Magnetic petrology of igneous intrusions: implications for exploration and magnetic interpretation

Clark, David

doi:10.1071/eg999005

Cited by 114 publications

(61 citation statements)

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“…[4] The remanent magnetization model is restricted to the oceanic lithosphere, not because of the lack of continental remanent magnetization [Clark, 1999], but because not enough is known of continental remanent magnetization to make a global model. The oceanic remanent model [Dyment and Arkani-Hamed, 1998] that we use is based on non-satellite magnetic input and has been subsequently calibrated using observed Magsat anomalies in the South Atlantic ocean [Purucker and Dyment, 2000].…”

Section: A New Global Magnetization Modelmentioning

confidence: 99%

“…We use magnetic susceptibility values of 0.035 SI for the continental crust and 0.04 SI for the oceanic lithosphere [Purucker et al, 1998]. The resulting maps are not strongly sensitive to the exact choice of magnetic susceptibility contrast between oceans and continents.[4] The remanent magnetization model is restricted to the oceanic lithosphere, not because of the lack of continental remanent magnetization [Clark, 1999], but because not enough is known of continental remanent magnetization to make a global model. The oceanic remanent model [Dyment and Arkani-Hamed, 1998] that we use is based on non-satellite magnetic input and has been subsequently calibrated using observed Magsat anomalies in the South Atlantic ocean [Purucker and Dyment, 2000].…”

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confidence: 99%

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The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Purucker

Langlais

Olsen

et al. 2002

Geophysical Research Letters

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[1] A global model is developed for both induced and remanent magnetizations in the terrestrial lithosphere. The model is compared with, and well-described by, Ørsted satellite observations. Interpretation of the observations over North America suggests that the large total field anomalies, associated with spherical harmonic degrees 15 -26 and centered over Kentucky and the south-central United States, are the manifestations of the magnetic edges of the southern boundaries of cratonic North America. The techniques and models developed here may be of use in defining other cratonic boundaries.INDEX TERMS: 1545 Geomagnetism and Paleomagnetism: Spatial variations (all harmonics and anomalies); 1219 Geodesy and Gravity: Local gravity anomalies and crustal structure; 7218 Seismology: Lithosphere and upper mantle; 9350 Information Related to Geographic Region: North America Background[2] The launch of the Ørsted high-precision geomagnetic field satellite [Neubert et al., 2001] has invigorated efforts to understand the magnetic field of the earth's lithosphere. Early attempts [Langel and Hinze, 1998] to model the lithospheric field relied on forward and inverse approaches over local regions. After the realization that much of the lithospheric magnetic signal might be obscured by overlap with the long-wavelength magnetic field from the core [Meyer et al., 1985], recent work has explored the potential of global forward [Cohen and Achache, 1994; Dyment and ArkaniHamed, 1998] and inverse models [Purucker et al., 1998]. Our work elaborates on but differs from previous work which 1) included remanent magnetizations associated only with the Cretaceous Quiet Zones [Cohen and Achache, 1994;Purucker et al., 1998], 2) did not consider induced magnetizations at all [Dyment and Arkani-Hamed, 1998], 3) used a model of induced magnetization that had fewer geologic and geophysical inputs [Purucker et al., 1998]. A New Global Magnetization Model[3] The global model of induced magnetization is based on an estimate of the volume of the magnetic crust and its magnetic susceptibility. We assume that induced magnetizations are restricted to the crust [Wasilewski and Mayhew, 1992] and utilize a global seismic tomography model [Nataf and Ricard, 1996] for estimating crustal thickness. The model also contains a tectonicbased subdivision of the crust into three categories, each of which has an associated geotherm. These geotherms, when coupled with an assumption about the magnetic mineral(s) responsible for the bulk of the magnetization, allow for the calculation of a depth to the Curie isotherm. We assume here that the magnetic mineral is magnetite or low-Ti magnetite. The magnetic layer thickness is calculated as the thickness of the igneous crust above the magnetite Curie isotherm. We utilize a sediment thickness model [Laske and Masters, 1997] to account for the presence of effectively non-magnetic sediment or sedimentary rock which serves to decrease the effective magnetic layer thickness. Although we calculate our induced magnetization mod...

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Section: A New Global Magnetization Modelmentioning

confidence: 99%

mentioning

confidence: 99%

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Purucker

Langlais

Olsen

et al. 2002

Geophysical Research Letters

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“…NRM directions were scattered and intensity values had an average of 0.032 A/m. These values are low for a typical gabbro (Clark, 1999). Although 24 samples from the gabbro constitute a relatively small sample set, the aeromagnetic data show that the TMI over the gabbroic rock unit is closer to background values compared to the other rock units on Leka.…”

Section: Magnetic Property Datamentioning

confidence: 81%

Geophysical expression of the Leka Ophiolite, Norway, modeled from integrated gravity, magnetic and petrophysical data

Michels¹,

McEnroe²,

Fichler³

2018

NJG

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The ca. 497 Ma Leka Ophiolite Complex (LOC) comprises oceanic lithosphere that formed near the margin of Laurentia in a suprasubductionzone setting. The LOC was obducted onto Laurentia in the Early Ordovician and later thrust onto Baltica during the Scandian continent-continent collisional orogeny (ca. 430 Ma), and now forms part of the Uppermost Allochthon. The LOC contains superb exposures of partially serpentinized mantle rocks, crustal cumulate layered series and wehrlites, sheeted dykes and pillow basalts, including exposures of the petrologic and geophysical paleo-Moho. 564 specimens were collected and measured for density, magnetic susceptibility and natural remanent magnetization. These form the constraints for three profiles forming a new 2.5D gravity and magnetic model. This is the first study of the LOC that models both gravity and magnetic data, whereas previous models were based on gravity data alone. The Bouguer-corrected anomalies express a distinct high correlating with the topographic highs in the center of the island, as well as an additional high in the southern part of the island, indicating an increasing depth of the LOC southwards. New high-resolution aeromagnetic data were used to characterize the nature of the contacts between the rock units, and the orientation of the major normal fault between the ultramafic units and the gabbro to the east. We suggest that this major fault is serpentinized, which accounts for the distinct magnetic anomaly along the trace of the fault (magnetic anomaly up to 2800 nT). Three model sections that transect the island from east to west were created. These sections indicate that the LOC has a bowl-shaped, synformal structure. Extensive serpentinization was found in samples in the uppermost portion of the LOC. The new models suggest that the deepest extent of the complex is approximately 4 km and that the volume of the LOC is approximately 200 km 3 .

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“…A área de ocorrência desses granitoides é delimitada a sul pela faixa de greenstone belts de Sapucaia e a nordeste pelas rochas má-ficas do Diopsídio-Norito Pium e por aquelas associadas à Suíte Plaquê. O termo Petrologia Magnética vem sendo utilizado para integrar estudos de propriedades magnéticas de rochas, aliados àqueles da mineralogia e petrologia clás-sicas, com o intuito de definir os processos que originam e modificam os minerais magnéticos (Wasilewski e Warner, 1988;Frost, 1991;Clark, 1999), bem como para avaliar as condições geológicas que controlam as propriedades magnéticas de uma rocha, tais como estado de oxidação do magma, alteração hidrotermal e metamorfismo. A assinatura magnética de uma rocha está diretamente relacionada à composição, ao tamanho e ao conteúdo dos óxidos.…”

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“…Ela é controlada principalmente pela distribuição dos íons de Fe entre as fases silicatos e óxidos, fortemente influenciada pelo grau de oxidação do magma. A partir da determinação da natureza e dos fatores que controlam o equilíbrio das fases minerais opacas (Buddington e Lindsley, 1964;Haggerty, 1981; Spencer e Lindsley, 1981), nas quais se inclui a magnetita, principal mineral ferromagnético, pode-se procurar definir a evolução da oxidação dos óxidos de Fe e Ti durante a cristalização magmática.O Grupo de Pesquisa Petrologia de Granitoides vem desenvolvendo, durante as últimas duas décadas, diversos trabalhos nessa linha de pesquisa, envolvendo dados de suscetibilidade magnética (SM) e de minerais opacos em rochas graníticas da Província Carajás (Magalhães e Dall 'Agnol, 1992;Dall'Agnol et al, 1997a, 1999, 2005Oliveira et al, 2002Oliveira et al, , 2010aNascimento, 2006;Almeida et al, 2007). Essas considerações são baseadas no contraste entre a SM dos minerais óxidos de Fe e Ti e contribuíram para a definição da tipologia e das condições de fugacidade de oxigênio nas quais evoluíram diversos granitoides arqueanos e proterozoicos.…”

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Petrologia magnética dos granodioritos Água Azul e Água Limpa, porção sul do Domínio Carajás - Pará

Gabriel

Oliveira

2013

Geol. USP, Sér. cient.

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Disponível on-line no endereço www.igc.usp.br/geologiausp -89 - ResumoOs granodioritos Água Azul (GrdAA) e Água Limpa (GrdAL) afloram no extremo sul do Domínio Carajás como dois corpos alongados segundo o trend regional E-W, anteriormente inseridos no Complexo Xingu. O GrdAL é formado essencialmente por biotita-anfibólio granodioritos e muscovita-biotita granodioritos, além de anfibólio-biotita tonalitos subordinados; no GrdAA, epídoto-anfibólio-biotita granodioritos são dominantes, epídoto-anfibólio-biotita tonalitos e (anfibólio)-epídoto-biotita monzogranitos, subordinados. Essas rochas mostram assinaturas geoquímicas afins dos sanukitoides arqueanos. O estudo de suscetibilidade magnética (SM) mostrou valores relativamente baixos para o GrdAL (média de 17,54 × 10 -4 SIv) e o GrdAA (média de 4,19 × 10 -4 SIv). Os estudos dos minerais opacos mostram que a magnetita e a hematita são as fases comuns e que a ilmenita está ausente nessas rochas. O GrdAL contém titanita associada à magnetita, enquanto o GrdAA contém pirita, calcopirita e goethita. No GrdAL, a magnetita é mais abundante e desenvolvida que no GrdAA, justificando, assim, sua SM mais elevada. A martitização da magnetita e a oxidação dos sulfetos, gerando goethita, ocorreram a baixas temperaturas. A correlação positiva entre os valores de SM e os conteú-dos modais de opacos, anfibólio, epídoto + allanita e quartzo + K-feldspato, assim como a correlação negativa de SM com biotita e máficos observadas nessas unidades, denunciam uma tendência no aumento de SM no sentido anfibólio tonalitos/anfibólio granodioritos à biotita granodioritos/biotita monzogranitos. Os dados geoquímicos corroboram esse comportamento, com correlação negativa entre os valores de SM e Fe 2 O 3 T, FeO e MgO, refletindo para as duas unidades uma tendência de aumento nos valores de SM paralelamente à diferenciação magmática. As afinidades geoquímicas e mineralógicas entre essas rochas e os sanukitoides do Domínio Rio Maria sugerem condições de fugacidade de oxigênio entre os tampões HM e FMQ para os granitoides estudados.Palavras-chave: Granitoides; Arqueano; Magnetita; Carajás; Petrologia magnética. AbstractThe Água Azul and Água Limpa granodiorites (AAGrd and ALGrd, respectively) outcrop in the extreme southern of the Carajás Domain as two elongated bodies following the EW regional trend and were previously included in the Xingu Complex. The ALGrd consists mainly of biotite-amphibole granodiorites and muscovite-biotite granodiorites, with subordinate amphibolebiotite tonalites; the AAGrd contains dominant epidote-amphibole-biotite granodiorites, epidote-amphibole-biotite tonalite and restricted (amphibole)-epidote-biotite monzogranites. These rocks show geochemical signatures like of archaean sanukitoids. The magnetic susceptibility (MS) values obtained in the ALGrd (average 17.54 × 10 -4 SIv) and AAGrd (average 4.19 × 10 -4 SIv) are relatively low. The main opaque minerals are magnetite and hematite, and ilmenite is lacking in these rocks. The ALGrd contains titanite associated with...

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Magnetic petrology of igneous intrusions: implications for exploration and magnetic interpretation

Cited by 114 publications

References 7 publications

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Geophysical expression of the Leka Ophiolite, Norway, modeled from integrated gravity, magnetic and petrophysical data

Petrologia magnética dos granodioritos Água Azul e Água Limpa, porção sul do Domínio Carajás - Pará

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