The Kunene complex, Angola/Namibia: a composite massif-type anorthosite complex

1994 ◽  
Vol 131 (5) ◽  
pp. 579-591 ◽  
Author(s):  
Lewis D. Ashwal ◽  
David Twist
Keyword(s):  
Block Structure ◽  
Layered Intrusion ◽  
Coarse Grained ◽  
Ultramafic Rocks ◽  
Intrusive Complex ◽  
Grenville Province ◽  
Rock Types ◽  
Ti Oxides ◽  
Field Area ◽  
Anorthosite Complex

AbstractThe c. 15000 km2, c. 2 Ga Kunene complex of southern Angola and northern Namibia is one of the world's largest occurrences of anorthositic rocks, rivalled only perhaps by the Lac St Jean massif of the Grenville Province in Quebec. We report here the results of a detailed field and laboratory study of a 100 km2 area in the northern part of the complex. Coarse grained (av. 1–2 cm) anorthosite and leucotroctolite are the predominant rock types, although colour index varies between 0 and 50, averaging about 10. Ultramafic rocks are totally absent. Typical mineralogy is: plagioclase (An57–76) + olivine(Fo64–71) + orthopyroxene(En65–74) + clinopyroxene ± Fe—Ti oxides ± apatite. Textures are dominantly massive, although weak, impersistent, magmatic lamination with nearvertical dip and unsystematic strike is also present. Other magmatic features include plagioclase and orthopyroxene megacrysts, block structure, mortar texture, and anorthositic dykes. Metamorphic effects are minimal or absent. All of these attributes are similar to those found in typical massif-type anorthosites. Since comparable features are present over large areas elsewhere in the complex, we suggest that Kunene should be considered analogous to a large, composite, massif-type anorthositic intrusive complex, rather than to a large, single or composite mafic layered intrusion such as Bushveld, as has been previously suggested or assumed. This interpretation is supported by satellite imagery, which shows the outlines of several individual anorthositic plutons, one of which clearly encompasses our field area.

The oxygen isotope composition of the surface crystalline rocks of the Canadian Shield

1978 ◽  
Vol 15 (11) ◽  
pp. 1773-1782 ◽  
Author(s):  
Yuch-Ning Shieh ◽  
Henry P. Schwarcz
Keyword(s):  
Isotope Composition ◽  
Crystalline Rocks ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Baffin Island ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Northern District ◽  
Basic Rocks

The average 18O/16O ratios of the major rock types of the surface crystalline rocks in different parts of the Canadian Precambrian Shield have been determined, using 47 composite samples prepared from 2221 individual rock specimens. The sampling areas include Baffin Island, northern and southwestern Quebec, Battle Harbour – Cartwright, northern District of Keewatin, Fort Enterprise, Snowbird Lake, Kasmere Lake, and Saskatchewan, covering approximately 1 400 000 km2. The granitic rocks from the Superior, Slave, and Churchill Provinces vary only slightly from region to region (δ18O = 6.9–8.4‰) and are significantly lower in 18O than similar rock types from the younger Grenville Province (δ = 9.2–10.0‰). The sedimentary and metasedimentary rocks have δ18O = 9.0–11.7‰ and hence are considerably lower than their Phanerozoic equivalents, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the original sediments. The basic rocks in most regions fall within a δ18O range of 6.8–7.6‰, except in northern and southwestern Quebec where the δ-values are abnormally high (8.5–8.9‰). The overall average 18O/16O ratio of the surface crystalline rocks of the Canadian Shield is estimated to be 8.0‰, which represents an enrichment with respect to probable mantle derived starting materials by about 2‰.

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

2019 ◽  
Vol 175 (1) ◽  
Author(s):  
Aku Heinonen ◽  
Heli Kivisaari ◽  
Radoslaw M. Michallik
Keyword(s):  
Host Rock ◽  
Compositional Evolution ◽  
Al Content ◽  
Rock Types ◽  
Global Comparison ◽  
Different Types ◽  
Anorthosite Complex ◽  
High Aluminum

AbstractThe 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, euhedral-to-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% Al2O3), 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~60–70) and rims (En~50–70) of the HAOMs than in the host rock (En~45–60) 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.

Liquid immiscibility and the origin of alkali-poor carbonatites

1988 ◽  
Vol 52 (364) ◽  
pp. 43-55 ◽  
Author(s):  
B. A. Kjarsgaard ◽  
D. L. Hamilton
Keyword(s):  
Liquid Immiscibility ◽  
Raw Material ◽  
Coarse Grained ◽  
Rock Types ◽  
Liquid Fractionation ◽  
Ring Complexes ◽  
The Common ◽  
Melt Cooling ◽  
Carbonate Melt ◽  
Common Sequence

AbstractThe work on liquid immiscibility in carbonate-silicate systems of Freestone and Hamilton (1980) has been extended to include alkali-poor and alkali-free compositions. Immiscibility is shown to occur on the joins albite-calcite and anorthite-calcite at 5 kbar. These results make it possible to interpret ocellar structure between calcite-rich spheroids in lamproite or kimberlite host rock as products of liquid immiscibility. The common sequence of rock types found in carbonatite complexes of melilitite-ijolite-urtite-phonolite is interpreted as being the result of both fractional crystallization and liquid fractionation, the corresponding carbonatite composition changing from nearly pure CaCO3 (±MgCO3) progressively to natrocarbonate. A carbonate melt cooling in isolation will suffer crystal fractionation, the residual liquid producing the rarer ferrocarbonatites, etc., whilst the crystal accumulate of calcite (dolomite) plus other phases such as magnetite, apatite, baryte, pyrochlore, etc., are the raw material for the coarse-grained intrusive carbonatites commonly found in ring complexes.

Mineralogy and cryptic layering of the Kunene anorthosite complex of SW Angola and Namibia

1992 ◽  
Vol 56 (384) ◽  
pp. 319-327 ◽  
Author(s):  
Zenaide C. G. Silva
Keyword(s):  
Low Temperatures ◽  
Gabbroic Rock ◽  
Rock Types ◽  
Northern Half ◽  
Western Area ◽  
Southern Half ◽  
Mineral Compositions ◽  
Anorthosite Complex

AbstractThe gabbro-anorthosite complex of SW Angola and Namibia (Kunene Complex) is dominated by anorthosite-troctolite cumulates. Other broadly gabbroic rock types are subordinate. An-rich plagioclase (max. An85) and Fo-rich olivine (max. Fo79) are common in the western area of the complex with plagioclase becoming gradually less anorthitic (min. An45) and olivine less forsteritic (min. Fo62) toward the east. This cryptic change is more pronounced in the northern half of the complex where rocks are darker, fresh, and the rhythmic layering is also more conspicuous. Within the white 'massive' anorthosite type, which is largely restricted to the southern half of the intrusion, cryptic layering is less pronounced. Textures indicate that rocks cooled very slowly and the co-existing mineral compositions indicate re-equilibration to usually low temperatures.

Metamorphic evolution of the Karimnagar granulite terrane, Eastern Dharwar Craton, south India

2010 ◽  
Vol 148 (1) ◽  
pp. 112-132 ◽  
Author(s):  
D. PRAKASH ◽  
I. N. SHARMA
Keyword(s):  
Granulite Facies ◽  
Dharwar Craton ◽  
Ultramafic Rocks ◽  
Chemical Compositions ◽  
Eastern Dharwar Craton ◽  
Geochronological Data ◽  
Rock Types ◽  
Wide Range ◽  
The Subject ◽  
Lower Crustal

AbstractThe Karimnagar granulite terrane is an integral part of the Eastern Dharwar Craton (EDC), India, having been the subject of much interest because of the only reported granulite facies rocks in the EDC. It shows a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockites (Opx+Pl+perthite+Qtz±Bt±Grt), gneisses (Opx+Crd+Bt+Pl+Qtz+perthite±Sil±Grt±Spl; Bt+Qtz+Pl±Crd±Hbl±Spl), mafic granulites (Cpx+Pl+Qtz±Opx±Hbl), quartz-free granulites (Spr+Spl+Bt+Crd+Kfs+Crn; Bt+Crd+Kfs±Crn±Spl±Krn; And+Bt+Kfs+Chl), granites (Qtz+Pl+Kfs±Bt±Hbl), altered ultramafic rocks (Chl+Trem+Tlc), metadolerites (Cpx+Pl±Bt±Qtz±Chl), banded magnetite quartzites and quartzites. Andalusite- and chlorite-bearing assemblages presumably suggest a retrograde origin. Investigation of quartz-free granulites of the area brings out some interesting and important observations, reflecting the presence of refractory phases. These granulites are devoid of sillimanite and contain corundum instead. Reaction textures in the gneisses include breakdown of garnet to form coronas and symplectites of orthopyroxene+cordierite, formation of cordierite from garnet+sillimanite+quartz and late retrograde biotite and biotite+quartz symplectites. In the mafic granulites, inclusions of quartz and hornblende within orthopyroxene are interpreted as being a part of the prograde assemblage. At a later stage orthopyroxene is also rimmed by hornblende. The quartz-free granulites display a variety of spectacular coronas, for example, successive rims on corundum consisting of spinel+sapphirine+cordierite±orthopyroxene, rare skeletal symplectitic intergrowth of sapphirine+cordierite+potash feldspar, and late retrograde formation of chlorite, corundum, spinel and andalusite from sapphirine±cordierite. Based on chemographic relationships and petrogenetic grids, a sequence of prograde, isothermal decompressive and retrograde reactions have been inferred. Quartz-free sapphirine granulites and mafic granulites record the highest P–T conditions (~7 kbar, 850°C), whereas the gneisses were formed at lower P–T conditions (~5 kbar, 800°C). In addition, the presence of andalusite-bearing rocks suggests a pressure of around 2.5 kbar. This change in pressure from 7 kbar to around 2.5 kbar suggests a decompressive path for the evolution of granulites in the study area, which indicates an uplift for the granulite-facies rocks from lower crustal conditions. The implications for supercontinent history are also addressed in light of available geochronological data.

Computerized geological mapping in the Grenville Province, Quebec

1970 ◽  
Vol 7 (6) ◽  
pp. 1357-1373 ◽  
Author(s):  
H. R. Wynne-Edwards ◽  
A. Nandi ◽  
M. M. Kehlenbeck ◽  
A. F. Laurin ◽  
K. N. M. Sharma ◽  
...  
Keyword(s):  
Processing System ◽  
Structural Data ◽  
Data Bank ◽  
Geological Mapping ◽  
Grenville Province ◽  
Geological Interpretation ◽  
Rock Types ◽  
Potential Map ◽  
Computer Based ◽  
Project Data

Since 1965, the Quebec Department of Natural Resources has conducted reconnaissance mapping in the Grenville Province, completing to date over 70 000 square miles (181 000 km2). In 1968 a computer-based data processing system was designed at Queen's University, and applied to this Grenville Project. Data are recorded in the field on input documents designed to recover a complete description of the structure and lithology of an outcrop in a standard and reproducible form. From these records a data bank, now with descriptions of over 5000 outcrops, has been constructed for the field seasons of 1968 and 1969. Lthological sorting programs based on the textural and mineralogical qualities used to define rock types have been written to provide listings of some 40 potential map-units. Outcrop maps identifying lithologies have been drawn on a computer-driven drum plotter to provide the geologist with a basic document from which to make geological interpretation. Structural data have been extracted and plotted in equal-area projections, and also plotted on maps directly by machine, using standard structural symbols for layering, foliation, and lineation. The system has upgraded the standard of data collection in the field, and provides a rapid and versatile means of handling data and of interpreting the geology. The data bank, when made publicly available, will enable users of government geological maps and reports to reinterpret the area in their own manner, or freely to add any proprietary data in the preparation of revisions.

A Grenville-age layered intrusion in the subsurface of west Texas: petrology, petrography, and possible tectonic setting

1995 ◽  
Vol 32 (12) ◽  
pp. 2159-2166 ◽  
Author(s):  
Hulusi Kargi ◽  
Calvin G. Barnes
Keyword(s):  
Tectonic Setting ◽  
Basaltic Magma ◽  
Layered Intrusion ◽  
Mafic Magma ◽  
Magma Source ◽  
Rock Types ◽  
Hornblende Gabbro ◽  
Tectonic Environment ◽  
Extensional Tectonic ◽  
General Order

The Nellie intrusion is a thick (more than 4420 m) mafic to ultramafic layered intrusion with a radiometric age of ~1163 Ma. Rock types change abruptly with stratigraphic height and include norite, pyroxenite, gabbronorite, hornblende gabbro, gabbro, anorthosite, harzburgite, and lherzolite. Norite is most abundant, but gabbro and hornblende gabbro are locally abundant. Rare olivine-rich layers are also present. The general order of crystallization was olivine, orthopyroxene, plagioclase + clinopyroxene, and hornblende. Mg#'s, expressed as 100 Mg/(Mg + Fe), range from 76.3 to 85.8 for olivine, 56.7 to 84.9 for orthopyroxene, 62.5 to 90.3 for clinopyroxene, and 52.4 to 82.8 for amphibole. Mg#'s vary with height and display abrupt reversals, which indicate open-system addition of new mafic magma. Eleven cyclic units were identified on the basis of evidence for injection of basaltic magma; these can be grouped into three megacyclic units. The abundance of orthopyroxene, and mineral compositional evidence for Fe enrichment within cyclic units, indicates that parental magmas were subalkaline and tholeiitic. Plagioclase in equilibrium with olivine ranges from An65 to An46, which precludes an arc-related magma source. Although the intrusion is approximately coeval with Keweenawan magmatism and with emplacement of diabasic dikes in western North America, it is dissimilar in detail to both suites of rocks. Nevertheless, its composition and geophysical setting are consistent with emplacement in an extensional tectonic environment.

Genetic interpretation of CSD for olivine through the dunite section of the Dovyren layered intrusion: linking with geochemistry and probable dynamics of the cumulate mush.

2020 ◽  
Author(s):  
Sergei Sobolev ◽  
Alexey Ariskin ◽  
Simone Tarquini ◽  
Ivan Pshenitsyn ◽  
Georgy Nikolaev ◽  
...  
Keyword(s):  
Parental Magma ◽  
Layered Intrusion ◽  
Coarse Grained ◽  
Main Body ◽  
Russian Science ◽  
Narrow Zone ◽  
Genetic Interpretation ◽  
Modal Layering ◽  
Lower Temperature ◽  
Log Linear

&lt;p&gt;The Yoko-Dovyren ultramafic-mafic intrusion (the northern Baikal region, Russia) has excellent outcropping as well as layering falls vertically. It`s age is 728 Ma. Length of the main body is 26 km. The modal layering of its central part (~3 km thick) includes a basal reversal (from chilled rocks to plagiolherzolites) followed with Pl-bearing to adcumulate dunite, troctolite and gabbroic sequence.&lt;/p&gt;&lt;p&gt;Over the past 20 years, several sections of the massif have been studied in detail. (Ariskin et al 2018) have determined two major types of parental magmas recorded in the FeO vs MgO trends for the Ol cumulates through the first 500 m of the cross-section. These two geochemically similar magmas are consistent with equilibrium olivine Fo88 and Fo86 in the range of temperatures from 1290&amp;#176;C to ~1200&amp;#176;C.&lt;/p&gt;&lt;p&gt;We present the results of quantification of CSD of olivine from the dunite succession, which argue for two types of olivine grain populations differing for the more primitive and relatively evolved magma.&lt;/p&gt;&lt;p&gt;The slope of the log-linear CSD function in the lower-temperature magmas has a less steep as compared to the higher temperature ones.&amp;#160; Both populations can be considered to represent intratelluric olivine crystallized at a pre-emplacement stage. At a stratigraphic level of 200 m from the lower contact, in some of the samples we observed changes in the CSD patterns, which evidence a coarsening of the populations within the Dovyren chamber. Starting from 350-400 m coarsening is noticeable everywhere, so that the CSD cease to be log-linear. In addition, in a narrow zone of 500-550 m dunite are found to display a pronounced bimodal (kinked) distribution of olivine. In a larger population, olivine has highest aspect ratio (up to 3-3.5) compared to other dunite samples. The origin of such dunite can be explained by the intrusion of hot portions of magma into the colder cumulus. In this case such elongated crystals may be due to the increased growth rate of the original olivine grains. The smaller population may be due to a new nucleation event after emplacement. CSD in cumulates above the &amp;#171;kinked dunites&amp;#187; demonstrate coarsening of olivine, with the most coarse-grained populations typical of highly contaminated dunite.&lt;/p&gt;&lt;p&gt;Thus, a rather narrow zone is distinguished in dunite, where we can observe primary intratelluric CSD, which is not substantially altered nither by peritectic reactions in the loose cumulus of the reversal sequence, where the temperature drops rapidly, nor by coarsening during long history of temperature oscillations close to the primary magmas condition above this zone.&lt;/p&gt;&lt;p&gt;This work support from the Russian Science Foundation (RSF, grant No. 16-17-10129)&lt;/p&gt;&lt;p&gt;Ariskin Alexey, Danyushevsky Leonid, Nikolaev Georgy, Kislov Evgeny, Fiorentini Marco, McNeill Andrew, Kostitsyn Yuri, Goemann Karsten, Feig Sandrin, and Malyshev Alexey. The dovyren intrusive complex (southern siberia, russia): Insights into dynamics of an open magma chamber with implications for parental magma origin, composition, and cu-ni-pge fertility.&amp;#160;Lithos, 302:242&amp;#8211;262, 2018.&lt;/p&gt;

Formation of the hour-glass structure in augite

1969 ◽  
Vol 37 (288) ◽  
pp. 472-479 ◽  
Author(s):  
D. F. Strong
Keyword(s):  
Glass Structure ◽  
Coarse Grained ◽  
Crystal Face ◽  
Thin Sections ◽  
Rapid Crystallization ◽  
Fine Grained ◽  
Rock Types ◽  
The Common

SummaryA study of augite in over three hundred thin sections of mainly alkalic rocks permits the distinction of two main types of hour-glass structure. The common ‘swallow-tailed’, sometimes skeletal augite crystals are found in the fine-grained groundmass of many rock types, and it is suggested that rapid crystallization alone accounts for their formation. Hence, this type of hour-glass structure has been called ‘quench hour-glass’. The hour-glass structures of larger augite crystals of porphyritic and coarse-grained rocks are commonly described as hour-glass ‘zoning’, as they result primarily from compositional differences between the different sectors. These were formed under conditions of relatively slower cooling than the ‘quench hour-glass’, and thus cannot be explained in the same way. They are thought to have formed by a process involving adsorption of impurities on a particular crystal face so as to impede growth of these faces, producing an initial skeleton of hour-glass form, which is completed by later crystallization of augite richer in FeO, Na2O, TiO2, and Al2O3. This hypothesis also explains the patchy zoning of other augite crystals, casting doubt on some petrogenetic interpretations of such zones as core zones.

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