MINERALOGICAL MAGAZINE, JUNE I980, VOL. 43, PP- 779-87

Spotted structures in gneiss and veins from Broken Hill, , Australia

BRYAN E, CHENHALL AND EVAN R. PHILLIPS* Department of , University of , Wollongong, NSW, 2500, Australia AND R. GRADWELL Deceased. Formerly Department of Geology and Mineralogy, University of , St. Lucia, Queensland, 4o67, Australia

SYNOPSIS SPOTTED eye-like structures, made up of densely eyes. Moreover, the compositions of corresponding packed biotite 'pupils' mantled by quartz and feld- in these phases are very similar. For spar, constitute about Io % by volume of a high- example, plagioclase is consistently about An 35-40, grade quartzofeldspathic gneiss located on the road the biotites are all very iron-rich (Fe/(Fe + Mg) to Purnamoota Homestead about IO km north of 0.87) with compositions near siderophyllite, and K- Broken Hill. These spotted structures have an feldspar has the composition Or94Ab 6. uneven distribution within rocks exposed at this It has been suggested that the biotite spots locality. In some parts of the outcrop they tend to formed by alteration of garnet probably during a be closely spaced (fig. I), whereas elsewhere they are retrograde metamorphic event. However, studies of less abundant and the spotted gneiss merges into the microstructure of the biotite and garnet in the an essentially homogeneous gneiss composed of spotted gneiss show that biotite of the spots does quartz (34~. by volume), plagioclase (24%), K- not replace garnet. Furthermore, the chemical simi- feldspar (i 7 ~), biotite (20 %), and garnet, muscov- larity between the spots and the matrix is best ite, and opaques (5 %). This homogeneous gneiss is explained by an isochemical rearrangement of believed to have developed into a 'matrix' gneiss components of the matrix phase to form the eyes. phase holding the 'eyes' which have a felsic mantle Thus, a more likely origin is best related to the dis- to mafic clot ratio of 4: I. placement of matter along chemical potential The whole eye-like structure appears to be a gradients possibly induced by deformation in the distorted prolate spheroid with its axis set close to system--a process described by the term the foliation plane of the gneiss. The internal biotite metamorphic differentiation. clot forms another prolate spheroid with its axis Veins in the Purnamoota Road gneiss are of two inclined at approximately 70 ~ to that of the main contrasting types--regularly-disposed vein- enveloping quartz-feldspar mantle. Although most lets which are composed almost entirely of K- of the eyes are isolated within the matrix, some are feldspar and quartz, and irregularly shaped dis- linked to hold three or four biotite aggregates continuous trondhjemitoid variants rich in biotite (fig. S I) and rarely a vein-like patch which contains spots and carrying rare garnet porphyroblasts. The some or more partly linked spots of biotite may field relationship between these two main types of be found. In addition to the biotite, xenoblasts of veins is difficult to discern. In some parts of the out- almandine occur within some of the eye-like crop the veins occur adjacent to one another and structures. locally appear to merge. Veins with modal (and Whole-rock chemical analyses (including both chemical) compositions intermediate between the major oxides and trace elements) show that the spotted and the K-feldspar-rich veins have also homogeneous gneiss and the spotted gneiss are been recorded from the outcrop. very similar. The most notable feature of the geo- Despite a wide whole-rock chemical variation, chemistry of the spotted rocks is the close chemical the minerals in all the veins are similar chemically correspondence between the matrix and the whole and compare closely with corresponding minerals * Professor E. R. Phillips died on iIth May i98o. 780 B.E. CHENHALL ET AL.

FIG. SI. A general view of the spotted Purnamoota Road gneiss. The eye-like form of the spots and mantles is evident. The white quartzofeldspathic portions are elongate parallel to the main foliation in the gneiss, whereas the elongate biotite 'pupils' tend to be arranged at angles of about 5o 7o~ to the foliation. Most of the eyes contain one mafic spot but some have up to four closely associated biotite clots. in the host gneiss. One exception may be the plagio- places some doubt on the suggestion that partial clase in veins rich in K-feldspar where normative melting has played a signficant role in the genesis calculations indicate a composition near An14. of the veins. Interpretation of the trace element data counts Although mechanisms of vein formation such as against the veins having formed by partial melting igneous injection or metasomatism are considered and the variable K-feldspar contents place both as other alternatives for vein formation, we suggest main vein types well away from ternary minima in that metamorphic segregation (as proposed for the the system Q-Ab-Or. Calculations based on the origin of the spots in the gneiss) will also account compositions of co-existing biotite and garnet in for the development of the veins. the host quartzofeldspathic gneiss and in associated prograde pelitic schists indicate an equilibrium temperature of 65o~ 5 ~ ~ at pressures between [Manuscript received 20 June 1979, revised 19 November I979] 3 and 4 kilobars. This fact, together with trace ele- ment data and the calcic nature of the plagioclase in the homogeneous gneiss and the matrix phase, Copyright the Mineralogical Society SPOTTED STRUCTURES IN GNEISS 78x

SPOTTED STRUCTURESIN GNEISS AND VEINS FROM BROKEN HILL~ NEW SOUTHWALES t AUSTRALIA

Bryan E. Chenbe]l and Even R. Phillips

Department of Geology, University of Wollongong, gollongong, R.S.W., 2500, Australia

and

R. Gradwell

Deceased. Formerly Department of Geology and Mineralogy, University of queensIand, St. Lucia. QLD., 4067, Austra]ia.

QUARTZOFELDSPATNICgneisses (or 'granite' gneisses) are abendant throughcsJt the Wil]yama Complex of western New South Wales (Binns. 1964; Vernon, Ig69; Chenhall eta]., 19771. In the vicinity of Mine Mile Creek. about g to 11km nortIT-oFBroken Hill. some of these exhibit an unusual, spotted, eye- like structure described by Andrews (1922), Browne (1922) aed Chenhall et al. (19781. One of the best examples of this structure, together wi~ ~lated spotted and felsic veins, is seen in rocks located just east of the road to Purnamoota Homestead 2km MNE of the Nine Mile Mine (fig. 1; approx- imately grid reference 1773 OOOE 21238 BOON on the GeologicaI Map of the Broken Hill District published by the Australasian Institute of and Metallurgy in 19681. The rocks from this area are knowtl as the Purnamoota Road gneiss. The 'eyes' consist of '... a dark centre or nucleus [of biotite], generally with suboircular outline, surrounded by an elongated area of white minerals, the whole simulating with startling clearness the appear- ante of a rather slit-like eye, so that the name "augen" gneiss is sing- ular]y appropriate...' (Browne, 1922. p. 3311. Tbese eyes are dispersed through a biotite gneiss host crossed by veins which may be grouped into two types -- one containing biotite spots and tending to have a low K- feldspar content, the other usually being almost biotite-free but tending to be rich in K-feldspar. The distinctive spotted gneiss grades into a more even-grained, weIl-foliated but homogeneous 'platy' variant in which the spots are virtually absent. Both of these 9neissic varieties display, to the east. a sharp, irregular contact with a leucocratic (biotite-poor or biotite-free) quartzofeldspathic gneiss containing numerous, apparent]y isolated 'pegmatitic' elongate patches (fig. 2). Sillimanite schists with Fig. 1. Locality map shOWing major rock outcrops north of Broken Hill, incipient retrogression (MObbs eta|.. 1968 Vernon, 1969; ChenhalI. 1973) After the Geological Map of the Broken Rill District, Australasian Instit- occur along the western contac~--o~the quartzofeldspathic gneisslc vari- ute of Mining & Metallurgy. 1968. ants. aggregates range from 0.25 to 2ram in length. Many felsic crystals vary Browne (19221 suggested that the biotite spots within the eyes were from O.5 to 1.0ram across, indicating a slightly coarser grain-size than for derived by alteration of garnet. However, we note here the similarity of the homogeneous variety. the eyes to stictolithic, fleck or 'cigar' structures described elsewhere quartz sh~s undulatory extinction and cool,only occurs in aggregates (e.g. Loberg, 1963; Mehnert, 1968; Russell, 1969; Dahl. 1972; Fisher. 19731 of polygonal crystals. Most pIagioclase grains An~_~by opt cal deter- and to diffusion spheres (Atherton, 19761, and the hypothesis that the eyes mination; An39 by chemical analysis. Table 11, col~n~-6) are almost un- were derived by metamorphic differentiation is considered below. In add- altered and virtually unzoned with sharp, regular and pers steot albite ition, the association of such structures with quartzofeIdspathic veins twins. Apparently non-perthittc K-feldspar shows incipient cress-botched appears to be of fundamental importance (especially Loberg, 1963. p.102). twinning and contains lobate myrmekite. It occurs in lesser amount than in Since the genesis of sow~quartzofeldspathic veins and ]eucoso~s in mig- the homogeneous gneiss. Strongly pleochroic Biotite er straw ~t = ~= matites is again in dispute (Ashworth, 1977. 1979; Yardlny, 1977, 1979) or very dark brown; chum ca analysis, Table 1I. column 2) is identical in is the subject of detailed attention (e.g. OIsen, 1917; Yardlny, 1978; also colour and compesition to that forming spots within the eyes. Almandine Hedge. 1972; Fershtater. 19771. we be]ieve that the time is opportune to (Table [I. columns 13-15) occurs as sporadic, individually roughly spher- present an account of both the eye-like structures and the veins in the ical crystals or as aggregates of grains 3 to 5mm in diameter. Most of the Purnamoota Road gneiss. larger almandine xenoblast~ contain numerous Inclusions of quartz (O.lmm Descriptions across) and rarer biotite flakes. Secondary muscovite (usually in the pIagioclase) and accessory iImenite cOmplete the minerals of the matrix. The 'homogeneous' phase of the Purnamoota Road gneiss The eyes. The eyes are usually about 2cm long 1 to 1.5cm deep and are This is very similar in its mineralogy and chemistry to the spotted sharply demarcated from the gneissic host. Further. the black biotite-rich gneiss (into which it gradually changes) described below. In hand-specimen centres of the eyes are very clearly separated from their biotite-free the homogeneous gneiss has an overall speckled grey colour, being co~*pesed quartzofe]dspathic mantles. Most eyes are isolated within the matrix but mainly of quartz, feldspar and biotite -- the latter showing a preferred some are ]inked to Bold two or three biotite aggregates; uncommonly a vein- orientation with individuaI fIakes and aggregates of flakes imparting a like patch which contains some ten or more partly-linked clots of biotite foliation. Modal and chemicaI data appear in Table I, column 1. iS seen (fig. 3C). In thin-section all the feIsic grains are seen to be xenoblastic. The The margin of the mantle appears to have the shape of a distorted subhedral biotite flakes vary from 0.25 to lmm in length (with some agg- prolate spheroid with its axis lying in or near the foliation plane (fig. regates about 2mm long) whereas polygona] quartz crystals, which are co@m~ 3A and D). Sections both close to an elliptical print pa sect on and only found in granoblastic aggregates 1 to 2mm across range in size from normal to the foliation have a roughly lenticular outline. Normal to such about 0.25 to D.gmm. The fe dspars are similar in size to the quartz but sections, within the foliation plane, the outline is more evenly ellipt- some plagioclase grains are up to 2.Ommacross. The plagioclase is unzoned ical and sections perpendicular to both of these are roughly circular. The andesine (An4~ by optical determination An ~ from a chemical analysis form of the clots of biotite may be that of a pro]ate spheroid as well. Table II, coFumn 5 , shows well-formed alO te tw ns. and ls cocnmonly The major axes of the black e11ipses (as the clots appear in vertical partially altered to muscovite. Someof it is locally antiperthitic and sections) and the elongated closures of the whole eyes have acute angles rare albite rims occur adjacent to K-feldspar. The K-feldspar (OrgAAbg. varying from g4o to 72* with many close to /O*. This latter angle is taken by chemical analysis, Tab]e IT, column 10) shows no perthitic structure to be close to the difference in direction between the two axes of the under the microscope, but poorly-developed cross-hatched twinning and an prolatm spheroids. Optical examination of the orientation of the biotite undu]atory extinction are evident. It is commonly free from turbid a]ter- flakes within the aggregates indicates that most flakes have their cleav- ,ation products although some crystals are partia]ly replaced by lobate ages subberallel to the axis of the internal prolate spheroid. blabs of mprmekite. The biotite has ~= straw.jg, = ~r = deep brown and its chemical composition is given in Table IT, column I. Flakes of muscovite Enlarged tracings and photostat copies of the eyes were made from over are found with the biotite or occur in matted aggregates between feldspar 50 sawn surfaces and the areas representing the dark portions and grains. Minor aheaedine, usually intergrown with muscovite, is present and the light mineral portions were cut out and weighed. In this analysis accessory minerals include iheenite (mainly in the biotite), zircon and random sections across the foliotion plane were used so that s~ sections apatite. ref~resented a maxi~ content of biotite whereas other sections had no biotite. The biotite was found to constitute 19% of the eyes giving a The spotted phase of the Purnamoota Road gneiss ratio of light minerals : dark minerals of about 4:1 which is similar to and associated veins that of the homogeneousgneiss and matrix. An average chomical analysis of General. The rock type described in this section is a spotted, well-fol- a number of whole eyes appears in Table I, column 4. iated gneiss in which the spots are eye-like, the margins having rough]y a The quartzofeldspothic mantles consist almost wholly of quartz, plag- lens-shaped outline in most cross-sections normal to the foliation. The ioclase and K-feldspar (Table I, column 51. quartz grains, tdlich 1ocally eyes, which from measurements on sawn surlaces constitute 10% of the whole tend to aggregate close to the biotite clots are polygonal in shape and rock, are composed of a central black portion of densely-packed biotite vary n s ze from 0.05 to O.75mm; and the feldspars are of slmllar dimen- (and minor ilmenite) forming a core or pupil) about which white felsic sions. The plagioclase An~ 42 by optical determination An39 from a mantle domains are developed fig. 3A . These eyes are set in a biotite chemical ana ysis. Tab e IT, ~,iffn 7) has virtually the same composition gneiss matrix (or,host) and they occur irregularly, some being close to- as that of p]agioclase in the host and homoge~e~s gneisses. Non-perthitic gether or even merging, others being relatively far apart. An average K-feldspar grains are irregularly distributed through so~e mantles, being chemical analysis of the whole spotted gneiss (matrix pIus eyes) is listed sparsely developed in certain sections but clustering in other parts. in Table I, column 2. The gneiss contains quartzofeldspathic veins; those with biotite spots and tending to be low in K-feldspar are grouped together The 'pupils' of the eyes are composed almost entirely of suhhedral as vein type 1 whereas those low in or without biotite but tending to be densely-matted biotite flakes (0.2 to rarely 2.Dram in length) with a chem- rich in K-feldspar are designated as vein type 2. The matrix, the eyes. ical composition (Table II, column 3) and pleochrolc scheme the same as and the veins are described below. those of the biotites of the host gneiss and the homogeneous gneiss. Minor asmunts of llmenite (apparently in one or two anhedral grains abOUt The matrix or host. The gneissic host to the eyes is similar in mineralogy Imm long) occur near the core of some pupils. Its chmical analysis app- and microstructure to the homogeneous phase described above (TabIe I, ears in Table II, column 16. The ilmenite appears to be missing from many column 3). Quartz, plagioc]ase and K-feldspar occur as xenoblastic grains pupils but this may be the effect of random thin-section cuts. Rare sub- varying in size from 2mm down to O.Imm and subhedral biotite flakes and hedral muscovite flakes form the third mineral in the pupils. 782 B.E. CHENHALL ET AL.

[~ Quartzofeldspathic gneisses ~'~ Leucocratic quartzofeldspathic gneiss ~ Pegmatite ~ Localitygneisses of spotted

Amphibolite [~ Retrogressed sillimanite schists ~ Major retrograde zones

Fig. 2. Geological sketch map of the Nine Mile Creek area north of Broken Hill where the Purnamoota Road gneiss outcrops. Almandine grains morphologically and chemically identical to those in the subtype richer in K-feldspar are more regular in form hut sparser in found in the host gneiss, occur oca y both n the fe sic mantles or development than those in the troedhjemitoid veins, girtual]y all the i~inging onto the biotite clots. Rare isolated garnets may also be partly biotite associated with vein type 1 is present as dense spots and no mafic- mantled by felsic domains. However, the roughly equldimensional habit of rich selvages bordering these veins were seen (of. White, 1966, fig. 3; the almandines is markedly different in morpbology from the elongate bio- Hedge. 1972. fig. 2; Olsen. 1977, fig. 3). Although it was not possible to tite pupils (fig. dA and g). Also. the garnet is usually quite fresh and do accurate micremetric analyses along the length of the veins, there can shows no alteration to biotite. be little doubt that the proportion of felsic to mafic minerals in the veins is greater than that in the eyes. Vein Vpe I with biotite spots. Veins composed essentially of quartz. plagioclase, K-feldspar and clots of biotite (fig. 3B, D and E) appear to In thin-section, some parts of the veins (but not necessarily marginal be closely related to the eyes described above. This is especially so for port ons) show a grain-size for the felsic minerals similar to, or only those eyes linked together along the foliation plane to form composite slightly larger than, that for the felsic minerals of the mantles or the vein-like patches (fig. 3C). In addition, apart from the comparable miner- matrix of the spotted gneiss. AS the biotite of the clots is similar in alogy in both developments, the similarity between the orientation of grain-size as we the c ose comparison to the eyes is most marked bere. biotite clots in the veins and in nearby eyes suggests a close structural However. other parts of the veins (usually in the trondhjemitoid variant) relationship (fig, 3D), show an increase in grain-size with both quartz ~nd feldspar xenoblasts up to 3mm across. K-feldspar in veins of the second subtype co.only forms We can distinguish at least two subtypes of vein type 1: one (the grains tending towards the larger size. Someof the smaller biotite clots more abundant rock) is almost free fromK-feldspar and is a trondhjemitoid are attenuated, appearing to be c~mpacted between felsic grains; others variant (Table I, column 6); and another (which appears to be spatially have less definite margins with extensions of biotite splaying outwards related to an example of vein type 2 described below, see fig. gEl has into the quartz and feldspar In add tion although subhedral muscovite quartz, plagioclase, K-feldspar and biotite all as essential minerals (also flakes are present, ilmenite appears to be ~arer than in the biotite cots Table I, column 7). of the eyes. Veins of the troedbjemitoid subtype have an irregular distribution and Throughout both subtypes, almandine grains may occur either within a appear to be made up of the partial coalescence of irregular, apparently biotite clot or adjacent to it and large isolated porphyroblasts of garnet isolated, tapering vein-like felsic patches containing biotite clots; some (up to 10mm across) are locally well-developed in the trondbjemitoid vari- of these patches look like aggregations of eyes and near their margins some ant. Optical measurements on the plagioclase (which is commonly slightly veins seem either to merge with or to envelop eyes. For the most part. the altered to sericite) gave a composition close to AnAo also Table El, veins run subparallel to the foliation and can be traced discontinuously column 8). Cross-hatched K-feldspar has the c~bosition Org 4 Ab 6 Tab e over distances of 1 to 2m. Their width is irregular, varying from about If, column 11). lcm up to 5cm. An example of the subtype richer in K-feldspar, illustrated in fig. 3E. has, however, a more regular form with almost parallel sides Vein type 2. Veinsof this type are rich in quartz and feldspar: at one extreme, they are composed almost totally of K-feldspar and quartz (with and a tabular disposition. It is also more even in width, varying from minor~scovite and virtually accessory myrmekite and biotite, some forming about 1 to 2cm across. rare clots, e.g. Table I. column 9); at the other, plagioclase is an The biotite clots (Table II, column 4) have an irregular distribution additional important phase and biotite flakes are more abundant but are within most veins and vary from ellipsoida] spots about Zmm across (which still only in minor amount (e.g. Table I. column 8). In form the velns locally nest together in the trondhjemiboid subtype) up to blebs 1.5cm rich in K-feldspar are usually quite regu at; they have subparallel sides long. Many of the~ are between 3 and lOmm in length and the overall range and can be traced continuously over distances of 2 to 3m or more. The vein in size in the troedhjemitoid variant is greater than that observed for illustrated in fig. 3E is about 2cm thick and locally shows an almost biotite clots within the eyes. In general, it seems that the biotite spots pty~atic style of folding. It holds no discrete plagioclase grains (Table I, analysis 9). Another example is 3 to 4cm thick and strikes across the foliation, giving the appearance of a very narrow dyke. It carries more Fig, 3A. Detail of the spotted Purnamoota Road gneiss showing the foli- p agioclase as ma be discerned from its mode and chemical analysis ation and the elongation of the mantles and biotite spots. Roundedallan- fable I co umn 10~. Vein type 2 may also form irregular patches or thin dine borphyroblasts are present. The bar represents a length of ZOmm. discontinuous ve ns w tb n the host gneiss and it is such patches which Fig. 3B. gein type 1, trondhjemitoid variant. Note the discontinuous usually {but not exclusively) have significant amounts of plagioclase. nature and irregular width of the vein and the variability in size and Locally, the K-feldspar-rich vein illustrated in fig. 3E has very thin disposition of the biotite spots. The direction of elongation of the biotite-enriched partial selvages but this property is not well-developed. biotite spots in the veln and in the enclosing gneiss is similar. Thin-sections of the K.feldspar~uartz veins show that the grain- Fig. 3C. Elongate patches showing the extension of the white mantle along s ze is about 3 to 4g~ and- that a the crystals are xenoblastic. The the foliation direction and around multiple biotite spots. Many of the cross-botcbed twinned K-feldspar has a c~position Org4 Ab 6 (Table El, biotite clots have a preferred orientation with their elongation set sub- column 12). Relatively coarse apatite, is the. accessory mineral. ~ The parallel to that for the single spots illustrated near the bboto~ of the 'dyke' represented by analysis iO, Table I is s11gntly llner-gralne~ and photograph. seemed regular enough in texture for the mode listed to be measured, lhe plagioclase in this rock s An37 fronmptical determinations E microprobe Fig. 3D. Detail of vein type I, trondhjemitoid variant. Although the analysis of plagioclase in a discontinuous vein rlcher in plagloclase gave vein is irregular in width its general confomitywith the foliation in the a compos tion close to An37 (Table II. column 9). However plagioclase spotted gneiss is evident. The subparallel nature of the elongation of all compositions suggested by norm calculations are more sedic (Table I, co - the biotite spots is well displayed. The bar represents a length of 10~. umns 8.g,~o). Pig. 3E. Veins type 1 and 2. fbe spotted vein is the subtype which The detailed field relationship between vein types 1 and 2 is diff- contains K-feldspar as well as plegloclase, quartz and biotite. The vei~ icult to discern. Certainly. they may be quite separate in their develop- type 2 (with virtually no biotite) is composed essentially of K-feldmper ment, but the association of the veins illustrated in fig. SO, apart frc~ and quartz. It is almost ptyg~tic in style. The dark. fine-grained areas strikingly demonstrating obvious differences such as the presence or are due to weathering and this has obscured part of vein type Z and so~ of absence of biotite clots, presents an additional problem. These two veins the spotting in the surrounding gneiss. The coi~ has a diameter of 18mm. SPOTTED STRUCTURES IN GNEISS 783 784 B. E. CHENHALL ET AL.

z 3 ~ s ~ s 9 ~0 ~ LZ e,o~e= (vol.s)

~=~z ~ ~.e 3S 3~.Z 4~ .... 34 ~ Z7 elegl~tase ~4 ~9.1 29 30.O 37 .... 12 ]2 %3 ~ 1Z'C3 12.~Z 1Z.73 1Z.63 I5.ZZ 18.~3 B.75 9.41 1.58 3.81 Z.34 1.29

81ott te ~ z],) 22 1~.o ..... 3 ? 4 ...... o~ .... ~:~ ...... }]}}]., .... s~ s z.o z 1.6 z .....

T~Oz 0.73 0.~0 0.73 ~.75 0.03 0.38 0.3] 0.23 0.06 0.10 0.27 0.08 A~203 13.98 14.12 1~.03 13.93 14.Z4 16.03 13.47 13.9Z 14.34 14.14 14.S0 14.03 u ~ T" ~S Z4 I[0 93 114 130 G, Zl ZO ZO ~o ~z

2.el Z.~9 Z.7S ~.70 3.24 4.03 ].90 1.97 OJ8 1.05 0.95 0.57 ~s ,; ~ ~?o~ <:~

~0 3.~ 3.35 3.3O 3.40 z.~ 1.51 4.61 4.69 9.*~ ).Z6 6.36 8,58 Cr 191

6~Z ~ZZ SR~

z. ~ole s~t~ Pu~a Road 9~s avera~ of 3). 3. ~tr 9of ~he e~m~ta eoad gneiss (a,e~age of Z). %0. ~otherexile of .ein ty~ 2 (the 'dyke' wIL~ I~ plaHioclaseacturrln9 in

6. u t~pe I (plagl~las~rtr es~nt~ally K-feldspar-f~ *et~s with quart~ a~

7" ~cu~Ve~etY~n 1~(velnS~h e P~r Ro,dPIaei~IaSegness. K-feIes ar q~rtz and with bioLite s~tl) 8. u type Z (veins r 9 plagl~lase K-feldspar quartz a~d s~ biotite but n~ as Spots or162 ~9 n ~e ~ta e~ad 9~ss. Ana~sts: R.H.FI~ a~ S.[. Sh~. ~cqua~ie~,iverslt~.

locally coalesce as a somewhat irregular patch of vein material. Although these elements in their enclosing gneiss. Rb. Ga and Cu values are quit~ the identity of both vein types could be recognized in some of this patch, similar. our overall impression was that they essentially merged. Modal and normative data indicate that the K-feldspar content of the Rocks associated with the Purnamoota Road gneiss pomogeneous gneiss is greater than that of the spotted gneiss, the matrix As noted in the introduction, a leucocratic gneiss with its associated and the whole eyes, with piagioclase exhibiting a reverse relationship. 'pegm~tites' has a sharp easterly contact with the Pur~a~ta Road gneiss. Normative and modal biotite contents remain fairly constant throughout, Neither the leucognelss nor its enclosed 'po!~natites' exhibit spotting and although biotite is possibly more abundant in the matrix of the spotted mapping indicates that they have not influenced the development of eyes in gneiss. A s~pathetic relationship between K-feldspar and plagioclase is the Purnamoota Road gneiss. Both leucogneiss and pegmatites stop abruptly also evident between the two vein types exhibiting extrBne compositional - at the contact with the Purnamoota Road gneiss (and may even predate it). differences. In both. mesonormbiotite contents are diminished relative to and no field connection between the eyes and the contact is evident. the enclosing spotted gneiss. Chemical and modal data on these two rock types are included in Table I Consideration of the mineral analyses (Table II) leads to the observ- (columns 11 and 12) mainly for comparative purposes. It will become evi- ation that all the biotites are Fe-rich (near siderophyIlite) with molar dent from the discussion below that such rocks need play no part in a FeO/(FeO + MgO) greater than 0.84. In particular, very close agreelnent metamorphic segregation model for the genesis of vein type I. Field rel- exists between the compositions of biotite fr~ the eyes and from the ations count against them forming anatectic melts which introduced vein matrix of the spotted gneiss. Molar distribution coefficients (Kretz. type 2 material into the Purnamoota Road gneiss, although the possibility K biotite matrix-spots of the derivation of 'pegI~atite' from its host leucogneiss by partial 1959), D(Fe-Mg) are close to I. melting is not denied. In general, there is good agreement between compositions of plagio- Chemical and modal data clases determined by microprobe and by optical methods and these also generally correspond with normative plagioclase compositions calculated Major element analyses (Table I) show that the homogeneousphase of from the whole rock analyses (Table I). The disparity evident for the the Purnamoota Road gneiss, the whole spotted gneiss, the matrix and the An/CAb + An) values in analyses 8-10. Table I. (ex~npies of vein type 2) as whole eyes are virtually identical apart from slight variation in SiO , compared with measured plagioclase compositions+ of about An37 can be ex- CaO. Na~O and.. to a lesser..extent , K20. Moreover. their trace element plained by the cross-association of normatlve Ab contained in the usually geochemlstry Is al)~ very slmilar. Values for Rb/Sr lle In the range 0.90- abundant K-feldspar with the minor normative An held in the relatively low 1.02 with Rb x 10 /K. 6.05-6.58; Ba/Rb. 3.04-3.86 and Ba/Sr, 2.88-3.8Z. amounts of modal plagioclase. C~mpositio~al variatia~ in the veins is a consequer~eofthC~Lal criteria by which the vein types were defined. Almandines fr~n the hOmogeneous and spotted gneiss phases are inter- esting because they exhibit a marked degree of compositional diversity not For these veins, it may be noted that trace element analyses of the only between grains in the one rock sample but also within individual troedhjemitoid subtype show that Y, Sr. Cu, Mn and Ba are greater than. and grains. This chemical variation is expressed in terms of difference in Zr and Rb less than, those elements in the host. Pb. Ga, Zn and V are Pe Ca arK] Mn contents and individual crystals appear to have an irregular. present in aporoximately similar amounts. The K-feldspar-rich subtype of patchy 'zoning' cf. Chenhall, 1976). The 'zoning has not yet been exam- vein type 2 has Sr, Pb and Ba greater than, and Y, Zr, Zn and Ti less than, ined in any detail.

Fig. 4A. Photomicrograph of the spots, mantle and so. matrix, showing, distribution. The garnets appear to be unaltered. Plane-polarized ~ight. in particular, the elongation of the central spots (composed of densely- The bar represents a length of 5mm. matted biotite flakes, many of which are arranged subparalle) to the spot elongation) as Opposed to the rounded equidimensional habit of the alman- Fig. 4B. Detail of the botto~left area of Fig. RA showing subhedral dine crystals. Garnet grains occur in part of the central spot (botteRB- porphyroblastic garnet which is unaltered and the preferred orientation left), within a mantle and in the matrix (top-r ght) in a random manner, displayed by the biotite flakes in the spots. Partly crossed nicols. shying that they are independent of spots, mantle and matrix in spatlai SPOTTED STRUCTURES IN GNEISS 785

T~le It. Electr~-mlCrO~be ,~al~s of m~als ~ the Pur~ta Road g~$s ~r ~lated rc~s.

Biotite Pl~l~la~ ~. feldspar Gl~t lignite Wt$ 1 Z 3 4 5 G ? 8 g 10 11 12 13 14 15 16

SlO2 33.40 3e,ZS 3Z.Z6 33.~ $9.~ 57.~ $l*gZ 59.03 ~,53 ~.~ ~.B~ ~.lZ 3S.~ 3S,41 35.59 - SIOz T~O~ ~.~4 Z,S3 ~.~ 3.0~ ~9.1e TIOz AI203 17.89 17.46 17.31 17.77 Z6.6Z ~6.~ 27.07 ZT.0Z ZS.69 tg.~ 18.93 18.~ W.71 ~.63 ~.67 0.16 AI2~ eeo ~ 3o,91 ~.~ ~o.~4 ~o.~ 37.21 32.39 31.7Z ~.S3 VeO"

N~o o.z9 0.~ 0.~4 0,~4 z.ez ~.lz e.~s z.40 7.~ o.ez o.~z ~ ~zo ~zo 9.4s ~.1~ g.l~ ~.3~ o.ze o.zs o.ls o.zz o.za le.oa ls.~ le.o~ r~o TOT~C 97.55 94.98 94.76 96.Z6 101.48 99.99 100.Z6 101.33 99.6~ 99.81 100.46 99.65 100.~1 9B.9S 99.6Z 97.50

lo.4e ^~1,

Fee* 4. I SJl 4.11 ~.15 4.13 5.15 4.01 5.61 2,56 2.Z4 Z.I8 2.~ Fe~+

O.I~ O.a ~.53 ~.11 R

An ~*15 39.17 39.1~ 35.~4 37.~ i0. Fr~ t~ '~s' phlseof th~ pu~n~ta Ro,dgoeiss. e,~ ,el. ty~ I ~curmngi, the pu~ta Roadg~s. 3. Frm 'eYes' i~ the Purna~taRoad g~ss averageof 5. ~ Fe~ veinty~ 2 (veinsrich iA K-feldsp.r but a~st

9.

K-feldspar in all sampIRs is ve~ low in Na and this accounts for its ients can be initiated by differences in pressure, temperature, ionic apparent non-perthitic nature. concentration, or even differential stress between domains in a rock system (Vernon, 1976). Accepting that temgerature and pressure were essentially Discussion the same throughout the gneiss, and noting the similar composition and The origin of the eyes in the Purnamoota Road gneiss modal proportion of the minerals, differential stress may well be the main parameter initiating diffusion in these rocks. Further. we suggest that It has been suggested that the biotite in the eyes of the Purnamoota eye formation took place during the metamorphism that produced the matrix Road gneiss formed by pseudomorphous replacement of almaedine during sub- and not after it (see. however, Fisher, 1970, p. 96). sequent alteration (Browne, 1922, p. 331). Notwithstanding that the form- ation of the quartzofeldspathic portions of the eyes is not readily ex- The relationship in orientation between an internal biotite clot and plained by alteration, the following microstructural features indicate its enveloping mantle and the preferred orientation of biotite within the that the biotite spots do not replace garnet: spots (fig. 4) might suggest that the biotite in the eyes was concentrated under the influence of one set of conjugate shear planes in a stress Field (a) unaltered, equant almandine occurs adjacent to elongate biotite with~ 1 (the 9re~ compressive stress. Turner and Weiss. 1963. p. 261) clots showing that neither mineral replaces the other and that the outline normal to the foliation of the g~eiss. Another possibi]ity is that orig- of the spots and the almaedine crystals have a completely different morph- inally spherical biotite spots were subsequently deformed selectively ologY (fig. 4); and during a later shortening directed almost parallel to the foliation. In (b) essentially unaltered almandines, with the same morphology as their present form, the directed structure of the biotite spots within the thoSe associated with the biotite in the spots, occur randomly dispersed Purnamoota Road gneiss contrasts with the isotropic fabric in the segre- throughout both the matrix of the host gneiss (clearly without quartzo- gations described by Loberg (1963). feldspathic mantles) and in the mantles of the eyes. If the biotite were Diffusion and nucleation of the spots could also be promoted by lower- formed by alteration of garnet during retrogression, matrix garnets should ing the grain boundary energy of the system. This may occur by a reduction also be affected, producing some unmantled biotite spots. These have not in the number of grain boundaries between like minerals (although we have been found. (A similar independence in microstructure between garnet and not been able to conclusively demonstrate this), by an increase in grain- biotite spots is seen in type 1 veins, again suggesting no retrograde size of the felsic matrix of the spotted gneiss, and by the gevelop~nent of connection between these two minerals.) planar, low angle, rational interfaces (decussete aggregates) in the bio- There is no microscopic evidence for the former presence of cordierlte tite spots. in the Purnamoota Road gneiss and tourmaline andalusite and sillimanlte The origin of veins in the Purnamoota Road gneiss have not been observed in these rocks. Hence, both the proposal of Russe (196g) that the formation of flecks in gneisses from the Vastervik area was Genesis of the veins within the Purnamoota Road gneiss is problemat- due to the porphyroblastic growth of cordierite, followed by the break- ical, as indeed is the origin of many quartzofeldspathic veins in high down of this mineral to give biotite and muscovite or anda}usite and sill- grade metamorphites. Someworkers (e.g. Ashworth. 1976, 1977) hold that imanite, and the subsequent hypothesis of Dahl (1972) that the existence of similar veins formed by anatexis, whereas others (e.g. Yardley lg77, 1978) either muscovite or anda]usite-sillimanite in the same ~geks depends on suggest that they originated by metamorphic segregation with igneous vapour fugacity or pH cannot be applied to the Purnamoota Road gneiss. injection or external metas~atism as additional possibilities). Related topics in this regard, as applied mainly to migmatites, are summarized and An additional fact makes it un]ikely that biotite in the clots is of discussed, for exa~le, by White (1966), Mehnert (1968). Olsen (1977) and retrograde origin. This is that all the biotites (including those in the Yandley (1978). who refer to much of the earlier work. homogeneous and matrix phases) are very similar chemically and presumably formed during the same progrede metamorphic episode. TwO points are important to any appraisal of the origin of veins in the Purnamoota Road gneiss: AS pointed out above, the composition of the homogeneous gneiss is similar to that of the spotted gneiss, but small differences in the pro- (a) the veins appear to be entirely enclosed by their host suggesting portions of K2O Na~O and CaD ref]ect differing K-feldspar to plagioclase an in situ formative process such as metamorphic differentiation or partial ratios. Of greater importance is the observation that chemical analyses of mel~n~ the enclosing gneiss; and the eyes themselves are in close agreement with analyses of the matrix and (b) the volume of the veins, although difficult to estimate, is of the whole spotted gneiss. Noting these differences, the homogeneous undoubtedly less than 6r of the total volume of the gneiss -- which implies phase appears to be a likely parent at least for the development of the that if the veins originated by anatexis then they must have segregated eyes and the matrix. It is conceivable that the veins within the spotted during the earliest stages of that process. Support for this suggestion is gneiss because of their isolated character and close spatial relationship obtained from temperature estimates using biotite and garnet compositions with the eyes (especially for vein type 1}, are also der ved from tb s in the spotted gneiss (.Table If. columns 1.2,13.14). Calculations based on homogeneous parent. Somepossible mechanisms leading to the formation of the procedure outlined in Ferry and Spear (197B) indicate t~eratures of the veins are discussed below. 6600C-~50~ at pressures between 3 and 4 kilobars and these temperatures are We believe that the eyes in the Purnamoota Road gneiss have for~din consistent with those found by us for progrode pelitic schists in the same situ by diffusion leading to metamorphic differentiation (of. Eoberg. 1963. area. Suchtemperatures indicate that only small amounts of minimummelt ~6 . The eyes simply represent a redistribution of material. It should could have formed. be noted that most of the eyes are so abed--they have no feeders to th~ Let us initially consider the proposition that the veins within the and hence cannot be igneous i~jected material. Nor do they appear to be Purnamoota Road gneiss werederivedby partial melting of their host. Data relict structures. In addition, the mineralogy and chemistry of the host from Table I, in particular the trace element results, are used to test and the eyes ar~ very similar, with the proportions of light to dark miner- this hypothesis. Equations explaining the theoretical behavlour of trace als being almost the same (and close to that ratio in the homogeneous elements during partial melting have been derived by Shaw (1970) and this phase . Furthermore we consider that the differentiation took place topic has been reviewed for example, by Arth (1976). Al1~gre and Hart essentially in the solid state because the m hera s show crysta oh astic (1978), Allegro and Minster 1978 . and Hanson (1978). microstructures. We follow the "batch u melting proposition of Arth (1976) because this The similar proportion of light to dark minerals in the eyes and in is the most geologically realistic model for the generation ~ large. the matrix indicates that ions probably have diffusod essentially within homogeneous bodies of magma where continuous equilibrium exists between volumes now represented by the eyes themselves. It is evident from the residual solid and liquid phases. Clearly. this model does not relate present distribution of minerals in the eyes that the ions Fe and Mg h~ve exactly to the small volu~ of material that must have formed the veins we been preferentially concentrated at the centres whereas elements such as Na are considering but it is preferable to medels involving either the contin- and Ca either tended to lag behind movement towards the centre or migrated uous removal of infinitesimal amounts of 11geld fr~ a restite o~ non- outwards to the mantle. equilibrium partial melting. The grounds for this preference are flrstly The driving force behind this differentiation is not fully understood. that the veins in the Purnamoota Road gneiss appear to have ford in situ Chemical potential gradients promote diffusion of matter and these grad- (thus allowing for eduilibrium to be continually maintained betwe~ 786 B. E, CHENHALL ET AL.

VEIN TYre 2

I, o VEIN TYPE 2 Ob..,,.~ ,.t~= --,a/Rb ~ rROND~ZM~rOIO VEIN ~ /S./Rb -- e./s, s, --s,

--rib/s,

~Rb/S, 024~ 03.44810 F "81~ F 024F6Bl'~ o 7 F~ e ,o

Fig. B. Plots of calculated Sr. Ba and Rb liquid concentrations ppm and Ba/Rb Ba/Sr and Rb/Sr liquid ratios both represented by CL versus the fract- on of me tng F. The d stribution coeff cients are taken from Table II of Hanson (1978). Observed va ues and ratios are indicated on the right hand side of each diagram.

and restite) and. secoedly, that repeated removals of melt are often con- the plagioclase compositions of the veins and the host are virtually ident- siderod to be physically unlikely EArth. 1976). ical and this must count against the proposition that the plagioclase of The equation for batch melting (from Hanson. 1978) is given by: these veins was derived by partial ~elting of plagioclase of the host gneiss (Milch, 1966k 9erBc~. 1976. p. 222; Yardley. 1977. 1978). CL/% = I/ [B o + PEt-PSi If metamorphic differentiation is accepted as the formative process where: CL = weight concentration of trace element in derived melt for the type 1 veins, it is clear from both chemical and mineralogical data that the veins, unlike the spots, represent localisod systems 'open' to the CO = weight concentration of trace elements in parent migration of ions. On mineralogical grounds, it appears that the most Oo = bulk distribution coefficient of a given trace element i~ortant aspect of vein type I evolution is the partial or complete elim- at onset of melting ination of K-feldspar with the concomitant production of piagioclase. F = weight fraction of melt relative to parent The origin of vein type 2. AS noted above, an origin by anatexis also seems unlikely for vein type 2. Two further points count against vein type P = bulk distribution coeffic)ent of minerals making up melt 2 being produced by anatexis. The example of vein type 2 with the high K- n feldspar content (Table I, column 9) contains almost 55% normative Or and and DO = ~=1 xl Kdl on a plot of normative q-Ab-Or it falls well away fro~ ternary minima or n . . eutectics (Luth etal., 1964). Thus it appears difficult to assume that and P = ~=1 pl Kd l such a rock for~d as a result of crystal~.~ liquid equilibria. Those varieties of vein type Z which hold plagioclase (e.g. Table I, column B) where: Xi = weight fraction of a given mineral i in parent present a further barrier to the anatectic model. The plagioclase (e.g. Table IS, column g} has a composition close to An)7. which is virtually the KdI = mineral/melt distribution coefficient for a given trace same as the plagioclase in both the homogeneous~d spotted phases of the element for mineral I Purnamoota Road gneiss. Such a calcic plagioclase can hardly have been pi = normative weight fraction of mineral i in melt. derived from these gneissesbyperhial melting (yardley, 1977). A factor contributing to the uncertaint~ in C,/C as a function of F Other modes of origin, such as igneous injection or external meta- is the magnitude of the Kd's. Values for Kd's are~ge#erally determined by somatism, may also be feasible. HOWever. these mechanisms divorce vein studies involving phenocrysts to groundmass trace element abundances in formation fremeye formation and as such destroy the apparent entity of the volcanic rocks (e.g. Schnetzler and Philpotts, 1970) and at best can be Purnamoota Road outcrop which may be best regarded overall ~s a closed regarded as approximations when applied to other rock systems. Further. Kd system. So~0e examples of vein type 2 (especially the 'dyke', Table I, varies as a function of temperature and mineral and melt composition (Han- column 10 and the vein illustrated in fig. 3E (also Table I, column 9)) son, 19785 and such information is not presently available to us. appear to disrupt the host gneiss and might have been emplaced by lateral movement involving fluid introductions. On the other hand, fig. 3E shows In all our calculations (the results of which are presented essent- that varieties of veins 1 and 2 may occur in pairs and. as noted above. ially as fig. B) we have assUmed that the Kd's of Ba. Rb aed Sr for quartz these two veins appear )Bodily to merge. Such a close association suggests are close to zero (Hansen, 1978). Cn is taken to be those analytical that both veins have similar origins and we favour metamorphic different- values obtained for the parent bca,ogen~ous gneiss. The distribution co- iation as the mode of origin for vein type 2. Furthermore, it is likely efficient data are taken from Hanson (1978, Table Ii5. Theoretical trace that the very K-feldspar-rich veins represent segregation products comple- element modelling for both vein types reveals some similarities in trace mentary to the trondhjemitoid veins with biotite spots. The most important ele~nt distribution patterns. Theoretical Rb values decrease with and Sr aspect of vein type 2 formation is thus the partial or complete elimination values increase with. the increase in the fraction of melting. Ba shows an of plagioclase (and formation of K-feldspar) by diffusion. We are unsure increase with greater F for both veins but this pattern is much more of the role of biotite in vein type 2 formation, but mineralogical observ- marked for the K-feldspar-rich subtype of vein type 2. ations suggest to us that this phase is also removed by diffusion. For the troedhjemitoid variety of vein type I, the observed trace Lastly. the subtype of vein type 1 with an appreciably high K-feldspar element abundances do not match the theoretical values obtained from the content (Table I columh7) and the variety of vein type 2 with substantial melting model. The Ba value for this vein is greater than. and the Rb plagioc ase e.g. Table I, column B) are chemically similar and intermed- value less than, the calculated concentration irrespective of the magnitude iate in composition between the trondhjemitoid vein type I and the K- of F (fig. 5). The actual Sr concentration of 267 ppm is obtained for feldspar-rich representative of vein type 2. Some of these intermediate approximately 80% partial melting of the host which is clearly unaccept- rocks may represent the products of incomplete reactions which produced the able. The theoretical Ba/Rb ratio does not correspond with the observed ~r~ strikingly diverse veins. value and for the calculated Rb/Sr and Ba/Sr ratios to approach the obser- ved values. F must exceed 40% (fig. B). Conclusion Calculations for the K-feldspar-rich subtype of vein type 2 show that We have attempted in this paper to present mainly a description of the the observed Ba level is in excess of any value determined by the model spotted gneisses and related veins fr~ Purnamoota Road, Broken Hill and to with Sr matching values for F at 6~. The tabulated Rb value is c~at- outline processes leading to their formation. Some features of these rocks ible with the model for low degrees of partial melting. HOWever, observed deserve special mention and should be considered in a general dissert- Ba/Rb and Ba/Sr ratios do not match the calculated ones for F of anymag- ation on spotted gneisses and veins. nitode and an F value of 50% is necessary to produce Rb/Sr of B.71 (fig. B). (I) The spots in the Purnamoota Road gneiss are unusual because the mafic clots within the mantles are virtually monomineralic, being composed almost These tentative results lead us to reject the proposition that any of entirely of a dense aggregate of biotite. Other flecks, for example from the veins in the Purnamoota Road gneiss were derived by partial melting of Vastervik, Sweden have biotite as only one of the main minerals in a core their host. surrounded by a mafic-free mantle. The origin of vein t pe 1. Structural observations and data on the gross (Z) The mineralogical relationship between spots, mantle and matrix seems chemistry suggest ~e following. AS noted above, a close relationship to be relatively simple, involving essentially only quartz, plagioclase, K- exists between vein type 1 and the eyes in space, structure and overall feldspar and biotite with minor amounts of muscovite and ilmenite. The form, and it is reasonable to consider that these veins have developed by a composition of these phases is remarkably similar, no matter in what set- process similar to eye formation involving linking of eyes. Because of ting they occur. this we favour a process involving diffusion as the mechanismwhicb for~d vein type 1. (35 Although almandine odrphyroblasts are developed throughout all domains of the spotted gneiss and in vein type 1, it is not the retrogression of On structural grounds it is difficult to imagine how partial melting the garnet that has formed the biotite spots. could firstly produce biotite spots and secondly maintain them as coherent bodies once they were for~d. (it can be argued, in fact, that the partial (4) Irrespective of their origin, the obvious structural relationship melting will tend to lead to the removal of biotite to form melts which between the whole eyes and the central biotite clots and their arrangement will crystallize K-feldspar (Yardlay, 1978, p. 9425). In addition, the in the foliation stands in contrast to the isotropic fabric of the flecks presence of the biotite spots must preclude lateral movement along a vein described by Loberg (1963). of arq material derived by igneous injection (Yardley, 19785 from outside (55 The linking of the eyes to form vein-like patches with multiple bio- the gneiss as here again the spots would surely have been dispersed by tite pupils appears to be an initial process in the development of the such a methedof transport. veins with spots. It is also notable that the biotite forms clots con- The chemical composition (and mineralogy5 of the type 1 veins is tained within the veins rather than developing as mafic selvages on the different fr~ that which might be expected as a result of anatexis. vein margins. Because the normative Or is low in the trondhje~Itoid subtype, the plot of (6) The production of complementary veins of trondhjemitoid and K-feld- Q-Ab-Or falls well away from ternary minima and eutectlcs (Luth et al., spar-rlch aspect is apparently unusual. Both can be most readily explained 19645. Although the subtype richer in K20 (Table I, analysis 7T-pTots by metamorphic segregation. nearer the ternary minima and eutectics, it falls in the opposite direction from the higher pressure data presented by Luth etal. (1964). This is Acknowledgements. We thank Professor A.C. Cook, Mrs. H. Bunyan. Mr. A.J. contrary to what would be expected under high grade metamorphism. Further. Kantsler, Mr. D. Martin and Drs. A.J. Wright and R.A. Facet of the Depart- SPOTTED STRUCTURES IN GNEISS 787 ment of Geology at the University of Wollongong for help with the prepar- Oobl (O.). 1972. Geol. fSren. Stockholm FOrh. 94, 69-82. ation of this paper, Mr, J.B. Cameron of the Departtaent of Geology and Ferry (I.N.) and Spear (F.S.). 1978. Contr. Mineral. & Petrol. 6.66, MineralogY, University of Queensland, kindly supplied us with some of Dr. 113-117. Gradwell's original specimens. Dr, S.E. Shaw and Or. R.M, Flond at the School of Earth Sciences, Macquarie University carried out most of the Fershtater (6.B.), 1977. Geochemistry International 14, 63-72. whole rock analyses and Mr. N. Ware from the Research School ~ Earth Fisher (G.W,), 1970. Contr. Mineral, & Petrol. 29, 91-103, Sciences, Australian National University helped with the microprobe anal- yses. Professor Dr, K.R. Mehnert of the Free University of Berlfn intro- Fisher (G.W,), 1973. Am. J. SCI. 273, 897-924. ducs(t us to some of the European literature on the stictolithic structure. Hanson {G.N.), 1978. Earth and Planetary Science Letters 38, 26-43. Much of this paper was completed and written up while Phillips was on study leave at the Department of Mineralogy, British Museum (Natural Mist- Hedge (C,E.), 1972. Geol. Soc. Am. Mem. 135, 65-72. ory). He would like to thank the Keeper of Mineralogy, Dr. A.C. Bishop, Hobbs (g.E.), Ransom (D.M.), Vernon (R.H.) and Williams (P.F,), 1968. and His staff, in particular Mr. g.J. S~es and Mrs. J.E. Bevan, for their Mineral. Deposlta 3, B93-316. valuable assistance. The granting of study leave to Phillips and the financial aid of a University of WoIlongong Special Research Grant are also Kretz (R.), 1959. J. Geol. 67, 371-402. gratefully acknowledged. Lober9 (B.), 1963. Geol. f~ren. Stockholm F~rh~85, 3-109. REFERENCES Luth (W.C.), Jobns (g.H.) and Tuttle (O.P.), 1964. J. Geoph. Res. All~gre (C.J,) and Hart (S.R.)(eds.), 1978. Trace Elements in Igneous 69, 759-773. Petrology. Elsevier, Amsterdam. Mehnert (K.R.), 1968. Miqmetites and the Orlqin of Granitic Rocks. Al1~gre (C.J.) and Minster IF,), 1978, Earth and P~qn.etary Science Elsevier, Amsterdam. Letters ~, 1-25. Misch (P), 196G. Contr. Mineral. & Petrol. I/, 1-70. Andrews (E.C,), 1922. Mem. Geol. Surv. New South Wales ~, Olsen (S.N.), 1977. Geol. Soc. Am. Bu11. 88, 1089-1101. Arth (J.G.), 1976. J. Res. U.S. Geol. SurveyS, 41-47. Reed (S.J.) and Ware (N.G.), 1973. X-Ra;/ Spectrometry 2, 69-74. Ash~x)rth (J.R.), 1976. Mineralog. Ma9. 40, 661-~2. Russell (R.V.), 1969. Geol. f~ren, Stockholm F~rh, 91, 217-Z82. Ashworth (J.R.), 1977. Mineralo~. Ma9. 41, 295-296. Schnetzler (C.C.) and Philpetts (J.A.), 19/0. Geochim. Cosmochim. Acta Ashworth (J,R,), 1979. Geol. Soc. Am. Bull. 90, 887-888. 34, 331-340. Atherton (M.P.), 1976. Phil. Trans. R. Soc. Lond. A, 283, 2SB-Z/O. Shaw (D.M.), 1970, Ge~Chim. Co~ochim. Acta 34, 237-243. Binns (R.A.), 1964. J.Geol. Soc. Australia l l, 283-330. Turner (F.J.) and Weiss (L.E.), 1963. Structural Analysis of Metamorphic Tectonites. McGraW~Htll, New York. Brow~e (W.R.),tnAudrews (E.C.), 1922. Mem, Geol. Surv. New South Wales Vernon (R.H.) in Packh~ (G.M.) (sd.), 1969. J. Geol. Soc. Australia ~, 20/55. Chenhall (B.E.), iglS. Ph.D. thesis, Univ. of (unpubl,). Vernon (R.H.), 1976. Metamorphic PrOcesses. Wiley, New York, Chenhall (B.E.), 1976. J. Geol. Soc. Australia 23, 235-242. White (A.J,R.), 1966. Chem. Geol. 1, 165-200, Chenhall (B.E.), Pemberton (J.W.), Phtllips (E,R,) and Stone (l.J.), 1977. Mineraloq. Meg. 41, ~,20, Yardiey (B.W.D.), 1977. Nineralo~. Ma9. 41, 292-294. Chenhali (8.E.), Phillips {E.R,) and Gradwell (R,), 1978. X] General Yardley (8.W.D.), 1978. Geol. Soc. Am. Bull. 89, 941-951. Meeting of International M hera 09ica] Association Abstracts {gol. Yardley (B.W.U.), 1979. Geol. Soc. Am. Bull. 90, 8~. I L), H~vosibirsk.