelectedAveraged Microprobe Analyses ofPhenocrystal Ferroaugites and FerrohedenbergitesFrom Rhyolitesand Trachytes

i 2 3 4 5 6 7 8

sioe 50.12 49.37 49.25 48.97 47.30 48.19 47.54 48.28 TiOe 0.58 0.49 0.54 0.29 0.38 0.43 0.30 0.32 A1203 1.13 1.!5 0.85 0.49 0.63 0.90 0.83 0.24 Fe203* 1.33 1.17 1.46 0.57 0.25 • 2.50 0.92 FeO* 19.27 20.37 22.13 25.50 30.93 30.25 28. I7 30.12 MnO 0.69 0.71 0.81 0.66 0.78 0.75 1.11 0.81 MgO 6.90 5.87 4.84 4.12 0.21 0.20 0.03 0.03 CaO 20.26 19.91 19.99 18.63 18.46 19.43 19.35 18.04 Na20 0.48 0.52 0.47 0.29 0.25 0.38 0.64 0.84 • 100.75 99.56 100.35 99.61 99.19 100.53 100.47 99.60 1, 3•Fraser Island trachytes(samples 36367 and 38986);2--Glass Housestrachyte (sample 38669)'•; 4, 5---Mt. Gilliesrhyolites (33030, 9570); 6•Mt. Flinderstrachyte (38702); 7•Minerva Hills tra6hyte (38739c);8---Mt. Alford rhyolite (387!0c). * Calculatedon ba_sig•Af0 -- 6; cations-- 4. '• Samplenumbers listed in Tables2-8 referto collectionsin Departmentof Geology,University of Queensland. 10250 EWART: QUEENSLANDTERTIARY ANOROGENICSILICIC VOLCANICS

TABLE 3. SelectedAveraged Microprobe Analysesof Phenocrystal ;ogenesisof certaingroups of thesoutl•ern Queensland silicic Orthopyroxenes(1-3) and Ferropigeonite(4) From Rhyolites magmaswill resultin pyroxeneNa-enrichment; this presum- I 2 3 4 ably must representan advancedstage of fractionationof thesemagmas. The stageat whichNa-enrichment actually oc- SiOn_ 48.30 48.12 49.39 49.95 TiO•_ 0.23 0.19 0.53 0.55 curs,however, is likelyto be controlledby additionalparame- A19_O3 0.94 0.36 0.89 0.74 terssuch as fo2 and asio: liquid (i.e., is dependent on the Fe203* 2.46 0.38 -- 2.11 assemblagespresent). In theseQueensland silicic magmas it F eO* 34.18 40.64 35.28 26.87 will be subsequentlyshown that crystallizationoccured at an MnO 0.69 0.80 0.75 0.72 fo2below the QFM buffer,which is evidentlyconsistent with MgO 12.08 8.37 12.57 15.50 CaO 0.71 0.91 0.54 3.03 the developmentof pyroxeneNa-enrichment only in extreme Na•_O 0.14 0.14 0.04 0.24 Fe-enrichedpyroxene compositions. Y. 99.73 99.81 99.99 99.71 Olivines. Compositionsrange from approximatelyFa7o to 1--Springbrook rhyolite (sample 31052); 2--Mt. Alford rhyolite pure fayalite,most analyzed compositions being between Faro (38990); 3, 4--Mt. Gillies rhyolites(samples 33.033, 33037). and Faloo.Olivines occur throughoutthe trachyte-rhyolite * Calculated on basis of O = 6; cations = 4. compositionalrange only as phenocrystphases, but are very rare in comendites. Figure 10b illustratesthe relationshipbetween a^l:o3Uq uid and aNaFeSi:O6py...... as calculatedfrom the secondreaction Amphibolesand and utilizingestimates of {gA1203- liquid for the comenditesand the Three distinct occurrencesof amphibole (Figure 11 and Mt. Gillies metaluminous rhyolites from the reactions and Table 5) can be recognizedas follows,using the nomenclature data listed in Ewart et al. [1976]. It is further assumedthat of Ernst [1968]: (a) pure Fe-hastingsite,occurring as pheno- aFe203itme,it• •_ 0.03, with the activitiesof the other components crystsonly in certain of the Minerva Hills trachytes.This am- as above.The relativelyhigher aNaFeSi:O6 py...... calculatedfor phibole showslittle chemicalvariation. A typical formula is the comenditicpyroxenes is thus consistentwith observation, (Cal.77Nao.73Ko.34)(Fea.56Mno. loTio.34Alo. lo) (All.63Si6.37) althoughit shouldbe noted that reaction(2) is not strictlyap- (OHi.71Fo.29);(b) fluorarfvedsonite,characteristic as a ground- plicable to thosecomendites not containingilmenite. mass phase within the peralkaline extrusives. They are In summary, the reactionsconsidered suggest that the pro- stronglyMg depletedand contain significantZrO2; (c) inter- gressivedevelopment of decreasingaA1205 liquid during the pet- mediate Na-Ca ,classified as ferrorichterites,oc-

NQ

••(•)/Micrøsylnites t t ooloAo•odbt;oß , , -• ...:..: fibl, , , , ; t , ' k

MO 50 Fe + Mn - Na Fig. 9. Compositeplots, based on microprobedata, comparingthe levelsof Na enrichmentwithin the pyroxenesoccur- ring within the microsyenites,trachytes and rhyolites, and peralkaline extrusivs.Atomic percent. EWART:QUEENSLAND TERTIARY ANOROGENIC SILICIC VOLCANICS 10251

TABLE 4. SelectedAveraged Microprobe Analyses of GroundmassSodic Pyroxenes From Peralkaline Trachytesand Comendites I 2 3 4 5 6 7 SiO2 51.59 50.80 52.04 52.80 51.58 48.64 51.76 TiO2 1.43 0.81 3.58 1.16 0.45 0.21 1.18 ZrO2 2.92 2.85 0.56 0.77 2.43 0.49 0.18 AI•O3 0.23 0.17 0.22 0.47 1.01 0.68 0.34 Fe:O3* 22.43 18.50 25.87 29.01 24.28 8.52 29.79 FeO* 6.45 11.04 3.87 1.72 5.79 20.03 1.89 MnO 0.36 0.63 0.35 0.15 0.26 1.01 0.55 MgO -- 0.01 -- • 0.03 0.54 0.06 CaO 2.61 5.35 0.43 0.93 2.61 16.54 1.54 Na:O 11.69 9.56 13.08 13.20 11.59 3.31 12.55 • 99.71 99.72 99.99 100.22 100.02 99.96 99.85 1, 2•Glass HouseMountains comendite and trachyte(38672, 38605); 3•Binna Burracomendite (15779);4•Mt. Alfordcomendite (538); 5, 6---WarrumbungleMountains trachyte (38993), aegirine and aegirineaugite; 7--Nandewar Mountains trachyte (39000). * Calculated on basis of O -- 6; cations -- 4. curring as interstitial phases within the intrusive micro- droxyl exchange.Coexisting Fe-Ti oxidesare absentin these syenites.These amphiboles are more magnesian than the biotite-bearingrhyolites. above-describedamphiboles, in conformity with the associ- Fe- Ti Oxides ated more magnesianpyroxenes in the microsyenites. Biotite is a very rare phenocrystphase in the silicicmag- Nonperalkalineeruptives. Within the lower SiO2 trachytes mas, occurringin certain rhyolites(Figure 12; Table 5). The (<66%), titanomagnetite(ulv/Sspinel 40-75 mol. %) is the sole compositionsare againvery Fe-rich and approachannite. F is oxide phase (including phenocrystsand groundmass).Co- present,but has probably been reducedby posteruptionhy- existing,discrete phenocryst titanomagnetite and ilmenite do occur in some,but by no means all, of the more siliceoustra- chytes;only in the vitreousor extremely fine-grainedlavas are the oxides homogeneous.These spinel phasesare character- ized chemically (Table 6) by very low Mg, V, and Cr levels, and variable Mn. Within the rhyolites,there is an almost completeabsence of a phenocrystspinel phase; the only oxidephase is normally an almost stoichiometricphenocryst ilmenite (Table 7). One ex- ception is found in a single Binna Burra rhyolite flow that a containstrace quantities of almostpure ulv/Jspinel(Usp. 95- •-• Cornendites •.• 99 mol. %; seeEwart et al. [1977]). Peralkaline eruptives. Fe-Ti oxides are commonly absent, -9 -7 -5 -3 -1

lOOO Na .c_• o :?.:...... •ø K mendires 1200øK

-2

50 1200øKMt. Gillies 1000 Rhyolite

-3 -11 -9 -7 -5 -3 -1 Liquid loga AI203 KEY Fig. 10. Schematic'•lotsillustrating the calculatedrelationships //• a Microsyenites / . Comendites betweenaNaF½Si206 clinopy...... andaA1303 liquid, utilizing reactions (1) (Figure 10a)and (s) (Figure10b) given in the text. In Figure 10athe data are plottedfor the threefo: buffers,each calculated at I bar for / ATrachyte/ ,/ (non/ peralkaline)/ two temperatures(1100 ø and 1200øK).Based on thermodynamicdata Mg Atomic % from Stull and Prophet[1971], Nicholls and Carmichael[1969], Marsh [1975],and Robie et al. [1978]. Estimates ofa^•2o 3hqmd' ' forthe rhyolitic Fig. 11. Plot of amphibolecompositions, in termsof Mg, Na, and and comenditicmagmas is basedon reactionsgiven in Ewart et al. Ca (atomic%), from microsyenites,trachytes, and comendites.Micro- [1976]. See text for further details. probe data. 10252 EWART:QUEENSLAND TERTIARY ANOROGENIC glLICIC VOLCANICS

withinpyroxene. Modal abundancesare estimatedto be _•0.001%.Identification aschevkinite has been confirmed by X ray diffraction, as discussedby Izett and Wilcox [1968]. These authors also describe the occurrence of chevkinite in various Cenozoic ash depositsin the western . Partialmicroprobe analyses are presented in Table8 of two chevkinitesoccurring in Mt. Gillies and Mt. Afford rhyolite Fig. 12. Compositionsof phenocrystbiotites, expressed in terms samples.The mostnotable feature is the huge enrichmentin of A1, Mg, and Fe + Mn (atomic%), from two rhyolites.Microprobe LREE togetherwith significantconcentrations of Th, Zr, Nb, data. and Zn. The compositionsare generallysimilar to thosesum- marizedby Viasoy[1966]. but where'present they are mostfrequently represented by il- Allanite. This rare-earthcalcium aluminosilicate occurs as menitcin the comehalites,or leõscommonly a titanomagnetite discrete,nonmetamict, euhedral, stubby prismatic to equant in the peralkaline trachytes. In two comendites, dis- microphenocrysts.It is found only in thoserelatively rare rhy- equilibrium-coexistingFe-Ti oxide phaseshave been ob- 0litesand trachytesof the regioncontaining Fe-biotite or

served.The oxidesin the peralkalinemagmas are highly de- hastingsiticamphibole. Three of theseamphibole-allanite- ß pletedin Mg but maycontain significant Zn concentrationsbearing trachytes give Fe-Ti oxide equilibrationtemperatures (Table 7). of 920ø-980øC,which are significantlyhigher than the upper 2øC- foe data. Only relativelyfew estimateshave been limit of 763øC reportedfor allanitc occurrencein the Bishop made on accountof the paucityof suitablecoexisting Fe-Ti by Hildreth [1979]. Partial microprobe analysesof se- oxidephases. Available data (using the Buddington and Lind- lected allaniresare presentedin Table 8 and are closelycom- sley[1964] geothermometer) are plottedin Figure13, no data parable to thosepresented by Hildreth [1979]. They exhibit extreme LREE enrichment,with significantTh contents. beingavailable , for the pe3alkalinemagmas. Equilibration temperatures,for whichspecific estimates have beendeter- Zircon. Euhedral prismatic microphenocrystsof zircon mined, lie between 885ø and 980øC, with a single micro- are ubiquitous in the nonperalkalinetrachytes and rhyolites syenitegiving 840øC. All data plot betweenthe QFM and but, as previouslydiscussed, are conspicuouslyabsent from WM buffercurves. It hasbeen previously argued JEwart et al., the peralkalinelavas. Microprobe analyses of selectedzircons 1976]that the presence within the rhyolitesof almoststoichio- (Table 8) indicateHREE enrichment(based on Y data), sig- metric ilmenite without a coexistingspinel phase indicates nificant Fe, and Zr/Hf ratios that most commonlyrange be- theirequilibration at an foeclose to theWM buffer,and anal- tween 41 and 48 but decrease to 26 in zircon from the Binna ogywas drawn with the Skaergaardintrusion [Lindsley et al. Burrarhyolite. These Zr/Hf ratioscorrelate with the available 1969].Thus, these Queensland silicic magmas are interpreted data for the whole rocks,which range from 34 to 45 for most to be relativelyreduced, which is consistentwith the occur- rhyolites,44 to 50 for the comendites,but decreaseto 24 in the reneeof very Fe-enrichedferromagnesian phenocryst silicate most highly fractionatedBinna Burra rhyolite. assemblages. PETROGENESIS TraceMicrophenocryst Phases Theanorogenic volcanic suite described is strongly bi- Chevkinite. This rare-earth titanosilicate is restricted to modal. Ewart et al. [1980]have discussedthe possibledevelop- the ferrohedenbergite-fayaliterhyolites in whichit occursas ment of the marie magmas,concluding that althoughorigi- nonmetamict, euhedral, deep-brown, prismatic micro- nally mantle derived, these have undergone lower-crust phenocrysts;it may occur as discrete or asinclusions fractionation within the region of the crust-mantleboundary.

TABLE 5. SelectedAveraged Microprobe Analyses of Biotites(1, 2), CalcicAmphiboles (3-5), and GroundmassArfvedsonites (6-11) 1 2 3 4 5 6 7 8 9 10 11 Sioe 34.63 34.13 38.43 38.58 49.32 49.25 49.45 49.30 49.15 48.34 48.41 TiO:, 5.64 4.77 2.65 2.76 2.27 0.58 0.60 0.85 1.08 0.90 1.28 ZrO2 ..... 1.33 0.78 0.56 0.32 0.77 0.55 A1203 12.5912.68 8.39 8.94 0.63 0.66 0.47 0.53 0.38 0.47 0.73 FeO* 29.98 32.69 33.33 33.00 28.61 34.00 34.}5 34.27 34.05 33.28 33.25 MnO 0.22 0.17 0.68 0.65 0.55 1.19 0.81 1.16 0.68 1.68 0.89 MgO 2.90 2.01 -- 0.05 3.65 0.05 0.06 0.09 -- 0.15 0.66 CaO -- • 9.69 10.02 4.68 2.38 1.55 2.00 1.18 1.81 3.76 NaeO 0.45 0.46 2.16 2.27 6.21 7.59 7.90 7.86 8.14 7.97 6.70 K20 8.51 8.55 1.48 1.63 1.27 1.35 1.38 !.26 1.35 1.17 1.27 F 0.30 0.77 0.49 0.53 n.d. 2.17 2.32 2.44 2.62 2.78 n.d. •- 95.09 95.91 97.09 98.21 97.19 99.64 98.49 99.29 97.85 98.15 97.50 l•FlindersPeak rhyolite (sample 38971), phenocrysts; 2•Binna Burrarhyolite (38675), phenocrysts; 3, 4--Hastingsite,Minerva Hills trachytes (samples 38739a and b), phenocrysts; 5--Ferrorichterite, mi- crosyenite(38979), interstitial phase; 6, 7, 8•GlassHouses comendites (samples 38672, 38606, 38605); 9•BirmaBurra comendite (15779); 10•Warrumbungle Mountains peralkaline trachyte (38994); 11- NandewarMountains peralkaline trachyte (39000). * Total Fe as FeO. •- Less0 for F; n.d., no data. EWART:QUEENSLAND TERTIARY ANOROGENIC SILICIC VOLCANICS 10253

TABLE 6. SelectedAveraged Microprobe Analyses of TitanomagnetitePhenocryst Phases Occurring in Trachytesand Rhyolites 1 2 3 4 5 6 7 0 SiO2 0.09 0.09 0.13 0.27 0.07 0.21 0.07 TiO2 19.24 20.91 21.13 21.44 22.95 22.60 22.11 A1203 0.58 0.07 0.16 0.94 0.98 1.28 0.72 V203 .... 0.04 m 0.17 Cr203 0.01 0.01 m • 0.02 0.04 0.05 Fe203* 31.32 28.31 27.03 24.31 23.96 22.34 25.23 FeO* 48.12 48.10 49.69 50.01 51.77 50.91 50.94 MnO 0.66 0.63 0.86 0.72 0.62 1.13 0.45 MgO 0.12 • 0.02 0.04 0.12 -- 0.14 CaO ..... 0.06 • ZnO 0.37 2.13 n.d. • 0.38 n.d. 0.30 NiO ...... • 100.51 100.26 99.02 97.74 100.91 98.57 100.18 Mol.%Usp 54.6 59.7 61.0 62.8 64.3 65.2 62.5 1, 2raGlassHouses trachytes (38669, 38668); 3--Fraser Island trachyte (38986); 4, 5•Flinders Peak trachytes(38702, 28678); 6--Minerva Hills trachyte(38739a); 7•Mt. Gilliesrhyolite (33030). * Calculatedon ulvospinelbasis; n.d., no data.

A number of possiblepaths of evolution may thus be appro- abundancesdo occur between the silicic and the exposed priate for the silicic magmas.These include: (a) fractional mafic lavas. crystallizationfrom parental basalts;(b) as previously, but (c) Normative compositionsof the trachytesand rhyolites with subsequentmodification by crustal assimilation/equili- plot closeto the two-feldsparboundary curve and the quartz- bration either during or after fractionation;(c) crustalmelting feldspar'minima' in the (Ab-Or-Q)97An3phase system, which with or without subsequentcrystal fractionation. The problem again is consistentwith crystal-liquidfractionation. is complicated by the presence of peralkaline and non- (d) The continuityof phenocrystcompositions through the peralkalineeruptives in the region,often occurringwithin the trachyte-rhyolite compositions,especially notable within the same eruptive center. pyroxenesand olivines that extend to the pure Fe end-mem- The following lines of evidenceare relevant to these alter- bers. An analogy can be drawn here, for example, with the natives: late stagesof fractionation of the Skaergaardintrusion [Brown (a) Within those eruptive centers containing silicic and and Vincent, 1963]. mafic lavas, the siliciceruptives are always subordinatein vol- (e) Fe-Ti oxide equilibrationtemperatures of the trachytes ume by at least a 1:7 proportion. and rhyolites range from 885ø to 980øC. Application of the (b) Trace elementabundance patterns provide convincing olivine-clinopyroxene geothermometer [Powell and Powell, evidence ...... far frne. tinnnl t'•retalli•at;'•n,u,,.+ua.a.,,,a. •,a.u vUutl US I,IIK; 111y Ullt•: 1974] suggeststemperatures (i bar) of 890ø-1100øC, averag- and comendite chemistriesespecially. Examples are the ex- ing 976øC for the trachytesand 942øC for the rhyolites.These treme depletionsof Sr, Ba, V, Mg, P, Cr, and Eu togetherwith increaseto 920-1130øC if 5 kbar is assumed.Finally, Ewart et the variable enrichment of Rb, U, Th, Nb, Y, and Zn, etc. The al. [1976] estimated equilibration temperaturesof approxi- trachytesexhibit thesetrace elementpatterns to lesserdegrees, mately 900ø-1000øC for the Springbrookhypersthene rhyo- favoring an interpretationthat they representerupted mag- lites, basedon calculationsof the ilmenite-orthopyroxeneex- mas at progressivelyless advanced stages of the fractionation changereaction. Although varying in their likely reliability, processes.Nevertheless, discontinuities in the trace element all three temperatureestimates are consistentin indicatingtel-

TABLE 7. SelectedAveraged Microprobe Analysesof Ilmenite PhenocrystPhases Occurring in Trachytes,Rhyolites, and Comendites 1 2 3 4 5 6 7 8 9 10

SiO2 .... 0.!2 • • • TiO2 50.23 50.47 48.69 48.24 51.18 51.54 51.44 50.96 50.52 51.57 A1203 0.02 0.06 0.03 0.08 • 0.06 0.09 0.04 0.04 0.03 V203 ..... 0.02 0.27 • -- • Cr203 m • 0.05 -- • 0.01 0.04 • • • Fe203* 5.00 4.62 6.61 6.09 2.20 0.83 2.39 2.59 3.53 1.38 FeO* 43.86 44.17 42.11 31.29 45.19 45.19 43.24 44.95 43.24 44.39 MnO 1.14 0.73 1.59 11.90 0.96 0.73 0.48 0.72 1.06 1.14 MgO m 0.22 • • • 0.17 1.39 0.01 • • ZnO 0.17 0.09 n.d. n.d. n.d. 0.13 0.06 0.15 1.27 0.94 NiO ...... • 100.42 100.36 99.08 97.60 99.65 98.68 99.40 99.42 99.65 99.45 Mo!.%R203 4.8 4.5 6.5 6.1 2.1 0.9 2.7 2.5 3.4 1.4 1--Glass Housestrachyte (38668); 2mFlinders Peak trachyte(38678); 3, 4--Minerva Hills trachytes (38739a;38739b, single grain); 5•Mt. Alfordrhyolite (38710c); 6--Binna Burrarhyolite (38675); 7• Springbrookrhyolite (38679); 8•Mt. Gilliesrhyolite (9570); 9--Glass Housescomendite (38606); 10• Binna Burra comendite(15779). * Calculated on R203 basis;n.d., no data. 10254 EWART: QUEENSLANDTERTIARY ANOROOENICSILIClC VOLCANICS

-8 of specificsilicic compositionsby crystal fractiona- '•Microsyenit• J , i , , i tion. Theoretically, it has been found feasible to derive the -I0 ß TrachytesJ /_ comendites,trachytes, and rhyolites from various silica-satu- rated and undersaturatedhawaiite magma compositionsfrom - ©Rhyolites J • the region [e.g., Ross, 1977; Ewart et al., 1977]. The mineral -12 assemblagesnormally required are plagioclase+ olivine + aluminousaugite + apatite :t: titanomagnetite+_ ilmenite :t: alkali feldspar.Calculated weight fractions of derivativerhyo- -14 litic or comenditicliquids range between0.05 and 0.18, with o plagioclase being the most important subtracted mineral phase(amounting to between0.4 and 0.5 weightfraction), ex- -16 cept where alkali feldsparis alsoinvolved. Little differenceis noted in thesecalculations as to the proportionsor types of mineralsthat can producethe peralkalineor nonperalkaline -18 , I I I , 7OO 800 900 I 000 derivatives,with the notable exceptionof those solutionsin- T øC volring alkali feldspar.This mineralphase always comprises a Fig. 13. T(øC)-fo2 data deduced from coexisting Fe-Ti oxides proportionately higher weight fraction in the feasible solu- [Buddingtonand Lfndsley,1964], from a microsyenite,trachytes, and tionsinvolving the peralkalinederivatives. Trace elementdata rhyolites.The hatchedarea is the possibleregion of equlibrationof a do not, however,fully supportthese least squaresmixing cal- singleBinna Burra rhyolite sample,in which the Fe-Ti oxidesdo not culations.For example,considering the Binna Burra rhyolites, give a unique solution JEwart et al., 1977]. Three buffer curvesare which exhibit the most extreme trace element fractionation plotted (1 bar), representingquartz-fayalite-magnetite, nickel-nickel oxide, and wfistite-magnetite [after Eugsterand Wones,1962; Hewitt, abundancepatterns in the region, it is possibleto readily ac- 1978]. count for the almost completeSr depletion (causedby the high weightfractions of feldsparrequired in the calculatedso- lutions),but it is not possibleusing the same(or evensimilar) atively high temperaturesof phenocrystequilibration, further leastsquares solutions, to simulatethe complementarylevel of implying that the magmaswere stronglywater undersaturated Ba depletionnor the observedhigh levelsof Rb or Pb concen- during phenocrystequilibration. This is again consistentwith trations. Similar results are obtained for the more fractionated the general paucity of pyroelastics,the paucity of hydrous comendites,although the calculationsdo reproducethe high phenocrystphases, the relatively reducing fo2 buffer condi- Zr concentrations.Clearly, therefore, the leastsquares mixing tions estimatedto have existedduring phenocrystequilibra- calculationsare not wholly duplicatingthe inferred crystal tion, and the occurrenceof extremelyFe-enriched phenocryst fractionationprocesses, and the traceelement data may be in- compositions. dicating the occurrenceof additional fractionationphenom- Application of least squares linear mixing calculations ena, such as liquid-state differentiation, as advocatedto ac- [Bryanet al., 1969]have been undertaken to testpossible com- count for similar trace elementbehavior in the BishopTuff binationsof parental magmasplus mineral compositionsand [Hildreth, 1979].This is, perhaps,emphasized further by the assemblagesthat can in principle accountfor the production observedEu depletion in the Binna Burra rhyolites, which

TABLE 8. SelectedAveraged Microprobe Analyses of MicrophenocrystalZircon (1-5), Chevkinite (6-7), and Allanite (8-10), Occurringin Trachytesand Rhyolites

I 2 3 4 5 6 7 8 9 10

SiO• 32.44 32.59 31.61 32.38 32.44 19.28 18.41 30.40 30.25 30.54 TiO2 0.16 0.05 0.05 0.12 0.05 19.58 18.63 2.95 1.72 2.41 ThO2 0.19 0.07 0.11 0.25 0.17 1.04 0.43 0.36 0.84 0.37 ZrO2 65.5 65.4 65.9 64.2 65.4 0.80 0.44 -- -- 0.05 HiD2 1.33 1.19 1.41 2.19 1.24 n.d. n.d. n.d. n.d. n.d. Al•O3 ..... 0.68 0.14 11.47 12.88 12.25 La203 -- 0.02 -- 0.01 • 12.6 14.3 8.0 5.9 7.5 Ce•O3 0.07 0.03 0.02 0.03 • 20.8 22.2 12.2 11.7 12.2 Nd•O3 0.03 0.03 0.03 0.04 • 8.2 7.6 4.0 4.9 4.2 Y203 0.15 0.33 0.50 0.40 0.41 0.56 0.43 0.19 0.55 0.19 Nb•Os ..... 0.73 0.94 0.05 -- 0.04 FeO* 0.30 0.26 0.15 0.45 0.64 9.87 11.04 17.35 16.27 16.63 MnO ..... 0.10 0.15 0.42 0.23 0.21 MgO ...... 0.19 0.19 CaO ..... 3.50 2.64 9.37 9.45 10.18 Na20 ...... 0.03 0.09 0.13 0.13 F ..... 0.34 0.28 0.25 0.20 0.23 • 100.17 99.97 99.78 100.07 100.35 98.08 97.66 97.10 95.21 97.32 Zr/Hf 42.9 48.0 40.6 25.6 46.1 ..... 1, 2--Mt. Gillies rhyolites(33033, 33030); 3--Mt. Alford hypersthenerhyolite (38990); 4---Binna Burrarhyolite (38675); 5--Springbrook rhyolite (31052); 6--Mt. Gillies rhyolite(33033); 7•Mt. Alford fayaliterhyolite (38710c); 8•Minerva Hills trachyte(38739c); 9---Birma Burra rhyolite (38675); 10-- Finders Peak rhyolite (38971). * Total Fe as FeO; n.d., not determined. EWART: QUEENSLANDTERTIARY ANOROGENIC SILICIC VOLCANICS 10255 basedon REE analysesof coexistingfeldspars and total rock, magmas have been controlled by fractionation processes, is calculatedto require at least 95% feldsparfractionation whichare presumedto be dominatedby crystal-liquidequili- [Ewart et al. 1977]. brium;viscosity considerations, however, suggest some poten- Althoughthe abovediscussion has dealt with possiblefrac- tial difficulties.Using the data of Shaw [1972], the viscosities tionationprocesses in the developmentof the silicicmagmas, of the averageBinna Burra rhyolite and comehalite(Table 1) the previousdata presented cannot be readilyused to evaluate are extrapolatedto be approximately10 •ø and 10s'7 poises at crustalcontributions caused by equilibrationor even fusion 900øC, respectively,assuming anhydrous compositions.If processes.The followingis a summarytabulation of available 0.5% water is assumedto be present,these values drop to ap- isotopicdata for thevarious Queensland magma types [Ewart proximately109'• and 10s'3, respectively. These viscosities are et aL, 1977;and unpublisheddata, 1981]: high,and it seemsdoubtful as to whetherany mechanismof fractionationinvolving crystal settling could occur (simple •O •s•ø(Feldspars) (S7Sr/S6Sr)o 2ø•pb/2ønpb calculationsindicating, for example, settling rates <1 cm Hawaiites, 5.6-7.0 0.7036-0.7046 17.78-18.70 yr-•). One alternativemechanism might be the mechanical tholeiitic segregationof phenocrystsand liquid duringflowage through vertical feeder dykes (flowage differentiation;Komar [1972, Trachytes 4.9-8.7 0.7043-0.7059 17.78-18.65 1976])caused by the phenomenaof grain dispersivepressure, R_hyolites 6.9-10.4 0.7071-0.7086 18.40-18,68 Peralkaline 6.2-10.9 0.7048-0.709 17.78-18.57 which appearsto be favoredby increasingfluid viscosity,pro- lavas viding the dyke width is sufficientlysmall [Barri•re, 1976].In principle, this processwould seem to allow a more uncon- It isclear that considerable isotopic variability exists within straineddegree of crystal-liquidseparation than possible in eachmagma type, but a generalincrease in •80 andinitial Sr conventional,closed-system crystal settling. Further possi- ratiosoccurs with increasing silica. Although these isotope bilities, as previously noted, include liquid-state differenti- dataremain to be fully interpreted, the shift of oxygen and Sr ation[Hildreth, 1979]. Such additional processes seem to be compositionscertainly suggest theincorporation ofsome upper requiredby the extreme fractionation exhibited by the trace crustalcomponents into the magmas either during or after elementabundance patterns within certain groups of thesi- theirdevelopment [e.g., Taylor, 1980]. Moreover, the rela- licicmagmas. tively unradiogenicPb compositionsoccurring within certain of the lavas suggeststhat a moderatelyold and relatively U- Acknowledgments.Most of the analyticalwork undertakenduring depletedcrustal component may also have been at least par- this project has been supportedby grants from the Australian Re- tially involved. searchGrants Committee, including support to use the microprobe facility at the University of Melbourne. The generoushospitality of With respect to the rhyolites, it is still uncertain whether the Department of Geology at the University of Melbourne is grate- they representoriginal crustal melts, or are essentiallyfrac- fully acknowledged.Thanks are also due to A. S. Bagley (University tionatedfrom parental marie magma.A possibleapproach is of Queensland)for continued support with X ray analytical work. provided by the Zr experimentalresults of Watson[1979, p. Thoughtful reviewsof the manuscriptwere provided by W. P. Nash and W. P. Leeman. 417], who notesthat in magmaswhose origins involve crustal partial fusion,their Zr abundancesshould be bufferedat a rel- atively low and constantlevel by residualzircon. Thus the rel- REFERENCES atively lower abundancesof Zr in the Binna Burra rhyolites, Barri•re,M., Flowagedifferentiation: Limitation of the 'BagnoldEf- the hypersthenerhyolites, and to a lesserextent, the Mt. Gil- fect'to thenarrow intrusions, Contrib. Mineral. 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