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CanadianMineralogist Vol. 19,pp.47-63(1981) GRANITE PETROLOGY OF THESTHATHBOGIE BATHOLITH: A CORDIERITE-BEARING G. NEIL PHILLPS'!. VICTOR J. WALL AND JOHN D. CLEMENS Departmentof Earth Sciences,Monash University, Clayton 3168, Victoria, Australia ABSTRACT Sovrvernn The Strathbogie batholith is a high-level. discorLe batholithe de Strathbogie (Sud-Est de l'Ausdant, composite granitoid intrusion in southeastern tralie), dont le toit se trouve ir faible profondeur, Ar:stralia. Granitic rocks containing magmatic gar- est discordanl et composite. l.es roches granitiques net, cordierite and biotite predominate in the batho- qui contiennent grenat, cordi6rite et biotite magmalith. Intrusion involved marginal faulting. roof tiques y sont dominantes. L'intrusion Jest produite par glissement le long de failles marginales, souldlifting and "shouldering aside" of the country rocks. which include the Violet Town volcanic com- vement du toit et repoussement.lat6ral des roches plex of similar Late Devonian age. Textural evid- encaissantes. dont le complexe volcanique Violet ence indicates early crystallization of biotite to- Town. aussi d'Aee D6vonien tardif. D'aprEs les gether with garnet, plagioclase (- Anno) and some textures, la biotite a cristallis€ t6t, avec grenat. quartz. The bulk of the quartz, cordierite and Kpla-gioclase(- Anas) et un peu de guartz. Quartz, feldspar crystallized later during emplacement. Early cordi6rite et feldspath potassique ont. en maieure magmatic cfystallization occurred with P = 4 kbar, partie, cristallis€ plus tard, lors de la mise en place. T > 800oC and water contents in the magma of Le stade initial de la cristallisation a eu lieu i une about 4 tnl, Vo, Thus the Strathbogie magma was pression d'au moins 4 kbar, i une tempErature de relatively hot and water-undersaturated. Emplace800"C ou plus et i partir d'un magma contenant enment occurred at P -0.5 kbar with initial temper- viron 4Vo d'eau (€n poids). Le magma 6tait donc atures well in excess of the wet granite solidus. relativement chaud et sous-satur6 en eau. Pendant Oxygen fugacity was ( QFM throughout the mag- la mise en place, d une pression d'environ 0'5 kbar. ma's history. The Strathbogie rocks have SiO, con- la temp6rature initiale surpassait le solidus du systdme granitique satur6 en eau. La fugacit6 d'oxytents in the range 7A-76 wt. Vo with high K/(K Na * Ca), normative corundum around 2 wt, Vo gane est rest6e inferieure au tampon O.FM pendant toute l'6volution du magma. Les granites contiennent and Me/(Me * Fe) - 0.4-0.5. CaO. MgO, TiO, and P2O5 decrease linearly as SiO: increases. These de 70 i 767o de SiOr (en poids), et possBdent un chemical trends cannot he fully explained by re- rapport K/(K + Na * Ca) 6lev6. environ 2Vo rnoval or variable incorporation of restite materials, de corindon normatif et un rapport Me/(Mg + as inclusions of any kind are scarc€ in all variants. Fe) situ6 entre 0.4 et 0.5. Les concentrations de Differentiation mechanisms involving separation CaO, MgO, TiO: et PzOr d6croissent lin6airement and accumulation of early magmatic biotite. plagio- quand SiO2 augmente. Ce comportement ne peut clase and accessory phases are favored over batch s'expliquer entierement par 6limination ou incorpopartial-melting processes to model the chemical ration variable de mat6riaux rdsiduels. vu la raret6 variation. Magma chemistry, together with inferred des x6nolithes dans tous les facies. On pr6fdre les m6canismes de diff6renciation qui impliquent la early magmatic conditions, and the mineralogy of s6paration et I'accumulation, dans le bain fondu, de inclusions in the granites suggest initial formation biotite, plagioclase et phases accessoires. toutes of the Strathbogie magma bv partial fusion of biotite-bearing quar?ofeldspathic and pelitic rocks hitives, aux processus discontinus de fusion parunder granulite-facies conditions. These source tielle pour expliquer la variation de composition. rocks are apparently below the presentlv exposed D'aprbs la chimisme du macma, les conditions magmatiques initiales et la min6ralogie des x6nolithes lower Paleozoic sequencesof southeastern Australia. The transitional orogenic-anorogenic setting of the dans les granites, une fusion partielle de roches batholith is compatible with the partial-fusion event quartzofeldspathiques i biotite et de roches p6litihaving resulted from intrusion of mafic magma ques, 6quilibr6es dans le facies granulite, serait i I'origine du magma. Ces roches forment le socle into the deep crust. sous-jacent des s6ries du Pal6ozoique inf6rieur qui Keywords: peraluminous granite, granite petro- affleurent i pr6sent dans le Sud-est de l'Australie. genesis, granite geochemistry, granite emplace- Le milieu de transition orog6nique-anorog6nique du ment, cordierite, garnet, Australia, Strathbogie. batholithe concorde avec la fusion partielle qui fut *Now at the Department of Ceology, University le r6sultat de I'intrusion d'un magma mafique dans la cro0te profonde. of Western Australia, Nedlands 6009, W.A., Australia. (Traduit par la R6daction) 47 48 THE CANADIAN MINERALOCIST Mots-cles: granite peralumineux,p6trogendsegra- covite. On the basis of studies of granites in nitiQue,gEochimiedu granite, niise en place des southeastern New South Wales, Chappell & granites,cordi6rite,grenat,Australie. Strathbogie. White ( 1974) have proposed the term "S-type" for these chemically and mineralogically distinctive rocks, inferring their origin as a result INtnopucrroN of partial melting of metasedimentarymaterial. Peraluminous granitoid and volcanic rocks Recent field and petrological studies (Birch & are widespread in the Tasman fold belt of Gleadow 1974, Flood & Shaw 1975) have sugeasternAustralia. These dominantly post-tectonic gested that the peraluminous phases (cordierite batholiths and volcanic complexes contain one and garnet) are largely of residual metamorphic or more of the peraluminousphasescordierite, character (i.e., restite) derived from the source garnet, biotite, andalusite,sillimanite and mus- region of the partial melts. Here we describe a Ftc. l. Regional geological setting of the Strathbogie batholith and other felsic igneous complexes of central Victoria. PETROLOGY OF THE STRATHBOGIE BATHOLITH 49 contrasting situation in yhich textural features in a high-level pluton, subject to little solid-state readjustment, indicate a magmatic origin for the bulk of the cordierite, garnet and biotite. The implication is that moderately peraluminous melts may be generated in the lower crust and evolve by processeslargely independent of the presence of restite phases. sidered as part of the batholith but separated from the rest of the Strathbogie mass by a narrow zone without outcrop. The Strathbogie batholith, which is markedly elongate in an E-W direction, cuts across major structures in the Paleozoic sedimentary rocks. A broad contact-metamorphic aureole surrounds the batholith and is particularly well developed in the Paleozoic sedimentary rocks. At Bonnie Doon, spotted slate, biotite-muscoFIELD RELATIoNS vite, cordierite-muscovite and cordierite-K-feldspar zones have been delineated in the 3-kmGeneral setting wide aureole (Phillips & Wall 1980). Contactmetamorphic effects are present in the Violet The Strathbogie batholith is a large, discord- Town volcanic complex up to several hundred ant, compositebody more than l5O0 km, in area metres from the granite contact (Birch et al. situated 150 km north-northeastof Melbourne 1977, Clemens& Wall 1979). (Fig. 1). The batholith, of Late Devonian age (365 -+ 5 Ma, K/Ar, J.R. Richards, writ- Emplacement ten comm. 1975) intruded folded Siluro-Devonian sedimentary rocks of the Melbourne The undeformed Violet Town volcanic comTrough (Phillips & Wall 1980) and on its north- plex postdatesmajor folding, low-grade regional ern margin, the Violet Town volcanic complex metamorphism and erosion of the Siluro-Devo(Birch er al. 1977) of Late Devonian age. The nian sedimentaryrocks (Birch er al. 1977); all Barjarg granite of White (1954) is here con- these processesare presumably associatedwith + I SIRATHBOGIE GRANITES [T*l c*r."-g- ined sronite Nl rorptyr;ric micrcgronite € Kerrisdsle Cordieriie Mioqdonellite tl Tolhngollook llomblende Migqdarellite (l-tycc.) o locolion . r * o{ *\ (XRf) onoly*d 6 !*****\-#** sanples -/++++++ ++++a+++ + + + + + + + + + '{+ * El ***?***n*********" t' i+ + + + | + + + + + + !. ++ + + + + / F + + +-+ + + + + + a + + + + I o* + + + + + + + + + + + + + + t uf+ + + + + + + + + + + + a+ +/f +++ /\ a ll -- ltyv p pe e (Ct *+*a* + +++ I lf t ** ** ******************************** 1 I t.l.:.:.l.l.l.l.l.l.lri.li.) >t=f $llil; ' T T a-T ++++++La++++ + + J\_+ + + la+ +t +7 -a- ----?+? + + ? ?+? + + { '/ I Bonnie Doon T ? ? + + + R***a 20 KII,OMETBES Flc._2. D-istribution of granite types within the Strathbogie batholith. Cordierite and garnet are present in all variants except the I-type Tallairgallook hornblende microadamellite. 50 THE CANADIAN MINERALOCIST the Tabberabberan orogeny (Middle-Late Devonian: Talent 1965). Similar volcanic rocks in the Cerberean cauldron in Victoria have been dated as post-Tabberabberan(McDougall et al, 1966). Thus, at least the northern margin of the Strathbogie batholith, which intruded the subaerial volcanic pile (present thickness 3O0 m: Birch et al. 1,977),was probably emplaced at a very high level, i.e., pressures( 1.0 kbar. The character of the contact-metamorphic assemblages at Bonnie Doon also suggestsvery Iow pressures(O.5-1.0 kbar) and indicatespeak metamorphic temperatures of around 600oC (Phillips & Wall 1980). Ifence, taking into account the temperature drop (approximately 150oC) across the intrusive contact (Jaeger 1957), the presently exposed granite was emplaced at shallow depth (ca. 3 km) and relatively high temperatures. On a regional scale, the batholith is bounded by several nearly straight E-W and N-S contacts (Fig. 2). Major structuresin the Paleozoic sedimentary rocks on the southern side of the granite are truncated by the contact. However, the strike of the sedimentary rocks appears to be deflected near the western end of the batholith (Vandenberg & Garratt 1976), with the divergence becoming apparent within 20 km of the granite contact (Fig. 1). On a mesoscopicscale, abrupt right-angled steps occur along the contact. Between these steps the contact is straight for distances of l0 to 100 m. The contacts are generally near-vertical, with marginal zones of finer-grained granite up to 10 m wide. Rare aplitic apophysespenetrate the country rock. The development of high-grade contact metamorphic rocks adjacent to the granite appears to rule out major postemplacementfaulting as an explanation of the geometry of the contact. Penetrative deformational fabrics are completely absent from the granite and hornfels. Faulting during emplacement (brittle country-rock deformation) may, however, account for the stepped nature of the contact. Marginal faulting is well shown in the northern region where adjacent volcanic rocks are block-faulted and tilted away from the granite. In view of the paucity of counry-rock xenoliths, stoping is unlikely to be the major mechanism of granite emplacement. The composite character of the batholith is also evidenceagainststoping(White et al. 1974).From the above evidence we infer that emplacement was dilational in character, with some "shouldering aside" of the country rocks, marginal faulting and lifting of the roof of the magma chamber. This scheme differs from proposals of wrenchfault-conrolled emplacement of other discordant batholiths in southeasternAusfralia. In summary, the Strathbogie batholith is a discordant, high-level, post-tectonic batholith of composite character. The northern and eastern marginal portions (at least) representnear-roof zones, as they intruded the Violet Town volcanic complex. Clemens et al. (1979) suggestedthat the Violet Town volcanic complex and the Strathbogie batholith are comagmatic in view of their close temporal and spatial association and similar geochemistry. Prtlocnepuv Granitoid rocks We have subdivided the Strathbogie batholith into four mappable variants on the basis PETROGRAPHY Rock rype Porphyrltic micrcgmnlte "? Eiffiifi, 34.0 CoaEe-gralned 65.0 Kerrlsdale Cordlerlte MlcmadmlI ite Tallangall@k Hornblelde lillcmad@lllte <0.L Fabrlc Pherccrysts Gmundnass Accessorles porphjritlc usually with L0 - 259 euhedral phenocrys6 quartz,5-10m p l a g i o c l a s e( A n q q - r r ) .5 - 1 0 m orthoclase,4-l0m cordlerlte, <6 m blotlte, <5 m garnet (Type A) 0.5-1.5m quartz,orthoclase plagloclase ninor blotlte cordierite and gamet (TypeB) llrenlte pyrrhotite apatlt€ zlrmn mmzlte xenotlm mstly equlgranular 4-10m sm orthoclase,30 - 50 m quartz plagioclase(An5q1e) orthoclase biotlte cordlerlte garret (Type A) nJnor dltto quarrz orthoclase p l a g l o c l a s e( A n 5 a - 2 s ) blotlte (mlnor) quartz, plagloclase orthoclase,blotlte cordlerl te dltto quartz, 6 m p l a g l o c l a s e , 4m - 0.8 m quartz,orthoclase plaqloclase(An,.) irornblende, Sloilte "Porphyritlc' - contlnuus rangeln grainslze .5 - 5 m wlth flner sized donimnt porphyrltlc vlth - & euhedral phamcrysts Averageplagioclase composltionsare quotedcore to rln. sphene mgnetlte llrenlte apatlt! zlrcon 5l PETROLOGY OF THE STRATTIBOCIE BATHOLITH of textural and mineralogical characteristics (Table 1). Although outcrop is good, contacts between the major variants have not been observed, and hence their temporal relations are not clear. The distribution of the four variants is shown in Figure 2, The map shows two dreas of porphyritic microgranite, one in the centre of the batholith and another smaller body in the southwest. flhe terms "granite" and "adamellite" used in this papr are taken from the normative classificationof O'Connor (1965).1 The latter microgranite body appears to have intruded coarse grained granites. Such granites occnpy two areas in the east and the south..The north-sotrth boundary between these two main rock types is marked by a prominent scarp and a zone of no outcrop that is probably the expression of a fault. Areas delineated as coarse grained granite contain small (- 1 km'z) scattered bodies of porphyritic microgranite, particularly in the eastern half of the batholith. A small, separate phlton, the Kerrisdale cordierite microadamellite, granite in IUGS '(Streckeisen terms 1973)" lies at the extreme southwestern end of the batholith. The Tallangallook hornblende microadamellite, the only metahlminous variant observed in the Strathbogie batholith, forms a small (< I km wide) stock intruded into coarse grained cordierite granitesin the east (Fig. 2). Modal and chemical variation throughout the batholith is limited. All outcrops (except the Tallangallook hornblende microadamellite) exhibit cordierite and peraluminous biotite, al; though garnet is lesscommonly developed.Tables I and 2 and Figure 3 show the main modal and textural variations observed. All rocks are granites according to the IUGS classification. In all the granites some of the quartz forms Iarge euhedra, commonly with hexagonal-bipyramidal (6-quartz) habit. Inclusions of biotite and plagioclase are found in this type of large phenocrystal quartz. Plagioclasehas a tabnlar habit and shows complex twinning' and slight normal zoning with superimposedfine-scale, low-amplitude oscillatory zoning. Plagioclase compositionsare given in Table 1. Aggregates of plagioclasesuggestaccumulation of this phase during magmatic evolution. Quartz and biotite are the only inclusion phasesfound in the plagioclase;Orthoclase microperthite forms large, simply twinned subhedra that contain inclusions of cordierite, biotite, quartz and plagioclase. Red-brown biotite forms large, discrete euhedral booklets in all granite variants. Cordierite and garnet types in the granites are listed in Tables 3 and 4 (also see Figs. 3d, 4). Nearly all the cordierite forms euhedral prisms (type A), rarely twinned and inclusionpoor, although some encloses small, euhedral biotite flakes. In the coarse grained granites, cordierite is similar in size (4-10 mm; Fig. 3d) to plagioclase, biotite and quartz. In the finer grained Kerrisdale cordierite microadamellite, cordierite, like all other phases,has a grain size of 1-5 mm (Fig. 3b). Cordierite in the aplitic rocks also has a grain size similar to that of other phasespresent (0.5-1 mm). Many of the large, corroded (type-A) garnets have been partly pseudomorphedby cordierite that preserves the shape of the original euhedral, inclusion-free garnet. Textural and chen'ricalfeatures of the type-B garnets are summarized in Tables 3 and 5. The more leucocratic porphyritic microgranites, especially near the edges of the batholith, contain patches of tourmaline intergrown with the groundmass phases. This feature is most common in variants with a relatively coarse grained g4oundmass. A late-stage magmatic origin is favored for much of the tourmaline, as it rarely exhibits replacement textures. In some porphyritic microgranites, the groundmass contains small patchesof microgranophyre,i.e., eutectoid-like intergrowths of quartz and orthoclase. S-type granites hitherto described (Flood & Shaw 1975, White & Chappell 1977, Chappell 1978, Hine et al. 1978) display textural features interpreted by these authors as indicating that much of the cordierite, garnet" biotite and, perhaps, plagioclasein theserocks is residual (i.e., restite) material. The corroded. highly calcic cores of plagioclasecrystals in some granites are consideredby White & Chappell (1977) to be restite grains mantled by magmatic feldspar. TABLE2. MODAL ANALYSES l4l neral orthoclase 25 29 25 2A 19 22 plagloclase 28- 19 16 24 32 34 I 4 Tr Tr cordierltel6534' biotite95L7983 garnet Tr hofnbiende - 4 sphene - 1 1. typical porphyrltlc nicrogranjt€ (864); 2. cordierite-rich porphyrlt'jc mJcrogranite (756); 3. garnet-rJch porphyrltic nicroqranite (889)i 4. coarse gralned gran'ite (average of 801 and 803)i 5. Kerr{sdale cordierite nicroddare'llite (765); 6. Tallangallook (861). All rcdes are based on >1000 hornblende microadaml'lite polnts. Both phenocryst and groundmss phases were counted. 52 THE CANADIAN MINERALOGIST 1,C?i.rn d ffi# PETROLOGY OF THE STRATHBOGIE BATHOLITH TABLE 3. GARTFT fiPES Conposltlon (ilol. g) granltic rccks rcunded, corrcded crystols (< 15 m) wlth cordierlte-blotite reactlon corcnasi ilrenlte, apatlte, zlrcon I nc lus 1ons 167554 I - 2 m anhedro lntergmn rlth gmundmss felslc phases ln porphyrltlc' mrcroqranl res lncluslonsof high-grade reglonal mtamrphlc rck ver:y smll embayedcrystals ln quartz and feldsDar 53 5 8013 Relative Abundance the mst comn type scattereddistributlon 2 comn lncluded early mgmtlc laie mgmtlc or subsolldus rare restlte, readJusted restlte or xeml ithlc Iype A an-dB garnets ln tie Strdthbogle granltes are analogous to Blrch and cleadox's (1974) lyp€ z afr r gamets (respectiw'ly) fm upper lhvonlan acid volcanlc rccks ln the ceriereair cauldrcn, central victoria. Plagioclase in the Strathbogie granite does not contain obviously distinctive cores. Indeed, the progressive oscillalory zoning exhibited by the plagioclase (e.g., Fig. 3b) appean to be incompatible with a metamorphic origin for this feldspar. Features expected in restite micas (sillimanite inclusions and coarse, foliated aggregates) are entirely lacking in the Strathbogie biotites. Cordierite from metamorphic terranes is commonly anhedral, ovoid in shape and inclusion-rich. Such metamorphic cordierites are a component of the rare gneissic inclusions in the Strathbogie batholith. The euhedral, inclusion-free character of the type-A cordierite in the Strathbogie rocks and the correlation between grain size of the cordierite and the enclosing rock are compelling evidence for emplacement-stagemagmatic crystallization of the cordierite. The initial euhedral shape and inclusion-free character of the type-A garnets suggest a magmatic origin for this phase also. The grain shape, composition and lack of reaction of type-B garnets with the magma are compatible with a late-stage magmatic or subsolidus origin. Thus, the textural evidence indicates that the great bulk of the garnet, cordierite, biotite and plagioclasehas an entirely magmatic origin in the Strathbogie granites. TAELE4. 0currence Cordl erite Type gran'ltic rccks b ct Liquidus phase-relationsof a particular magma composition depend on variables such as P, T, l(O.) and the H:O content of the magma; hence, the inferred sequence of crystallization of a rock can yield information on these parameters. In deducing the sequenceof crystallization in the Strathbogie granite (Fig. 5), inclusion relations, reaction relations and grain shape are critical. We have assumed that small euhedral inclusions in the core of a large euhedral phenocryst are likely to have nucleated no later than the enclosing phenocryst (Vernon 1977). Inclusion relationships in the Strathbogie rocks are shown in Table 6. It has further been assnmed that crystals growing suspended in a magma are likely to grow as euhedran whereas during the later stages of magmatic crystallization crystals will mutually interfere, forming anhedral grains and interstitial fabrics. In the Strathbogie rocks, quartz, plagioclase and biotite are early-crystallizing phases that form large euhedral crystals with few inclusions. Apatite, rarecarth phosphates,zircono ilmenite and pyrrhotite found included in the biotite suggestthat these accessoryphasesalso crystallized early. Cordierite began crystallization before K-feldspar, which is commonly interstitial and anhedral. Quartz and alkali feldspar domi- CORDIERITE TYPES Fom and Paragenesls l4gl(tlg+Fe) Relatlve 0 r l gl n Abundance euhedra(< 6 m) wlth rare lncluslons of blotlte, llmnlte and zircon + 60. 0.56 alrays present primry mgmtlc lafge anhedrawlth blotlte and/or silllmnlte and greensplnel incluslons - 80" 0.46 rare readJusted restlte smll anhedraor, rore rarely, euhedral ln aggregatesconprlsingreactlon 116 on Type A garnets smll scattered anhedraderived by dlssaggregatlon of Type Cl 'lncluslonsof hlgh-grade regioml mtnmrphlc rcck rZV^ larle and smll anhedrawlth jnclusions of blotlte and,/orsllllnanlte and zincian hercynlt4 - 80' - 80. 0.34 acconpanlesType A) garners ] ) of&n present ) mgmtlc ) ln all enclavesof readjustedrestlte hlgh-graderegloml or xerclithlc mtmrphic rcck 54 THE CANADIAN MINERALOGIST AiIALYSES TABLE5. I{INERALCHEfiTSTRYf: RESIILTS OF ELECTR0II-||ICR0PR0BE splnel Rocklto. 872 S.f0: Tlo2 41203 57.32 Feor 32,44 nno 0.09 Zno 8.03 nso z.LZ Cao Bao n.d. t{az0 n.d. KuO ct(A) ct(A) nF{*) .16l 846 1589. 846 0.21 37.08 37.46 0.06 56.64 21.48 21.03 30.47 33.56 33.64 0.42 2.80 2.00 9.86 n.d. n.d. 1.37 3.82 4.41 1,27 1.35 0.62 n.d. O.42 0.05 u6 Bt Br et 7r.4 1589 754 831 35.43 4.75 17.86 23.48 0.09 n.d. 6.33 11.79 4.47 17.04 25.65 0.04 n.d. 5.02 34.m 4,41 18.14 24.14 0.11 n.d. 5.76 n,d. 0,42 8.94 n.d. - 5.381 2.619 0.578 2.9s2 0.544 0.012 5.349 2.651 0.530 3.396 0.532 0.005 36.08 35.88 0.1, 20.68 20.69 35.19 35.76 5.33 3.55 n.d. n.d. 1.69 1.37 1.02 0.62 n.d, n.d, 8.72 or 1618}1rr rrn"?Lron,f(A) cd(A) cd(A) cd(A) cd(B) "l:l*!;.* 1597.- 846- 35.26 n,d, 4.41 0.71 17.95 n.d. 23,t7 22.34 0.20 0.21 n.d. n.d. 6.51 6.72 0.14 n.d. n.d. n.d. n.d. 0.09 n.d. 8.96 9.08 872 754 35.43 4.49 X6.58 24.45 - 49.n 872 831 1597 846 1599 ,846 49.2e 49.13 49.20 48.44 49.29 48.5 n,d. 7.sl 33.42 10.24 0.r3 n.d. 7.01 33.49 9.46 0.18 n.d. 7.61 n.d. n.d. n.d. 32.59 9.97 O.47 n.d. 6.a} 32.49 9.90 0.37 n.d. 6.58 0.04 n.d. n.d. 32.69 11.85 0.50 n.d. 5.90 9..27 13.09 0.75 n.d. 4.61 32.9 U.a 0.c) n.d. 5.11 n.d. n.d. n.d. 9.66 88 Al(tv) At(vl) Fe Tl iln Zn ug Ca Ba l{a K r.952 0,784 0.002 0.171 0.091 - 0.006 2.970 2.992 0.030 0.008 1.961 1.997 t.972 0,74A 2.245 2.248 0.004 0.011 0.190 0.135 0,2L4 0.060 0.455 0.525 0.109 0.116 2.945 0.055 L.934 2.402 - 2.939 0.051 1.936 2.449 0.007 0.368 0.385 5.ai2 2.668 0.635 3.119 0.512 0.014 5.375 2.625 0.601 2,954 0.523 0.027 0.040 0.206 0.167 1.433 1.184 x.326 1.502 1.718 0.090 0.054 0.027 1.732 L.76L L.766 t.766 l,lql(lilotFe) 0.10 O.o7 0.17 0.19 .total Fe expressedas Feo 5.404 2.596 0.915 3.n4 - 1.890 5.413 5.029 5.016 5.020 5.r08 4.987 5.125 5.017 0.013 2.587 0.400 4.020 4.015 4.U7 3.977 4.030 3.955 3.961 3.124 .874 .805 .852 .860 r.020 1.380 1.098 0.516 0.01r 0.016 0.041 0.033 0.044 0.066 0.044 I 596 1.066 1.X54 1.040 1.018 0.9M 0.714 0.880 1.883 0.34 0.55 0.35 0.08 0.06 0,33 0.26 0.30 0.34 S all analyses mmallzed to l00u ercept those of biotlte 0.59 0.55 0.54 'bW=='d , ffi,ffis !::l.t f:ild i"$ :iF !#6ffi ii,;,i , .: L: .Tt* ,gs :: !iE$ l :l !:tirsi !: airairi'\r nii;,: ll rnrn 0.47 0.34 0'45 PETROLOGY OF THE STRATHBOGIE BATHOLITH EARLY 55 batholith, demonstratesour scheme of mineralogical evolution. This rock has a high phenocryst content (5O7o) set in a fine grained groundmass. Relatively high concentrations of type-A garnets and biotite crystals are present, and plagioclasephenocrystsare rather calcic (Anru-ro). Cordierite occurs mostly in reaction rims on garnets, although some euhedral phenocrystal cordierite is also developed. K-feldspar phenocrysts are rare and the groundmass comprises fine grained alkali feldspar and quartz. The groundmass appears to represent chilled (pressure-quenched?)liquid. Thus K-feldspar did not reach saturation in the 889 magma until the emplacementstage, when this composition was still more than SOVoliquid. The high concentration of garnet and biotite in 889 suggests that these phases represent accumulations and emphasizestheir importance in the differentiation of the Strathbogiemagma(s). Inclttsiotts in the granitic rocks Although generally scarce (- l%o) in the tATE Strathbogie batholith, three types of inclusions Ftc. 5. Schematic sequenceof crystallization for have been recognized. Micro-adamellite incluthe Strathbogiegranites. sions are the most numerous. with those of high-grade regional metamorphic rocks next in nated the final stagesof magmatic crystallization. abundance. Near the margins of the batholith, The role of garnet varies. The large, corroded the granitic rocks contain rare angular xenoliths (type-A) garnets are apparently early and have of quartz-rich and pelitic hornfels. These acundergoneconsiderabletextural and some chem- cidental hornfels inclusions are derived from the ical modification. The small, groundmass (type- surrounding country rock. B) garnets are a late-crystallizing phase in the The micro-adamellite inclusions are rounded. porphyritic microgranites. The deduced sequence l0-lo0 mm in diameter, composedof large (5 of crystallization is portrayed in Figure 5. mm) interlocking anhedra of quartz and orthoAlthough sequences of crystallization for clase, sieved with inclusions of plagioclase laths granitic magmas based on the above criteria (Anrr) and small euhedral biotite flakes. Acare commonly equivocal, the grain shapes and cessory zircon, ilmenite, apatite and rare sorgrain-boundary relations of the Strathbogierocks dierite euhedra are also present. The microindicate little subsolidus readjustment. Further- adamellite inclusions are interpreted as accumumore, the crystallization sequence deduced for Iations of small, early-formed biotite, plagioclase the Strathbogie magma closely matches that in- and cordierite crystals in interstitial liquid that ferred for the related Violet Town volcanic suite crystallized as quartz and orthoclase.These rocks (Clemenset al. 1979). Strathbogiespecimen889 probably represent a chilled phase of the granite (Tables 2 and 7), the most mafic variant of the formed during the early stagesof crystallization and later dislodged from the wall or roof of the TABLE6. INCLUSION RELATIONS IN STRAIHBOGIE !,IINERALS magma chamber and reincorporated in the main INCLUSION PHASE HOSTPHENOCRYSI bulk of the magma (c1., Bateman & Chappell Accessories Ct(A) BicdPlQ 1979, pp. 474-475). The micro-adamelliteincluct(A) sions resemble the Kerrisdale cordierite microBI adamellitein both texture and composition. x cd(A) The inclusions of high-grade regional metaPI morphic rocks (10-100 mm in diameter) are rounded in shape, fine- to medium-grained, and q always have a pronounced foliation (Fig. b). Kfs Biotite and cordierite are segregatedinto melano- 56 THE TABLE7. CANADIAN BULK-RoCK CHEMICAL 0ATA(XRF)At{DCIPHiloRtits si02 69.78 72.2r 74.63 71.83 72.78 T102 0.57 0.34 0.09 0.36 0.33 0.30 41203 14.56 L4.@ 13.66 14.25 14.0t 14.22 2.42 1.05 2.73 2,32 t.8t o.o1 0.04 0.71 7!.& Feo* 3.66, ltno 0.01 ifgo 1.40 1.02 0.59 1.04 0.95 Cao 1.85 r.67 L.29 2,14 !,57 1.85 MzO 2.60 2.86 2.94 3.08 2.65 4,25 K20 4.74 4.50 4.91 3.93 4,52 4,69 P20S 0.19 0.22 0.22 0.28 0.19 0.09 sog 0.10 0.08 0.13 0.10 0.06 0.06 L o s s* O.7l 0.47 0.61 0.50 0.54 0.33 Totn l 100.17 100.45 100.12 100.25 99.93 M9/(MgiFe) .50 .40 .42 .41 34.12 23.46 28.47 32.1,2 2.24 2.57 0r 28.t9 26.60 29.20 23.29 26.72 27.72 Ab 22.17 24.20 24.96 26,L4 22,42 35.96 An 7.99 6.85 4.96 8.84 6.55 5.88 tn 1,.7L ?1 A7 r,70 - DI Inclusions of country-rock hornfelses are recognizableby their modal and mineralogicalsimilarity to the Paleozoic rocks in the contact aureole (Phillips & Wall 1980). In contrast to the regional metamorphic inclusions, the hornfels inclusions lack well-developed foliations. The paucity of hornfels xenoliths and their restriction to marginal variants in the Strathbogie batholith imply that high-level cohtamination is of little importance in Strathbogie petrogenesis and that stoping of wall rocks is not a major factor in the emplacement of the Strathbogie magmas. GEocHEMrsrRY ee.es Mineral chemistry a c la MINERALOGIST 2.28 Hy 9.32 6.42 3.25 7.03 6.10 3.52 I1 1.08 0.65 0,r7 0.68 0.63 0.57 Ap 0.45 0.52 0.52 0.66 0.45 0.21 * Total Fe expressedas Feo. * Fusion loss. l. Speciren889, the mst mflc variant of the Strathbogie granite; 2. specjren 717, representativecoarse gralned granite, Strathboglebathollth; 3. speciren 754, representatlve porphyritic nicrogranlte, strathbogie bathollth; 4. speciren 765, Ksrisdale cordierlte nlcrcadarellite; 5. averageof 2l S-type Strathbogie granltoid rocks; 6. averageof 2 analysesof Tallangallook hornblendeadarelllte. cratic layers separatedby layers of felsic phases. Significant assemblages developed include (a) Or-Gt-Bi*Cd-Pl-Ilm-Ap-Po-Q and (b) Hc-Cd -f Sill. [Abbreviations are listed in Appendix l.l Descriptions of the garnet and cordierite types from these inclusions are given in Tables 3 and 4. Garnet is very rare, although at one time, it was probably present in all the regional metamorphic inclusions, as evidenced by pseudomorphs of garnet by minerals of assemblage (b) above. Readjustment of the regional metamorphic mineral assemblage by equilibration with the enclosing granitic magma has occurred, as indicated by the low-pressure assemblages. The alkali-poor, silica-depleted character of these inclusions is compatible with a restite origin, although their low-pressure mineralogy precludes estimation of source-regionconditions. Ferromagnesianphasesand plagioclase in the porphyritic microgranites and coarse grained granites were analyzed using the T.P.D. energydispersive electron microprobe at the Research School of Earth Sciences. Australian National University. Some analyses were performed on the Jeol JXA-SA and the ARL EMX microprobes at the Departments of Geology, Melbourne and Monash universities, respectively. The compositionsof the various garnet and cordierite types are of particular interest. All Strathbogie garnets are almandine-rich pyralspites (almandine: 74-80 mol. %). The Mg and Mn contents show the greatest variation (Table 3). Garnet in the fine grained groundmass, free of reaction coronas (i.e., type B), is relatively spessartine-enriched,with only minor . pyrope and grossular (e,g., specimen 754). Large garnets with reaction rims (type A) are relatively enriched in pyrope and grosstrlar, with only 4.57o spessartine.Rims of typeA garnetsare Fe-Mn-enriched and Mg-depleted. The reason for the Mn enrichment is the strong partitioning of this element into the garnet relative to the cordierite in the reaction rim. Euhedral (type-A) cordierites generally have Mgl(Mg + Fe) in the range 0.54-0.59. (Throughout this paper Fe and FeO refer to total iron in the sample. Structural formulae of the ferromagnesian phasesindicate that Fe3+ is minor in most minerals.) The chemistry and optical properties of type-A and type-B cordierites are quite different (Table 4). Type-B cordierites and cordierites in gneissic inclusions are more Fe-rich [average Mg/(Mg + Fe) 0.461 and contain more Mn than type-A cordierites. Type-C cordierites are similar in optical properties to type B but have the lowest Mg,/ PETROLOGY OF THE STRATHBOGIE BATHOLITH 57 (Mg * Fe) (average- O.34) of the cordierite Table 7. Most of the batholith comprises coruntypes. The significance of these points will be dum-normative (usually ) l%o C) S-type grandiscussedin detail in the section on petrogenesis. ites. whereas less than O.lVo of. the batholith Biotites in the Strathbogiegranites do not vary consists of the diopside-normative Tallangallook greatly in composition; they are all rather high hornblende microadamellite (I-type). in siderophyllite-annite, with an average comCompositions of all S-type granitoid rocks position (K,Na) r.e(Fes.rMgr.rTio.sAle.6) (Alr.oSis.r) are highly silicic (70-76% SiOr) and moderOuo(OH,Cl,O,F). Semiquantitativecrystal-spec- ately periluminous. All have rather high K/(K trometer analysesshowed that fluor- and chlor- + Na + Ca) ratios, reflected in the high proine-biotite substitutions are minor ( < 5 mol. portion of modal alkali feldspar (Table 2). 'To). Mg/(Me * Fe) is near 0.30, falling The ratio Mg/(Mg * Fe) is generally in the between the values for type-A cordierite and range 0.4G-0.50 and shows no distinct trend garnet and slightly lower than the bulk-rock with SiO: content. Two exceptionally Fe-rich values (Table 7). The biotite in the cordierite- rocks [Mg/(MS * Fe) = 0.31 and 0.191have biotite reaction rims surrounding type-A garnet significantly higher MnO and lower MgO and is similar in composition to other biotites ex- CaO. cept for its very low Ti content. Normalization Harker plots show strong correlations between of the Ti-rich (primary, magmatic) biotites to several oxides and SiOs content (Fig. 6). The 16 cations, assuming no octahedral vacancies more silicic variants are markedly depleted in (Bohlen et al. 198O), suggestsextensiveTiOz P,O', TiOg, CaO, FeO and MgO. From - (MgFe)(OH)z substitution,a featurein com- 7O7o to 76%ioSiOt there is a 3OVo decreasein mon with other high-temperature biotites PrOs and about a 70Vo decreasein the other (Bohlen et al. l98O). four oxides. No significant change in AlzO:r or Green spinel (Tables 4, 5) contains consider- K/Na is observedwith increasing SiOr, although able amounts of the hercynite and gahnite com- a large scatter of molar K/Na ratios (0.7-1.4) ponents in solid solution: hercynite 72/o, gah- is present. nite 2lVo. Some Mg and Mn are also present, Porphyritic microgranites are commonly more but the magnetite component comprises ( 2 silicic (average 73.4Vo SiO:) than the coarse mol. Vo. Similar zincian hercynites have been grainedtypes(average72.3%SiOa).Spatialchemreported in inclusions in acid volcanic rocks of ical variations within the batholith are also the Cerberean cauldron and Violet Town vol- evident. The most silicic rocks are the porphycanic suite in Central Victoria (Birch et al. ritic microgranites from the northwest (average 1977't. 74.6Vo SiOr). Coarse grained granites from the east have much lower SiO: (average 71.4Vo) Rock geochemistry and have a different average composition from Selectedbulk-rock analyses(XRF) and CIPW the coarse grained granites in the west (average norms of the Strathbogie rocks are presented in 73.O% SiO). 2.4 2.O 0.6 o.4 CaO 0.8 o.4 o.2 0.1 1.6 1.2 0.8 0 ' 6r 'r--l1-l rugfirvdF"r o.4 70 71. 72 73 SlO2 wt % 74 75 76 69 70 71 72, 73 74 75 76 SiO2 wt % Ftc. 6. Harker variation diagrams for 23 analyzed specimens of Strathbogie granite. Large dots are the two analyzed l-type rocks. 58 THE CANADIAN Although the Strathbogie I-type micro-adamellites are not well distinguished on the Harker diagrams of Figure 6, their higher NarO/KzO and lower (FeO * MgO) are distinctive. The I-type granitoid rocks are diopside-normative. PrrnoceNesls Cottditions of emplacement Pressure:Low confining pressureduring emplacement of the exposed granites is inferred from two lines of evidence. The granites intrude the Violet Town volcanic complex, which appears never to have had any significant cover (Birch et al. 1977). Hornfels developed on the south side of the batholith contains assemblagesindicative of pressures€ | khar (Phillips & Wall r980). Temperature' ancl HzO content of melt: E,mplacement temperatures san be limited by assemblagesdevelopedin the contact aureole. The high-grade cordierite-K-feldspar assemblages in hornfels adjacent to the granite imply temperaturesof the order 580-620'C (Phillips & Wall 1980). Using the methods of Jaeger (1957), this is equivalent to intrusion temperatures of 750oC (above the wet granite solidus) to 82O'C, if the sountry rocks are assumed to have been at temperatures between 100 and 200oC (Phillips & Wall 1980). Emplacement temperatures can be further limited by utilizing the crystal-melt equilibria determined experimentally by Clemens& Wall (1981). These experimental studies of the crystallization phase relations of the porphyritic microgranite (specimen 889) were carried out at P - 11 kbar, T = 600-900oC, a(HzO) = 0.11.0 and log l(O,) - QFM to QFM-0.5. This rock was chosen for experimental study because of its well-characterized sequence of crystallization and its apparently little-fractionated composition. However, it now appears to us that specimen 889 may have undergone enrichment in phenocrysts of garnet and biotite, although this would have little effect on the conclusions we reach below. Specimen 889 was inferred (above) to have been at least 5OVo liquid with few if any Kfeldspar crystals present when the magma was intruded. Experiments at I kbar (for near-saturation H:O contents in the melt of 3.5-4.5 wt. 7o) show that the K-feldspar saturation boundary occurs at about 750'C. This suggeststhat the emplacement temp€rature was at least MINERALOGIST 750oC. The absence of orthopyroxene (or obvious pseudomorphs after this phase) indicates that the Strathbogie magmas were at temtr'eratures below about 8O0oC at the level of emplacement(Clemens& Wall 1981, Fig. l). Oxygen fugacity: The occurrencesof coexisting ilmenite and pyrrhotite as inclusions in the Strathbogie type-A garnets, the absenceof magnetite and the magmatic almandine-rich garnets all suggest that l(Or) was low in the Strathbogie magmas, probably around. QFM. Oxygen fugacity in S'type rocks cannot be calculated r.rsing the Buddington & Lindsley (1964) approach owing to the absence of coexisting FeTi oxides. Froese (1973) has tabulated thermochemical data for the reaction Alm * Oe = Cd * Mt * Q, which provides an upp€r limit for l(Or) (since magnetite is absent from the Strathbogie rocks). This yields. at P = 2 kbar, log l(Or) < -16.8 at 700'C and ( -14.6 at 80O'C. The low-pressure assemblagespinel-cd-sill is common as pseudomorphs, often around metamorphic assemblagesin some inclusions in the Strathbogie batholith. These phases can be related by the reaction Cd + O, - Mt + Sill * Q. Assuming that these inclusions equilibrated with the enclosing magma at around 2 kbar, this assemblagecan be used to evaluate l(Ou) from the data of Froese (1973). Cal' culations using the analyzed cordierite and coexisting spinel compositions, and assuming a(SiO') = 1.0, yield log l(O,) - -18.3 at 700'C and -15.5 at 8@oC. These figures are compatible with the upper limits for l(Oz) set above. The mole fraction of magnetite in the analyzed spinel is small, and this constitutes the major source of error in the calculation. However, an uncertainty of 3OVoin ar."nro'rcorrespondsto a changein log l(Or) of only O.3. Thus a reasonable estimate of l(Or) for the Strathbogie magma (at the time of equilibration of the cordierite-spinel-sillimanite assemblages) is abont an order of magnitude below the QFM buffer. Early crystallization Experimentsby Clemens& Wall (1981) also give an indication of the ranges of P, T and HzO content during early crystallization of the Strathbogie magmas. The sequence of early crystallization for rock 889 is Desl modeled by the experimentallly determined phase relations for P between 4 and 7 kbar, T between 800 and 850'C and water contents of the melt in PETROLOGY OF THE STRATHBOCIE BATHOLITH the range 3-5 wt. Vo. For HzO contents less than 3 wt. Vo, the biotite saturation boundary is depressedto temperatures below the K-feldspar saturation boundary. For water contents greater than SVo, the garnet saturation boundary is depressed to such an amount that the inferred sequence of crystallization cannot be duplicatedwith garnet as a nearJiquidus phase. Summary: conditions of crystallizatiotr The Strathbogie granite magma, prior to emplacement, was of relatively high temperature (750-850oC) and markedly water-undersaturated. Water saturation of the magma was attained in the 11 kbar range, with less than 5O7o crystallization; by this stagesome garnet, plagioclase, biotite and quartz had developed. Cordierite and K-feldspar formed Iargely at emplacementlevels. Origitt of chemical variation in the Strathhogie batholith Several mechanisms have been proposed to explain the chemical variation observed in the S-type batholiths (e.g.. White & Chippell 1977. Clemens et al. 1979). These mechanismsand their relevance to the Strathbosie batholith are examined below. "Unmixing" ol restite materiill: Chemical trends in granites have been explained by a processof progressive "unmixing" of restite (refractory phases) from a "minimum-melt" granitic liquid (Wyllie et al. 1976,Wyllie 1977,White & Chappell 1977, Chappell 1978). This process requires that the undifferentiated magma leave its place of origin carrying some residual solid material. For the Strathbogie rocks, possible restite phases include biotite, garnet, orthopyroxene, quartz and plagioclase. Progressive removal of restite plagioclaseand biotite during magma migration is essentiallythe same prosess described below in the crystal fractionation model, except that the phasesinvolved are considered to be restite material rather than products of niagmatic crystallization. Within the Strathbogie batholith there is no firm evidence for calcic plagioclase of restite origin (c1., White & Chappell 1977) or aggregates of restite biotite. The euhedral single crystals of biotite, in particular, are unlike the biotite segregations in migmatites (Harme 1965). Inclusions of any kind are rare and not markedly concentrated in the more mafic variants. The textural evidence has shown that the 59 bulk of the cordierite and garnet in the Strathbogie batholith is magmatic in origin. Cordierite with inclusions of spinel and sillimanite is easily recognized (texturally and chemically) as readjusted restite or xenocryst material, but comprises less than l7o of the granite. In such quantities, restite cordierite cannot have played a major part in the chemical evoltttion. Restite unmixing thereforeseemsunlikely to accountfor any significant chemical variation seen in the Strathbogie granite. Crystal fractionation: The petrographic data for the Strathbogie rocks indicate that biotite, plagioclase and quartz are the volumetrically significant early-crvstallizing phases. Numerous ilmenite and apatite inclttsions are present in the biotite. Based on the modal data, large amounts of cordierite and garnet appear unlikely at this staqe. Oualitatively.fractionation of plagioclaseand biotite with inclusions can explain the desrease in TiOg. P,O'. FeO. MgO and CaO as SiOr increases(Fie. 6). Quantitative mixinq calculations using rock and mineral compositionswere performed with the least-squarescomputer program PETMIX III (Taylor et al. 1973).'The Strathbogie primary magma was assumed to have a composition similar to the average of the 2l analyzedS-typerocks (SiO, - 73'ttt. 7o, Table 7). This composition was chosen as an approximationto the overall compositionof the magma near its level of emplacement. More mafic variants probably contain accumulations of early-formed phenocrysts,as discttssedabove. Separationof 8% plagioclase(An,u), 67o bio' tite, 3Eo quartz and O.7Voeach of ilmenite and apatite from the primary magma composition can account for the chemistry of the most felsic (SiO' = 76 wt. % ) Strathbogie granites. Given the Fe-rich biotite compositions in the rocks (Table 5), problems arise in explaining the large decrease in MgO with increased SiOr content.The liquidus biotite, however,was probably more Mg-rich, but re-equilibration with the magma at lower temperatures has cattsed a drop in the ratio Mg/(Mg * Fe). As cordierite and garnet have substantial effects on AleOs, FeO and MgO contents, the role of these phases has been minor. The lack of variation in MnO also suggests that garnet was not a major participant in the fractionation processes. The importance of fractionation processesin the chemical evolution of the Strathbogie magmas is further supported by the distribution of tourmaline in the batholith. Tourmaline, and hence boron. are enriched in the leucocratic 60 THE CANADIAN (high-SiOr) variants by at least an order of magnitude relative to the bulk of the batholith. Simple removal of even relatively large proportions of restite material could not have produced sush strong enrichment in incompatible elementssuch as boron. Ilowever, liquids mechanically separated from univariant or invariant crystalline mushes may exhibit exponential increases in incompatible elements, whereas the major-element contents of the liquids may not vary much. Extreme fractionation processes must therefore have been involved in the development of the highly silicic variants. Fractional partial melting: Progressive partial melting of a particular region of crustal material leads to the production of successively more mafic, less silicic magmas. Thus, removal and emplacement .of successivemelt fractions may lead to a compositionally zoned or composite batholith. Ideally, unless the site of melting shifts, this process produces a granitic suite in which the younger plutons are the more mafic. Melting and crystallization experiments on granitic compositions (Winkler 1974, Winkler et al. 1975,Wyllie 1977, Clemens& Wall 1981) provide some idea of the compositionsexpected in a fractional partial melting sequence.Near the solidus, liquids are rich in K-feldspar, plagioclase and quartz components and are Fe-MgTi poor. With increasing temperature, Fe-Mg phases become more soluble in the melt, and initially low Mg/ (Mg * Fe) ratios are expected. Only at high temperaturesdo more mafic melts, with ahigher Mg/(MB f Fe) ratio, form. Since garnet crystallization is favored by higher Fe, and cordierite crystallization by higher Mg, such trends would lead to more garnet in the silicic and more cordierite in the more mafic variants. The opposite trend is observed in the Strathbogie batholith. The separate fractions of partial melt should crystallize as plutons in which the mafic minerals have differing Mg/ (Mg + Fe) valuescharacteristicof each pluton. The major variants of the Strathbogie batholith all have the same suite of early magmatic phases. Suntmary Although re-equilibrated restite materials exist in the Strathbogie batholith, their low concentration, even in the mafic granites, suggests that restite unmixing was not a major process involved in the magmatic evolution. Fractional partial melting would produce chemical trends that run counter to those observed. Thus. to. explain the variation in the Strathbogie batholith, we prefer a differentiation mechanism involving separation of early crystallized biotite and plagioclase with minor garnet, cordierite, quafrz and accessory Phases. Nature ol the source The available Sr isotope data (Pidgeon & Compston 1965, McDougall el al. 1966,Brooks & Leggo 1973, Flood & Shaw 1977) indicate that initial 8?5r/865risotopic ratios of eastern Anstralian S-type granites and volcanic rocks have a mean value in excess of 0.710. These high 875r/865rratios, together with the peraluminous nature of the granitoid suite, suggest that magma formation involved partial melting of old metasedimentaryrocks (Chappell & White 1974"White & Chappell 1977'). As pointed out by Flood & Shaw (1977), S-type rocks with lower initial ratios (around 0.706) suggestthe involvement of vottnger or less-evolved crustal sources for the magmas. Experiments carried otrt bv Green (1976. 1977). and Kilinc (1969. 1972\ show that liquids formed by partial melting of sillimanite-bearingpelitic materials must crvstallize peraluminous minerals' Green's inferred granitic liquids have, in general, more normative corundum than the Strathbogie granites (Table 7). Experimentalresults reported bv Clemens & Wall (1981) for a natural granitic composition from the Strathbogie batholith (specimen 889) seededwith sillimanite show rhat a liquid of this composition is too poor in alumina for equilibrium with sillimanite.This suggests that sillimanite-bearing pelitic rocks are unlikely to be the dominant litholoey in the source region of the Strathbogie magma. A muscovite-bearingsollrce region is ruled out bv the high temperaturesand torv water contents inferred for the magma. The most direct evidence concerning the nature of the source region of the gr4nitic magmas comes from the granites themselves.Apart from hiehJevel hornfels and the magmatic micro-adamellite, the most common inclusions encountered in Strathbogie granites are hlghgrade pelitic gneisses.Provided that these inclusions represent either source-region material (Chappell & White 1974) or readjusted restite material from the source region, these inclusions indicate some of the probable assemblagesin the source area during the extraction of melt. In the Strathbogie batholith the rare gneissicinchrsionscontain quartz, biotite, sillimanite"plagioclase, minor orthoclase, cordierite and hercynite. and rare garnet. The cordierite-hercynite PETROLOGY OF THE STRATHBOGIE BATHOLITH 6l assemblagerepresents a low-pressure readjust- postulate an anatectic source-region, characterment of an original garnet-sillimanite assem- ized by quartzofeldspathic to p"l-itic,'high-grade blage by reactions similar to regional metamorphit rocks. Such rocks-are not presently exposed in central Victoria. The min: FeaAlzSi.rOrz 2Al2SiO6 FeAlsO+ 'r8 + * FezAlaSioo.eralogy of these source rocks would most likely Alm Sill Hc Cd involve quartz, orthoclase, plagioclase, almanIndeed, sorne cordierite-hercynite aggregatesap- dine-rich garnet, Fe-rich biotite and orthopypear to pseudomorphoriginal idioblastic garnets. roxene. Rocks containing sillimanite, cordierite Accessory phases include ilmenite, pyrrhotite, and hercynite may also have been present but apatite and zircon. Thus, the mineralogy of in minor proportions. Accessoriesin the source these inclusions suggeststhat some of the source rocks would include ilmenite, pyrrhotite, aparocks for the Strathbogie magma contained as- tite and zircon. Magnetite and hornblende are semblagesinvolving combinations of plagioclase, unlikely. Although the actual mineral assemquartz, garnet, K-feldspar, biotite and silliman- blages are unknown, this range of minerite along with ilmenite and pyrrhotite. alogy, the relatively high temperatures and the calculations based on reac- HrO-undersaturated nature of the granitic melts - Thermodynamic tions An Gr + Sill + Q (Ghent 1976) and produced are consistentwith granulite-faciesreAn * Fs - Gr + Alm * Q (Phillips 1978) gional metamorphic conditions in the source indicate that garnet with a composition similar region. Consideiing ( I ) the high geothermal to the early, type-A garnetsfrom the Strathbogie gradient required to generate the high temperagranites could have been in equilibrium with tures at the moderate crustal depths proposed plagioclase(- An'o) - sillimanite - quartz or pla- for the source region and (2) the transiiional gioclase(Anro) - orthopyroxene(Enso) - quartz orogenic-anorogenic setting of the Strathbogie at n 5 kbar and 800"C.'Any pyroxene formerly batholith, we suggest that this granulite-facies present in the magma or its restite inclusions partial-fusion event was brought about by emhas reacted out and is no longer found in the placement of mantle-derived material in the assemblages. lower crust (c1., Wells 1980). The l(O,) during early crystallization of the Srathbogie magma was calculated to be about AcKNowLEDGEMENTS an order of magnitude below the QFM buffer. Together with the inferred low oxidation ratio We are grateful for suggestionsand criticisms of the magma, this suggestsanatexis of highly made by I.A. Nicholls, A.J.R. White. W.D. reduced source rocks that may have contained Birch. and D.B. Clarke. R.H. Flood and R.A. graphite during part pf their nretamorphic Binns reviewed earlier manuscripts. A.R. Milnes history. gave valuable advice during the bulk-rock analPartial melting of a metapelitic rock ysis. Ms. G.E. Earl drafted the diagrams. will commonly yield K-feldspar-bearing restites owing to the high K/(Na * Ca) rarios of these RrrenENcrs compositions. Thus, magmas derived from such highly aluminous source rocks will usually be BarrvlN, P.C. & CHeppELL, B.W. (1979): Crystalsaturated with K-feldspar all the way from the lization, fractionation, and solidification of the source region (Clemens & Wall l98l). HowTuolumne intrusive series. Yosemite National ever, K-feldspar crystallizes late from the SrathPark. California, Geol, Soc, Amer. Bull. 90. 465-482. bogie magmas and plagioclaseis a near-liquidus phase. We would therefore infer that the bulk Btncs, W.D., Crn*rNs, J.D. & Pmr,rrps, G.N. of the Strathbogie source region was under(1977): Dev<inian acid igneous complexes in saturated with respect to K-feldspar at the time central Victoria. SecondAust. Geol. Conv. Exof magma extraction. Appropriate source rocks cursion Guide C. for the Stratbbogie magmas would include & GLEADow, AJ.W. (1974): The genesis weakly peraluminous metasedimentary rocks or of garnet and cordierite in acid volcanic rocks: other quartzofeldspathic rocks poor in K-feldEvidence from the Cerberean cauldon. central spar. Pelitic inclusions in the Strathbogie granVictoria, Australia. Cont. Mineral. Petrolosv ites thus probably represent highly refractory, 45. t-13. melt-depleted materials not typical of the bulk BoHLEN, S.R., Peacon, D.R. & EssrNb, EJ. (1980): composition of the source region of the magma. Crystal chemistry of a metamorphic biotite and In snmmary, for the Strathbogie and other its significance in water barometry. Amer. Mincenftal Victorian S-type granitic magmas, we eral. 65, 55-62. 62 THE CANADIAN Bnoors, C. & Lrcco, M.D. (1973): The local chronology and regional implications of a Rb-Sr investigation of granitic rocks from the Corryong district, southeastern Australia. Geol. Soc. Aust. "r. 19, 1-19. BUDDTNGToN, A.F. & LrNlslrv, D.H. (1964): Irontitanium oxide minerals and synthetic equivalents. J. Petrology 5, 310-357. CHernELr, B.W. (1978): Granitoids from the Moonbi district, New England batholith, eastern Australia. Geol. Soc. Aust. l. 25, 267-284. & Wntrs, A.J.R. (1974): Two contrasting granite types. Pacilic Geol.8, 173-174. CLEMENs, J.D., PHutps, G.N. & WnI,r, V.J. (1979): Petrogenesis of an S-type volcanic-plutonic association - the Strathbogie igneous complex, central Victoria. /rr Symposium on Crust and Upper Mantle of Southeastern Australia Bur. Mineral Rer. Rec. L979/2, 8-9 (abstr.). & Weu, V.J. (1979): Crystallization and origin of some "$type" granitic magmas. /n Symposium on Crust and Upper Mantle of Southeastern Australia. Bur. Mineral Res. Rec. L97g/2' l0-11 (abstr.). MINERALOGIST KILtNc, I.A. (1969) : Experimental Metantorphism and Anatexis of Shales and Graywac&es. Ph.D. thesis, Pennsylvania State Univ., University Park, Pa. (1972): Experimental study of partial melting of crustal rocks and formation of migmatites. Int. Geol. Cong. 24th, Proc.2, 109-113. McDouclr-1. I.. Coupsror, W. & BortNcEn, V.M. ( 1966): Isotopic age determinations on Upper Devonian rocks from Victoria, Australia: a revised estimate for the age of the DevonianCarboniferous boundary. Geol. Soc. Amer. Bull. 77. 1075-1088. O'CoNNoR,J.T. ( 1965): A classificationfor quartzrich igneous rocks based on feldspar ratios' U.^t. Geol. Surv. Prof . Pap. 5258,79-84. Purt-t-rps,G.N. (1978): Metutnorphisnt and Geoc'lretnistryof the Willyanru Contplex, Broken Hill. Ph.D. thesis. .Monash Univ., Clayton, Australia. & Wer-1, V.J. (1980): The geologY and metamorphism of the Bonnie Doon area, Victoria. Roy. Soc. Yict. Proc.9L, 33-42. R.T. & CovrpstoN, W. (1965): The age PTDGEoN, and origin of the Cooma granite and its associated metamorphic zones, New South Wales. J. Petrology 6, 193-222. (1981): Origin and crystalliza& granitic tion of some peraluminous ($type) magmas. Can. Mfueral. 19, 111-131. Srrr.crr,rsrN, A.L. (1973): Plutonic rocks: classiFrooo, R.H. & Snew, S.E. (1975): A cordieritefication and nomenclature recommended by the bearing granite suite from the New England baIUGS Subcommission on the Systematicsof lgnetholith, N.S.W., Australia. Contr. Mineral. Peous Rocks. Geotimes 18(10), 26-30. trology 52, 151-164. TelrNr, J.A. (1965): The stratigraphic and dia(1977): Two ..S-type,l granite & strophic evolution of central and eastern Victoria 8?Sr/80Sr ratios from the suites with low initial in middle Palaeozoictimes. Roy. Soc. Vict. Proc. New England Batholith, Aushalia. Contr. Min79,197-t95. eral. Petrology 6t, 163-173. TAyLoR, S.R.. Gonror*, M.P., Murn, P., N"wcr, FRoEsE, E. (1973): The oxidation of almandine W.B." RunowsKl, R. & Wenr, N. (1973): Comand iron cordierite. Can, Mineral, 11, 991-1002. position of the Descartes region, Lunar Highlands. Geochim. Cosmochim. Acta 37, 2665-2683. Gnrxr, E.D. (1976): Plagioclase-garnet-Al2Si0rquartz: a potential geobarometer-geothermoA.H.M. & Gennerr, MJ. (1976): VANDENBERc, meter. Amer. Mineral. 6L, 710-714. The M.elbourne Trough. In Geology of Victoria (J.G. Doucr.ns & J.A. FrncusoN, eds.) Geol. Glners, T.H. (1976): Experimental generation of cordierite- or garnet-bearing granitic liquids from Soc. Atrst. Spec. Publ. E, 45-62. a pelitic composition. Geology 4, 85-88. VrnNoN, R.H. (1977): Relationships between mi(l97Xl: Garnet in silicic liquids and its crostructures and metamorphic assemblages,Tecpossible use as a P-T indicator, Contr. Mineral. rottophys. 39, 439-452. " Petrology 65, 59-67. Wrtls, P.R.A. ( 1980): Thermal models for the (1965): Henue, M. On the potassium migmatites accretion and subsequent metamorphism of conof southern Finland. Comm. G6ol, Finlande Bull. tinentaf crust. Earth Planet. Sci. Lett. 46, 253219. 265. HINr, R., Wrr-r,rnvs, I.S., Cuerrnr.r., B.W. & Wruru, A.J.R. (1978): Contrasb between I- and $type granitoids of the Kosciusko batholith. Geol. Soc. Aust, J,25, 219-234. Jercqn, J.C. (1957): The temperature in the neighborhood of a cooling intrusive she.et,Amer. J. Jci. 255, 306-318. WHrrE, AJ.R. & CHerPEr.r, B.W. (1977): Ultrametamorphism and granitoid genesis.Tectonopltys. 43, 7-22. -, & Crrenv, J.R. (1974): Geological setting and emplacement of some Australian Palaeozoic batholiths and implications for intrusive mechanisms.Pacilic Geol. 8, 159-171. 63 PETROLOGY OF THE STRATHBOCIE BATHOLITH WHITE, D.A. (1954): The geology of the Strathbogie igneous complex, Victoria, Australia. Roy. Soc. Yict, Proc. ffi,25-52. AppnNorx WtNrrrn, H.C.F. (1974): Petrogenesis of Metam or ph ic .Roc&s (3rd ed.) . Springer-Verlag, Berlin. -, BoEsE, M. & MARcopour.os, T. (1975): Low temperature granitic melts. Neues lahrb. M ineral. Monatsh' 245-268. a Kfs Tenrn oF ABBREVTATToNSUSED MrueRALs Or PI An Musc Bi Gt Alrn Py Sp Wvrrrr, PJ. (1977): Crustal anatexis: an experimental review. Tectottophys. 48, 4l-71. -, HUANG, Wuu-Lrexc, SrrnN, C.R. & MAAL@E,S. (1976): Granitic magmas: possible and impossible sources, water contents, and crystallization sequences. Can. l. Earth Sci.18, 1007-1019. P kbar T'C (1970): Zscr, H.P. An erupted migmatite from log/(Oz) Cerro del Hoyazo, SE Spain. Contr, Mineral. X(H29;'at Petrolotrly 26, 225-246. FI QFIUI Received October 1980, revised manuscript accepted December 1980. l. aQ) Liq qtJartz potassium feldspar orthoclase plagioclase anortlite muscovite biotite garnet almandine pitrope spessartine Gr En Fs C Cd Sill Hc Mt Ilrn Ap Po grossular enstatite ferrosilite corundum cordierite sillimanite hercynite magnetite ilrnenite apatite pyrrhotite Pnvsrcocuprutcer.Syrrnors pressurein kilobars temperaturein degreesCelsius logrooxYSenfugacity in bars lrater content of melt supercritical fluid phase oxygen fugacity equivalent to that ol tlre qtartz-fayalite-magnetite buffer activity of species i magmaticsilicateliquid