Arunta Province Projects

Goscombe maintains a long-term interest in research on the multiply reworked Palaeoproterozoic granulites of the Alieron Province and high-grade Neoproterozoic to Ordovician metamorphics of the Irindina Province, in central Australia. Research started in 1984 with undergraduate and honours projects at Adelaide University with Dr's Robin Oliver and Pat James. Continuing with PhD research in the intensely sheared granulites of the NE Strangways Range with Prof's Roger Powell and Chris Wilson at Melbourne University. More recently, research has continued in collaboration with Prof Cees Passchier, Dr Martin Hand and students at Adelaide University and a two year mapping contract and subsequent work contracts with the Northern Territory Geological Survey in the eastern Arunta region.

Below are links to summaries of outcomes on different aspects of central Australian geology:

• Age of Strangways reworking [1]

• Metamorphic evolution of the Strangways Metamorphics [2]

• Structural evolution of the Strangways Metamorphics [3]

• Silica-undersaturated granulites [4]

• Metamorphic evolution of the Harts Rage region [5]

• Metamorphic evolution of an Alice Springs Orogeny shear zone [6]

• Metamorphic domains in the East Arunta region
[7]

[1] High-grade reworking in the Arunta Province: part of a collisional system in the central Australian Proterozoic?

Martin Hand, Christopher Clark & Ben Goscombe

Continental Evolution Research Group, Adelaide University, South Australia.

See Journal of the Geological Society of London, v164, 937-940.

The evolution of the Australian Proterozoic has been a source of debate over the past 20 years over whether the tectonic events defining the Proterozoic record reflect dominantly within plate (intracratonic) processes, or processes linked to and driven by interactions at plate margins (e.g. ªener et al., 2005). The notion that the Australian Proterozoic may largely reflect within plate processes has been based on several lines of evidence: (1) lack of direct evidence for subduction (e.g. high-P metamorphic belts) within the orogens, (2) an apparent lack of igneous rocks typically associated with subduction regimes and (3) direct geological evidence that many orogens contain reworked older crust. While the Australian Proterozoic is typically characterised by regionally extensive and somewhat enigmatic low-P high geothermal gradient metamorphic terrains, there is a growing awareness that a zone of comparatively high-pressure metamorphic rocks exists along the southern margin of the Arunta Province and equivalents in central and western Australia. These high-P rocks contrast markedly with the typical low-P high-T character of many Australian Proterozoic terrains. Within the Arunta Province in central Australia, the granulitic Strangways Metamorphic Complex (SMC) dominates the southeastern part of the province. Existing zircon U-Pb geochronological data (e.g. Moeller et al., 2002) suggests that the complex records a single low-P high-T tectonothermal event at around 1720 Ma, in keeping with typical Proterozoic thermal regimes in Australia. However in contrast to the geochronological data, structural and petrological relationships are consistent with the existence of two distinct high-grade metamorphic events (Goscombe, 1992). The second of these events dominates the structural character of the SMC, and was associated with transpressional SW-directed transport linked to the formation of mylonitic fabrics and large-scale sheath fold systems that reworked the initial low P granulites. In metapelitic rocks, the mylonitic assemblages are defined by the high-PT granulite association orthopyroxene-sillimanite-quartz ± garnet, which formed at around 9-10 kbar and 800°C. Electron microprobe Th-U-Pb analysis of monazite that forms part of the high-PT mylonitic assemblages give 1629 ± 7 Ma (n = 223, 95% confidence). Data from armoured inclusions within relic garnet and orthopyroxene that formed during the earlier low-P event gives ages around 1725 Ma, consistent with existing age data (e.g. Moeller et al., 2002). The inferred age of high-PT reworking in the SMC is similar to the timing of recently recognised high-PT metamorphism in the western Arunta Province, which is interpreted to record the docking of an outboard continental terrain to the southern margin of the Arunta and equivalents (Scrimgeour 2003). The data obtained in this study suggest that if terrain collision did occur c. 1640-1630 Ma, high PT reworking of the SMC represents a localised inboard response. The timing of this event is essentially identical to age of arc-style magmatism in the Gawler Craton in southern Australia, hinting that the central and southern Australian Proterozoic regions share a linked history in the late Palaeoproterozoic.

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[2] High-Grade Reworking of Central Australian Granulites: Metamorphic Evolution of the Arunta Complex

BEN GOSCOMBE
Geology Department, University of Melbourne, Parkville 3052, Victoria, Australia
See Journal of Petrology 33, 917-962 (1992).

The poly-metamorphic evolution of the Strangways Range granulites of central Australia has been constrained by the phase stability relationships of silica-saturated aluminous gneisses in KFMASH, in conjunction with geothermobarometry and equilibrium thermodynamics. Two contrasting, but overlapping, P-T paths are proposed. The first (M1, at 1800 Ma) had an 'anticlockwise' P- T path (i.e., increasing P/T with time) and was terminated by isobaric cooling from 850-950ºC, at 8-9 kb, to a stable crustal geothcrm (<700ºC). In contrast, the second granulite metamorphism (M2-M5, suggested to be at 1400-1500 Ma; Goscombe, 1992a) followed a 'clockwise' P- T path (i.e., decreasing P/T with time) terminated by decompression and cooling to - 6-7 kb on a stable crustal geotherm. M2-M5 occurred during reworking of M1 granulites by compressional orogenesis (Goscombe, 1992a). Initially, loading and prograde metamorphism accompanied non-coaxial ductile shear and fold repetition (D2-D3). Prograde metamorphism was followed by uplift and retrogression accompanying oblique transpression and shear zone development while still under compression (D4-D5) (Goscombe, 1992a). The poly-metamorphic evolution indicates that ductile deformation reworked the M1 granulites in an orogenic episode unrelated, both temporally and tectonically, to M, metamorphism (Goscombe, 1992b).

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[3] High-grade reworking of central Australian granulites. Part 1: Structural evolution

Ben Goscombe
Geology Department, University of Melbourne, Parkville 3052, VIC., Australia
See Tectonophysics 204, 361-399 (1992).

Four distinct deformational events (D2-D5) accompanying a granulite facies metamorphic Cycle (M2-M5), are shown to structurally overprint (rework) pre-existing granulites (M1) of the northeast Strangways Range in the central Arunta Block. The first metamorphic cycle (M1) at 1800 Ma, involved an anticlockwise P- T path peaking at 850-950 ºC at 8-9 kbar. M1 metamorphism involved widespread partial melting that produced stromitic migmatites and map-scale concordant granitic gneisses. Isobaric cooling after the peak, and accompanying hydration, gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace M, mineral parageneses. No kinematic structuring is associated with M,, and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress.

The second metamorphic cycle (M2-M5), possibly at 1400-1500 Ma, involved a clockwise P-T path with a maximum P of > 9-10 kbar and a thermal peak of approximately 800'C. This metamorphic cycle accompanied the major ductile deformations recognized (D2-D5), and has been labeled the "Proterozoic Reworking". The Proterozoic Reworking has been divided into two thermo-barometric and structurally distinct periods. D2-D3 involved regionally inclined, ENE-WSW non-coaxial shear of high bulk shear strains, giving rise to an intense (often mylonitic) pervasive fabric (S2-L2) and isoclinal and sheath folds on all scales (F2-F3). S2-L2 is the first recognized kinematic fabric and does enclose and partially recrystallize M, mineral parageneses, including late-M, metamorphic reaction textures. D2-D3 deformation was due to crustal shortening and gave rise to crustal over-thickening (loading) accompanying prograde metamorphism (M2). D4-D5 involved upright, open and asymmetrical folding (F,) and E-W-trending shear zone development (S5) within a regionally extensive system of inclined, oblique, sinistral transpression. Both D2-D3 and D4-D5 episodes occurred under the same approximately E-W-directed compressive stress, and are considered sequential episodes in the one tectonothermal cycle. F4 folding accompanied significant melt formation and the peak of metamorphism of the M2-M5 metamorphic cycle. S5 shear zones accompanied 3-4 kbar of decompression with cooling, presumably during uplift in isostatic response to crustal over-thickening in D2-D3.
Progression from inclined non-coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening, as is typical of many compressional orogens. However, the Proterozoic Reworking is atypical of simple linear or arcuate mountain belts because late-stage shearing (S5) is aligned sub-parallel, not at a high angle, to the transport vector of the earlier ductile deformations (D2-D3). Consequently, this region is modelled in terms of crustal shortening directed along the length of an E-W-trending orogen confined to the north and south by relatively stable crustal blocks. When E-W shortening could no longer be accommodated by crustal thickening, strain was partitioned by sinistral transpression between the north and south bounding crustal blocks.

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[4] Silica-undersaturated sapphirine, spinel and kornerupine granulite facies rocks, NE Strangways Range, Central Australia

B. GOSCOMBE
Department of Geology, Melbourne University, Parkville, Victoria 3052, Australia
See Journal of Metamorphic Geology 10, 181-201 (1992)

Small pods of silica-undersaturated Al-rich and Mg-rich granulite facies rocks containing sapphirine, pleonastic spinet, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene-rich aluminous granofels and metapelitic gneisses containing sapphirine, spinet or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper.
Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900-950ºC and 8-9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the-sequential crossing of the following reactions in the system FMAS:
cordierite + spinel + corundum = sapphirine + sillimanite,
cordierite + spinet = orthopyroxene + sapphirine + sillimanite,
sapphirine + spinel + sillimanite = orthopyroxene + corundum,
sapphirine + sillimanite = cordierite + orthopyroxene + corundum.
Phase stability relationships in FMAS and MASH indicate an anticlockwise P-T path terminated by isobaric cooling. Such a path is exemplified by early low-P mineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase in aH2O and aB2O3 with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1 P-T path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.

Key words: Arunta Block; granulite facies rocks; kornerupine; P-T paths; sapphirine.
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[5] METAMORPHISM AND CRUSTAL CONSIDERATIONS IN THE HARTS RANGE AND NEIGHBOURING REGIONS, ARUNTA INLIER, CENTRAL AUSTRALIA

R.L. OLIVER, R.W. LAWRENCE, B.D. GOSCOMBE, P. DING,
W.J. SIVELL and D.G. BOWYER
Department of Geology and Geophysics, University of Adelaide, Australia
See Precambrian Research, 40/41, 277-295 (1988)

The Harts Range region consists of several segments, or terrains, which have behaved essentially independently, though some overlap of history, permitting correlation, is apparent. Some analogy with the 'terrane' concept is envisaged. The Harts Range terrains are: (1) the Strangways Orogenic Belt (SOB), south of the Muller Thrust, (2) the SOB of the Oonagalabi Tongue, (3) the SOB of the Entia Dome, together comprising a 'Basement', and (4) the Harts Range 'Cover' (HRC), all separated from each other by thrusts or faults.
The terrains consist of supracrustal sequences comprising metasediments and some metabasites, thought to have been deposited in extensional rift depressions, plus varying amounts of intrusive granitoids. Both granulite facies and amphibolite facies mineralogies characterise the basement terrain of the Oonagalabi Tongue and that south of the Muller Thrust. The Entia Dome and cover terrains have only amphibolite facies mineralogy.
A combination of thermobarometry and petrogenetic data suggests equilibration of the SOB south of the Muller Thrust at 700-800 ºC and 5-7 kbar and of the HRC in the Ruby Mine area at 680-780 ºC and 6-8 kbar. Some thickening of the supracrustal accumulations owing to compression is considered necessary for the development of these pressures. Comparison of the Harts Range terrains with the Strangways terrain shows some similarities and some differences.

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[6] PALAEOZOIC UPLIFT OF THE CENTRAL ARUNTA BLOCK GRANULITES, CENTRAL AUSTRALIA.

B. GOSCOMBE

Department of Geology, Melbourne University, Parkville, Victoria 3052, Australia.
Unpublished manuscript and published conference abtract (1991).

A structural and metamorphic study of the amphibolite facies Wallaby Knob Schist Zone, on the northern margin of the central Arunta Block, critically constrains the tectonic processes that caused uplift of the Central Australian granulites in response to the Palaeozoic Alice Springs Orogeny. Mineral parageneses, metamorphic reaction textures, geothermo-barometry and equilibrium thermodynamics from this zone in conjunction with previously published cooling ages accurately define a clockwise P-T-t path for the northern margin of the Strangways Range granulites during and subsequent to the Alice Springs Orogeny. North over south ductile thrusting over the Strangways Range granulites, gave rise to burial of these granulites from 6-7.5 kbar on a "normal" crustal geotherm to peak P-T conditions of 8-11 kbar and 550-650 ºC at approximately 390-430 Ma. Crustal over-thickening was terminated by rapid decompression to 2 kbar at 350 ºC within 40-70 Ma. The Wallaby Knob Schist Zone was reactivated by north down normal fault movements during the rapid decompression and was accompanied by regionally extensive partial hydrothermal retrogression of the shear zone and Arunta Block granulites. The northern margin of the Arunta Block granulites decompressed rapidly in isostatic response to the crustal over-thickening experienced during the compressional (over-thrusting) phase of the Alice Springs Orogeny. Late-stage cooling and decompression of the region was less rapid and resulted in exposure at the surface at some time in the Tertiary.

Geoscience Australia 1:100,000 geological map series

[7] Metamorphic domains in the East Arunta region

Ben Goscombe, March 2014, Unpublished ITAR Report.

Summarized from a report lodged with the NTGS in 2007.

The results of average PT calculations using THERMOCALC 3.1 from the East Arunta region are summarized below. All results show clustering very consistently with the known geology of the region. Each geological domain gives a population of PT calculation results that tightly cluster and represent the different events in that domain. Furthermore these results are all consistent with both petrology of the samples and previous PT estimates for different parts of the East Arunta in the literature (i.e. Mawby and Hand). The best considered PT conditions of the different events, in each domain, have been plotted on a simplified map (attached) and are listed below:

Albarta Metamorphic Complex (Proterozoic metamorphism of unknown age):

The typical metapelites of the Albarta Metamorphic Complex experienced peak metamorphism at 625ºC and 4.0 kb. Garnet core compositional isopleths in all samples centre on 530ºC and 2.8kb, indicating a shallow DP/DT heating with minor burial prograde path. Higher-grade samples in the Albarta Metamorphic Complex, possibly representing basement, experienced peak metamorphism at 700ºC and 4.8 kb. Two Albarta samples (138a and 188) give post-peak PT calculations of much higher pressures (up to 9kb) that peak metamorphic calculations. It is not known weather theses results represent disequilibrium or are some how recording a Larapinta Event over-print in the outer-rims of mineral grains in the Albarta Metamorphics.

Aloorajara Metamorphic Complex (Strangways M1 and M2 Orogenies):

Peak metamorphic conditions during the main phase, low-strain coarse-grained granulite phase in the Aloorajara Granulites were ≥840ºC and 9.2kb. The high pressures calculated for these granulites, is consistent with the cordierite-free parageneses in pelites and garnet-orthopyroxene mafic parageneses. These pressures are slightly higher than elsewhere in the Arunta for the Strangways M1 granulite metamorphism, where cordierite is common. Like elsewhere in the Strangways Metamorphic Complex, early hercynite and corundum inclusion parageneses indicate a low-P prograde history, suggesting an anticlockwise P-T path. Peak metamorphism was terminated by isobaric to decompressive cooling paths to post-peak PT calculated conditions.

Two Aloorajara metapelite samples (367a and 367b) give significantly lower results of 630ºc and 6.5kb, indicating post-peak re-equilibration of the minerals. The high-grade gneisses samples by helcopter to the west of Atulka formed at much lower grades than the Aloorajara Granulites, of 700ºC and 8.0kb. Dry granulite to upper amphibolite mylonite and ultramylonite zones representing reworking (M2) in the Aloorajara Granulites give a wide range of conditions centred on 750ºC and 8.6 kb.

North Aloorajara Shear Zone (Alice Springs Orogeny):

The North Aloorajara Shear Zone between Riddock Amphibolite and the Aloorajara Granulites experienced peak metamorphism at 700ºC and 8.0kb. This shear zone is interpreted to have been active subsequent to Larapinta Events because it overprints both Early and Late Larpinta Events in the Riddock Amphibolite. Consequently, the shear zone parageneses are interpreted to have formed at some stage in the Alice Springs Orogeny.

South Aloorajara Shear Zone (Alice Springs Orogeny):

The South Aloorajara Shear Zone that sheared the Bruna Granite, experienced mylonitization at conditions of 650ºC and 9.0kb. Similarly, this shear zones is interpreted to be of Alice Spings Orogeny age because the shear zone fabrics are grain-refining reworking fabrics in contrast to the annealed matrix polygonal granoblastic fabrics evident in both stages of the Larapinta Event.

Entia Dome Basement (Alice Springs Orogeny):

A single aluminous schist sample within the Entia Dome experienced peak metamorphism at 640ºC and 7.0kb, similar to earlier estimates by Mawby and Hand.

Hartz Range Metamorphic Complex (Early and Late Larapinta Events):

The Early Larapinta Event, coarse-grained clinopyroxene-garnet parageneses preserved in the Riddock Amphibolite calculate high-P peak metamorphic conditions of 770ºC and 10.0 kb. Identical peak metamorphic conditions of 770ºC and 10.0 kb are calculated from mineral core assemblages in the boudins of meta-dolerite in the Last Hope Metamorphics. The dry mineral parageneses of these boudins are interpreted to be preserving Early Larapinta Event conditions. These parageneses have apparently not been significantly re-equilibrated during Late Larapinta Event reworking, which was partitioned into fabric development in the enveloping gneisses around the boudins.

Consistent Late Larapinta Event conditions of 660ºC and 7.0kb are recorded from the main phase matrix fabric in the Riddock Amphibolite, Irindina Metamorphics metapelite, Brady Metamorphics metapelite and Last Hope Metamorphics metapelite that is host to the dolerite boudins. Garnet core compositional isopleths indicate prograde conditions clustering around 550ºC and 5.5kb in all Irindina Provence samples, suggesting burial with heating prograde paths.

Jinka Domain (Late Larapinta Event):

Of particular interest are the PT calculations from three localites in the "Jinka" domain to the east of the Entia Dome. These samples experienced conditions similar to that experienced elsewhere during Late Larapinta reworking event. The results of individual samples are listed below.

IC05BDG615 = 730ºC and 7.4kb.

IC05BDG662 = 662ºC and 7.2kb.

IC05BDG950D = 635ºC and 8.0kb.

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