all 1 : 25 000 sheets are 15 minutes x 7.5 minutes for the NSW standard 22-4/H54-15/4-4/1-I-N Vertical scale: 250

22-2/E52-02/2-3 Contour interval: 50

Titles in this newsletter: 'Australian crustal elements' map: a geophysical model for the tectonic framework of the continent A reality and a winner - automated AF demagnetisation comes of age in palaeomagnetic methodology at AGSO Is oil being generated beneath the northern Arafura Sea? Ocean-bottom-seismograph cruise on the North West Shelf NABRE's first field season facilitates the discrimination of sequence boundaries and maximum-flooding and transgressive surfaces in the Mount Isa-Lawn Hill region AGSO's 'fill-spill ' project: reducing exploration risk on the North West Shelf The Fundamental Gravity Network in Victoria Reworked Ordovician conodonts lead to an enhanced mineral and hydrocarbon potential in the southern Petrel Sub-basin, Western Australia The Broken Hill Exploration Initiative - seeking renewed prospectivity in an historically prospective area of New South Wales and South Australia

Titles in this newsletter: Recent NGMA mapping highlights the metallogenic potential of the East Kimberley Ocean outfall discharges of nutrients, offshore Sydney Applications of the laser Raman microprobe to the study of layered intrusions Preservation of old accumulations - key to Canning hydrocarbon exploration High-precision geochronology of Palaeoproterozoic layered mafic-ultramafic intrusions in the East Kimberley The Lakefield Basin: a newly named Permian basin in far north Queensland New mineral discoveries in the East Kimberley Late Palaeozoic magmatism in north Queensland: recent new perspective expanded by transfer-structure hypothesis Airborne gamma-ray spectrometry as a tool for assessing relative landscape activity and weathering development of regolith, including soils Palaeomagnetism suggests mid-Carboniferous convergence between Greater Australia and Altaids Magnetic reference field models for 1995 Geochemistry as an aid to interpreting relationships in the Narwietooma Metamorphic Complex, Central Province of the Arunta Block, central Australia Chemical oceanography of Port Phillip Bay Geochronology of an exposed late Archaean basement terrane in The Granites-Tanami region

all 1 : 25 000 sheets are 15 minutes x 7.5 minutes for the NSW standard 22-4/H54-15/4-4/2-II-N Vertical scale: 400

The Mount Aloysius massif comprises complexly folded high-pressure Mesoproterozoic granulites, typical of those making up much of the western Musgrave Block. The granulites host the well-studied mafic-ultramafic layered intrusions of the Giles Complex, but up to now have not been mapped in detail. Previous lithological mapping and airphoto interpretation produced conflicting synformal and antiformal interpretations of the structure of the massif. New mapping reported here resolves this conflict, and shows that the massif preserves four episodes of folding. The first formed small, initially recumbent (now reclined), F1 folds, which are earlier than any previously known in the massif. Two major episodes formed a large west-inclined isoclinal, gently doubly plunging, F2 antiform, which was subsequently folded early in D3 to a steeply doubly plunging antiform and then bent late in D3 to an arcuate shape. With each episode, the intensity of deformation decreased, from isoclinal F1 folds with a strong axial-plane granoblastic-textured foliation to open F4* folds with spaced axial-plane cleavage and local greenschist retrogression, reflecting decreasing ductility as the rocks cooled. Formation of the F2 antiform was helped by the presence of a stiff buttress of relatively massive granulite in the east of the massif, against which well-layered granulite to the west was squashed and flattened. Subsequently, the same near-massive granulite provided a stiff auge-like core, around which the well-layered granulite was wrapped during D3.

Layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block are confined to a southern granulite facies block thrust northward over an amphibolite facies block and southward over sediments of the Officer Basin. The Tomkinson Ranges to Jameson Range region, at the westernmost end of the southern granulite facies terrane, consists of 17 medium to large-scale faulted segments or intact layered mafic-ultramafic sills and lopoliths emplaced into felsic to intermediate or mafic granulite facies orthogneiss. Protoliths of these gneisses, giving ages of -1.55 and 1.3 Ga, were metamorphosed at -1.2 Ga. Emplacement of the mafic-ultramafic bodies into multiply deformed felsic granulites (D1 and D2-pure shear; T>750°C, P=5±1 kb), previously believed to have occurred at about 1200-1188 Ma, is now thought more likely to be of 1.08--1.06 Ga--coeval with the Tollu Group volcanics. The western Musgrave Block displays crustal zonation of near-contemporaneous units, from deep crustal ultramafic-dominated intrusions in the north (south of the Woodroffe Thrust), to gabbro-pyroxenite intrusions in the Tomkinson Ranges, to troctolite intrusions in the southwest, to upper crustal volcanics of the Tollu Group-i.e. a southward rise in crustal level. The layered intrusions include: (1) large olivine-clinopyroxene-plagioclase troctolite to troctolite-anorthosite bodies, with Fe-rich olivine and plagioclase as liquidus phases, crystallised from highly evolved silica-undersaturated liquids and representing high-pressure orthopyroxene fractionation prior to intrusion. These bodies are commonly magnetite-rich, representing high oxygen fugacities, and include little or no ultramafic component, e.g. Jameson, Blackstone, Cavenagh, and Bell Rock intrusions; (2) large orthopyroxene-clinopyroxene-plagioclase gabbronorite to norite intrusions, including a significant (up to about 30%) ultramafic component, e.g. Michael Hills, Mount Davies, Kalka; (3) small to medium-sized layered pyroxenite-peridotite-gabbro intrusions, e.g. Murray Range, Claude Hills, The Wart, Gosse Pile, Ewarara, and (4) stratiform anorthosites forming lenses and recrystallised tongues interlayered with felsic granulites, mainly around Teizi bore. Ultramafic increments crystallised from little-fractionated primitive basaltic magmas saturated with olivinespinel form late magmatic pulses injected into above-solidus resident gabbroic bodies. Intrusion was followed by isobaric cooling (Wingellina Hills: P=6±1 kb; Blackstone: ~4 kb). Near-coeval relations between the Giles Complex and the Tollu Group volcanics imply rapid uplift and erosion of deep crustal zones followed by volcanic activity. Feeders for the volcanics are represented by type-A dykes correlated with the ~1.05-1.07 Ga Kulgera swarm of the eastern Musgrave Block, and by extensive granite veining and related granulite facies recrystallisation of large sectors of the Giles Complex (D3-simple shear; early stage T=650-700°C, P=11 kb; late stage P=4.5±1.1 kb). Northward thrusting of the granulite facies block over amphibolite facies gneisses along the western extension of the Woodroffe Thrust -550 Ma was associated with elevated pressures along the fault zone (P=14.0±1.1 kb; T=750°C), contemporaneous with the Petermann Ranges deformation.

The Musgrave Block hosts about twenty major layered intrusions and several generations of compositionally diverse sills and dykes collectively referred to as the Giles Igneous Complex. All were emplaced in middle Proterozoic intermediate to felsic granulites and amphibolites. On the basis of crystallisation sequences within the cumulates, we distinguish between (1) ultramafic olivine-clinopyroxene rich cumulate sequences, (2) mafic clinopyroxene-plagioclase-rich sequences, and (3) evolved troctolitic olivine-plagioclase ± magnetite-rich sequences. The layered igneous rocks are intruded by several generations of dykes, whose phenocryst assemblages resemble the crystallisation sequences in the cumulates. On this basis, at least three discrete parental melt compositions are identified, namely (1) a near-primitive olivine±clinopyroxene saturated melt, (2) a slightly fractionated olivine-( orthopyroxene )-clinopyroxene-plagioclase saturated melt, and (3) a strongly fractionated fayalitic olivine-plagioclase ± magnetite saturated melt. These magmas represent derivatives of a parental liquid that experienced various degrees of polybaric orthopyroxene-clinopyroxene ± olivine fractionation prior to emplacement, at depths significantly greater than the emplacement levels of the Giles intrusions and the dykes.

The history of Mid-Proterozoic metasedimentary gneisses, felsic and mafic orthogneisses of the Tomkinson Ranges, western Musgrave Block, involved multiple low- to high-P metamorphic events and polyphase deformation. Isoclinal, gently inclined to recumbent D1/2 folding accompanied the intrusion of pre-S1 mafic and felsic orthogneisses and post-S1 , pre-S2 felsic orthogneisses under peak conditions of P=5±1 kbar and T-750 °C at ~1200 Ma. K-feldspar megacrystic granitoid stocks formed at ~1188 Ma. Post-S2 mafic to ultramafic magmatism resulted in the large layered intrusions of the Giles Complex at ~1080 Ma, whilst the terrane was at conditions of P=6±1 kb. At ~1080-1060 Ma, microgranitoid dykes and veins intruded Giles Complex sills in the Tomkinson Ranges, while comagmatic volcanics of the Tollu Group unconformably overlie other parts of the Musgrave Block further west. Type A mafic dykes, which may represent feeder veins to the Tollu Group, intruded rocks of the Tomkinson Ranges before a third deformation event, D3, resulted in a system of steep, southeast-trending mylonites with a granulite facies S3 foliation and a well-developed down-dip L3 stretching lineation. Type A mafic dykes and their host felsic granite gneiss show marginal to complete recrystallisation to granulite facies mylonitic S3 assemblages at conditions of P-11 kb and T-700°C. Near-isothermal decompression returned such parts of the Musgrave Block to conditions of P-4-5 kb late in D3. Parts of the Musgrave Block that are unconformably overlain by the Tollu Group probably remained closer to the earths surface during D3. Coarse-grained, ~800 Ma Type B mafic dykes and aphanitic, ~1000 Ma Type C mafic dykes cut S3, and are cut by a system of mylonite and retrograde shear zones, D4-7. East-trending ~550 Ma D6 ultramylonite pseudotachylite zones represent the effects of the Late Proterozoic to Cambrian Petermann Orogeny, which resulted in significant dislocation of parts of the Musgrave Block and the partial to complete recrystallisation of post-S3 dykes near the Woodroffe thrust zone at sub-eclogite facies conditions of P=14±1 kb and T=700-750°C. These late high-pressure assemblages suggest that the structural grain in rocks of south-central Australia resulted, to a major extent, from Late Proterozoic to Cambrian compression.

New geophysical data provide the first clear image of the marginal structure of the southern Musgrave Block and the northern Officer Basin of western South Australia. The data show that the Officer Basin and the Musgrave Block are underlain by a series of pervasive, north-dipping planar structures that predate development of the Officer Basin. Reactivation of these structures formed a wide thrust fault complex and developed a major homocline at the northern margin of the basin.