Charnockites in southern India range from low (5–6 kb) to high-pressure (7.5–8 kb) types. Interme... more Charnockites in southern India range from low (5–6 kb) to high-pressure (7.5–8 kb) types. Intermediate chemical compositions are infrequent, although a continuous series exists from mafic to tonalite-trondhjemite end members. With the exception of K2O, major element distributions reflect the original igneous parentage of the rocks. Rb, Cs, Th, U, and Pb are depleted in charnockites although the degree of depletion does not correlate well with inferred P-T conditions. These depletions appear to have developed during fluid-phase metamorphism. The lack of K depletion and of extreme Rb depletion may reflect the presence of a fluid phase with relatively high CO2/H2O ratios. Other trace elements including REE do not appear to have been redistributed by the fluid and reflect igneous fractionation trends of the protoliths. Although the distribution of some trace elements shows a geographic coherence, trace element distributions do not generally correlate with metamorphic grade.
... crustal evolution. In 1981, two important volumes were published in this same Elsevier series... more ... crustal evolution. In 1981, two important volumes were published in this same Elsevier series: Precambrian Plate Tectonics edited by Alfred Kroner (Kroner, 1981) and Archean Greenstone Belts by Kent Condie (Condie, 1981). In ...
... 50 Hovd 150 S Zimbabwe 2830 2668 162 Hug 750 480 270 Zimb1 2810 2750 60 Idermeg 700 370 330 Z... more ... 50 Hovd 150 S Zimbabwe 2830 2668 162 Hug 750 480 270 Zimb1 2810 2750 60 Idermeg 700 370 330 Zimb2 2823 2702 121 Lake 570 420 150 Zimb 3 ... The Oki and Southern Kita-kami belts in western Japan were originally part of the Yangtze craton and were rifted away ca. ...
Four first-order (Hadean, Archean, Proterozoic and Phanerozoic eon) and nine second-order (Paleoa... more Four first-order (Hadean, Archean, Proterozoic and Phanerozoic eon) and nine second-order (Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic era) units continue to provide intuitive subdivision of geological time. Major transitions in Earth’s tectonic, biological and environmental history occurred at approximately 2.5-2.3, 1.8-1.6, 1.0-0.8 and 0.7-0.5 Ga, and so future rock-based subdivision of pre-Cryogenian time, eventually by use of global stratotypes (GSSPs), will likely require only modest deviation from current chronometric boundaries (GSSAs) at 2.5, 1.6 and 1.0 Ga, respectively. Here we argue that removal of GSSAs could be expedited by establishing event-based concepts and provisional, approximate ages for eon-, era- and period-level subdivisions as soon as practicable, in line with ratification of an Ediacaran GSSP in 2004 and chronostratigraphic definition of the Cryogenian Period at c. 720 Ma in 2012....
Abstract In this study, seven isotopic databases are presented and analyzed to identify mantle an... more Abstract In this study, seven isotopic databases are presented and analyzed to identify mantle and crustal episodes on a global scale by focusing on periodicity ranging from 70 to 200 million years (Myr). The databases are the largest, or among the largest, compiled for each type of data – with an objective of finding some samples from every region of every continent, to make each database as global as conceivably possible. The databases contain zircon Lu/Hf isotopic data, whole-rock Sm/Nd isotopic data, U/Pb detrital zircon ages, U/Pb igneous zircon ages, U/Pb non-zircon ages, whole-rock Re/Os isotopic data, and large igneous province ages. Part I of this study focuses on the periodicities of age histograms and geochemical averages developed from the seven databases, via spectral and cross-correlation analyses. Natural physical cycles often propagate in exact integer multiples of a fundamental cycle, referred to as harmonics. The tests show that harmonic geological cycles of ∼93.5 and ∼187 Myr have persisted throughout terrestrial history, and the cyclicities are statistically significant for U/Pb igneous zircon ages, U/Pb detrital zircon ages, U/Pb zircon-rim ages, large igneous province ages, mean eHf(t) for all samples, mean eHf(t) values for igneous-only samples, and relative abundance of mafic rocks. Equally important, cross-correlation analyses show these seven time-series are nearly synchronous (±7 Myr) with a model consisting of periodicities of 93.5 and 187 Myr. Additionally, the similarities between peaks in the 93.5 and 187 Myr mantle cycles and terminal ages of established and suspected superchrons provide a framework for predicting and testing superchron periodicity.
Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more ... more Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more important of these are the following: (1) increasing proportion of basalts with "arc-like" mantle sources; (2) an increasing abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources; (3) onset of a Great Thermal Divergence in the mantle; (4) a decrease in degree of melting of the mantle; (5) beginning of large lateral plate motions; (6) appearance of eclogite inclusions in diamonds; (7) appearance and rapid increase in frequency of collisional orogens; (8) rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9) appearance of ophiolites in the geologic record, and (10) appearance of global LIP (large igneous province) events some of which correlate with global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere, and they may record the gradual onset and propagation of plate tectonics around the planet. To further understand the changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.
Journal of African Earth Sciences (and the Middle East), 1989
The Nsuze Group (-3000 Ma) in South Africa is comprised of volcanics and sediments that rest unco... more The Nsuze Group (-3000 Ma) in South Africa is comprised of volcanics and sediments that rest unconformably on older continental crust. Nsuze volcanics, which range in composition from basalt to rhyolite and include a significant proportion of andes~tes, have tholelitic to calcalkaline geochemical affinities. Incompatible element distributions in Nsuze volcanics are similar to those in volcanics associated with continental-margin arcs and extub~t a typical subduction zone component (SZC) (enrichment in LILE and relative to Nb-Ta). The volcanics can be related to each other by shallow fractional crystallization of average Nsuze basalt, together with up to 10% assimilation of granite with a composition similar to that of the underlying Mpuluzi granite. Geological data are consistent with an origin for the Nsuze Group in an intracratonic rift. The SZC in Nsuze volcanics may have been inherited from an older subcontmental lithospheric source.
One of the major obstacles to our understanding of the growth of continental crust is that of est... more One of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise <10%; the remainder comprises older, reworked crust. Most of the juvenile crust in orogens is found in continental arc assemblages. Our studies indicate that most juvenile crust preserved in orogens was produced during the ocean-basin closing stage and not during the collision. However, the duration of ocean-basin closing is not a major control on the fraction of juvenile crust preserved in orogens; regardless of the duration of subduction, the fract...
Evidence for depleted and enriched mantle do not become widespread until after 2.2 Ga Not all gre... more Evidence for depleted and enriched mantle do not become widespread until after 2.2 Ga Not all greenstones are ophiolites Modern tectonic settings cannot be easily identified in rocks older than 2.2 Ga Plate tectonics did not become firmly established on Earth until 2.2 Ga
The full-text may be used and/or reproduced, and given to third parties in any format or medium, ... more The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.
Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and ... more Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and magma evolution. These element ratios clearly indicate at least three sources must be available for some swarms and that few swarms can be accounted for by only one source. Some dyke magmas appear to reflect mixing of mantle plume sources with Ta-depleted Archean subcontinental lithosphere. Systematic secular compositional changes in Th/Ta or La/Yb ratios are not observed in dyke swarms ranging in age from 2.45 to 1.14 Ga from the southern Superior Province, and in the giant Mackenzie swarm (1.27 Ga) there is no change in either element ratio as a function of distance from an inferred plume source. Paleoproterozoic dyke swarms range from those dykes with low Th/Ta and La/Yb ratios, indicating a strong contribution of depleted or primitive mantle, probably in a plume source, to picrite/norite dykes with very high ratios, perhaps indicative of a source in the Archean subcontinental lithosphere. There is an overall shift in composition of dykes from high Th/Ta ratios in the Paleoproterozoic to low ratios in the Neoproterozoic, reflecting a decrease in importance of Archean subcontinental lithospheric sources and an increase in importance of plumes containing enriched mantle components (recycled sediments and oceanic lithosphere). The delayed appearance of enriched mantle components in dyke sources until the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere through the lower mantle, beginning in the Late Archean.
This chapter brings together eight of the great events in Earth history, many of which significan... more This chapter brings together eight of the great events in Earth history, many of which significantly influenced the evolution of the planet and of life. The first event is lunar origin. It includes a discussion of various models for the origin of the Moon with emphasis on the giant impactor model, as well as a summary of the Moon's thermal history. The origin of life is the second event, which includes discussion of the role of impacts, the RNA world, evidences for life, origin of photosynthesis, the first fossils, and extraterrestrial life. Event three is the onset of plate tectonics including discussion of plate tectonic indicators, global changes at the end of the Archean, and how and when did plate tectonics begin. The Great Oxidation Event at 2.35 Ga is event four and includes a discussion of geologic indicators of oxygen and mass-independent fractionation of sulfur isotopes. Event five is the Snowball Earth in the Neoproterozoic and event six the mass extinction at the end of the Permian. Events seven and eight are the Cretaceous superplume event and the K/T mass extinction.
... in Western Australia is the best documented example of a pre-2.9-Ga province with dome andkee... more ... in Western Australia is the best documented example of a pre-2.9-Ga province with dome andkeel structural architecture. ... increased anoxia related to CH4 input; (4) a small peak in deposition of banded iron formation at 2.7 ... 6. Thick lithosphere underlies many Archean cratons. ...
Nd isotopic data indicate that most Archean igneous rocks including compositions ranging from kom... more Nd isotopic data indicate that most Archean igneous rocks including compositions ranging from komatiite to tonalite are derived from undepleted or depleted upper mantle sources. If sampling is representative, only a few require enriched sources. A major unresolved question is the fate of the material removed from the upper mantle leaving early depleted sources as residue. One possibility is that widespread depletion of the early mantle resulted from a period of early degassing and magmatism. Rare gas isotopic data, in particular 129Xe/130Xe ratios, seem to demand that the upper mantle was extensively degassed at or before 4.4 b.y. and this led to rapid growth of the atmosphere and oceans. The lower mantle, however, was not significantly degassed during this event. It is likely that such widespread degassing and magmatism of the upper mantle transferred significant quantities of incompatible elements into the uppermost mantle or crust. Once formed, such an enriched fraction should resist recycling into the mantle and collect at or near the Earth's surface. One possibility is that it collects chiefly in a zone of partial melting, analogous to the present low-velocity zone at the base of the lithosphere.
Charnockites in southern India range from low (5–6 kb) to high-pressure (7.5–8 kb) types. Interme... more Charnockites in southern India range from low (5–6 kb) to high-pressure (7.5–8 kb) types. Intermediate chemical compositions are infrequent, although a continuous series exists from mafic to tonalite-trondhjemite end members. With the exception of K2O, major element distributions reflect the original igneous parentage of the rocks. Rb, Cs, Th, U, and Pb are depleted in charnockites although the degree of depletion does not correlate well with inferred P-T conditions. These depletions appear to have developed during fluid-phase metamorphism. The lack of K depletion and of extreme Rb depletion may reflect the presence of a fluid phase with relatively high CO2/H2O ratios. Other trace elements including REE do not appear to have been redistributed by the fluid and reflect igneous fractionation trends of the protoliths. Although the distribution of some trace elements shows a geographic coherence, trace element distributions do not generally correlate with metamorphic grade.
... crustal evolution. In 1981, two important volumes were published in this same Elsevier series... more ... crustal evolution. In 1981, two important volumes were published in this same Elsevier series: Precambrian Plate Tectonics edited by Alfred Kroner (Kroner, 1981) and Archean Greenstone Belts by Kent Condie (Condie, 1981). In ...
... 50 Hovd 150 S Zimbabwe 2830 2668 162 Hug 750 480 270 Zimb1 2810 2750 60 Idermeg 700 370 330 Z... more ... 50 Hovd 150 S Zimbabwe 2830 2668 162 Hug 750 480 270 Zimb1 2810 2750 60 Idermeg 700 370 330 Zimb2 2823 2702 121 Lake 570 420 150 Zimb 3 ... The Oki and Southern Kita-kami belts in western Japan were originally part of the Yangtze craton and were rifted away ca. ...
Four first-order (Hadean, Archean, Proterozoic and Phanerozoic eon) and nine second-order (Paleoa... more Four first-order (Hadean, Archean, Proterozoic and Phanerozoic eon) and nine second-order (Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic era) units continue to provide intuitive subdivision of geological time. Major transitions in Earth’s tectonic, biological and environmental history occurred at approximately 2.5-2.3, 1.8-1.6, 1.0-0.8 and 0.7-0.5 Ga, and so future rock-based subdivision of pre-Cryogenian time, eventually by use of global stratotypes (GSSPs), will likely require only modest deviation from current chronometric boundaries (GSSAs) at 2.5, 1.6 and 1.0 Ga, respectively. Here we argue that removal of GSSAs could be expedited by establishing event-based concepts and provisional, approximate ages for eon-, era- and period-level subdivisions as soon as practicable, in line with ratification of an Ediacaran GSSP in 2004 and chronostratigraphic definition of the Cryogenian Period at c. 720 Ma in 2012....
Abstract In this study, seven isotopic databases are presented and analyzed to identify mantle an... more Abstract In this study, seven isotopic databases are presented and analyzed to identify mantle and crustal episodes on a global scale by focusing on periodicity ranging from 70 to 200 million years (Myr). The databases are the largest, or among the largest, compiled for each type of data – with an objective of finding some samples from every region of every continent, to make each database as global as conceivably possible. The databases contain zircon Lu/Hf isotopic data, whole-rock Sm/Nd isotopic data, U/Pb detrital zircon ages, U/Pb igneous zircon ages, U/Pb non-zircon ages, whole-rock Re/Os isotopic data, and large igneous province ages. Part I of this study focuses on the periodicities of age histograms and geochemical averages developed from the seven databases, via spectral and cross-correlation analyses. Natural physical cycles often propagate in exact integer multiples of a fundamental cycle, referred to as harmonics. The tests show that harmonic geological cycles of ∼93.5 and ∼187 Myr have persisted throughout terrestrial history, and the cyclicities are statistically significant for U/Pb igneous zircon ages, U/Pb detrital zircon ages, U/Pb zircon-rim ages, large igneous province ages, mean eHf(t) for all samples, mean eHf(t) values for igneous-only samples, and relative abundance of mafic rocks. Equally important, cross-correlation analyses show these seven time-series are nearly synchronous (±7 Myr) with a model consisting of periodicities of 93.5 and 187 Myr. Additionally, the similarities between peaks in the 93.5 and 187 Myr mantle cycles and terminal ages of established and suspected superchrons provide a framework for predicting and testing superchron periodicity.
Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more ... more Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more important of these are the following: (1) increasing proportion of basalts with "arc-like" mantle sources; (2) an increasing abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources; (3) onset of a Great Thermal Divergence in the mantle; (4) a decrease in degree of melting of the mantle; (5) beginning of large lateral plate motions; (6) appearance of eclogite inclusions in diamonds; (7) appearance and rapid increase in frequency of collisional orogens; (8) rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9) appearance of ophiolites in the geologic record, and (10) appearance of global LIP (large igneous province) events some of which correlate with global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere, and they may record the gradual onset and propagation of plate tectonics around the planet. To further understand the changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.
Journal of African Earth Sciences (and the Middle East), 1989
The Nsuze Group (-3000 Ma) in South Africa is comprised of volcanics and sediments that rest unco... more The Nsuze Group (-3000 Ma) in South Africa is comprised of volcanics and sediments that rest unconformably on older continental crust. Nsuze volcanics, which range in composition from basalt to rhyolite and include a significant proportion of andes~tes, have tholelitic to calcalkaline geochemical affinities. Incompatible element distributions in Nsuze volcanics are similar to those in volcanics associated with continental-margin arcs and extub~t a typical subduction zone component (SZC) (enrichment in LILE and relative to Nb-Ta). The volcanics can be related to each other by shallow fractional crystallization of average Nsuze basalt, together with up to 10% assimilation of granite with a composition similar to that of the underlying Mpuluzi granite. Geological data are consistent with an origin for the Nsuze Group in an intracratonic rift. The SZC in Nsuze volcanics may have been inherited from an older subcontmental lithospheric source.
One of the major obstacles to our understanding of the growth of continental crust is that of est... more One of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise <10%; the remainder comprises older, reworked crust. Most of the juvenile crust in orogens is found in continental arc assemblages. Our studies indicate that most juvenile crust preserved in orogens was produced during the ocean-basin closing stage and not during the collision. However, the duration of ocean-basin closing is not a major control on the fraction of juvenile crust preserved in orogens; regardless of the duration of subduction, the fract...
Evidence for depleted and enriched mantle do not become widespread until after 2.2 Ga Not all gre... more Evidence for depleted and enriched mantle do not become widespread until after 2.2 Ga Not all greenstones are ophiolites Modern tectonic settings cannot be easily identified in rocks older than 2.2 Ga Plate tectonics did not become firmly established on Earth until 2.2 Ga
The full-text may be used and/or reproduced, and given to third parties in any format or medium, ... more The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.
Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and ... more Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and magma evolution. These element ratios clearly indicate at least three sources must be available for some swarms and that few swarms can be accounted for by only one source. Some dyke magmas appear to reflect mixing of mantle plume sources with Ta-depleted Archean subcontinental lithosphere. Systematic secular compositional changes in Th/Ta or La/Yb ratios are not observed in dyke swarms ranging in age from 2.45 to 1.14 Ga from the southern Superior Province, and in the giant Mackenzie swarm (1.27 Ga) there is no change in either element ratio as a function of distance from an inferred plume source. Paleoproterozoic dyke swarms range from those dykes with low Th/Ta and La/Yb ratios, indicating a strong contribution of depleted or primitive mantle, probably in a plume source, to picrite/norite dykes with very high ratios, perhaps indicative of a source in the Archean subcontinental lithosphere. There is an overall shift in composition of dykes from high Th/Ta ratios in the Paleoproterozoic to low ratios in the Neoproterozoic, reflecting a decrease in importance of Archean subcontinental lithospheric sources and an increase in importance of plumes containing enriched mantle components (recycled sediments and oceanic lithosphere). The delayed appearance of enriched mantle components in dyke sources until the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere through the lower mantle, beginning in the Late Archean.
This chapter brings together eight of the great events in Earth history, many of which significan... more This chapter brings together eight of the great events in Earth history, many of which significantly influenced the evolution of the planet and of life. The first event is lunar origin. It includes a discussion of various models for the origin of the Moon with emphasis on the giant impactor model, as well as a summary of the Moon's thermal history. The origin of life is the second event, which includes discussion of the role of impacts, the RNA world, evidences for life, origin of photosynthesis, the first fossils, and extraterrestrial life. Event three is the onset of plate tectonics including discussion of plate tectonic indicators, global changes at the end of the Archean, and how and when did plate tectonics begin. The Great Oxidation Event at 2.35 Ga is event four and includes a discussion of geologic indicators of oxygen and mass-independent fractionation of sulfur isotopes. Event five is the Snowball Earth in the Neoproterozoic and event six the mass extinction at the end of the Permian. Events seven and eight are the Cretaceous superplume event and the K/T mass extinction.
... in Western Australia is the best documented example of a pre-2.9-Ga province with dome andkee... more ... in Western Australia is the best documented example of a pre-2.9-Ga province with dome andkeel structural architecture. ... increased anoxia related to CH4 input; (4) a small peak in deposition of banded iron formation at 2.7 ... 6. Thick lithosphere underlies many Archean cratons. ...
Nd isotopic data indicate that most Archean igneous rocks including compositions ranging from kom... more Nd isotopic data indicate that most Archean igneous rocks including compositions ranging from komatiite to tonalite are derived from undepleted or depleted upper mantle sources. If sampling is representative, only a few require enriched sources. A major unresolved question is the fate of the material removed from the upper mantle leaving early depleted sources as residue. One possibility is that widespread depletion of the early mantle resulted from a period of early degassing and magmatism. Rare gas isotopic data, in particular 129Xe/130Xe ratios, seem to demand that the upper mantle was extensively degassed at or before 4.4 b.y. and this led to rapid growth of the atmosphere and oceans. The lower mantle, however, was not significantly degassed during this event. It is likely that such widespread degassing and magmatism of the upper mantle transferred significant quantities of incompatible elements into the uppermost mantle or crust. Once formed, such an enriched fraction should resist recycling into the mantle and collect at or near the Earth's surface. One possibility is that it collects chiefly in a zone of partial melting, analogous to the present low-velocity zone at the base of the lithosphere.
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