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THE PRECAMBRIAN ROCKS of TASMANIA, PART VII NOTES on the PETROLOGY of SOME ROCKS from the PORT DAVEY-BATHURST HARBOUR AREA by A

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THE PRECAMBRIAN ROCKS of TASMANIA, PART VII NOTES on the PETROLOGY of SOME ROCKS from the PORT DAVEY-BATHURST HARBOUR AREA by A PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 99. THE PRECAMBRIAN ROCKS OF TASMANIA, PART VII NOTES ON THE PETROLOGY OF SOME ROCKS FROM THE PORT DAVEY-BATHURST HARBOUR AREA By A. SPRY' and W. E. BAKER' (With two plates and two text figurea.) ABSTRACT No formal stratigraphic terms will be used in Petrological examination SUggests ,that the rocks this paper and it seems that, until formal strati­ of the Port Davey-Bathurst HaDbour area may be graphic terms can be useti in an ,accurate, under­ divided into .two main groups: the regionally meta­ standable manner, the stratigraphic names for this morphosed schists, quartzites, phyllites and amphi­ area may have to be abandoned. bolites which are probably older Precambrian, and the essentially unmetamorphosed sediments which PRINCIPLES are probably younger Precambrian. The latter can be divided into three main types; subgreywacke The rocks are divided into a number of litho­ sandstones and slates (Ua. Bay, Bramble Cove and logical assemblages using the amount of meta­ north eastern Bathurst Harbour), greywacke sand­ morphism and deformation as criteria following stones, slates and conglomerates (Joe Page Bay) Spry (1962a) because the structural and strati­ and orthoconglomerates and quartzites (Mts. graphic relations are not yet clear from field Rugby, Berry, &c.). studies. The low ,to medium-grade metamorphics strongly resemble rocks at Frenchmans Cap, the upper Mersey-Forbh area and Ulverstone, and INTRODUCTION are assigned to the Older Precambrian (Spry The geology of the remote south-western corner 1962a) , Moderately deformed but unmeta­ of Tasmania is complex 'and observations by morphosed sediments resemble rocks between Twelvetrees (1906, 1908, 1909), Baker (957), Zeehan and Corinna and along the North-West Stefanski (1957), and Taylor (1959) have left Coast (Spry 1957a, 1964) and may be Younger major problems of structure and stratigraphy un­ Precambrian. Slightly deformed conglomerates, answered. Jennings (1960) summarized the in­ greywackes &c. do not resemble Precambrian rocks formation available and made suggestions as to elsewhere in Tasmania closely but are probably poss.ible stratigraphic relations as a basis for Younger Precambrian. further work. Investigation of Precambrian rocks in various parts of Tasmania (Spry 1962a, 1962b, ROCKS OF OLDER PRECAMBRIAN TYPE 1963'a, 1963b) has shown that a study of the structural petrology of ,these rocks is a useful guide These rocks are distinguished by ,their meta­ to the stratigr,aphy although use of the degre,e of morphism and deformation. They ,are divided into deformation 'and tectonic style to subdivide the (1) a Schist-Quartzite-Amphibolite Assemblage rocks of an area is by no means infalli!ble (Spry containing regionally metamol'phosed sediments 1963a). The following notes are based on field and basic igneous rocks belonging to the quartz­ work (W.E.B. and A.S'> and petrology (A.S'). albite-epidote-almandine subfacies of the Green­ This work is generally in agreement with that of schist Facies and possibly the Almandine Amphi­ Jennings (960), but like it, is only intended to be bolite Facies (Turner and Verhoogen, 1960). (2) A an aid to detailed field investig·ations in the future. Quartzite-Phyllite Assemblage containing low grade DespitJe the fac,t that ,the stratigraphy and regionally metamorphosed sediments belonging to structure of this area is almost completely un­ the quartz-albite-moscovite-chlorite subfacies of known, no less than thirty-five stratigraphic terms the Greenschist Facies. have been used in a formal sense (Stefanski 1957, Petrologioal examination shows that the meta­ Baker 1957, Taylor 1959). With the possible ex­ morphism was accompanied by two periods of ception of Jennings' (1960) terms, few of the deformiation and in the following descriptions the fDrmal names are used according to the Australian criteria and terminology of Spry (1963a 19'63b) Code of Stratigraphic NDmenclature and at present are used; So is bedding, the first deformation F, it is practically impossible to understand what produced a foliation S, and a lineation L,' the some terms mean. Formations and groups have second deformation produced 8 2 and L2 &c.' been named without reference to type areas, the 1. Schist-Quar,tzite-Amrphibolite Assemblage. same rock is given different names of different Dark grey schists outcrop extensively on the authors (or even by the same author), uncon­ wave-cut platform extending northwards from formities or cDnformable successions are postulated Kelly Basin beyond Bond Bay. The schists to the with no supporting evidence,and the same strati­ graphic name is used by different authors to. (') Department of Geology, University of Tasmania. apply to different rocks. (') Broken Hill University College. R.S.-3 17 ..... 00 :::0 ---A Mt. King ? , o /60 .> Mtns S ~o· e. w ;>;" ~ :r" H S Pi t'J 'b" '1j (5 H, ~ t'J »C"l ~ i:t: S:;. t!) ro 22 ~ » ~ 2: en ;t' ~ , o g ~ g: PO RT [f) ro '1j ~ o nI DAVEY H :i Forrel >­ tl ito Ul I ~ "ro » "' Z '"to ! ;; 8'" "rf,~ ~ l=:il------- ~ 5:\ '\, ~ ~ ,;­ :-. ~ QUARTZITE- PHYLLITE ASSE~! BLAGE 4936' SPEC;MEN NUMBER DIP & STRIKE CF BEDDING » ;( PARACONGLOMERATES ETC [IJJ SCHIST-QUARTZiTE -AMPHIBOLITE LiTHOLOGiCAL CONTIl.Cr D!P & STRIKE OF FOLIATiON ASSEMBLAGE " ----------- ~J GREYWACKE:S, SLATES, CONGLOMERATES -- POSSIBLE FAULT PLUNGE OF MiNOR FOLD AXIS SANDSTONES,SLATES A. SPRY AND W. E. BAKER 19 (0) ... FOLD AXES I Fj? I A F, OR F, FOLD AXIS • POLE TO NEAR-AXIAL FOLIATION IS,? I X F, FOLD AXIS X POLE TO BEDDING (So+Sjl @ F3 FOLD AXIS • POLE TO S 'L F, LINEATION FIG. 2.--Struc.turaI data-- (a) Folds with di.3cordani non-axial foliation, Davey­ Crossing Haddle. (b) Fold axes, lineations and foliations in the Bond Bay area. south are rich in garnet whereas those to the north syntectonic but the internal surfaces (S) are contain abundant albite. Minor gal'net-quart.zites discordant with 8 e (the foliation in the surround­ outcrop at Kelly Basin and amphibolite bodies ing matrix). Some albite contains simple helicitic occur 1 mile north of Bond Bay. The foHation structure discordant with Se· The mica which out­ strikes at between 100' and 160 0 and dips are lines the foliation wmps around both garnet and moderate to the south and south east. albite which are .thus older than the foliation Sm.all "isoclinal folds of a colour banding (S,.?) (Le. pretectonic). The 81 within albite and garnet occur within layers parallel .to the main foliation are thus taken to be an old foliation 8,. The gaTnet , (S2! and plunge at about 40" towards 2200 (Plate is the same age as Si and is syntectonic to F ; the r., No. fig. 2). These may be F, folds in which albite is younger than 8 , and is post-,tectonie to the surface cleavage (S,) is paralled to, and Fl. The main foliation is thus S,. Some albite is masked by, S2, but are more likely to be F2 folds. lenticular along 82 and contains no Si, this is probably syntectonic and of the same age as 82. Lineation (ribs in quartzite, elongate envelopes The muscovite and biotite which outline S, are to garnets) is not widespread but is strongly interwoven and lenticular; these are taken to be developed in places and is regarded as L2. Measure­ both largely of similar age GO 8 2, and thus syntec­ ments show that it plunges fiatly towards about tonic to F 2 ; both micas occur in pressure shadows 220" (fig. 2). around garnet. Quartz forms elongate blobs along Kink zones, crenulations, and folds up to ten or both Sl and 82 and is probably syntectonic to both twenty feet across have a different tectonic style. F, and F,. are essentially parallel folds and disturb The foliation 8 2 has been crenulated with the S", and S,; they have no axial-surf·ace foliation incipient development of a fracture cleavage S, and ~,.o mineral growth is associated with them along which some biotite may possibly have (Plate 1., 5 and 6). Both macroscopically and grown. Chloritization of garnet is a very late micros(',opically these folds have the characteristics process as the chlorite is randomly oriented. of J:o'"'. folds (Spry, 1963a, 1963b). These structures are strongly developed just east of Bond Bay where The coarse .. grained schists (such as 4935 and they are associated with faulting. A few rather 4944) consist of albite porphyroblasts about 1 cm. open similar folds plunging fiatly towards 145 0 near across separated by irregular layers of contorted Kelly Basin probably belong to F,;. mica (about 1 mm. across). The albite is undulose Fine-grained schists (such as 4936", 4946, 4947, and somewhat shattered, it contains helicitic structure with 8i discordant with Se and the mus­ 4950) consist of small (1 mm.> porphyroblasts of in a foliated matrix of muscovite flakes covite wraps around it. Albite is post-tectonic to mm. long) with lesser biotite; albite occurs an early phase but pre-tectonic to the main foli­ as lenticular crystals about 0.4 mm. long and in ation which is thus S2. It is so abundant in some fine-grained lenticular aggregates. The garnet * N umbers refer to' specirl1ens in the collection (If the Geology contains snowball structure (Plate II., NO.3) and is Department, University of TaSITlania. 20 THE PRECAMBRIAN ROCKS OF TASMANIA specimens (2363, 2368) that inter-tectonic meta­ are post-tectonic). This is indicated by the ran­ somation is suggested (Gee, 1963; 8pry 19'63b, p. dom disposition of the biotite within the foliation 117). The analysis in Table II shows 4% Na20. layers in 4944 'and by the form of the muscovite The garnets 'are very small (less than 0.1 mm.), are in 4935. In this rock the mica flakes ,are not bent structureless and mostly contained within the around the hinges of the crenulations as they albite although they also occur outside it.
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