• East Yilgarn metamorphism
• West Yilgarn metamorphism
• West Yilgarn extension and lithospheric delamination
• Global Archaean metamorphic events
• Metamorphism in Archaean granite-greenstone terranes
• Proterozoic effects on Yilgarn Craton margin

 

[10] Neoarchaean metamorphic evolution of the Yilgarn Craton: a record of subduction, accretion, extension and lithospheric delamination.

 

Goscombe, B., Foster, D.A., Blewett, R., Czarnota, K., Wade, B., Groenewald, B. and Gray, D. 2019. Precambrian Research 335, 105441.

The greater part of the metamorphic record of the Yilgarn Craton formed during a protracted middle Neoarchaean orogenic cycle, spanning from about 2750 to 2620 Ma. At least six distinct metamorphic events are defined by parageneses in different tectono-stratigraphic settings. This review characterizes the middle Neoarchaean metamorphic response of the entire craton using large databases. Rock descriptions from ~60,000 sites in government databases and ~1,040 PT determinations from all sources, are used to construct metamorphic map patterns for each of these metamorphic events. P-T paths and timing of each metamorphic event are characterized using petrology and the well-constrained PT determinations from this study (n=287), combined with metamorphic age determinations from all sources (n=114).  The spatial patterns and metamorphic conditions of each event, provides critical constraints on tectonic settings and changes in tectonics over 130 million years. The record of metamorphism in conjunction with stratigraphic, kinematic and magmatic constraints shows that the Neoarchaean craton evolved from an accretionary crustal growth phase (Ma and M1), to thermal reworking of the crust during massive influx of granitic intrusions (M2), and lithospheric extension after the termination of subduction (M3). These events were followed by lithospheric delamination, resulting in a craton-wide, diffusion-delayed thermal pulse that coincided with renewed contraction (M4). M1 metamorphism at 2748±19, 2727±8 and 2706±10 Ma experienced high-P/moderate-T hairpin, clockwise P-T paths during burial of magmatic arc margins during terrane accretion events. M2 regional-contact type metamorphism at 2671±6 Ma tracked low-P/moderate-T clockwise P-T paths in back-arc settings during voluminous felsic magmatism related to shallow subduction. M3 metamorphism at 2656±5 Ma experienced low-P/moderate-T anticlockwise P-T paths in post-volcanic rift basins formed during lithospheric extension. M4 metamorphic peaks at 2644±4 and 2629±7 Ma experienced low-P/high-T clockwise P-T paths during regional-scale thermal pulses resulting from lower-crust and mantle lithosphere delamination at 2665-2668 Ma.

 

[9] East Yilgarn Craton Metamorphism, Amalgamation, Lithospheric Extension and Gold

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

A metamorphic database covering the entire Yilgarn Craton has been compiled from pre-existing mapping and literature with 52,896 sites of qualitative metamorphic information. In addition 7,065 key sites with detailed quantitative metamorphic data including P, T, T/depth ratio, P-T paths and metamorphic geochronology have been compiled from newly generated data and literature. The derived temporal and spatial patterns for the East Yilgarn (east of the Ida Fault) contrast with previous tectonic model and long-standing metamorphic paradigm that assumed invariant crustal depth and a single collisional metamorphic event. In particular, there are large variations in peak metamorphic crustal depths (12 to 31 km), and five broad metamorphic periods / styles can now be recognised.

Metamorphic patterns during each event have been temporally and spatially integrated with the new deformation framework (Blewett and Czarnota, 2007c) by a process of metamorphic domain analysis and using metamorphic field gradients. The continual evolution with time of fundamental metamorphic parameters throughout the entire history have been constructed as evolution curves and integrated with the deformation, magmatic, stratigraphic and mineralization history. The evolution in metamorphic response can be related to the tectonic evolution of the East Yilgarn Craton. From crustal growth phase (Ma and M1), to thermal priming of the crust at termination of volcanism and massive granite influx (M2), leading to runaway lithospheric extension (M3a) and finally a phase of inversion and reactivation (M3b).

The East Yilgarn crust experienced five distinct thermal regimes at different times in its evolution (Goscombe et al., 2007, 2009). M1 is recorded by moderate-P (7.5 kb), low-T/depth ratio (20 ºC/km) metamorphism at approximately 2710-2695 Ma accompanying formation of the volcanic sequences and before the high-Ca granite bloom. M1 parageneses are spatially restricted to crustal shear systems and are interpreted to have formed during partial downward advection of magmatic arc margins during docking events, followed by rapid isothermal decompression. Significantly these parageneses predate the granite bloom and doming events and formed in a horizontal tectonics setting (i.e. plate tectonics) and not vertical tectonics settings such as keels adjacent to diapiric domes. Rare low-P (2.5-5.0 kb) granulite parageneses (Ma) are interpreted to have formed in high heat flow settings within the magmatic arcs.

Age determinations from high-P, low-T/depth M1 mineral parageneses fall into two groups. The youngest is centred on 2709±47 Ma from locations in the Kalgoorlie-Kurnalpi Superterrane and is interpreted to represent arc-docking events within these two terranes. The oldest is centred on 2727±16 Ma from locations at the west Burtville margin and may be related to arc docking and burial in the Hootanui Shear Zone. These older M1 ages are both temporally and spatially distinct from the younger M1 parageneses within the Kalgoorlie-Kurnalpi Superterrane indicating westward propagation of docking events. Together, both docking events closed the Kalgoorlie-Kurnalpi back-arc basin. Significantly, these events occurred 25-40 Ma before through going subduction, felsic volcanism and the granite bloom with accompanying M2 metamorphism at 2685-2655 Ma. Consequently, all M1 parageneses formed before granites were emplaced and before granite domes could possibly form. As a result, models for M1 parageneses involving vertical tectonics, diapirism or partial convective overturn for giving the high-P parageneses in sinking keels, must be discounted.

A regional M2 thermal anomaly produced low-P (3.5-5.0 kb), high-T/depth ratio (30-40 ºC/km) metamorphism (M2) associated with emplacement of large volumes of high-Ca granite at 2685-2655 Ma. M2 is associated with termination of volcanism and through-going subduction, with compression at 2670 Ma resulting in minor crustal thickening and tight clockwise P-T paths. The regional M2 thermal anomaly softened the crust, priming it for a marked response from a stress switch to extension at 2665-2645 Ma (M3a). M2 crustal thickening was insufficient to induce gravitationally driven extension, consequently M3a lithospheric extension is interpreted as far field imposed stress such as slab roll-back. Similar horizontal to vertical s1 stress switches are a common feature of Phanerozoic accretionary orogens due to changes in outboard subduction zone dynamics. M3a parageneses indicate a low-P (4.0 kb), very high-T/depth ratio (40-50 º/km) thermal anomaly superimposed on regional M2. The M3a thermal anomaly is spatially restricted to arcuate zones associated with post-volcanic siliciclastic rift basins in the upper-plate of crustal scale extensional shear zones. The extensional upper-plate setting is confirmed by M3a parageneses tracking anti-clockwise P-T paths.

It could be argued that the East Yilgarn Craton would not be as endowed in world class gold mineralization without the late orogenic switch from compression during M2 to lithospheric extension during M3a. All of the processes, architectures and temporal and spatial juxtapositions required for world-class gold deposits, formed during the M3a period and are a natural response to this significant stress switch. M3a lithospheric extension thinned the crustal lithosphere resulting in a higher flux of energy from the mantle lithosphere that then drove many of the key processes leading to mineralization. Lithospheric extension was heterogeneous, resulting in basinal rifts and partitioned zones of high heat flux resulting in steep thermal gradients that drove fluid flow. The extensional stress field also modified fluid flow trajectories, giving both horizontal and downward paths in part (Sheldon et al., 2007), allowing the mixing of multiple fluid sources in the upper crust. Extensional rifting dropped basinal fluids into the upper crust, significantly increasing the ambient fluid volume and introducing a new fluid type into the crust. Lithospheric extension and associated deep penetrating structures facilitated both the generation of mafic granitoid, syenite, lamprophyre and carbonatite melts and tapped dry strongly reduced fluids from the mantle (Walshe et al., 2008a) and transported these up into the upper crust. These mantle derived melts and fluids are possibly a major primary source of the gold and mixing of this distinct fluid type with ambient hydrous fluids in the upper crust ultimately resulted in precipitation and mineralization. Lithospheric extension both created and reactivated older crustal shear zones, generating suitable structural architectures such as extensional growth faults and footwall domes. These architectures allowed the focusing of fluids from different sources facilitating mixing and ultimately mineralization, both during M3a lithospheric extension and during subsequent reactivation of these structures in D4 and D5 with remobilization associated with low-grade hydrothermal alteration events (M3b) to 2620 Ma. M3a lithospheric extension and consequent high heat flow and decompressing lower-plate domes, facilitated crustal melting and emplacement of low-Ca granites in the upper-crust, which extended the M3a thermal anomaly and perturbed isotherms, further influencing fluid flow and mineralization.

The key event that initiated runaway lithospheric extension was sudden termination of through-going subduction at the margin of the Yilgarn Craton. This switch may have been caused by an ad hoc geological event such as choking of the subduction zone by an oceanic plateau (Blewett and Czarnota, 2007c). An alternative first-order cause could be the sudden global reorganization of tectonic plate trajectories following attainment of a metastable plate configuration. An example of the latter scenario is the sudden stress field reorganisation induced at the culmination of assembly of supercontinents. Reconfiguration of tectonic plate trajectories followed the culmination of Gondwana assembly generated stress switches at ~520 Ma (Foster et al., 2009), and again at the culmination of accretionary growth with slab rollback at the east Gondwana margin at ~440 Ma (Squire and Miller, 2003). If subduction termination and lithospheric extension in the East Yilgarn signifies a global plate reconfiguration event, similar fundamental switches in tectonics may be recorded in other Neoarchaean terranes world wide at ~2665 Ma. It is also probable that global stress switches at this time may have initiated similar geological responses leading to Au-mineralization, with the possibility of a global self-similar mineralization period between 2665 and 2615 Ma.

Conceptually, the first-order Neoarchaean evolution of the East Yilgarn follows a sequence of metamorphic periods that are the natural outcome of earlier thermo-mechanical states that established the foundation for, and initiated the subsequent metamorphic period. This cycle of ongoing cause and effect also strongly influenced crustal processes and thermo-mechanical evolution in the West Yilgarn (see below). M2 high heat flow associated with subduction and granite influx heated and thermally softened the upper crust (4-6 kb) with accompanying convergent stress field. The high T/depth M2 period pre-prepared the crust for runaway lithospheric extension at the termination of through-going subduction and initiation of slab hinge rollback. Lithospheric extension causes the second superimposed thermal anomaly as ribbons of high-heat flow, giving M3a parageneses at 3.5 kb.  The slow dissipation of both M2 and M3a thermal anomalies, and later superimposed far-field stress (D4-D5) gave new shear fabrics and parageneses that were formed at high T/depth conditions and 3 kb, during early M3b mineralization events [Au1 to Au3]. Lithospheric extension and possibly also the later superimposed transpression and transtension events (D4-D5), initiated delamination of the lower crust resulting in sudden crustal isostatic response. Rapid uplift and exhumation during late M3b mineralization events gave parageneses at lower pressures of 1 kb and very high T/depth ratios. Delamination of the lower crust may have occurred immediately after M3a extension with thermal conductive lag through the crust taking 15-20 Ma, coinciding with the youngest mineralization events and low-Ca granites. Isostatic rebound due to delamination is recorded in the rock record by vertical s1 flattening structures such as flat-lying crenulations (D6). Metamorphic parageneses record ~2.5 kb or 8.8 km of uplift and exhumation in response to detachment of the dense lower crust. This resulted in very high average thermal gradients in the youngest [Au4] gold events because of the very low pressures, while still maintaining a high heat flow from the lower crust.

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[10] West Yilgarn Craton Metamorphic Patterns and Evolutions

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

West of the Ida Shear Zone, the West Yilgarn shares some regional metamorphic patterns and timings of metamorphic events in common with the East Yilgarn. However, metamorphic conditions and histories are dissimilar and interpreted geodynamic settings for M1, M2, M3a and M3b metamorphic periods are not the same in all parts of the Yilgarn Craton. Furthermore, additional metamorphic periods have been identified in the West Yilgarn. The West Yilgarn preserves a very large range in metamorphic conditions, and five distinct tectono-metamorphic terranes are identified with self-similar metamorphic evolutions: Southern Cross Domain, Ravensthorpe Belt, Murchison Domain, Southwest Terrane and Narryer Terrane. These are described below.

The Southern Cross Domain shows a near-typical regional-contact style metamorphic pattern, with similarities intermediate between the EYC and Murchison Domain. In general, the greenstone belts show increasing metamorphic grade (T) from synformal cores to margins adjacent to the granite-gneiss domes. Regional metamorphic parageneses are consistently of high T/depth conditions (>35 ºC/km) and formed at low pressures of 3.0-3.5 kb, intermediate between the EYC and Murchison Domain. There is no systematic pressure variation recorded across the synforms and no systematic difference in pressure has been recorded between the synforms and the granite-gneiss domes. This is similar to the Murchison Domain, but dissimilar to parts of the EYC. This metamorphic pattern is consistent with a regional thermal anomaly associated with large volumes of granites during the 2670-2680 Ma granite bloom. Consequently, the regional metamorphic pattern is broadly similar and coeval with regional-contact M2 metamorphism in the EYC and Murchison Domain.

However, the first-order regional-contact metamorphic patterns in the Southern Cross Domain are modified by later thermal anomalies in the Diemal Belt and west margin outlined by Bullfinch - Southern Cross - Cheritons - Forrestania Belts. The Diemals Belt contains a younger clastic sequence, called the Diemal Formation, deposited at some time in the range ~2675-2640 Ma. These sediments were deposited after formation of the regional-contact metamorphic pattern in the older volcanics. The Diemals Formation was metamorphosed at greenschist to lower-amphibolite facies grades of <525 ºC and 1.5-2.3 kb, with anticlockwise P-T path. Consequently, these rocks preserve evidence for a second high T/depth thermal anomaly superimposed on the regional-contact M2 metamorphic pattern at some stage after 2640 Ma. This event shares similar high T/depth conditions, anticlockwise P-T path and association with extensional basin as M3a in the EYC, but occurred ~10 Ma later.

The Bullfinch - Southern Cross - Cheritons - Forrestania Belts were metamorphosed at middle- to upper-amphibolite facies, with temperatures higher than elsewhere in the Southern Cross Domain. Furthermore, peak metamorphism in these belts was post-kinematic and occurred between 2640-2620 Ma, indicating a second metamorphic anomaly after regional-contact metamorphism associated with the granite bloom. This relatively young and higher-grade metamorphism does not show as marked variation in metamorphic grade across the greenstone belts, as preserved in older regional-contact metamorphic patterns elsewhere in the Southern Cross Domain. The late-stage thermal overprint in this western margin gave peak metamorphic conditions ranging 540-610 ºC and 2.5-4.6 kb, and mostly followed clockwise P-T paths. This late thermal anomaly was broadly coeval with late-stage anticlockwise metamorphism in the Diemal Belt. Consequently, it is interpreted that at approximately the same time that late-stage extension-related metamorphism was occurring in the Diemals Belt, the western margin to the south was experiencing a relatively high-grade high heat flow thermal anomaly accompanying minor contraction.

Granite-gneiss domes in the Youanmi Terrane crystallized at low pressures (<6 kb) commensurate with metamorphism in the greenstones. There are small differences in crystallization pressures across the Ghooli Dome in the Southern Cross Belt. Core of the dome crystallized at highest pressures of 5.4-6.2 kb and the north and south margins at 4.0-4.6 kb. The 1.6 kb pressure differential between core and margins corresponds to 5.6 km exhumation of the core. The Ghooli Dome is concentrically zoned and the core and margins were emplaced at different times, recording pressures at different times in the evolution of the crust. This is consistent with the P-T evolution of the adjacent Southern Cross Belt, which experienced maximum-pressures during early prograde metamorphism of 4.6-5.8 kb, before decompressing to 2.8-4.6 kb at the peak of metamorphism during clockwise evolutions. On its own, this barometric evolution of the crust is insufficient independent evidence for either a diapiric or extensional doming mechanism for the Ghooli Dome. Nevertheless, the small differential exhumation of 5.6 km recorded is insufficient to be considered partial convective overturning of any significance. Furthermore, the complete absence of relatively high-P peak metamorphic parageneses (>5 kb) anywhere in the Youanmi Terrane, indicate that diapiric or partial convective overturn geodynamics did not operate at anytime in the evolution of this terrane.

The Ravensthorpe Belt has the most complex metamorphic pattern in the Southern Cross Domain, with evidence for at least two regional metamorphic events. A few localities within the central Ravensthorpe Terrane and Carlingup Terrane in the east, contains early amphibolite facies parageneses of 545-575 ºC and 3.7-4.0 kb, with anticlockwise P-T paths. These relict parageneses are interpreted to be the result of early extension and basin formation. All other metamorphic parageneses, in all terranes in the Ravensthorpe Belt, are interpreted to have formed in a single regional metamorphic event within a thrust complex. The regional metamorphic pattern varies from greenschist facies in the central south to middle amphibolite facies towards both the northeast and western margins, and transitions into granulite facies conditions into the Southwest Terrane to the west. Regional middle amphibolite facies conditions experienced, were 530-575 ºC and 4.1-4.9 kb, with clockwise P-T paths. The western margin of the Ravensthorpe Terrane preserves higher-P parageneses with conditions of 540-580 ºC and 6.0-7.5 kb, indicating a deeply buried thrust sheet. Similarly, the Chester Formation in the northeast margin contains high-P parageneses with conditions of 510-600 ºC and 6.8-8.0 kb, also indicating a deeply buried thrust sheet.

The Murchison Domain shows regional-contact style metamorphism that is typical of granite-greenstone Archaean cratons worldwide. In general, the greenstone synforms show increasing metamorphic grade (T) from the cores to the margins adjacent to the granite-gneiss domes. These regional metamorphic parageneses are consistently of high T/depth conditions and formed at low pressures of 1.5-3.0 kb, lower than in any other part of the Yilgarn Craton. Metamorphic evolutions followed tight shallow clockwise P-T paths. Relatively high-grade parageneses at Big Bell and other gneissic margins are interpreted to represent the highest-grade portion of the regional metamorphic pattern. There is no systematic pressure variation recorded across the synforms and no systematic difference in pressure has been recorded between the synforms and the granite-gneiss domes. This metamorphic pattern suggests that metamorphism is the result of the regional thermal anomaly associated with influx of large volumes of granites during the granite bloom. The larger volume of granite was emplaced between 2690 and 2660 Ma, at similar times to the granite bloom in the EYC. Consequently, the Murchison metamorphic pattern formed in the same way and at broadly the same time as regional-contact M2 metamorphism in the EYC. There is no evidence for late-stage extensional tectonics or anticlockwise P-T paths, though a late-stage 2638-2619 Ma thermal anomaly (M3b) is recorded by U-Pb titanite age determinations. The simple regional metamorphic pattern is complicated by older parageneses in very thin, high-grade contact aureoles (Mc) at the margins of mafic-ultramafic intrusive complexes. These formed in at least 14 separate events, in 7 distinct periods, ranging in age from 2825 to 2673 Ma across the Yilgarn Craton, with most in the Murchison Domain. Windimurra contact aureoles preserve very low-P conditions ranging up to granulite conditions of 709 ºC, 0.2 kb and 844 ºC/km within a raft on the upper surface.

The Southwest Terrane has a simple regional metamorphic pattern of widespread granulite facies parageneses across most of this vast region. Granulite facies assemblages are pervasive throughout the entire terrane with a transitional margin of upper-amphibolite facies conditions to the north, northeast and east of only ~40 km width. This transitional margin appears to be a smooth gradient that transitions into the regional amphibolite facies conditions experienced in the Murchison and Southern Cross Domains. Only the western 20-30 km wide margin of the Southwest Terrane shows more typical lower to middle amphibolite facies regional-contact metamorphic conditions, such as in the Boddington greenstone belt. Peak metamorphic conditions in the granulites are 745-835 ºC at 4.3-6.3 kb in the Lake Grace Terrane and 780-875 ºC at 5.5-7.5 kb in the Jimperding Belt. Metamorphic evolutions predominantly followed tight recumbent clockwise P-T paths, though anticlockwise paths are also widely distributed. Regional granulite facies metamorphism occurred between 2663 and 2644 Ma and a secondary peak at 2638-2624 Ma, and possibly as young as 2615 Ma. The peak metamorphic period is broadly correlated with M3a and the secondary peak with M3b events in the EYC. Both regions experienced very different metamorphic grades (T), though share similar high T/depth ratios and similar timings, and may be geodynamically related. M3a metamorphism in the EYC was the direct result of lithospheric extension in this region and this event may have triggered lithospheric delamination giving slightly later high-grade metamorphism across the Southwest Terrane and moderate-grade metamorphism in parts of the Youanmi Terrane, such as the Bullfinch - Southern Cross - Cheritons - Forrestania Belts region.

The Narryer Terrane has a simple regional metamorphic pattern of widespread granulite facies parageneses in the south and west and amphibolite facies in the north and northeast. The granulites experience peak metamorphic conditions of 730-805 ºC at 4.5-6.0 kb, with tight shallow clockwise P-T paths. There is insufficient information to document a transitional gradient between the granulite and amphibolite facies regions. It is probable that the transition is marked by sharp jumps in metamorphic grade across fault zones. The regional metamorphic pattern is interpreted to be the result of metamorphism at 2660-2620 Ma. This metamorphic period is correlated with the M3a-M3b metamorphic period in the EYC, though the two widely separated regions were metamorphosed at very different conditions and experienced contrasting P-T evolutions, clockwise in Narryer Terrane and anticlockwise in EYC. Poorly constrained relict Palaeoarchaean metamorphic age determinations have been reported from the Narryer Terrane, but these cannot be correlated with any specific metamorphic parageneses and the distribution of these events are unknown. Superimposed on the simple regional Neoarchaean metamorphic pattern are retrograde shear zones with Palaeoproterozoic mineral parageneses. These range in scale from m-scale hydrous retrograde shear zones to km-scale reworked Jack Hills Belt. Palaeoproterozoic mineral parageneses formed at greenschist to lower-amphibolite facies grade and followed shallow clockwise P-T paths, indicating burial during orogenic events in the Gascoyne Orogen to the north.

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[11] West Yilgarn Craton Extension and Lithospheric Delaminaton

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

There is evidence for late-stage lithospheric extension in the Southern Cross Domain, significant enough to generate a rift basin and thermal anomaly. The Diemals Belt contains a younger clastic sequence called the Diemal Formation and deposited at some stage between ~2675-2640 Ma. These sediments were deposited after the regional metamorphism in the older volcanics. The Diemals Formation was metamorphosed to greenschist to lower-amphibolite facies conditions and followed an anticlockwise P-T path. Consequently, these rocks preserve evidence for a second high T/depth thermal anomaly superimposed on the regional metamorphic pattern at some stage after 2640 Ma. This second metamorphic event shows many similarities to the second high T/depth M3a metamorphism in the EYC that also experienced anticlockwise P-T paths and was associated with extension and rift basin formation. In the EYC the peak of M3a metamorphism occurred at ~2650 Ma, approximately 10-15 Ma after initiation of extension and deposition in post-volcanic turbiditic basins from 2665-2660 Ma. The M3a metamorphic anomaly in the EYC is clearly related temporally and spatially with lithospheric extension of the Kalgoorlie-Kurnalpi Superterrane. In contrast, the second thermal anomaly in the Diemals Belt occurred after ~2640 Ma, which is 10 Ma younger than M3a metamorphism in the EYC. Consequently, late-stage extension in the Southern Cross Domain is correlated with the later M3b metamorphic period in the EYC.

These timing relationships indicate that the respective early clockwise and later anticlockwise events in the two halves of the craton cannot be directly correlated across the Ida Shear Zone, but are separated by ~10 Ma. Nevertheless, the existence of two self-similar cycles of anticlockwise rifting metamorphism following on from clockwise regional metamorphism, suggest this progression may be a natural consequence of thermal softening of the crust by the earlier high heat flow event, as suggested for the M2-M3a transition in the EYC (see above). There is no systematic pressure variation recorded across greenstone belts in the Youanmi Terrane, and no systematic difference in pressure between greenstones and the granite-gneiss domes. The absence of large pressure differences in the Southern Cross Domain contrasts strongly with the EYC and indicates that M3a-M3b lithospheric extension events and exhumation of the middle-crust by crustal telescoping, are processes that diminished significantly to the west of the Ida Fault. Consequently, late-stage extension during M3a-M3b events was greatest in the EYC, while the Youanmi Terrane acted as a hinterland with a lot less extension experienced.

The majority of dated high-grade metamorphic parageneses in all parts of the Southwest Terrane have age determinations <2660 Ma, in the same range as the late-stage M3a-M3b thermal anomaly in the EYC. Recognising that the regional granulite grade metamorphic events in the Southwest Terrane are coeval with lithospheric extension and subsequent thermal anomaly in the EYC is a crucial observation that connects the first-order tectono-metamorphic response across the whole craton. Though high T/depth metamorphism in the two regions is broadly time equivalent, the tectonic setting and driving mechanisms of metamorphism are interpreted to be very different. M3a metamorphism in the EYC was the direct result of lithospheric extension of the EYC region and this event also possibly triggered widespread lithospheric and probably also lower crust delamination, giving rise to high-grade high T/depth metamorphism slightly later in the Southwest Terrane. Anticlockwise P-T paths are widely distributed across the Southwest Terrane and coincide with the range of parageneses with 2663-2624 Ma age determinations. This pattern is in contrast to the EYC where M3a parageneses and thermal anomalies are highly arcuate and associated with crustal-scale shear zones. Consequently, though time equivalent and potentially geodynamically related somehow, the tectonic setting and driving mechanisms of metamorphism are interpreted to be very different in the Southwest Terrane and the EYC.

The very broad regional distribution of high-grade metamorphism in the Southwest Terrane, suggests a very large thermal flux from the mantle lithosphere coeval over a very large region. The relatively low metamorphic pressures and P-T paths involving either tight clockwise loops with little burial or anticlockwise P-T paths, suggests the widespread thermal metamorphism did not involve crustal over-thickening such as experienced in collisional belts. In addition, high-grade metamorphism of the Southwest Terrane is not thought to be due to lithospheric extension. Unlike typical extensional settings, the high-grade Southwest Terrane thermal anomaly is widespread and accompanied by a mixture of clockwise and anticlockwise P-T paths. Crucially the Southwest Terrane does not appear to be traversed by a network of crustal-scale extensional shear zones nor is there any evidence for extensional rift basins. Furthermore, this terrane is apparently devoid of mantle penetrating structures and magmas tapped from the mantle such as syenites and lamprophyres and mafic granites. Nevertheless, the late-stage low-Ca granites and a number of charnockite and high-HFSE granites require high temperatures in the lower crust. Collectively these granites are widespread across the Southwest Terrane and lithospheric delamination is the most plausible source of the widespread elevated temperatures. Delamination below the West Yilgarn Craton is evident in mantle tomography (Blewett et al., 2009) and is consistent with the high T/depth metamorphic parageneses developed in the Youanmi, Narryer and Southwest Terranes.

There are four U-Pb metamorphic titanite age determinations from the Murchison Domain that fall in the M3b period, ranging 2638-2619 Ma. Similarly, peak metamorphic granulite facies parageneses in the Narryer Terrane have been dated at 2640-2620 Ma by U-Pb monazite. These ages are identical to post-kinematic peak metamorphism in the Southern Cross Domain (2640-2620 Ma) and the second high-grade metamorphism of the Southwest Terrane (2638-2624 Ma). This late metamorphic event occurred across all of the West Yilgarn Craton, and significantly, this event post-dates main phase metamorphic events (M2-M3a) in the East Yilgarn Craton by approximately 10 Ma. The most plausible scenario for a widespread high-T, high T/depth thermal regime, with little barometric response, is by delamination of a very large region of lithosphere and probable eclogitic lower crust. The late-stage widespread thermal anomaly experienced across the Southwest, Narryer and Youanmi Terranes is interpreted to be due to a conductive thermal pulse after delamination. A model involving delamination propagating from east to west would give the observed younging in peak metamorphism of the late-stage thermal anomaly from ~2650 Ma in the EYC to ~2640-2620 Ma in the WYC. Post-kinematic low-Ca granite age determinations also young from east towards the west, indicating delamination propagated towards the west.

Because of the delay in conducting heat from the base of the crust, delamination must have occurred earlier than the 2640 Ma onset of the late-stage widespread thermal anomaly. The maximum limiting age constraint is; delamination must have occurred after regional-contact metamorphism associated with the high-Ca granite bloom in the Murchison Belt, and thus must be younger than 2668 Ma. The dense lower crust was formed by extraction of large volumes of partial melt over a long period of time, contributing melt fractions to the high-Ca granites that pooled in the upper crust. Delamination could not have occurred until after this process had been operating for a significant period of time, producing a dense eclogitic lower crust, and possibly coincides with termination of this process. Consequently, delamination must have occurred between 2668 and 2640 Ma and most probably at the older end of the range. Resulting in a ≤28 Ma conductive thermal delay for transport of heat from the asthenosphere to the upper crust. Mantle lithosphere tomography shows a very large detached slab below the Southwest Terrane (Blewett et al, 2008). Similarly the regional gravity image of the Yilgarn Craton shows a marked low gravity anomaly and gradient across the Southwest Terrane from NE to SW. The higher metamorphic grade in the Southwest Terrane suggests that a relatively thicker section lithosphere was delaminated compared to elsewhere in the Yilgarn Craton. Delamination provides an "instantaneous" transport of heat into middle crust levels by the upwelling of asthenosphere up into much shallower levels. Consequently, conduction of heat across the shorter distance into upper crustal levels, from the now relative shallow asthenosphere, was relatively fast. Relatively fast heat transport is evident when compared to other tectonic settings, such as the long thermal delay in collisional orogens due to radiogenic heat production and conduction through an over-thickened crust (England and Thompson, 1984).

The coeval timing of metamorphism in the Southwest, Narryer and Youanmi Terranes and M3a-M3b thermal anomalies in the EYC, suggest a probable geodynamic inter-relation between lithospheric extension in the EYC and delamination in the WYC. There are at least two models for the initiation or triggering of this delamination.

[1] The internal mechanism model, proposes triggering is a consequence of the lower crust reaching a critical density and gravitational threshold, after an extended period of intermediate melt extraction, resulting in delamination of the eclogitic lower-crust. This mechanism is a natural consequence of the magmatic crustal differentiation process that operated during the M2 granite bloom phase. This internal mechanism for delamination of the lower crust below the WYC may have precipitated sufficient rebound of the crust to propagate stress back into the EYC. Such a feedback is considered highly unlikely because extensional metamorphism in the EYC (M3a) occurred before metamorphic events in the WYC (M3b). Furthermore, feedback from exhumation of the WYC would not be sufficient to produce the marked runaway lithospheric extension that stretched the EYC crust, generating 15 km deep turbiditic basins, extreme crustal thinning leading to the M3a thermal anomaly and tapping of mantle melts and fluids. Consequently, lithospheric extension of the EYC, pre-dates delamination elsewhere in the Yilgarn Craton.

[2] The external mechanism model, proposes that the delamination process in the West Yilgarn may have been induced instead by a far-field effect, such as D3-M3a lithospheric extension or D4 transpression in the EYC. The preferred far-field hypothesis involves D3-M3a lithospheric extension focused predominantly into the previously closed back-arc setting of the EYC, possibly resulting from subduction hinge rollback. D3-M3a extensional stress also propagated west of the Ida Shear Zone into the hinterland, and minor extension has been documented in the Southern Cross Domain (see above). The westward propagation of the D3 extensional stress field at ~2665-2660 Ma, may have been sufficient to initiate decoupling between crust and mantle lithosphere or between middle and lower crust, giving delamination throughout the entire West Yilgarn. The timing of delamination in the West Yilgarn is constrained to the older end of the 2668-2638 Ma range, and extension of the Southern Cross Domain is very approximately constrained to within the 2675-2640 Ma range. Both constraints on extension and initiation of delamination in the West Yilgarn are entirely consistent with the well-constrained initiation of lithospheric extension in the EYC at 2665-2660 Ma.

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[12] Global Archaean Metamorphic Events Shared With Yilgarn Craton

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

Many Archaean Cratons worldwide share metamorphic events of similar age to those in the Yilgarn Craton. However, only four cratons have metamorphic events spanning the same period as the crucial stress switch from convergent M2 to extensional M3a metamorphism in the Yilgarn Craton. The stress switch to lithospheric extension was crucial to gold mineralization and speculatively interpreted to be induced by far field effects and not internal mechanisms. Consequently, other cratons showing similar bimodal thermal histories at the same times may share geodynamic histories in common and a global mineralization event. A probable scenario for global connectivity of stress switching may be the reconfiguration of crustal plate trajectories on formation of a supercontinent at ~2665 Ma. The four cratons showing strongest temporal similarity to the Yilgarn Craton are discussed below. Probable fruitful research and exploration in these cratons would be to test for mantle penetrating extensional structures govern the development of late-stage extensional basins and superimposed thermal anomalies.

The few metamorphic age determinations available from the Aldan Superterrane in the Siberian Craton span the transition from M2 to M3a and M3b periods in the Yilgarn Craton. The Aldan Superterrane was metamorphosed at 2660 Ma, coinciding with lithospheric extension and M3a metamorphism in the Yilgarn Craton. The Tungus Province was metamorphosed 2623-2649 Ma, which is similar to the M3b metamorphic and gold mineralization period at 2620-2640 Ma in the Yilgarn Craton. The Zimbabwe and Tanzania Cratons have no metamorphic age determinations, but do have bimodal Neoarchaean granite distributions. Granite peaks in the Zimbabwe Craton centre on 2705 Ma and 2645 Ma, corresponding to the high-Ca and low-Ca granite peaks in the Yilgarn Craton. Bimodal granite distributions suggest these cratons share similar thermal histories, corresponding to the M2 and M3a metamorphic events in the Yilgarn Craton. Furthermore, maximum deposition ages in the Zimbabwe Craton indicate deposition of young sedimentary sequences at <2643 and <2661 Ma. These late-stage sequences correlate in age with the D3-M3a extensional-depositional-metamorphic event and later M3b reactivation-metamorphism-mineralization events in the Yilgarn Craton. The strongly similar thermal histories and stratigraphic indications of late-stage extension, makes the Zimbabwe Craton highly probable to share similar stress switching, and the gold mineralization model may be similar to the Yilgarn Craton. The Zimbabwe and Yilgarn Cratons share some post-Archaean mafic intrusive episodes in common. The youngest Sebanga dykes are of 2408 Ma age, identical to the 2410 Ma Widgiemooltha dykes. Dolerite sills of the Umkondo LIP in the east margin of the Zimbabwe Craton is of 1105 Ma age, approximately similar to the Giles Event in the Musgrave Orogen and Warauma LIP in the Yilgarn Craton of 1075 Ma age. Consequently, both cratons may share more than temporal similarity of thermal and geodynamic histories, but may also have been contiguous and spatially connected.

Globally, the Superior Province shows the greatest similarity in both geodynamic history and timing of metamorphic events as the East Yilgarn Craton. Both regions are disproportionally endowed with gold mineralization and these similarities can only be drawn because deformation and metamorphism have been intensively investigated in both cratons. Across the whole Superior Province, three metamorphic periods are variably represented in different blocks and belts. The first two periods of 2723-2730 and 2693-2711 Ma are associated with granulites, regional metamorphism and granites, and these timings coincide with the two arc-docking events in the East Yilgarn. The third metamorphic period of 2667-2684 Ma age, is associated with widespread regional-contact metamorphism, and metamorphic ages of 2680-2684 Ma in the Hemlo Belt correlate exactly with the M2 event in the Yilgarn Craton. Metamorphism of the Hemlo Belt is of typical regional-contact type, with low pressures of 4.0-4.4 kb and high T/depth ratios ranging from 34 to 42 ºC/km towards greenstone margins. Extensional detachments are documented at the base of the greenstone sequences in the Abitibi and Hemlo Belts, with granite domes in the footwall showing similar architecture to those in the East Yilgarn Craton. Early uplift of granite domes is synchronous with sedimentary basins filled with granite detritus, and gold mineralization occurred during the latest stages of uplift and extension.

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[13] Metamorphic Evolution of Archaean Granite-Greenstone Terranes Worldwide

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

Granite-greenstone terranes make up the majority of Archaean terranes worldwide. These show remarkable metamorphic similarity worldwide, and over a very long period of time (~900 Ma) from the Palaeoarchaean to the Neoarchaean. These terranes also show commonality in volcanic stratigraphy, granitic magmatism, crustal geometries and structural style and probably share first-order geodynamic settings and histories in common. An exhaustive, but far from comprehensive, global database of Archaean metamorphism allows the following generalizations.

Metamorphism in greenstone belts shows the same narrow range of metamorphic conditions and identical metamorphic patterns worldwide. Typically metamorphic grade (T) ranges from sub-greenschist (~200 ºC) in the synformal cores of greenstone belts, to upper amphibolite facies (≤660 ºC) at margins adjacent to granite and granitic orthogneiss domes. Metamorphic patterns show concentric metamorphic isograds that parallel the granite-greenstone margins. Pressures are universally low (2.0-5.0 kb) and field gradients show only very shallow increase towards the greenstone margins. Near uniform pressure across greenstone belts indicates a systematic increase in thermal regime towards the granite-gneiss domes, with T/depth ratios characteristically high throughout (30-60 ºC/km). Increase in both temperature and thermal regime (T/depth) towards granitic margins, as well as concentric isograds parallel to granite-greenstone margins, indicate regional-contact type of metamorphism with conducted heat sourced from large volumes (~60-80 %) of granite pooled in the middle- to upper-crust. Where reported, P-T paths are typically tight recumbent clockwise paths with maximum prograde pressures in the order of ~6 kb, indicating that only minor crustal thickening accompanied contraction.

Partial convective overturn or diapiric processes have been commonly proposed as a pervasive and uniquely Archaean geodynamic process. However, there is very little empirical metamorphic evidence compatible with this model. The typical greenstone metamorphic pattern described above, with both low-pressures and little pressure variation, is incompatible with this model. Diapirs exhume high-P rocks into upper crustal levels. Consequently, the cores of putative diapiric granite-gneiss domes should record pressures significantly higher than the surrounding greenstone belts. The highest pressure reported from granite-gneiss domes is 7.5 kb in tonolitic orthogneiss from the Abitibi Belt. Globally, the pressure differential reported between granite-gneiss domes and adjacent greenstones is small and typically no more than 1-2 kb. Diapirs can also potentially exhume high-P rocks from deeply buried greenstone keels that have been accreted to their margins. In this scenario, the accreted high-P rocks would share high T/depth ratios because of proximity to the hot granitic diapir, and should be decoupled from pressure variation in the adjacent greenstone belts. Examples fitting these criteria were not found in the global metamorphic database, indicating that diapiric processes, if present, were not common in the Archaean. All documented examples showing relatively high pressures, formed at conditions of 490-640 ºC and 6-9 kb, indicating only moderate thermal regimes of 18-26 ºC/km, much lower than 30-60 ºC/km thermal regimes associated with contact metamorphism at the margin of hot granitic diapirs.

Archaean granite-greenstone terranes with pressures significantly higher than normal (i.e. >6 kb) and with relatively low T/depth ratios (i.e. <25 ºC/km) are very special cases that are only rarely reported. These fall into two groups, those with moderate T/depth ratios (Barrovian series) and those with low T/depth ratios (Subduction series). Most upper-amphibolite gneisses found at greenstone margins are limited to pressures ≤7 kb, and show high T/depth ratios contiguous with the regional-contact metamorphic field gradients. Anomalous moderate-P, moderate T/depth metamorphism on the margin of greenstone belts, indicate an advection dominated tectonic setting that is otherwise incompatible with subduction settings. Barrovian series conditions are reported from the East Yilgarn, with moderate-P (7-9 kb) and moderate T/depth (18-20 ºC/km) M1 parageneses formed during partial burial of magmatic arc margins during accretionary closure of a back arc setting. These M1 parageneses became exposed and available for sampling only because of fortuitous extensional reactivation exhumed these mid-crustal assemblages in the footwall of extensional detachments. Other examples of Barrovian series metamorphism are rare in the literature and all are from the Barberton Belt, with domains showing 6.0-7.7 kb and 21-26 ºC/km conditions.

Other special cases showing much higher pressures up to 15 kb and T/depth ratios as low as 12 ºC/km are reported from the Archaean, and interpreted to indicate subduction settings. The best example of probable subduction parageneses is the Palaeoarchaean Inyoni Shear Zone of 3227-3237 Ma age in the Barberton Belt. Metamorphic parageneses attained peak pressure conditions of 542-675 ºC, 13-15 kb and 12-13 ºC/km. These low T/depth ratios are consistent with a subduction zone setting and the pressures indicate only moderate depths of burial in the subduction system. Other probable Archaean subduction parageneses are in the Ameradik domain of the Isua Supracrustal Belt, metamorphosed to 650-700 ºC, 11-16 kb and 13-17 ºC/km, at ~3630 Ma. High-P granulite in the Grindino melange of the Belomorian Belt in the Baltic Shield, experienced conditions of 814 ºC, 16.6 kb and 14 ºC/km, at ~2720 Ma. Relict eclogite boudins from the Sutam Block in the Siberian Craton, experienced peak metamorphism at ~700 ºC, 12.5 kb and 16.0 ºC/km. UHT eclogites from the Sharyzhalgai Complex in the Siberian Craton, experienced peak metamorphic conditions of 975 ºC, 26.5 kb and 10.5 ºC/km. Preservation and exhumation of subduction zone parageneses involves a fortuitous and rare combination of geodynamic events, and these rocks are rarely exposed at any time in the geological record. Consequently, the preservation and exposure of even rare subduction parageneses of Archaean age, is strong evidence that asymmetric plate tectonics operated possibly as far back as 3630 Ma. Because of the problem of incomplete preservation of subduction parageneses, it is impossible to estimate the relative importance of subduction versus other geodynamic processes in the Archaean. Asymmetric oceanic crustal plate subduction may not have been widespread or the universal geodynamic process in the early Archaean, but the rock record shows that these processes were at minimum sporadic.

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[14] Proterozoic Metamorphic Effects on Yilgarn Craton Margins

 

Goscombe, B., Blewett, R., Groenewald, B., Foster, D., Wade, B. and Wyche, S., 2015.

Metamorphic evolution of Gondwana 1. Metamorphic evolution of the Yilgarn Craton and its orogenic margins: with comparison to Archaean metamorphism globally. Geoscience Australia Record 2015/XX, (in review).

 

The Yilgarn Craton is surrounded by Proterozoic orogenic belts; the Pinjarra Orogen in the west, Albany-Fraser Orogen in the SE, the Gascoyne Orogen in the north and Musgrave Orogen in the NE. Despite these major orogenic margins and numerous post-Archaean orogen events, the Archaean structure, metamorphic parageneses and character is remarkably well preserved in almost all parts of the craton. Superimposed on the first-order Archaean nature of the Yilgarn Craton are some reworked margins and also margins with amalgamated juvenile Proterozoic crust. The SE margin of the Yilgarn Craton is variably reworked by events in the Albany Fraser Orogen. The Munglinup Gneiss is the most intensely reworked part of this margin and was a rifted Archaean crustal block incorporated within and totally recrystallized, in the Albany Fraser Orogen. The dry Narryer Terrane was not pervasively reworked in the Proterozoic but was reactivated by retrograde shear zones. The western margin of the Yilgarn Craton contain two strongly sheared juvenile Proterozoic terranes, the Chittering Belt and West Balingup Terrane, that were previously interpreted to be reworked Archaean Crust.

The anhydrous granulites of the Narryer Terrane are cut by retrograde shear zones of all scales, from dm- to km-scales. Within the highest-grade portion of the Narryer Terrane these shear zones are up to middle-amphibolite facies grade and typically only dm- to m-scales. The Jack Hills Belt in the lower-grade north Narryer Terrane constitutes a large km-scale dextral shear zone (Occhipinti et al., 2004; Spaggiari et al., 2008). Peak metamorphic conditions in the Jack Hills Belt have a relatively large pressure range of 2.4-4.3 kb at 495-540 ºC, indicating isothermal decompression and clockwise P-T paths. Ar-Ar thermochronology indicate that the main foliation of Jack Hills Belt is of 1856-1740 Ma age, associated with the Palaeoproterozoic Capricorn Orogeny in the Gascoyne Province (Occhipinti et al., 2004; Spaggiari et al., 2008). The smaller shear zones to the south are interpreted to be of the similar age. The Palaeoproterozoic clockwise metamorphic evolutions indicate burial and partial reactivation of the northern Narryer Terrane.

The latter stages of orogenesis of the Pinjarra Orogen (<600 Ma) occurred while in contact with the Yilgarn Craton, giving rise to a wide thermal over-print that disturbed isotopic systems. Earlier stages of orogenesis (>600 Ma) in the Pinjarra Orogen may have occurred outboard of the Yilgarn Craton. Internal terranes within the Pinjarra Orogen are the Northampton Block with low-P granulites and the Leeuwin Complex in the south, with moderate-P upper amphibolite facies gneisses formed at 680-720 ºC and 6.8-7.0 kb. The Chittering Belt is dominated by psammopelite schists with very high-P and low T/depth metamorphic conditions entirely distinct from elsewhere in the Yilgarn Craton. Metamorphic monazite ages are 677-681 Ma, and no relict Archaean grains have been identified, indicating juvenile Neoproterozoic crust. P-T evolutions in the Chittering Belt involved near isothermal loading from pressures as low as 4.0-5.5 kb to maximum-P conditions of 555 ºC and 11.5 kb, followed by isothermal decompression through the peak of metamorphism. Maximum pressures indicate ~40 km depths of burial and low T/depth ratios of 14 ºC/km, both of which are compatible with subduction zone settings. The West Balingup Terrane experienced two distinct metamorphic events: early low-strain lower-P assemblages and higher-P mylonitic reworking parageneses. Peak metamorphism during reworking was 635-675 ºC and 6.0-7.0 kb in the south and 625-660 ºC and 7.0-8.2 kb in the north. P-T paths were tight clockwise loops involving burial of the early parageneses, followed by isothermal decompression. Reworking was at low T/depth conditions of 18-28 ºC/km, indicating burial in a probable collisional setting. Metamorphic monazite and zircon give a spectrum of ages ranging ~600-660 Ma, consistent with reworking, and no evidence for relict Archaean grains indicating juvenile crust.

The Albany Fraser Orogen has a large range in regional metamorphic conditions in different fault bound terranes. Overall the Albany Fraser Orogen has a very high-grade core centred on the granulite facies Fraser Complex and successively lower-grade terranes towards the north. Peak conditions in the Fraser Complex, based on phase stability relations in olivine-spinel-bearing ultramafic granulites is 7.4 kb and 830 ºC. The north margin of the orogen has high-P upper amphibolite facies conditions of 700-790 ºC and 10.0-11.4 kb in the Dalyup Gneiss, and 730-770 ºC at 6.5-9.0 kb in the Munglinup Gneiss. The contact between Dalyup Gneiss and the Yilgarn Craton is a thin schist zone, of lower amphibolite facies grade. All terranes south of the Fraser Complex are upper-amphibolite facies migmatitic gneisses with peak metamorphic conditions of 660-740 ºC at 5.0-9.0 kb in the Nornalup Gneiss and 630 ºC at 5.8 kb in the Coramup Gneiss.

The Munglinup Gneiss at the southern margin of the Yilgarn Craton is late Neoarchaean crust strongly reworked in the Mesoproterozoic. Munglinup Gneiss represents a crustal fragment rifted off the Yilgarn margin during Palaeoproterozoic basin development. This terrane and the remainder of the Albany Fraser Orogen were metamorphosed in main phase Mesoproterozoic events outboard from the craton. Re-amalgamation of the Munglinup Gneiss, and juxtaposition of other terranes with the Yilgarn Craton, occurred late in the Albany Fraser history. Re-amalgamation occurred with little loading of the cratonic margin, only weak thermal overprint of the craton in a very narrow halo, and with no significant reworking of the Yilgarn Craton. The thermal overprint is expressed by secondary muscovite growth restricted to only a 4 km wide halo along the southeast margin of the craton. No disturbance of isotopic systems have been recorded from along the southern Yilgarn margin, suggesting main phase orogenesis in the Albany Fraser Orogen occurred before juxtaposition with the Yilgarn Craton. Two Mesoproterozoic generations of granulite facies metamorphism in the Fraser Complex have no thermal expression, at any metamorphic grade, within the Yilgarn Craton. Consequently, the Albany Fraser Orogen does not represent a major collisional margin with the Yilgarn Craton. The absence of evidence for burial, metamorphism, pervasive deformational reworking and significant thermal effects on the southern margin of the Yilgarn Craton during the Proterozoic, is entirely inconsistent with the conventional collisional model for this orogen. These metamorphic, deformation and temporal constraints suggest main phase orogenesis and metamorphism in the Albany Fraser Orogen was distant from and independent of the Yilgarn Craton, and the two were brought together along a strike-slip margin, with only a very minor component of convergence leading to insignificant burial and thermal effects along most of this cratonic margin.

Palaeoproterozoic metasediments outcrop on the southern margin of the Yilgarn Craton, and are interpreted to be remnants of rift basin infill along this margin. Maximum deposition ages from detrital zircons indicate deposition <2016 Ma in the Stirling Range Group, <1790 Ma in the Mount Barren Group and deposition of the Woodline Formation at ~1650 Ma. The Mount Barren Group experienced Barrovian series metamorphism at ~1025-1120 Ma, with conditions of 575-600 ºC and 6.5-8.0 kb, equivalent to approximately 25-28 km depth. These rocks sit on the cratonic margin, yet no burial of the Yilgarn Craton occurred and no thermal or deformational overprint is evident. The Archaean Ravensthorpe Belt below the Mount Barren Group, records no petrologic or geochronologic evidence for significant Proterozoic burial or metamorphism. These observations suggest that the Mount Barren Group was a thin slice, thrust from depth out of the Albany Fraser Orogen and obducted onto the craton margin after metamorphism of the Mount Barren Group itself.

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