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Thematic Metamorphic Projects

The strategic agenda of almost all ITAR projects is the metamorphic response to orogeneses. The practical approach taken is the application of currently available metamorphic tools in characterising metamorphism in different terranes and domains that make up an orogenic system. These results form the basis for constraining the spatial variation in metamorphic parameters across whole orogens and the evolution of these parameters with time in different tectonic settings. Currently active metamorphic projects investigating the metamorphic response to orogenesis in a wide range of tectonic settings, include; the Yilgarn Craton, Damara Orogen, Malawi Granulites, Arunta Province, Himalayas, Zambezi Belt and Oman. Outcomes can be found in project sheets listed by geographic location. Additional ongoing metamorphic projects are investigating multiple granulite events in the Albany-Fraser Orogen and Malawi Mosaic, seafloor alteration on Macquarie Island and metamorphic response in the Damara Orogen and provinces of the Archaean Yilgarn Craton.

Below are links to summaries of thematic metamorphic projects:

• Thermal gradients and metamorphic field gradients

• Metamorphic analysis of rock flow

• Granulite grade reworking of granulites

• Silica-undersaturated sapphirine granulites

• Re-equilibration without re-crystallization

• Tasmanian eclogite

[1] Classification of metamorphic gradients and their utilization as indicators of tectonic regimes.

Ben Goscombe, David Gray, Chris Carson
SGTSG conference, Townsville (2005)

Metamorphic gradients, where thought to be time equivalent, are first order variables that are dependent upon the tectonic setting and crustal architecture of orogens. Consequently, metamorphic gradients offer a unique way to encompass a whole orogenic system and have the potential to act as discriminatory tools to determine palaeo-tectonic setting in metamorphic belts. Metamorphic gradients are simply the variation in metamorphic variables (pressure–P, temperature–T and average thermal gradient–G) with respect to a length scale. Metamorphic gradients can be divided into two broad types. (1) Average thermal gradient (G - ºC/km) is the ratio of temperature over depth and not to be confused with instantaneous thermal gradient. (2) Metamorphic field gradients, which are the variation in a metamorphic parameter with respect to a horizontal length scale, either along an orogen (DT/Dy, DP/Dy and DG/Dy) or across an orogen (DT/Dx, DP/Dx and DG/Dx).

To illustrate their utility we have determined metamorphic gradients from metamorphic belts across a large range of inferred palaeo-tectonic settings (Kaoko Belt, Damara Orogen, Semail Ophiolite Sole, East Himalayas and East Yilgarn Craton). Average thermal gradient is a single variable that can be determined at a site, but nevertheless encapsulates the vertical distribution of temperature and offer critical constraints to plausible tectonic settings of metamorphism (i.e. Spear, 1993). For example; systems with G<15 ºC/km are dominated by advection of material over conduction, and can only occur in a subduction zone-type setting. Systems with G between 15 and 30 ºC/km occur where there is a balance between heat conduction and heat advection such as in typical collisional orogenic belts. Systems with G>30 ºC/km are dominated by heat conduction in settings that may be pervaded by magma, involve crustal extension, thermal blanketing and/or mantle under-plating.
Metamorphic field gradients, in contrast, document spatial patterns and illustrate the metamorphic variation in a horizontal plane throughout the metamorphic belt. As a consequence, metamorphic field gradients are dependent on first-order orogenic variables such as degree of convergence obliquity, as well as the nuance of adhoc, orogen specific, variables such as magma history, orogen architecture, and basinal sequence thickness, among many others. Metamorphic field gradients across and along orogens therefore offer a way to pool these many and varied second-order variables and uniquely characterise an orogenic system.
Analysis of orogens of known tectonic setting, suggest metamorphic gradients can be used predictively to generate diagnostic constraints on tectonic regime. This is useful for the many old metamorphic belts that are typically dismembered and isolated domains without boundary conditions or preserved tectonic context. Where metamorphic conditions can be constrained, the average thermal gradient (G) can be utilized to narrow down plausible tectonic regimes that would be otherwise unknown. Furthermore, where there is spatial variation in metamorphic conditions, metamorphic field gradients can be used to extract further tectonic information that is otherwise not available. For example, the developed metamorphic patterns will mirror the crustal-scale architecture of the orogen and distribution of radiogenic elements or other heat sources such as mantle lithosphere thickness and magma accretion. In addition, co-temporal composite metamorphic belts develop metamorphic field gradients that can indicate degree of convergence obliquity, the relative rate of transport of material through the crustal column and polarity in field gradients will indicate the direction of tectonic vergence.

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[2] Metamorphic response in orogens of different obliquity, scale and geometry.

Ben D. Goscombe, David R. Gray

Abstract from: Gondwana Research, volume 14, 2008.

Investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets. To illustrate the problems in documenting flow vectors we present integrated structural-metamorphic datasets from two transpressional systems; the Kaoko Belt in Namibia and the Kalinjala Shear Zone in South Australia. These orogens experienced widely differing metamorphic responses to transpressional deformation. Integration of kinematic and metamorphic datasets from the Kaoko Belt indicate shallow up-plunging extrusion trajectories in the orogen core, and show that the maximum stretching direction pattern matches the inferred flow vectors. High-grade domains (800-840ºC and 7.0-8.0 kb) in the orogen core developed low-angle upward-verging maximum stretching direction trajectories, whereas a low-grade domain (575-600ºC and 5.0-5.5 kb) in the orogen core has downward-verging lineation trajectories. The barometric differential between these high-grade and low-grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high-grade domains that were extruded obliquely, for the amount of lateral shear estimated for the orogen core. The Kalinjala Shear Zone in South Australia contrasts strongly with the Kaoko Belt. In this example, the high-grade and high-strain shear zone core of the orogen, experienced high-T/high-P metamorphism with low thermal gradients of 21-26 ºC/km and steep decompressive P-T paths. The lower-grade external domains experienced lower-T/lower-P metamorphism with high thermal gradients of 35-37 ºC/km. Sub-horizontal maximum stretching directions do not match the vertical extrusional flow in the high-grade core that is indicated by the metamorphic data. This comparison shows that in general and on a gross-scale, maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis. In some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow. Consequently, information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record. These two datasets must be used in concert to obtain realistic constraints on first-order material flow trajectories at orogenic-scales. The horizontal component of flow is typically best recorded by structural fabrics (maximum stretching direction and sense of shear), whereas the vertical component is typically best recorded by metamorphic information, such as P-T paths, temperature over depth ratio and metamorphic field gradients (i.e. DT, DP and DG) across the orogen.

Diagrammatic models comparing inferred flow vectors and developed metamorphic field gradients in two contrasting examples of transpressional orogenesis. (a) Kaoko Belt example has a co-temporal composite metamorphic belt geometry and is centred on a wider and longitudinally complex Orogen Core. The structural and metamorphic datasets are mutually consistent, and the real flow vectors in the Orogen Core can be resolved to some degree, involving oblique, shallow up-plunging trajectories (Goscombe et al., 2005). (b) Kalinjala Shear Zone example is centred on the high-grade orogen core that experienced extrusional flow vectors, most plausibly oblique and steeply up-plunging. However, information on the vertical and horizontal components of flow are decoupled; the horizontal component recorded by structural fabrics only and the vertical component recorded by metamorphic data only. The true flow vectors experienced cannot be resolved. Note that peak metamorphic field gradients are interpreted to be approximately time equivalent across these orogens. The extruded high-grade orogenic cores in these two examples have very different metamorphic field gradients for variation in pressure and temperature over depth ratio.

The broad classification of transpressional orogens, with distinct groupings shaded and based on a simplified suite of parameters in geometric and metamorphic space. The data is derived from transpressional orogens documented in the literature and is compared to examples of high-angle convergent orogens such as the Himalayas. Orogen geometry is summarized by the α/θ ratio between general plunge of the maximum stretching direction within the orogen core (α) and the inclination of the orogenic grain (θ). Patterns in metamorphic response are summarized by the gradient in temperature over depth ratio (ΔG) from the low-grade external domain to the high-grade orogen core of the different metamorphic belts.

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[3] 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 (Ml, at 1800 Ma) had an 'anticlockwise' P- T path (i.e., increasing PIT with time) and was terminated by isobaric cooling from 850-950'C, 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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[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 900-950'C 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] Tectonometamorphic evolution of the Chewore Inliers:
Partial re-equilibration of high-grade basement during the Pan-African Orogeny.

Ben Goscombe, R. Armstrong1 and J.M. Barton2
Geological Survey of Zimbabwe, P.O. Box CY210, Causeway, Zimbabwe.
1Research School of Earth Science, Australian National University, Canberra, Australia.
2Geology Dept. Rand Afrikaans University, P.O. Box 524, Johannesburg, Republic of South Africa.

The Chewore Inliers are isolated outcrops of the Zambezi Mobile Belt within the Mesozoic Lower Zambezi Rift Valley in Zimbabwe. Detailed mapping has recognised four terranes the Zambezi, Quartzite and Granulite Terranes and the recently recognised Ophiolite Terrane (Johnson et al., 1996). Apart from the Ophiolite Terrane, all are dominated by supracrustal gneisses with concordant granitic orthogneiss units of 1071±8 and 1083±8 Ma age. These terranes experienced low-P/high-T metamorphism (M1) terminated by isobaric cooling at 945±34 Ma. M1 assemblages of sillimanite-spinel-garnet, garnet-orthopyroxene and two pyroxene mafics are recorded in the Granulite Terrane, and conditions of formation were 4.4±1.7 kb and >800 ºC. M1 mineral parageneses and associated ductile deformation structures dominate the Granulite Terrane, but M1 mineral parageneses are only preserved as sillimanite-spinel inclusions in garnet cores in the other terranes. The Zambezi, Quartzite and Ophiolite Terranes were almost totally recrystallized during reworking in the M2 metamorphic cycle. M2 metamorphism accompanied NE over SW directed transport during Pan-African orogenesis of the Zambezi Belt at 524±16 Ma. Average peak M2 conditions, calculated using THERMOCALC V2.0b (Powell & Holland, 1988), were 7.9-8.6 kb and 590±95 ºC, 630±95 ºC and 717±95 ºC from the south and north Zambezi Terranes and Quartzite Terrane respectively. M2 involved a clockwise P-T path from the chloritoid stability field with matrix assemblages crystallised in the kyanite-staurolite field or at the kyanite/sillimanite transition and near isothermal decompression occurred through the peak of metamorphism into the sillimanite field. In contrast the Granulite Terrane was incorporated within the Zambezi Belt as a thrust-bound slab and experienced only minor structural reworking during M2. Granulite Terrane samples within 2 km of the basal thrust margin, preserve M1 mineral assemblages but these minerals were chemically re-equilibrated without recrystallization during M2 at conditions of 5.6±1.5 kb and 631±100 ºC. Granulite Terrane samples were totally recrystallized in shearzones at the margin of this terrane. These samples equilibrated at conditions identical to the peak of M2, at 7.7±1.9 kb and 590±110 ºC. The re-equilibrated and recrystallized sample sets define two points on the clockwise P-T path experienced by the Granulite Terrane during further burial and reworking in the Pan-African Orogeny, and is consistent with the M2 P-T path documented for the other terranes.

Keywords: Equilibrium thermodynamics, Geochronology, Metamorphism, P-T paths, Reworking, Pan-African Orogeny.
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[6] Equilibrium thermodynamics of the Lyell Highway eclogites

B. D. Goscornbe
Division of Mines and Mineral Resources Tasmania Report 1990/19

One sample of the Lyell Highway eclogite and one sample of the enclosing aluminous schist have been studied using equilibrium thermodynamics (Powell and Holland 1988). The resultant average P-T estimate of both cores and rims in the eclogite is very well constrained at 15.2 ± 1.05 kb and 698 ± 28ºC. However, only one rim assemblage of the enclosing schist was in equilibrium and gave a pressure estimate of 16.4 ± 1.3 kb (at 700ºC). These estimates are consistent with those of Kamperman (1984) derived by traditional geothermometers and geobarometers. Close correlation between P estimates in eclogite and the enclosing schist suggests their juxtaposition prior to the peak of metamorphism. Rim assemblages equilibrated at only slightly higher temperatures than cores (difference of 11-21ºC). In contrast to Kamperman (1984), a small decrease in P from core to rim is recorded, but this is not considered a significant P-T vector indicator.

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