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The Mineralogy and Chemistry of the Anorogenic Tertiary Silicic Volcanics

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The Mineralogy and Chemistry of the Anorogenic Tertiary Silicic Volcanics 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 Geology& Mineralogy,University of Queensland,St. Lucia,Brisbane, Queensland 4067 The Late Oligocene-EarlyMiocene volcanismof this regionis chemicallystrongly bimodal; the mafic lavas(volmetrically dominant) comprise basalts, hawaiites, and tholeiiticandesites, while the silicic eruptivesare mainly comendites,potassic trachytes, and potassic,high-silica rhyolites.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 plagioclase(albite-andesine), calcic anorthoclase, sanidine, quartz, ferroaugite-ferrohedenbergite,ferrohypersthene, fayalitic olivine, ilmenite, titano- magnetite,and rarely biotite (near annite) and Fe-hastingsiticamphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite-bearingrhyolites only), and allanite (amphibole and botite rhyo- lites only). The comenditesgenerally contain Ca-poor anorthoclase-sanidine,quartz, fluorarfvedsonite, aegirine and aegirine-augite(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 magmas arethus interpreted to havebeen strongly water undersaturated during phenocryst 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-rhyolite 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 Volcano. This again constitutes the eroded remnants of a shield volcano 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 comendite, 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. basalt 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 trachyte 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 dacites;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 feldspars(0.5 to 1.7). boundary,and
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