A Plate Tectonic Story from the Joyner’s Find
Description: A Plate Tectonic Story from the Joyners Find Greenstone Belt in Western Australia By Des Lascelles The Joyners Find Greenstone Belt lies close to the northern margin of the Yilgarn Craton in Western Australia It is one of the smallest
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slide1. A Plate Tectonic Story from the Joyner’s Find Greenstone Belt in Western Australia By
Des Lascelles<br>
slide2. The Joyner’s Find Greenstone Belt lies close to the northern margin of the Yilgarn Craton in Western Australia
It is one of the smallest greenstone belts being only 45 km long and 10 km wide (exposed portion) and this assisted in determining its relatively simple geology The Western Australian Craton<br>
slide3. A. The Joyner’s Find geology interpreted from regional aeromagnetic data. B. Sample of the geology interpreted from field mapping (area shown as white rectangle in A). The detailed surface geological mapping of the Joyner’s Find greenstone belt produced new insights into the early Archean history of plate tectonics. Geological Mapping<br>
slide4. Stratigraphy of the Joyner’s Find greenstone belt The greenstone belt consisted of a west-facing steeply dipping approximately 5 km thick continuous sequence of mafic to ultramafic volcanic rocks with interbedded banded iron formations (BIF) overlain by 4-5 km of siliciclastic sediments with interbedded mafic volcanic rocks (Table. 1). Stratigraphy and structure<br>
slide5. The greenstone belt was interpreted as a small portion of an extensive deep sea-floor stratigraphic sequence of mafic to ultramafic lavas and hydrothermal chemical deposits (BIF) probably deposited directly on the mantle.
Neither the base nor the top of the sequence was seen due to anatexis with the remnant forming a roof pendant in the granite batholith. The presence of iron formation turbidity current deposits in the Yaloginda Formation suggests that the JFGB was initially close to the spreading centre and source of the Murrouli Basalt while the thick deposit of siliciclastic sediment at the top of the sequence indicates proximity to an older granitic craton during the closing stage. Stratigraphy and structure<br>
slide6. The sequence of rocks in the greenstone belt comprised a small portion of the west-facing limb of a very large fold system with only minor repetition of strata due to subsidiary folding in the BIF units. Contacts with the surrounding granite showed anatectic in situ replacement and the mineralogy indicated purely thermal metamorphism to greenschist facies rising to amphibolite facies and granulite facies close to the granite. Numerous N-S trending shear zones enclosed blocks of undeformed strata apart from tight isoclinal minor folds in the BIF units with no observable evidence of subsidiary folding in the mafic and ultramafic volcanic rocks Cross-section of the Joyner’s Find Greenstone Belt<br>
slide7. Replacement of the down-folded ocean crust by granite and erosion of the upper parts of the folds reduced the mass of the fold belt forcing uplift of the belt leaving a craton thickness of around 30-35 km
The numerous shear zones postdate the granite and are associated with movement on the Ida Fault - a major suture between the Youanmi Terrane and the Eastern Goldfields Terrane Present cross section of lithosphere beneath the Joyner’s Find Greenstone Belt<br>
slide8. INTERPRETATIONS
AND
CONCLUSIONS<br>
slide9. Partial melting of the hot mantle in an upwelling convection current causes production of ultramafic magma which erupts over the surface of the mantle and cools to form a crust.
Overturn of the mantle convection current drags on the solidified crust causing rifting and the formation of crustal plates.
Interaction between seawater and hot lava occurs producing hydrothermal vents precipitating mounds of iron silicates and iron hydroxides around the vents. (Estimated depth of ocean ~2 km) 4. The mounds collapse producing turbidity currents (TC) carrying the coarser particles and density currents carrying colloidal particles.
5. The turbidity currents drop their loads close to the source but density currents flow all the way across the sea floor.
6. The presence of turbidites (Granular Iron Formations) indicates deposition of the Yaloginda Formation close to the hydrothermal vents Stratigraphic interpretation<br>
slide10. 1. Granular iron formations give way to thin banded iron formations (density current deposits, DC ) in the upper Yaloginda Formation. 2. Followed by a thick sequence of mafic to ultramafic lavas with minor felsic flows in the Meekatharra Formation. 3. Thick banded iron formation (DC) units of the Wilgie Mia Formation are deposited interbedded with mafic lava flows. As the crustal plate moves away from the rift the character of sedimentary and volcanic deposition changes.<br>
slide11. At the final stage of deposition of the Joyner’s Find greenstone belt, the oceanic sediments are buried beneath the Glen Group sequence of siliclastic turbidites and gravels derived from an ancient continental craton. The mantle loses heat through the crust as it flows away from the upwelling convection current and becomes cooler and denser, finally sinking back down.<br>
slide12. It was assumed that the tight isoclinal folding in the BIF units reflected the overall style of the major fold but remarkably, given the intensity of the folding, there was no evidence of dynamic metamorphism or cleavage development. Extrapolation of the fold from the partial limb suggests that an overall thickness of the strata of 10 - 15 km was involved and constituted the whole of the oceanic crust down to the mantle, thus giving the folds a total amplitude of up to 100 km.
The folded crust would be deeply depressed into the mantle by isostatic compensation with consequent partial melting and separation of granite from a dense ultramafic restite. Structural interpretation<br>
slide13. As the plates above two converging mantle convection currents collide the ocean crust is deformed into multi-kilometre scale folds.
The converging currents of cooler and denser mantle material are deflected downward completing the convection current cycle.
The folded crust is deeply depressed into the mantle by isostatic balance Application to lithosphere evolution
The origin of Archean Cratons<br>
slide14. The cooled mantle is still hot enough to cause partial melting of the depressed folded crust but only affects the least refractory felsic components resulting in the separation of granitic magma from a dense restite.<br>
slide15. Prolonged erosion of the folded crust and separation of the dense restite allows uplift of the newly formed granite. The cooler restite and the mantle that has lost heat through the crust sinks down to the base of the mantle and terminates the mantleconvection.<br>
slide16. Erosion reduces the surface to a low lying landscape and the craton stabilises. Remnants of the folded ocean crust remain as greenstone belts on top of the craton.<br>
slide17. Archean cratons to modern continents
The change to modern subduction plate tectonics Sediment from subaerial erosion of the uplifted craton surface including mafic lava flows is deposited on the thin passive margins of the craton.
The added mass causes stress that is relieved by faulting and depression of the craton margin.<br>
slide18. The surface of the basin is depressed to the level of the ocean floor and thick sequences of ocean sediments and volcanic rocks are deposited, including banded iron formations.
(vertical scale greatly exaggerated)<br>
slide19. The marginal basin becomes a fixed part of the craton.
Continued uplift of the craton due to erosional unloading
causes the supply of ocean floor sedimentation to be cut off
and the marginal basin is filled by shallow water sediments.<br>
slide20. Continued uplift starts erosion of the marginal basin as well as the craton and a thick pile of sediments accumulate on the ocean floor forming a major sedimentary basin (geosyncline) and depressing the ocean crust.<br>
slide21. The deep sedimentary basin is compressed and then accreted on to the continent. The ocean crust is then forced beneath the buoyant sedimentary basin and continent and subducted down into the mantle thus initiating the subduction phase of plate tectonics.<br>
Des Lascelles<br>
slide2. The Joyner’s Find Greenstone Belt lies close to the northern margin of the Yilgarn Craton in Western Australia
It is one of the smallest greenstone belts being only 45 km long and 10 km wide (exposed portion) and this assisted in determining its relatively simple geology The Western Australian Craton<br>
slide3. A. The Joyner’s Find geology interpreted from regional aeromagnetic data. B. Sample of the geology interpreted from field mapping (area shown as white rectangle in A). The detailed surface geological mapping of the Joyner’s Find greenstone belt produced new insights into the early Archean history of plate tectonics. Geological Mapping<br>
slide4. Stratigraphy of the Joyner’s Find greenstone belt The greenstone belt consisted of a west-facing steeply dipping approximately 5 km thick continuous sequence of mafic to ultramafic volcanic rocks with interbedded banded iron formations (BIF) overlain by 4-5 km of siliciclastic sediments with interbedded mafic volcanic rocks (Table. 1). Stratigraphy and structure<br>
slide5. The greenstone belt was interpreted as a small portion of an extensive deep sea-floor stratigraphic sequence of mafic to ultramafic lavas and hydrothermal chemical deposits (BIF) probably deposited directly on the mantle.
Neither the base nor the top of the sequence was seen due to anatexis with the remnant forming a roof pendant in the granite batholith. The presence of iron formation turbidity current deposits in the Yaloginda Formation suggests that the JFGB was initially close to the spreading centre and source of the Murrouli Basalt while the thick deposit of siliciclastic sediment at the top of the sequence indicates proximity to an older granitic craton during the closing stage. Stratigraphy and structure<br>
slide6. The sequence of rocks in the greenstone belt comprised a small portion of the west-facing limb of a very large fold system with only minor repetition of strata due to subsidiary folding in the BIF units. Contacts with the surrounding granite showed anatectic in situ replacement and the mineralogy indicated purely thermal metamorphism to greenschist facies rising to amphibolite facies and granulite facies close to the granite. Numerous N-S trending shear zones enclosed blocks of undeformed strata apart from tight isoclinal minor folds in the BIF units with no observable evidence of subsidiary folding in the mafic and ultramafic volcanic rocks Cross-section of the Joyner’s Find Greenstone Belt<br>
slide7. Replacement of the down-folded ocean crust by granite and erosion of the upper parts of the folds reduced the mass of the fold belt forcing uplift of the belt leaving a craton thickness of around 30-35 km
The numerous shear zones postdate the granite and are associated with movement on the Ida Fault - a major suture between the Youanmi Terrane and the Eastern Goldfields Terrane Present cross section of lithosphere beneath the Joyner’s Find Greenstone Belt<br>
slide8. INTERPRETATIONS
AND
CONCLUSIONS<br>
slide9. Partial melting of the hot mantle in an upwelling convection current causes production of ultramafic magma which erupts over the surface of the mantle and cools to form a crust.
Overturn of the mantle convection current drags on the solidified crust causing rifting and the formation of crustal plates.
Interaction between seawater and hot lava occurs producing hydrothermal vents precipitating mounds of iron silicates and iron hydroxides around the vents. (Estimated depth of ocean ~2 km) 4. The mounds collapse producing turbidity currents (TC) carrying the coarser particles and density currents carrying colloidal particles.
5. The turbidity currents drop their loads close to the source but density currents flow all the way across the sea floor.
6. The presence of turbidites (Granular Iron Formations) indicates deposition of the Yaloginda Formation close to the hydrothermal vents Stratigraphic interpretation<br>
slide10. 1. Granular iron formations give way to thin banded iron formations (density current deposits, DC ) in the upper Yaloginda Formation. 2. Followed by a thick sequence of mafic to ultramafic lavas with minor felsic flows in the Meekatharra Formation. 3. Thick banded iron formation (DC) units of the Wilgie Mia Formation are deposited interbedded with mafic lava flows. As the crustal plate moves away from the rift the character of sedimentary and volcanic deposition changes.<br>
slide11. At the final stage of deposition of the Joyner’s Find greenstone belt, the oceanic sediments are buried beneath the Glen Group sequence of siliclastic turbidites and gravels derived from an ancient continental craton. The mantle loses heat through the crust as it flows away from the upwelling convection current and becomes cooler and denser, finally sinking back down.<br>
slide12. It was assumed that the tight isoclinal folding in the BIF units reflected the overall style of the major fold but remarkably, given the intensity of the folding, there was no evidence of dynamic metamorphism or cleavage development. Extrapolation of the fold from the partial limb suggests that an overall thickness of the strata of 10 - 15 km was involved and constituted the whole of the oceanic crust down to the mantle, thus giving the folds a total amplitude of up to 100 km.
The folded crust would be deeply depressed into the mantle by isostatic compensation with consequent partial melting and separation of granite from a dense ultramafic restite. Structural interpretation<br>
slide13. As the plates above two converging mantle convection currents collide the ocean crust is deformed into multi-kilometre scale folds.
The converging currents of cooler and denser mantle material are deflected downward completing the convection current cycle.
The folded crust is deeply depressed into the mantle by isostatic balance Application to lithosphere evolution
The origin of Archean Cratons<br>
slide14. The cooled mantle is still hot enough to cause partial melting of the depressed folded crust but only affects the least refractory felsic components resulting in the separation of granitic magma from a dense restite.<br>
slide15. Prolonged erosion of the folded crust and separation of the dense restite allows uplift of the newly formed granite. The cooler restite and the mantle that has lost heat through the crust sinks down to the base of the mantle and terminates the mantleconvection.<br>
slide16. Erosion reduces the surface to a low lying landscape and the craton stabilises. Remnants of the folded ocean crust remain as greenstone belts on top of the craton.<br>
slide17. Archean cratons to modern continents
The change to modern subduction plate tectonics Sediment from subaerial erosion of the uplifted craton surface including mafic lava flows is deposited on the thin passive margins of the craton.
The added mass causes stress that is relieved by faulting and depression of the craton margin.<br>
slide18. The surface of the basin is depressed to the level of the ocean floor and thick sequences of ocean sediments and volcanic rocks are deposited, including banded iron formations.
(vertical scale greatly exaggerated)<br>
slide19. The marginal basin becomes a fixed part of the craton.
Continued uplift of the craton due to erosional unloading
causes the supply of ocean floor sedimentation to be cut off
and the marginal basin is filled by shallow water sediments.<br>
slide20. Continued uplift starts erosion of the marginal basin as well as the craton and a thick pile of sediments accumulate on the ocean floor forming a major sedimentary basin (geosyncline) and depressing the ocean crust.<br>
slide21. The deep sedimentary basin is compressed and then accreted on to the continent. The ocean crust is then forced beneath the buoyant sedimentary basin and continent and subducted down into the mantle thus initiating the subduction phase of plate tectonics.<br>