Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

“…However, most anorthosite provinces seem to be associated with major lineaments (Corrigan and Hanmer, 1997;Scoates and Chamberlain, 1997;Duchesne et al, 1999;Ryan, 2000;Bogdanova et al, 2004), and a consensus is emerging that anorthosites were emplaced in continental arc environments in a post-collisional regime Zhang et al, 2007;Vander Auwera et al, 2011;Bybee et al, 2014).…”

Section: Crystallization Conditions Of Anorthositesmentioning

confidence: 99%

“…The major contention concerning the petrogenesis of anorthosite is whether parental magmas are produced by melting the mantle (Emslie, 1978;Ashwal, 1993;Bybee et al, 2014) or by melting a mafic lower crust Longhi et al, 1999;Longhi, 2005). Rb-Sr and Sm-Nd isotopic studies (e.g.…”

Section: Composition and Source For Parental Magmas: Mantle Vs Lowermentioning

confidence: 99%

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

Charlier

Namur

Bolle

et al. 2015

Earth-Science Reviews

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a b s t r a c t a r t i c l e i n f oMagmatic rocks containing economic concentrations of iron, titanium, vanadium and phosphorous are commonly associated with massif-type anorthosites and related rocks. This rock association is part of the anorthositemangerite-charnockite-(rapakivi-)granite suites that are restricted to the Proterozoic. Understanding the geochemistry and emplacement mechanisms of ilmenite, magnetite and apatite ore deposits is crucial for exploration, efficient mining operations and ore processing. This review discusses the controlling factors on the grade of an ore, its mineralogy, and its major and trace element distribution. We present petrogenetic models of currently mined deposits (Lac Tio, Tellnes, Damiao) and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide (Grenville, North China Craton, East European Craton, Rogaland, Laramie). Models of formation of anorthosite and related rocks are presented, as well as the nature of the possible parental magmas of the suite. A mineralogical classification of Fe-Ti ores is proposed: (1) Gabbro-noritic ilmenite ore ± apatite ± magnetite; (2) Ti-magnetite-dominated ore; (3) Nelsonite (Fe-Ti oxides + apatite); and (4) Rutile-ilmenite ore. The stability of ilmenite and magnetite is then critically reviewed and the influence of various factors, particularly oxygen fugacity and crystallization pressure, is examined. We discuss liquidus compositions of Fe-Ti oxides and the behavior of important trace elements such as Cr and V, both of which are sensitive to fO 2 variations. Postcumulus evolution of both oxides can occur due to re-equilibration with trapped liquid, re-equilibration with ferromagnesian silicates, exsolution, oxidation, reaction between ilmenite and magnetite, and metamorphic overprinting. These various processes are described and their effects on the oxide geochemistry are emphasized. Several potential ore-forming processes have been invoked and can explain the formation of huge concentration of ilmenite, ±magnetite, ±apatite. Fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals. Silicate liquid immiscibility can segregate conjugate Si-rich and Fe-rich melts, the latter being enriched in Fe-Ti-P. Magma mixing can produce hybrid magmas located in a single-phase field of the phase diagram and precipitate a pure ilmenite cumulate. Alternative processes are also described, such as ejection of Fe-Ti-enriched residual melts by filter-pressing and compaction, solidstate remobilization of ilmenite in veins, and hydrothermal transport of Fe and Ti from the host anorthosite followed by concentration in veins and lenticular ore bodies. The magnetic properties of Fe-Ti ore deposits present contrasting signatures, depending on whether the natural remanent magnetization is dominated by hemo-ilmenite or multi-domain magnetite. Micro-and macro-scale deformation features of ore rocks are intimately correlated with magma emplacement, and...

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“…The formation of the parent magmas for massif-type anorthosites is controversial: Some authors propose that they form after mantle melting [89][90][91], whereas others point to melting of the mafic lower crust [92][93][94]. According to [30], it is unlikely that a large volume of calcic plagioclase, like in the KIC, cumulated from crustal melts, which is supported by Sr and Nd isotopic compositions of the anorthosite, which suggest a mantle source.…”

Section: Genesis Of the Fe-ti(-v) Oxide Bodies In The Kicmentioning

confidence: 99%

Fe–Ti(–V) Oxide Deposits of the Kunene Anorthosite Complex (SW Angola): Mineralogy and Thermo-Oxybarometry

Villanova-de-Benavent

Torró

Castillo-Oliver

et al. 2017

Minerals

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Abstract:The Kunene Intrusive Complex (KIC), in NW Namibia and SW Angola, is one of the largest Proterozoic anorthosite massif-type exposures in the world. A geochemical, mineralogical and petrological study of four Fe-Ti(-V) oxide bodies located in the understudied Angolan part of the KIC has been performed. The massive Fe-Ti(-V) oxide bodies, locally apatite-rich (nelsonites), are lenticular or dike-like. They consist mostly of titaniferous magnetite, ilmenite and minor aluminous spinel, apatite, olivine and graphite. Titaniferous magnetite displays a wide variety of subsolvus features, including aluminous spinel-magnetite-ulvöspinel exsolutions and ilmenite (Trellis) exsolutions. This work estimated the composition of the titaniferous magnetite prior to the exsolution, in order to calculate the temperature and oxygen fugacity of the different lithologies of each ore body. The thermo-oxybarometry results obtained range from 600 • C to 820 • C and f O 2 from 10 −24.7 to 10 −14.7 . These values do not correspond to magmatic crystallization in equilibrium, but to a later re-equilibration. In addition, the mineralogical and geochemical results indicate that the studied ore bodies contain economic reserves of Ti, V, and possibly of P and REE.

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Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

Cited by 71 publications

References 72 publications

Laurentia-Baltica-Amazonia relations during Rodinia assembly

Laurentia-Baltica-Amazonia relations during Rodinia assembly

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

Fe–Ti(–V) Oxide Deposits of the Kunene Anorthosite Complex (SW Angola): Mineralogy and Thermo-Oxybarometry

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