Crustal evolution and the temporality of anorthosites

Ashwal, Lewis D.; Bybee, G. M.

doi:10.1016/j.earscirev.2017.09.002

Cited by 112 publications

(78 citation statements)

References 191 publications

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“…This stage was followed by an extended period of emplacement, whereby magmatism in individual massifs commonly lasted up to 100 m.y. (Charlier et al, 2010;Ashwal and Bybee, 2017). Many anorthosites are spatially and temporally related to the convergent margin along the Laurentia-Baltica side of the Columbia-Rodinia supercontinent-an example of a 'Goldilocks period' in the Earth history when the combination of amalgamated continental lithosphere, high ambient mantle temperature and location of the active margin produced conditions that were 'just right'.…”

Section: The Metamorphic Record and Time's Cyclementioning

confidence: 99%

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Brown

Johnson

2019

MinMag

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Although the thermal evolution of the mantle before c. 3.0 Ga remains unclear, since c. 3.0 Ga secular cooling has dominated over heat production—this is time's arrow. By contrast, the thermal history of the crust, which is preserved in the record of metamorphism, is more complex. Heat to drive metamorphism is generated by radioactive decay and viscous dissipation, and is augmented by the influx of heat from the mantle. Notwithstanding that reliable data are sparse before the Neoarchean, we use a dataset of temperature (T), pressure (P) and thermobaric ratio (T/P at the metamorphic ‘peak’), and age of metamorphism (t, the timing of the metamorphic ‘peak’) for rocks from 564 localities ranging in age from the Cenozoic to the Eoarchean eras to interrogate the crustal record of metamorphism as a proxy for the heat budget of the crust through time. On the basis of T/P, metamorphic rocks are classified into three natural groups: high T/P type (T/P >775°C/GPa, mean T/P ~1105°C/GPa), including common and ultrahigh-temperature granulites, intermediate T/P type (T/P between 775 and 375°C/GPa, mean T/P ~575°C/GPa), including high-pressure granulites and medium- and high-temperature eclogites, and low T/P type (T/P <375°C/GPa, mean T/P ~255°C/GPa), including blueschists, low-temperature eclogites and ultrahigh-pressure metamorphic rocks. A monotonic increase in the P of intermediate T/P metamorphism from the Neoarchean to the Neoproterozoic reflects strengthening of the lithosphere during secular cooling of the mantle—this is also time's arrow. However, temporal variation in the P of intermediate T/P metamorphism and in the moving means of T and T/P of high T/P metamorphism, combined with the clustered age distribution, demonstrate the cyclicity of collisional orogenesis and cyclic variations in the heat budget of the crust superimposed on secular cooling since c. 3.0 Ga—this is time's cycle. A first cycle began with the widespread appearance/survival of intermediate T/P and high T/P metamorphism in the Neoarchean rock record coeval with amalgamation of dispersed blocks of lithosphere to form protocontinents. This cycle was terminated by the fragmentation of the protocontinents into cratons in the early Paleoproterozoic, which signalled the start of a new cycle. The second cycle continued with the progressive amalgamation of the cratons into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Neoproterozoic. This cycle represented a period of relative tectonic and environmental stability, and perhaps reduced subduction during at least part of the cycle. During most of the Proterozoic the moving means for both T and T/P of high T/P metamorphism exceeded the arithmetic means, reflecting insulation of the mantle beneath the quasi-integrated lithosphere of Columbia and, after a limited reorganisation, Rodinia. The third cycle began with the steep decline in thermobaric ratios of high T/P metamorphism to their lowest value, synchronous with the breakup of Rodinia and the formation of Pannotia, and the widespread appearance/preservation of low T/P metamorphism in the rock record. The thermobaric ratios for high T/P metamorphism rise to another peak associated with the Pan-African event, again reflecting insulation of the mantle. The subsequent steep decline in thermobaric ratios of high T/P metamorphism associated with the breakup of Pangea at c. 0.175 Ga may indicate the start of a fourth cycle. The limited occurrence of high and intermediate T/P metamorphism before the Neoarchean suggests either that suitable tectonic environments to generate these types of metamorphism were not widely available before then or that the rate of survival was low. We interpret the first cycle to record stabilisation of subduction and the emergence of a network of plate boundaries in a plate tectonics regime once the balance between heat production and heat loss changed in favour of secular cooling, possibly as early as c. 3.0 Ga in some areas. This is inferred to have been a globally linked system by the early Paleoproterozoic, but whether it remained continuous to the present is unclear. The second cycle was characterised by stability from the formation of Columbia to the breakup of Rodinia, generating higher than average T and T/P of high T/P metamorphism. The third cycle reflects colder collisional orogenesis and deep subduction of the continental crust, features that are characteristic of modern plate tectonics, which became possible once the average temperature of the asthenospheric mantle had declined to <100°C warmer than the present day after c. 1.0 Ga.

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Section: The Metamorphic Record and Time's Cyclementioning

confidence: 99%

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Brown

Johnson

2019

MinMag

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“…Massif anorthosites and related igneous rocks are focussed, particularly in terms of overall area of anorthosite, in the period 1.8-0.8 Ga, but with minor examples extend the overall range from ca. 2.6-0.5 Ga (Ashwal and Bybee, 2017). They are spatially and temporally linked to overall convergent continental plate margins, occurring in syn-subduction environments but well inboard of the inferred plate margin, or in postsubduction, collisional orogenic settings (Ashwal and Bybee, 2017;McLelland et al, 2010;Whitmeyer and Karlstrom, 2007).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

“…2.6-0.5 Ga (Ashwal and Bybee, 2017). They are spatially and temporally linked to overall convergent continental plate margins, occurring in syn-subduction environments but well inboard of the inferred plate margin, or in postsubduction, collisional orogenic settings (Ashwal and Bybee, 2017;McLelland et al, 2010;Whitmeyer and Karlstrom, 2007). Massif anorthosite formation is related to the ponding of magma of basaltic composition at depths of 30-40 km, at the base of the crust, where plagioclase rich mushes accumulate through flotation separation, and which then rise over protracted periods through the crust, were variably contaminated, prior to emplacement at mid-crustal levels (Ashwal and Bybee, 2017, and references therein).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

“…3A, blue curve) would have facilitated massif anorthosite formation. Ashwal and Bybee (2017) note that massif anorthosites pre-dating middle age and extending back to 2.6 Ga may relate to secular cooling, which resulted in increased lithospheric strength and crustal thickness (Fig. 3A, red line) suitable for the ponding and slow crystallization of basaltic magmas at the base of the thick stabilized Archean cratonic crust.…”

Section: Accepted Manuscriptmentioning

confidence: 99%

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Continental crustal volume, thickness and area, and their geodynamic implications

Cawood

Hawkesworth

2019

Gondwana Research

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Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3 Ga range from < 10 % to > 120 % of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3 Ga. By 3 Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40 % of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3 billion years. New continental crust was relatively thin and mafic from ca. 4-3 Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3 Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5 Ga to 1.8 Ga, and that it remained relatively constant through Earth's middle age (1.8-0.8 Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15 %. This

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“…Proterozoic massif-type anorthosites are large intrusions of anorthositic rocks (plagioclase > 75%, Fe-Mg silicate minerals < 25%; Ashwal 1993) commonly associated with coeval ferroan (or A type) granitoids in the so-called AMCG (anorthosite-mangerite-charnockite-granite) suites (Emslie 1978;Ashwal 1993;Heinonen 2012). Questions concerning the sources and nature of parental magmas (e.g., Frost and Frost 1997;Longhi et al 1999;Heinonen et al 2015), intrusion and emplacement mechanisms, depth(s) of crystallization and emplacement, tectonic setting (e.g., Scoates and Frost 1996;McLelland et al 2010), and temporal restrictions (Ashwal 2010;Ashwal and Bybee 2017) of anorthosites have puzzled and perplexed petrologists for decades.…”

Section: Introductionmentioning

confidence: 99%

High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites

Heinonen

Kivisaari

Michallik

2019

Contrib Mineral Petrol

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The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite-massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedralto-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4-7.6 wt% Al 2 O 3), through the rims (1.3-3.6 wt%), into the host rock (0.5-1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En~6 0-70) and rims (En~5 0-70) of the HAOMs than in the host rock (En~4 5-60) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65-1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas.

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Crustal evolution and the temporality of anorthosites

Cited by 112 publications

References 191 publications

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Continental crustal volume, thickness and area, and their geodynamic implications

High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites

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