Petrology and geochemistry of the fine-grained, unbrecciated diogenite Northwest Africa 4215

The mineralogy, petrology, and geochemistry of an igneous-textured clast in the Peace River L6 chondrite meteorite was examined to determine the roles of nebular processes, accretion, and parent-body metamorphism in its origin. The centimetre-scale clast is grey and fine grained and is in sharp contact with the host chondrite. Two sub-millimetre veins cut across both the clast and host, indicating that the clast formed prior to the impact (shock) event(s) that produced the numerous veins present in the Peace River meteorite. The clast and host are indistinguishable in terms of mineral compositions. In contrast, there are differences in modal mineralogy, texture, as well as trace element and oxygen isotope composition between the clast and host. These differences strongly suggest that the clast was formed by impact melting of LL-group chondritic material involving loss of Fe–FeS and phosphate components, followed by relatively rapid cooling and incorporation into the Peace River host meteorite. Subsequent metamorphism on the Peace River parent body caused recrystallization of the clast and homogenization of mineral compositions and thermally labile element abundances between the clast and host. Shock metamorphism, including formation of shock melt veins, occurred post-metamorphism, during fragmentation of the L chondrite parent body. The results suggest that the formation of the Peace River parent asteroid included the incorporation of material from other asteroids and that the pre-metamorphic protolith was a breccia. Accordingly, we propose that the Peace River meteorite be reclassified as a polymict breccia.

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Late Quaternary Climatic Variations in Northwest Africa Deduced from East Atlantic Sediment cores

Quaternary Research ◽

10.1016/0033-5894(76)90056-9 ◽

1976 ◽

Vol 6 (2) ◽

pp. 299-314 ◽

Cited By ~ 47

Author(s):

Liselotte Diester-Haass

Keyword(s):

Late Quaternary ◽

Sediment Cores ◽

Climatic Conditions ◽

Terrigenous Material ◽

East Atlantic ◽

Humid Climate ◽

Fine Grained ◽

Senegal River ◽

Northwest Africa ◽

High Desert

Characteristic variations in glacial and interglacial periods are shown by the CaCO3 contents, amounts and grain size of terrigenous material (>40 μm), quartz/mica ratios, and desert quartz numbers in East Atlantic sediment cores, collected during Meteor cruise 25 off Sahara and Senegal, 15–27°N. The following results were obtained. During Holocene an arid climate with eolian supply from the continent prevailed throughout the region (fine grained, slight terrigenous input; high CaCO3 values; high desert quartz numbers) except in the Senegal area. Here terrigenous muds indicate river supply and hence humid climate. During upper and lower Würn the climate was humid in the present day Sahara north of 20°N (low desert quartz numbers: input of large quantities of coarse terrigenous material; low CaCO3 values). South of 20°N the climate was arid, the Senegal river input disappeared during upper Würm, desert quartz numbers are as high as in Sahara dune samples, and wind strength was stronger than in the Holocene. Climate during middle Würm resembled Holocene climatic conditions. Climatic conditions during Eem (Riss) were probably similar to those during Holocene (Würm).

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Plagioclase-rich microgranular inclusions from the late-Caledonian Galway Granite, Connemara, Ireland

Mineralogical Magazine ◽

10.1180/002646100549030 ◽

2000 ◽

Vol 64 (1) ◽

pp. 113-120 ◽

Cited By ~ 2

Author(s):

N. T. Graham ◽

M. Feely ◽

B. Callaghan

Keyword(s):

Contact Zone ◽

Microprobe Analysis ◽

Major Element ◽

Mixing Zone ◽

Host Granite ◽

Fine Grained ◽

Major Element Chemistry ◽

Element Chemistry ◽

Petrology And Geochemistry ◽

Oriented Parallel

AbstractWe report on the occurrence, petrology and geochemistry of recently recognized leucocratic plagioclase-rich microgranular inclusions hosted by two granite facies in the late-Caledonian Galway Granite, Connemara, Ireland. They have been recorded at 66 localities along an ESE trending, 4 km wide corridor which incorporates the contact zone between their host granites (i.e. The Megacrystic Granite and the Mingling and Mixing Zone Granodiorite). The inclusions are discoidal in shape and oriented parallel to the general ESE trending foliation in the granites with the most elongate (6.0 × 0.6 cm) occurring in zones of strongest fabric intensity. Contacts between the inclusions and the host granite are sharp with no chilled margin visible. They display a fine-grained (<1 mm) interlocking texture with occasional crystals of plagioclase ranging up to 2 mm in length. Microprobe analysis shows that the plagioclase is essentially oligoclase (An22–32) in composition and is similar to that (i.e. An21–30) occurring in the host granites. Furthermore, the oligoclase accounts for between 61 and 78% of the mode which is reflected in the major element chemistry of the inclusions. Other minerals (in decending order of abundance) include K-feldspar, quartz, biotite and magnetite. The origin of the inclusions is unclear. However, the results of the microprobe analysis provide evidence of a link between them and their host granites.

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Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): an HED parent body impact melt

Geochimica et Cosmochimica Acta ◽

10.1016/s0016-7037(03)00092-9 ◽

2003 ◽

Vol 67 (20) ◽

pp. 3959-3970 ◽

Cited By ~ 60

Author(s):

J.A. Barrat ◽

A. Jambon ◽

M. Bohn ◽

J. Blichert-Toft ◽

V. Sautter ◽

...

Keyword(s):

Parent Body ◽

Impact Melt ◽

Northwest Africa ◽

Petrology And Geochemistry

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Petrology and geochemistry of high-Al chromitites from the MedellÍn Metaharzburgitic Unit (MMU), Colombia

Boletín de la Sociedad Geológica Mexicana ◽

10.18268/bsgm2020v72n3a120620 ◽

2020 ◽

Vol 72 (3) ◽

pp. A120620

Author(s):

Juan S. Hernández-González ◽

Lídia Butjosa ◽

Núria Pujol-Solà ◽

Thomas Aiglsperger ◽

Marion Weber ◽

...

Keyword(s):

Thermal Evolution ◽

Atomic Ratio ◽

Ore Deposits ◽

Chromian Spinel ◽

Tholeiitic Magma ◽

Fine Grained ◽

San Pedro ◽

Petrology And Geochemistry ◽

Compositional Variations ◽

Back Arc

The Medellin Metaharzburgitic Unit (MMU), emplaced onto the western continental margin of Pangea during Triassic time, is located in the Central Cordillera of Colombia and consists of metaharzburgites, minor metadunites and chromitite bodies (Patio Bonito and San Pedro ore deposits). The ultramafic rocks contain relicts of mantle-derived olivine, chromian spinel and minor orthopyroxene, and a later metamorphic mineral assemblage composed by tremolite, chlorite, talc, fine-grained recrystallized olivine, serpentine-group minerals, magnetite, and secondary chromian spinel, formed during the thermal evolution of the unit. The Cr# [Cr/(Cr+Al) atomic ratio] of the accessory primary chromian spinel in the metaperidotites ranges from 0.58 to 0.62 and overlaps those of supra-subduction peridotites from ophiolites. According to textural and compositional variations, the accessory chromian spinel in the metaperidotites can be classified into three groups: i) partially altered chromian spinel with an Al-rich core, ii) porous, Cr-Fe2+-enriched and Al-Mg-depleted chromian spinel, and iii) homogeneous Fe3+-rich chromian spinel. These variations can be related to superimposed medium-T metamorphism that reached amphibolite facies (ca. 600 ºC). Chromitite bodies associated with the metaperidotites have massive and semi-massive textures, and mainly consist of chromian spinel crystals, which show large unaltered cores surrounded by thin alteration rims of ferrian chromian spinel and chlorite. Chromitites are Al-rich (#Cr <0.6) and strongly depleted in platinum group elements (ΣPGE <41 ppb). The primary petrological and geochemical characteristics preserved in the metaperidotites and chromitites indicate that the MMU formed at shallow levels of a suboceanic lithospheric mantle related to a supra-subduction zone (back-arc basin/incipient arc scenario), and that the chromitites crystallized from a tholeiitic magma (back-arc basin basalt type).

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