COCKER,

teen Devonian, granitoid plutons inContact metamorphic aureoles range 200 m to 2 and Clarkemetamorphic assemblage cordierite-andalusite-biotite-quartz-potash feldspar is pre-served. Approximately 70 per cent by area of the granitoid terrain is composed ofgarnet and/or cordierite-bearing granites (eight plutons), 20 per cent biotitegranites (five including two altered plutons, and 10 per cent hornblende-

biotite granodiorites (three plutons). The granitoids were intruded by widespreaddolerite dykes of pre-Tertiary age. Cainozoic sedimentary sequences are thin andirregularly distributed. Mid-Tertiary sediments formed in lagoonal and shallow watermarine environments have been reworked during the Quaternary by alluvial and aeolianprocesses In the Rooks River tinfield the cassiterite, which was eroded frompervasively altered Rooks River granite, is thought to be a lag deposit derived byreworking of the Tertiary sediments. Potential areas of exploration includesouthern end of the Lee River valley near the strongly altered Rooksthe alluvial fans north and south of the altered Hogans Hill granite pluton.

INTRODUCTION

This report summarizes the regional geology of the southern Furneaux Group1), particularly the geology of the granitoid rocks. Some 20 islands in and southFranklin Sound are considered with emphasis on Cape Barren and Clarke Islands. Six­teen granitoid plutons outcrop over some 60 percent of the southern Furneaux Group(fig. 2). The Lower Palaeozoic Mathinna Beds which are poorly exposed, especially in­land, occur on Clarke and Badger Islands and in the northeast of Cape Barren Islandwhere they are continuous with outcrops on Tin Kettle Island and FlindersIsland. Marine, and aeolian Cainozoic sediments are poorly exposed in broad,low-relief, low altitude areas between granite mountains. Bedrock exposure is variablefrom excellent along some coastal sections to very poor, especially in the easternhalf Cape Barren Island where sand dunes and tea-tree marshes cover large tractsground.

(Blake 1947), discussion of mines prospectsCainozoic sediments

Other contributions to the Tertiaryby Johnston (1879), (1945) and

the Cainozoic geology and geomorphology(1973) briefly described the distribution ofFlinders Island.

geologicalsummary the regional geology(Keid and an outline ofwestern Cape Barren Islandgeology of the southern Furneaux

(1971); and Sutherland (1973)Flinders Island (fig. 1). Groves

textural granitoid types on

This work is based on reconnaissance field mapping and Figures 2, 3 and 4 areproduced from aerial photo layouts without ground control. Consequently, the universal

49

50

which are contact metamor­phosed byareMathinna Beds in north­eastern Tasmania. Theserocks are well exposedalong the north coast of

Barren Island, onIsland (fig. 2),

and along the northernand southern coasts of

I s I and (fig. 4).measurements and

aerial inter-structures

that the MathinnaBeds strike mainly in anortherly direction with

plunging foldsby steep

axial surfaces (vertical± 20°), long planar limbsand sharp closures. Awayfrom contact metamorphosed

The change in fold vergence eastas evidence for the disruption of the

As there is excellent , thecould be clearly defined if field

TASMANIA

40° 5-+--1+0

,--+---120-,-j--+--t---t-r--~

FIG. 1.- Locality map for the southern Furneaux Group.

areas, slaty cleavage is present in mostand west of Puncheon Point may beregional fold trends by the granitoidregional structure of the Mathinnastudies were carried out.

sedimentarysandstone rarelysedimentary~arable to thoseBadger Island and

aSedimentary structures com­

along the north coast ofBarren Islands.

Numerous coastal Mathinna Beds-granitoidsouthern Furneaux Group. Clearly exposed contacts are notedwidth of contact metamorphic zones varies from 200 m to

outcrop in the2 and 4. The

thin-section the

OQ(D

oi--4o

OQ

"<

tni--4

52

On a broad scale, the larger granitoid to rectangular inwith their largest dimension oriented north-south, parallel to the MathinnaBeds structure. This structural fabric is similar to that on the Tasmanianmainland. The were intruded following the folding of the Mathinna Beds, butmay have been by regional tectonic events which controlled the development ofK-feldspar megacryst foliations. variably developed, the foliations are wide-spread and commonly trend north-south. , straight and sharpcontacts and the lack of marked marginal deformation the Beds indicatesthat individual plutons were emplaced along fault controlled margins by roof-lifting. Minor stoping of country rocks also occurred during emplacement.sharp, transgressive contacts and lack of primary muscovite thethat these plutons were emplaced at high in the crustkm

The granitoid plutons will be considered in three groups distinguished by theirmafic mineralogy. Eight are characterized by the occurrence of garnetcordierite. Three small granodiorite occur in thethe southern Furneaux Group and biotite occurs as the mafic silicate ingranite plutons, including the strongly altered Rooks River granite. The intrusiverelations and common textural variants of each pluton are summarized in table 1. Themain features of each pluton are discussed below. The petrogenesis of the granitoidsis discussed in Cocker (1977a and in prep.).

MT KERFORD GRANITE

The Mt. KerfordCape Barren Islandages with the Kent Baygranite is intruded bytextural variantsphase, intruderock formsand ofexposed is dominantly

although well around the southeastern coast ofIsland, has poorly exposed external contacts. Relative

granites are unknown, but the Mt. Kerfordgranite and its associated The two

, a porphyriticinclusions one

of the

In hand specimen, quartz, K-feldspar, and biotite are theminerals with minor garnet and secondary Garnet is a widely distributedaccessory mineral .01%), especially in the southern half of , and is alsoconcentrated in spherical to lenticular masses. At FR/128 197, eastern side ofKent Bay, the garnet occurs in a crudely layered complex with a 0.5 m thick

Plutons

granite; intrudes MathinnaBedsIntruded by Rooks River Fine and mediumgranite; intrudes Mathinna equigranularBedsIntrudes Mathinna Beds Medium grained, equigranular

Intrudes Mathinna Beds Medium grained equigranulargranite

Intrudes Mathinna Beds Fine grained equigranulargranite

Kent Bay granite

Modder River

Key Bay

Clarke Island granite

Dover River granite

Hogans

Puncheon Point granite

Rooks River granite

Corner granite

Thirsty Lagoons granite

Intrudes Mathinna BedsDover River graniteModder River granite

Intrudes Mathinna Beds

variety ofand porphyritic

Medium grained, megacrysticand equigranular granitesFine grained, equigranular

Mt. Chappell Islandgranite

Vansittart Island granite Intrudes Mathinna Beds

grained, equigranulargraniteWide variety, equigranularand porphyritic phases

Cape Sir John granodiorite Intrudes Mathinna Beds Medium grained, equigranulargranodiorite

Badger Island granodiorite Mathinna Beds Medium grainedand megacrystic

Unicorn Point granodiorite Mathinna Beds Medium grainedmegacrystic

made interms. names have been

and may be retained following detailedcharacterization. terms after

basal garnet-rich layer (up to 60% garnet) overlain by a relatively thin (0.biotite-rich layer. Flow sorting and possibly gravitational settlingthe mode of concentration of the garnet. Approximately spherical

54

thelized includewith vermicular bookletthe Mt Kerford from An 40,

An The zoned portionsrim especially adjacent to

other phases and in the porphyriticrimming zoned Quartz probably has apluton as it is included in early formed garnet, is in the porphyriticand is commonly euhedral against K-feldspar in the equigranular rocks.

intergrown in 43232, 43235 andphase rock type (FR/249.223),

mm diameter, are an light-brownby white-mica sea-green biotite. amorphous com-cordierite which intergrown with other phases in the

rock.

The composition of the garnet in the Mt. Kerford granite and in other plutonsdiscussed below is restricted to the following range almandine 73 86% 6 -20%; grossular 3 5%; 3 - 5% (molecular). Similarly, the biotite these

is rich in Fe Al (Fe/(Fe+r-.1g) 0.73 - 0.93; Alvl = 0.72 1.33) (Cocker,prep.) .

HOGANS HILL GRANITE

Hogans Hill granite is a small composed of porphyritic granite andthat out around Hill. The largest area of outcrop is

the south is small areas of the Mt KerfordThe hill to west of Hogans Hil of a series of shallowlydykes oined by short vertical sections. to the northwest (nearSlml north-south striking dykes, to 30 m wide, and aresmaller dykes intruding the Mt granite within a of five from HogansHil. Contacts between the granite and the Kerford granite, although

exposed, are well especially those contacts the Mtgranite. Mafic inclusions not observed in this pluton.

Red garnet occurs as a to subhedral, partially pseudomorphed phenocrystphase, up to 10 mm in diameter. Unlike other garnet-bearing plutons, concentrations ofgarnets are rare and the of garnets is relatively low. The garnets tend tobe surrounded by red-brown , comparable to the phenocryst biotite andby pale green biotite in cracks. Quartz, red-brown biotite, apatite, zirconilmenite are included in the garnets. red-brown biotite occurs as individualbooklets up to 5 mm width and as aggregates of grains. The groundmass

100Group2 km wide, the by acontact migmatite, further north (ER/988.structure. The Bay granite-Madder granite(161 0 ), sharp Lascar on Barren Island.intrusive relations could not be this contact. Betweenand Clarke Islands this contact may have been displaced by a fault whichnortheast along the Dyas Corner River valley

The main rockgranular granite,common CapePreservationmegacryst-richexpressed asapproximately

the graniteup to per cent garnetare common around Keyon Clarke Island.

a coarse-to medium-grained,microgranites and porphyries

along strike to the opposite onalong the coast,

of rockof megacrysts,

Lascar Point.

MOODER RIVER GRANITE

on and Clarke Islands. an

layeredup to 40 m

layers whichis sub-

grainsBiotite­

granite, occurring as scattered individ-layers. Many of the inclusions are

the country rockOn Tin Island

to the contact. A planarof biotite grains was noted

plutonThese

with a fabricare paralleled by tabularhorizontal, but in oneand small inclusionsrich inclusions are relatively commonual inclusions and swarms associatedsimilar to hornfelsed Mathinna Beds andcontacts, therectangular to 100 mvertically definedon Clarke

Modder River granite isSubhedral plagioclase

and anhedral quartz. The K­groundmass around their margins.core compositions from An 35 tois partially replaced fine-

rarely and epidote.and for some specimens exceeds 25

is red-brown, although minorquartz, occurs as a fringe on

Zircon and two generationsThe K-feldspar is variably

spatiallysutured grain boundaries

and rare grains to 0 5

Cordierite,identified in three samples 43070, 43068). samplesern of the pluton and cordierite occurs with garnets incomplex near the Mathinna Beds contact on Tin Kettle Island

reveal a more widespread distribution

Cordieritethe fine-grainedmica. Cordierite,'spots' to rumA microgranite dyke (43870)as part of a granular

CLARKE ISLAND

PUNCHEON

--_._-\ ".,nlf)nWnl boundary

River Fault

rock which hasthe contact.intrude the Modder

commonlycontact Geology by JDCocker If

to dykes and thetact, Moddergranite has been dis-playing cream to pink feld- .. - TI1e Rooks River granite.

The eastern contactthe Dover River granite is poorly located and the medium-grained, equigranular

is an(fig.south where itoff the northeasternof Clarke Island, pos-sibly as narrowdyke-like body theMathinna Beds and the DoverRiver granite across theDover River valley. Thispluton intrudes the ModderRiver granite anirregular, steeplycontact with asome 500 m. The contactis offset by at one andpossibly two faults.ER/970.245 and ER/947.324the boundary between theRooks River granite and theModder River wellexposed,(up to 200 m long) ofstraight steeply dippingcontact, between the medium-grained, ModderRiver a granite

micro­former

rock type (43800). Theporphyry may be a chilled

to the medium-

60

characteristic textural grain sizefine- and rocks. rocks and

in the porphyries are commonly orientation.little biotite alignment and rare biotite layering is associated minor

Mafic inclusions are rare in this pluton. Quartz veins aredeveloped, steeply dipping and strike at 39 49 0

content, of

phenocrysts, with feldsparsthe porphyries

are extremely variable in theof different phenocrysts.

biotite booklets occurWhite-mica is

area forms per centThe aplite and dykes are similar in

A characteristic of the dykes thecavities commonly associated with stringers coarser-grained

ingrains

most common.the Battery Bay

groundmass feldspars.the main graniteoccurrence of(up to 10 mm) minerals.

enclose the which is com-boundaries is usually less than 1 mmmm in length. In the coarse-grained

the grain size of the groundmass ranges from to 3 mm.normally zoned grains with core in thethe phenocryst cores are altered to white-

plagioclase grains are Theoligoclase, poorly-zoned and commonly deformed

twins. The dustiness of variable,altered.

61

CORNER

The Corner is a medium- to coarse-grained, megacrystic granite exposedalong the coast near the Corner settlement around Long Island. The contact be-tween this pluton and the Modder River was not located in this work Bothplutons have similar characteristics the Corner granite tends to be coarsergrained and more K-feldspar megacrysts. The contact between this pluton andthe Cape Sir granodiorite is not

outcrop the Corner granite is heterogeneous short biotitecommon dykes and tourmaline nodules

a high proportion mafic inclusions. K-feldspar megacrystsare aligned (129 to and commonly biotite layers. OnLong land an 80 m layered K-feldsparmegacrysts, to layered complexes in shortbiotite layers are commonly cross-cutting and in some cases resemble sedimentary cross-bedding. Southwest of the Corner, mafic inclusions, some of resemble theMathinna Beds, are aligned in the .K-feldspar foliation. The tourmaline nodules tendto occur in Tourmaline is also a in thematites and is planes in the granite evidencethe activity of a fluid phase the intensely altered, red granite atER/862.307. In outcrop over some 50 all the phases of this rock are altered andreplaced by chlorite, and fluorite. In thin-section the Corner graniteresembles the Modder granite.

THIRSTY LAGOONS GRANITE

This rock type is poorly exposed on two rocky points and offshore rocks on thenortheastern of Cape Barren with the Mathinna Beds is notexposed, but small folds adjacent related to intrusion of the

The Lagoonsmuscovite granite. Thefeldspar. The K-feldspargranitoids the plagioclase is densely inclusions.

ISLAND GRANITE

Mtcoasts of Mtodiorite were not examined.grained, equigranular,

exposed around the northerncontacts with the Badger Island

Island granite is a uniformlyMuscovite aplites intrude this pluton and

6

FIG.4.- Geology of the Badger, Mt Chappell and Goose Islands.

VANSITTART ISLAND GRANITE

Vansittart Island on Vansittart, Great Dog and Little DogIslands at the northeastern end of Sound , 2) This pluton is com-posed of a variety of cream to pink granites with range of textures comparablewith of the Rooks River granite.

JOHN GRANODIORITE

is notstructure

veins, haveinclusions

byAn irregular cross-cuttillg

developed in most outcrops.

with biotite masses andmedium-grained with a variable

63

The foliationand the proportion

Annealed quartz wereAccessories both

UNICORN POINT GRANODIORITE

Island and GooseBadger transgressive

disconformable structurescommon rock type and

A

beddingCoastalcan be

coastaland commonly

movementsome dykes.

Barronand

sedimentsin and rubblemillimetres to 10 m.

in the granitoid rocksthe dykes

slickensided jointveins occupy oints in

dyke by a cm widevein (43943) which does not

and

ingsenvironment of tocarenites nearby. lagoonal sediments(Harris 1965) and the Rooks River valley have a1972). Based on the distribution of basement outcropsTertiary sequences may occur in nearby Lee River valleyCorner River valley.

Alkali-olivine basalt is exposed on a small tidal platform east of Lascar Point.

QUATERNARY SEDIMENTS

.. ..., .... £'" ..... £~f".,,-', inthe Tertiary

are widespread inBarren Island, and

to km long and sandyalluvial

thansand

Quaternary sand cover and alluvial sedimentsin central and easterncover includes parabolic

felsic minerals fromRiver, Lee River and

9 m thick and overlie Tertiary sediments. Aand gravel and Tertiary sediments is exposed in the Rookswhich the younger sediments occur above and infillings in mudrocks. To some extent the Quaternary sediments in the largerwind-sorted lag resulting from the of finerfields in the east. Most streams have limited accumulations of alluvial materialthe of the Modder River valley, and alluvial fans north and southHill alluvial sediments in the Modder River valley are widespreadusually less than 20 mthick. Bedrock occur in numerous streams.are no extensive alluvial sediments on Clarke

wind vectors similar to lunette dunesBarren Island and north-central

climb low hills and havecoastal

Group restrictedcarrying minor

, and a cassiterite-134) Both

Island

cavity-richgranite (ER/978.

Clarke Islandbeing assayed from

Clarke Island,

66

ExtensiveStandard 1972)The higher concentrationshas been mined, maycoastal sediments. Avalley. The sedimentsand a marine sedimentary

after the doleritesequences were

features inand blowouts on

the

geologicalTasmanian

grade regionalrock plutonsdykes, probablydykes the

REFERENCES

alluvialCompany,

Flinders

GEOLOGY OF TASMANIA, J. geol.

. Rep. Dep. Mines Tasm.,

the Tasmanian granitoids.

geology Mt William area, in, Cocker, J.D. and Jennings, D.. Bull.

Appleby, W.R. and McEwan, I. , 1966: Progress report ontin deposits in N.E. Tasmania and the Furneaux Group.Rep0Y't No. 139.

Banks, M.R., 1962: Silurian and Devonian Systems,inSoc. Aust. 9: 177-187.

Blake, , 1947: The Furneaux Group of Islands.1947 52-82.

Cocker J.D., 1977a:of Tasmania,

_____________, 1977b: The geology of the St. Helens petrologystructure, in The Blue Tier Batholith, by Groves, D.I. Cocker, J.D., andJennings, D.J. Bull. Surv. Tasm., 55.

in prep.: Petrogenesis of Devonian garnet and cordierite granites inTasmania.

Coscombe, P.W., 1965: Report on a geological reconnaissance of Cape Barren Islandwith proposals for drilling stanniferous leads. Utah Company~

Report No.Crespin, I., 1945: Middle Miocene limestone from Cape Barren Island, Furneaux Group,

Bass Strait. Pap. PY~oc. Soc. Tasm., (1944): 13-14.Groves, D. 1972: The zoned mineral deposits of Scamander-St.Helens district.

Bull. . Surv. Tasm., 53.1973 Notes on the reconnaissance of the granite rocks

Rep. Dep. Mines Tasm., 15 9-11.The geology, geochemistry and of the Blue Tier

in Blue Tier Batholith, by Groves, D.I., Cocker, J.D., and JenningsBull geol. Surv. Tasm.

Harris, W.K. 1965Group,

68

Streckeisen, A.,1-33.

Sutherland, F.L., 1973 geological development of the southern shores and islandsof Bass Strait. Proe. R. Viet., 85 111-144.

and Kershaw, ., 1971: geologyBass Pap. Proc. Soc. Tasm. 105: 151-176.

Williams, E., 1959 The sedimentary structures the Upper Scamander sequence andtheir significance. Pap. Proc. R. Soc. Tasm. 93: 29-32.